A Project Report On Study OfElectrical Power Systems, Its Protection at

Tarapur Atomic Power Station 3 and 4and

Online Purification of Tranformer Oil

Aayush ChaudharyB.Tech (Electrical Engineering)

Indian Institute of Technology, Bombay

June 13, 2014

Contents

1 Acknowledgement 6

2 Certificate 7

3 Introduction to TAPS-3&4 and India’s nuclear Programme 83. 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83. 2 How a Nuclear Reactor Works? . . . . . . . . . . . . . . . . . 83. 3 Types of Reactors . . . . . . . . . . . . . . . . . . . . . . . . . 93. 4 Importance of Atomic Power Station . . . . . . . . . . . . . . 103. 5 General Layout of TAPS 3&4 . . . . . . . . . . . . . . . . . . 113. 6 Unique features of the Plant . . . . . . . . . . . . . . . . . . 113. 7 India’s Nuclear Program . . . . . . . . . . . . . . . . . . . . . 11

3. 7.1 Nuclear Fuel Cycle . . . . . . . . . . . . . . . . . . . . 12

4 Electrical System 134. 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134. 2 Objectives of Electrical Systems . . . . . . . . . . . . . . . . . 134. 3 Clasification of Electrical System . . . . . . . . . . . . . . . . 14

4. 3.1 Class IV power Supply . . . . . . . . . . . . . . . . . . 144. 3.2 Class III power Supply . . . . . . . . . . . . . . . . . . 144. 3.3 Class II power Supply . . . . . . . . . . . . . . . . . . 154. 3.4 Class I power Supply . . . . . . . . . . . . . . . . . . . 15

4. 4 Description of Station Auxiliary Power Supply System . . . . 154. 4.1 Class IV Power Supply System . . . . . . . . . . . . . 154. 4.2 6.6 kV System . . . . . . . . . . . . . . . . . . . . . . . 164. 4.3 415 V System . . . . . . . . . . . . . . . . . . . . . . . 164. 4.4 Class III Power Supply Sytem . . . . . . . . . . . . . . 164. 4.5 Class II Power Supply Sytem . . . . . . . . . . . . . . 164. 4.6 Class I Power Supply System . . . . . . . . . . . . . . 17

4. 5 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . 184. 6 Auto-Transfer Scheme . . . . . . . . . . . . . . . . . . . . . . 18

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4. 7 Emergency Transfer Scheme . . . . . . . . . . . . . . . . . . . 204. 7.1 6.6 KV Class-III, 415 Class-III Bus Supply And Feeder

Restoration . . . . . . . . . . . . . . . . . . . . . . . . 214. 7.2 EMTR Initiation For 415 V Class-III Buses . . . . . . 214. 7.3 EMTR Initiation for 415 V Class-II Buses . . . . . . . 214. 7.4 Emergency Transfer Panels . . . . . . . . . . . . . . . 22

5 Electrical Protection Relays 235. 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235. 2 Shunt Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235. 3 Purpose of Electrical Protection . . . . . . . . . . . . . . . . . 245. 4 Basic Relay Terminology . . . . . . . . . . . . . . . . . . . . . 245. 5 Types of Relays . . . . . . . . . . . . . . . . . . . . . . . . . . 255. 6 Essential Features of Protective Relay . . . . . . . . . . . . . . 255. 7 Description of M.S. English Electric Relays . . . . . . . . . . . 275. 8 Working of Trip Circuit . . . . . . . . . . . . . . . . . . . . . 285. 9 MICOM P220 . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5. 9.1 Thermal Overload . . . . . . . . . . . . . . . . . . . . 295. 9.2 Short Circuit Protection . . . . . . . . . . . . . . . . . 305. 9.3 Earth Fault protection . . . . . . . . . . . . . . . . . . 30

6 Switchgear 326. 1 Generator Circuit Breaker(GCB) . . . . . . . . . . . . . . . . 32

6. 1.1 Operating Mechanism . . . . . . . . . . . . . . . . . . 346. 2 Air Circuit Breaker (ACB) . . . . . . . . . . . . . . . . . . . . 36

6. 2.1 Design Features and Working . . . . . . . . . . . . . . 366. 3 Vacuum Circuit Breakers (VCB) . . . . . . . . . . . . . . . . . 386. 4 SF6 Gas Circuit Breaker . . . . . . . . . . . . . . . . . . . . . 39

6. 4.1 Properties of SF6 . . . . . . . . . . . . . . . . . . . . . 406. 4.2 Operating Principle . . . . . . . . . . . . . . . . . . . . 406. 4.3 Types of SF6 Circuit Breaker . . . . . . . . . . . . . . 416. 4.4 Working of SF6 Circuit Breaker . . . . . . . . . . . . . 41

7 Generator 457. 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457. 2 Constructional Detals . . . . . . . . . . . . . . . . . . . . . . . 46

7. 2.1 Stator . . . . . . . . . . . . . . . . . . . . . . . . . . . 467. 2.2 Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . 467. 2.3 Retaining Rings . . . . . . . . . . . . . . . . . . . . . . 467. 2.4 Fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477. 2.5 Stroboscope . . . . . . . . . . . . . . . . . . . . . . . . 47

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7. 3 Generator Excitation and AVR . . . . . . . . . . . . . . . . . 487. 3.1 Power Circuit . . . . . . . . . . . . . . . . . . . . . . . 487. 3.2 Control Circuit . . . . . . . . . . . . . . . . . . . . . . 497. 3.3 Description and general Information with important

operating parameter . . . . . . . . . . . . . . . . . . . 497. 3.4 Power Supply System . . . . . . . . . . . . . . . . . . . 507. 3.5 Field Circuit Breaker . . . . . . . . . . . . . . . . . . . 507. 3.6 De-excitation . . . . . . . . . . . . . . . . . . . . . . . 507. 3.7 Cooling System . . . . . . . . . . . . . . . . . . . . . . 51

7. 4 Technical Data and nameplate Ratings . . . . . . . . . . . . . 537. 4.1 Generator . . . . . . . . . . . . . . . . . . . . . . . . . 537. 4.2 Cooling Media . . . . . . . . . . . . . . . . . . . . . . . 547. 4.3 Peak Short Circuit Current . . . . . . . . . . . . . . . 547. 4.4 No load short circuit ratio (saturated)= 0.46 . . . . . . 547. 4.5 Permissible unbalanced load= 8%. . . . . . . . . . . . 547. 4.6 Efficiency at p.f.-0.8, full load = 98.58% . . . . . . . . 547. 4.7 Stator winding: 6 terminal, double star winding . . . . 547. 4.8 Rated Field Current- 4463 A . . . . . . . . . . . . . . . 547. 4.9 Rated Field Voltage- 370 V . . . . . . . . . . . . . . . 547. 4.10Reactances . . . . . . . . . . . . . . . . . . . . . . . . 54

7. 5 Protections Of Generator . . . . . . . . . . . . . . . . . . . . . 557. 6 Nature of Faults in Generator and their protection . . . . . . . 55

7. 6.1 Stator Winding Faults and protection . . . . . . . . . . 557. 6.2 Overcurrent Protection . . . . . . . . . . . . . . . . . . 567. 6.3 Overvoltage Protection . . . . . . . . . . . . . . . . . . 577. 6.4 Undervoltage Protection . . . . . . . . . . . . . . . . . 577. 6.5 Rotor Earth Fault Protection . . . . . . . . . . . . . . 577. 6.6 Loss of Excitation . . . . . . . . . . . . . . . . . . . . . 587. 6.7 Unbalance Loading . . . . . . . . . . . . . . . . . . . . 607. 6.8 Overspeed Protection . . . . . . . . . . . . . . . . . . . 607. 6.9 Overfluxing . . . . . . . . . . . . . . . . . . . . . . . . 61

8 Transformers 628. 1 Generator Transformer (GT) . . . . . . . . . . . . . . . . . . . 62

8. 1.1 Bushings . . . . . . . . . . . . . . . . . . . . . . . . . . 628. 1.2 Transformer Tank . . . . . . . . . . . . . . . . . . . . . 638. 1.3 Conservator . . . . . . . . . . . . . . . . . . . . . . . . 638. 1.4 Gas Sealed Conservator . . . . . . . . . . . . . . . . . 638. 1.5 Construction and Operation . . . . . . . . . . . . . . . 648. 1.6 Bellow and Diaphragm Sealed Conservator . . . . . . . 648. 1.7 Thermosyphon Filter . . . . . . . . . . . . . . . . . . . 65

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8. 1.8 Cooling System . . . . . . . . . . . . . . . . . . . . . . 658. 1.9 Manholes . . . . . . . . . . . . . . . . . . . . . . . . . 658. 1.10Marshaling Box . . . . . . . . . . . . . . . . . . . . . . 658. 1.11Technical Data . . . . . . . . . . . . . . . . . . . . . . 66

8. 2 Start-Up Transformer . . . . . . . . . . . . . . . . . . . . . . . 668. 2.1 Technical Data . . . . . . . . . . . . . . . . . . . . . . 67

8. 3 Unit Transformer . . . . . . . . . . . . . . . . . . . . . . . . . 678. 3.1 Technical Data . . . . . . . . . . . . . . . . . . . . . . 68

8. 4 Transformer Faults . . . . . . . . . . . . . . . . . . . . . . . . 688. 4.1 Winding Faults . . . . . . . . . . . . . . . . . . . . . . 688. 4.2 Core Faults . . . . . . . . . . . . . . . . . . . . . . . . 688. 4.3 Tank Faults . . . . . . . . . . . . . . . . . . . . . . . . 69

8. 5 Transformer Protection . . . . . . . . . . . . . . . . . . . . . . 698. 5.1 Transformer Differential Protection . . . . . . . . . . . 698. 5.2 Gas actuated relay . . . . . . . . . . . . . . . . . . . . 718. 5.3 Pressure Relief Device (PRD) . . . . . . . . . . . . . . 728. 5.4 Magnetic Oil Level Gauge (MOLG) . . . . . . . . . . . 738. 5.5 Oil temperature indicator (OTI) and Winding temper-

ature indicator (WTI) . . . . . . . . . . . . . . . . . . 738. 5.6 Cooler System Failure . . . . . . . . . . . . . . . . . . 748. 5.7 Overfluxing Protection . . . . . . . . . . . . . . . . . . 748. 5.8 Earth Fault Protection . . . . . . . . . . . . . . . . . . 748. 5.9 Backup Protection . . . . . . . . . . . . . . . . . . . . 768. 5.10Lightning Arrestors . . . . . . . . . . . . . . . . . . . . 768. 5.11Dissolved gas analysis (DGA) . . . . . . . . . . . . . . 76

9 Online Oil Purification Method 799. 1 Importance of Transformer Oil . . . . . . . . . . . . . . . . . . 799. 2 Why Oil Purification is Required? . . . . . . . . . . . . . . . . 799. 3 How is Online Purification Different from Offline Mode? . . . 809. 4 Process for Oil Filtration . . . . . . . . . . . . . . . . . . . . . 809. 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

10 Switchyard 8210. 1400 kV GIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8210. 2Local Bay Controller . . . . . . . . . . . . . . . . . . . . . . . 8410. 3Components of GIS . . . . . . . . . . . . . . . . . . . . . . . . 84

10. 3.1Power Line Carrier Communication . . . . . . . . . . . 8410. 3.2Current transformer(CT) . . . . . . . . . . . . . . . . . 8510. 3.3Voltage Transformer . . . . . . . . . . . . . . . . . . . 8610. 3.4Capacitive Voltage Transformer (CVT) . . . . . . . . . 86

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10. 3.5Lightning Arrestor . . . . . . . . . . . . . . . . . . . . 8710. 3.6Earthing Switch and Disconnector Switch . . . . . . . 8810. 3.7Hot Line Washing . . . . . . . . . . . . . . . . . . . . . 8910. 3.8Station Billing and Metering System . . . . . . . . . . 9110. 3.9Bay Control Mode . . . . . . . . . . . . . . . . . . . . 92

11 DGs, CUPS, PUPS and Batteries 9411. 1Diesel Generator Unit . . . . . . . . . . . . . . . . . . . . . . 94

11. 1.1Design Criteria . . . . . . . . . . . . . . . . . . . . . . 9411. 2Diesel Engine and Auxiliary System . . . . . . . . . . . . . . . 95

11. 2.1Low Temperature Water System . . . . . . . . . . . . . 9511. 2.2High Temperature Water System . . . . . . . . . . . . 9611. 2.3Lube Oil . . . . . . . . . . . . . . . . . . . . . . . . . . 9611. 2.4Fuel Oil System . . . . . . . . . . . . . . . . . . . . . . 9711. 2.5Compressed Air System . . . . . . . . . . . . . . . . . 97

11. 3UPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9811. 4Modes of Operations . . . . . . . . . . . . . . . . . . . . . . . 100

11. 4.1Highest priority . . . . . . . . . . . . . . . . . . . . . . 10011. 4.2Lowest priority . . . . . . . . . . . . . . . . . . . . . . 10011. 4.3Test/Maintenance . . . . . . . . . . . . . . . . . . . . . 100

12 Appendix 103

13 Bibliography 105

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Acknowledgement

I wish to express my gratitude and sincere thanks to TAPS 3&4 managementfor accepting my request for undergoing practical training for a month . Iwould also like to thank Shri A.N. Thakur, SME(E) and all other officialswho helped me to undertake and guide me through the project.

The work culture and the willingness of all the officials and staff that helpedme in resolving my query, was truly appreciable and amazing.

In particular, I would like to thank

Sh. Tapas Kumar DeySh. Ashwin Kumar YadavSh. Rahul Kumar MishraSh. Ezaz AnwarSh. Pankaj JigiyoSh. Rahul SapkaleSh. AnandSh. M.M RautSh. Pradeep ThombreSh. Chandrakant WajeSh. Gajanan HaraoSh. Jayapaul JayaKumar

for giving me their valuable time. It was because of their unrelenting supportthat I was able to learn so much during the training.

Lastly, I would like to apologize in advance for any mistake that may haveinadvertently taken place while making this document.

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Certificate

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Introduction to TAPS-3&4 andIndia’s nuclear Programme

3. 1 Introduction

Nuclear Power Station, TAPS 3&4 has a twin unit module of 540 MW capac-ity. Each unit uses pressurised heavy water to rotate the turbines. Each unitis divided in two divisions, div-1 and div-2. Div-1 consists of odd numberedbuses while div-2 consists of even no. of buses. They are divided in sucha way that if component of any of the division fails, the load can then betransferred to the other division. Its operation is supported by the supply ofheavy water, used for cooling of reactor bundle and the fuel for nuclear reac-tors i.e. Uranium to generate electrical power using fission reaction. TAPS3&4 is located alongside the Arabian Sea for its continuous supply of waterwhich is required as a coolant for the fuel bundle.

3. 2 How a Nuclear Reactor Works?

A nuclear reactor is a source of heat, which is produced by self sustained andcontrolled fission reaction within the reactor core. The geometrical bound-aries within which the nuclear fuel, moderator, coolant and control rods arearranged to facilitate production and control of nuclear chain reaction toprovide heat energy at a desired rate is called reactor core.

The natural Uranium is used as a fuel to drive the heavy water reactors.The element has three isotopes, U-238, U-235 and U-234.Only U- 235 is usedfor energy production.

In a reactor core, the fuel is placed in a fuel bundle which is placed inside thereactor core. When the thermally excited neutron hits the nucleus of U-235,it undergoes fission giving a large amount of energy and two or more smaller

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nuclei. These fast moving nuclei are slowed down by the use of modera-tor (heavy water) and hence the probability of fission with U-235 increases.The process continues and the self sustained chain reaction is maintained.The energy produced using the above method is directly proportional to theamount of neutron density in the reactor core. Thus the reactor power isregulated by the use of moderators to absorb excess neutrons in the core.

The heat produced is used to generate highly pressurized steam which rotatesthe turbines and thus produces electrical energy.

Figure 3.1: Fission Chain Reaction

3. 3 Types of Reactors

1. LWR: Light Water 8These type of reactors uses light water both as a coolant and the mod-erator.These are further divided in two types:

• PWR: Pressurized Water ReactorThe primary circuit water in these reactors is pumped at rela-tively high pressure into the reactor vessel where it is heated andthen transferred to steam generators, where it boils the secondary

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circuit water, which in turns evaporates and passes throught heturbine.

• BWR: Boiler Water ReactorIn these, the water is allowed to boil and produce steam. Thesteam produced passes directly through the turbines. The fuelused is Uranium Dioxide. The major disadvantage of such reactorsis that for refuelling such type of reactors the plant has to be shutdown. Refuelling can’t be accomplished under load.

2. PHWR: Pressurized Heavy Water Reactor

This uses heavy water(D2O) moderator fuelled with natural uranium.Heavy water is used both as a moderator and coolant, and is collectedin a tubed tank called Calandria.

In advanced PHWRs, the heavy water is used only as a moderatorand light water as a coolant.

The fuel used is natural uranium oxide and unlike the LWRs, the fuelbundle can be replaced in the running condition. PHWRs also produceplutonium which can be used by other types of reactors.

3. LWGR or PTGR: Light Water Cooled Graphite or Pressurized TubeGraphite Reactor

It also uses uranium dioxide clad in Zr alloy tubes as a fuel and iscooled by H2O. The water flows through the numerous pressure tubesarranged in channels in the body of graphite which serve as a modera-tor.

4. AGR: Advanced Gas cooled Reactor

It uses CO2 as a coolant and outlet temperature is about 6500C. Theseuses enriched ceramic dioxide fuel in stainless steel shells and have muchhigher effective fuel burn up.

3. 4 Importance of Atomic Power Station

The states in the western zone are located far from the coal fields and coallinkages. Potential of development of hydro-electric power is also limited.

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Therefore, the development of electrical power using nuclear energy will con-tribute greatly towards the electrical power in western zone.

3. 5 General Layout of TAPS 3&4

The layout of the plant is such that both the units operate independentlywith seperate control room for each of them. All safety related systems andcomponents are placed in control building. A common fueling machine headcalibration and maintenance facility between two reactor units is provided,communicating via fueling machine air lock and passage leading to each Re-actor Body. This facility is located in service building, common to both theunits. reactor Auxiliary building is located very close to the reactor buildingto avoid long piping lengths.

3. 6 Unique features of the Plant

• 220KV & 400 KV GIS

• 400 KV used for Power evacuation system

• CB between generator and GT

• Totally independent EMTR for both the divisions

• System divided in two independent division, one fed by UT and theother by SUT.

• Four DG sets/unit

3. 7 India’s Nuclear Program

To utilize large Uranium and Thorium reserves in the country for electricitygeneration. India has been following a three stage nuclear power program:

1. PHWR based on natural uranium,

2. FBR utilising plutonium-uranium fuel cycle, and

3. Breeder Reactors for utilising thorium.

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Figure 3.2: TAPS 3& 4

3. 7.1 Nuclear Fuel Cycle

India has acquired comprehensive capability in the PHWR design, construc-tion and operation of associated plants/facilities covering the entire nuclearfuel cycle of the nuclear power program based on PHWRs.

Nuclear fuel fabrication for power and research is done at Nuclear Fuel Com-plex, Hyderabad and BARC respectively.

There are seven heavy water plants in the country that are based on ammonia-hydrogen exchange and hydrogen sulphide-water exchange. the later hadbeen developed indigenously. Through continuing research BARC has devel-oped heavy water upgrading technology on commercial scale. Based on thistechnology, at present 23 upgrading/final enrichment towers are in operationat various sites.

The Indian Nuclear Power generation program is based on closed cycle ap-proach that involves reprocessing of spent fuel and recycle of plutonium andUranium-233 for power generation. The development of fuel reprocessingtechnology had commenced from inception of DAE’s nuclear power program.DAE has a pilot plant for fuel reprocessing at Trombay and industrial scaleplants at Tarapur and Kalpakkam. BARC has successfully developed tech-nology for vitrification of radioactive waste.

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Electrical System

4. 1 Introduction

The electrical system is divided mainly into two sub systems. Main poweroutput system and station auxiliary power supply system. Main power out-put system transmits the power generated by 500 MWe generators at 21 KVfrom generator terminals to the switchyard through generator transformer,which steps up voltage upto 400 KV before evacuation to grid.

Station auxiliary power supply system provides power supply for variousstation loads required for start up operation, safe shutdown and maintainingthe unit. The main sources of power supply are from startup transformerinterconnected with 220 KV grid and unit transformers (2 No.) connectedwith output terminals of main generator. Station auxiliary power supplyis also divided into various classes namely CL-IV, CL-III, CL-II, and CL-I,depending upon requirement of availability / reliability of power supply forvarious loads. Diesel generators and battery banks are provided as backuppower supplies depending on the loads connected in a particular class.

4. 2 Objectives of Electrical Systems

The objectives of Electrical Power Systems are:

1. To evacuate the power generated from the turbo generators to the offsite grid connected to the station at 400 KV switchyard.

2. To provide required quality of power to the station auxiliaries throughstart-up transformer (SUT) and/ or GT/UT combination and in caseof emergency on site diesel generator sets and uninterruptable powersupply systems.

3. To provide emergency electric power supply to safety system of the

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station during simultaneous occurrence of postulated initiating eventsand single failure of any active/passive electric component/system.

4. To provide station emergency electric power system with reliable offsite power from at least two transmission lines preferably connected totwo generating stating stations.

5. To provide fast transfer systems, emergency transfer systems and loadshedding schemes so that electrical power supply is restored within theinterruption time permitted by the connected loads.

6. To provide necessary isolations, alarms and indications for safe opera-tion maintenance of electrical equipment.

7. To provide fire protection and safety.

8. To provide earthing of electrical system and equipment for personneland system safety and isolation of defective system.

9. To provide surge suppression, lighting protection.

10. To provide adequate lighting during plant operation and during emer-gency.

4. 3 Clasification of Electrical System

4. 3.1 Class IV power Supply

Alternating current power supply to auxiliaries, which can tolerate prolongedinterrupt without affecting safety of reactor, is classified as class-IV. Thissupply is the normal power supply drawn from switchyard through SUT andor GT/UT combination.

4. 3.2 Class III power Supply

Alternating current power supply to auxiliaries, which can tolerate shortinterruptions (up to one minute), is classified as class-III power supply. Undernormal conditions this power supply is derived from class-IV and on loss ofclass-IV power supply, on-site standby diesel generators provide the back up.

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4. 3.3 Class II power Supply

Alternating current power supply to auxiliaries, which require un-interruptedpower supply, is called class-II power supply. Under normal conditions elec-trical power is derived from class-III buses through AC/DC rectifier togetherwith DC to AC inverter. A battery bank provides direct back up power sothat class-II power is available even when supply to class-III or rectifier isnot available.

4. 3.4 Class I power Supply

DC power supply to loads which require uninterrupted direct current powersupply. Normally direct current power is derived through a AC to DC rectifierconnected to class-III. Battery backup is provided so that direct currentpower supply continues to be available even when class-III or rectifier fails.

4. 4 Description of Station Auxiliary Power

Supply System

Station auxiliary power supply system (SAPSS) provides power supply tovarious station auxiliary loads required for start-up, shut down and runningoperations of the unit. The class IV SAPSS has been divided into two di-visions, one division (Div I) supplied from unit transformers (UTs) and theother division (Div II) supplied from start up transformer (SUT). Intercon-nections are provided between Division-I and Division II at all voltage levelsexcept 415V CL.III and CI.IV to feed the loads belonging to the other di-vision in case of total or partial loss of power to that division. The buses,transformers and MCCs in Division-I are given odd numbers and Division-IIeven numbers. Supply sources in each division can independently meet theentire station demand under normal and abnormal conditions of one unitoperation.

4. 4.1 Class IV Power Supply System

Class IV Power supply system for each unit derives its power from UTs (twoNos.) and SUT (one No.). UTs are two winding transformers of 21 KV/6.9KV, 35 MVA rating each, connected to Generator terminals through a Gen-erator Circuit Breaker (GCB). SUT is a three winding transformer (70/35/35MVA, 220/6.9/6.9 KV and unloaded delta for suppression of harmonic cur-rents), which is connected to 220 KV grid and supplies power to class IV

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system at 6.6KV.

Under normal condition of operation, the power to all the auxiliary loadsis fed from class IV power supply system.

The class IV power supply has two levels of voltages supplying power at:

• 6.6 kV, 3-phase for motors of rating 200KWand above.

• 415 V, 3-phase for motors 200 KW rating.

4. 4.2 6.6 kV System

This system consists of four numbers of buses with each switchgear bus fedfrom UTs or SUT directly. The start-up/auxiliary power of the unit will bederived through GT/UT and/or SUT. Major loads connected on this systemare Primary Coolant pumps; Boiler feed pumps, Condensate pumps, CWpumps, Chillers etc. One 6.6 kV feeder will be provided for supplying loadsin waste management plant and D20 & Upgrading plant from unit-4.

4. 4.3 415 V System

This system consists of six numbers of buses supplied through six 6.6 kV/433V, 2 MVA transformers for feeding power to auxiliary loads. 415 V loadsin service building, CW pump house and DM plant will be supplied from415 V; Class IV local MCCs. MCCs located in DM plant will be suppliedfrom Unit-4. To maintain the continuity of the supply with minimum timeof interruption when any one of the six transformers fails, a hot standbytransformer is provided to supply the load of the affected bus, which will beswitched in manually.

4. 4.4 Class III Power Supply Sytem

This system derives its power from class IV, 6.6 kV system under normalcondition of operation. This system consists of four numbers (4 Nos.) of 6.6kV buses, each backed up by a DG set, and four numbers (4 Nos.) of 415 Vbuses.

4. 4.5 Class II Power Supply Sytem

Class II, 415V, emergency power supply system provides uninterrupted A.C.power to the loads connected to this system. This system consists of 2 Nos.

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of 415 V buses which derive power from power UPS.

4. 4.6 Class I Power Supply System

This system provides 220 V DC uninterrupted power to the DC loads. Thisis further divided into:

• 220 V DC power supply

• 220 V DC control supply

Table 4.1: Unit Auxiliary System Voltage Levels

Voltage Description

21 kV (ac) Input to unit Auxiliary transformer/ Unit generationvoltage

6.6 kV (ac) Unit main power buses, DG sets, motors above 200 KWrating & auxiliary transformers

415 V (ac) Distribution buses, motors below 200 KW rating240 V (ac) Single-phase loads like, control power supplies,

recorders, lighting, spaceheaters.220 V (dc) Control power to circuit breaker, DG controls, emer-

gency lighting etc.24 V (dc) Controls, annunciations, indications Involving main con-

trol room control

Table 4.2: Voltage level nomenclature

Voltage Alphabet

400 kV (ac) A220 kV (ac) B6.6 V (ac) C415 V (ac) D220 V (dc) E24 V (dc) F

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Table 4.3: Class nomenclatureClass Numerical

IV 4III 3II 2I 1

4. 5 Nomenclature

For designating equipments in div-1, odd numbers are used & for Div-2,even numbers are used. With reference to table 2 and table 3, consider thefollowing example:52410- BU- C 4 352410 implies System USIBU implies BusC implies 6.6 KV4 implies Class IV3 implies bus no. 3 i.e. in division 1

4. 6 Auto-Transfer Scheme

The purpose of auto transfer is to achieve automatic transfer of 6.6 kV, Class-IV auxiliary power supply, in the event of failure of either of the two feeds(from unit transformer or start-up transformer) due to faults in feeder. Theauto transfer scheme shall consist of:a) Fast transfer scheme andb) Slow transfer scheme.

Automatic Fast Transfer (SUT to UTs)Immediately after the tripping of SUT by protections, the loads fed fromSUT will be transferred to UTs by automatic fast transfer under the follow-ing conditions:

• No fault on bus undergoing transfer i.e. Bus incomer breaker lockoutrelay in reset condition.

• Angular difference between residual bus voltage and incoming voltageis less than preset value.

• Residual bus voltage is above preset value.

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Automatic Fast Transfer (UTs to SUT)After the tripping of UTs by protection, the loads fed from UTs will betransferred to SUT. Fast transfer is initiated only in case of faults in UT,GT and interconnecting bus duct up to GCB. In case of unit trip GCB willbe opened and the power to UT buses is maintained through GT/UTs.

Automatic Slow TransferIn the event of failure of fast transfer the slow transfer can be carried outafter the voltage of the bus undergoing transfer goes below 20%.In case othersystem conditions demand the disconnecting of loads during Slow transfer,the to the affected bus will be restored automatically after disconnecting theloads. Restoration of the loads will be carried out manually thereafter.

In the event of electrical faults in start-up transformer or in the zone cover-ing generator transformer and unit transformers, buses fed by one of thesesources are automatically transferred to healthy source by simultaneous trip-ping of faulty source and closing of incomer breaker from healthy source.The fast transfer scheme receives it initiating signals from the lockout relaysof GT, UT, SUT, 400 KV bus to which GT is connected, 220KV bus towhich SUT is connected etc. Auto transfer initiation from UT to SUT takesplace on energisation of relay UTX by 86A2-I, 86A2-II and 400KV BUS diff.Relay 4 along with GCB 52 X contact. Auto-transfer initiation from SUTto UT takes place on energisation of relay SUTX by SUT protection mainand backup lockout relays, 220 KV Bus diff. Protection and LINE-1, LINE-2protection Lock Out relays as shown in the scheme.

The scheme proposed for fast transfer shall be high-speed dead transfer witha dead time of about 2 cycles (40 ms) after considering the difference betweenclosing time and opening time of breakers. If the above minimum bus deadtime of 2 cycles is not achieved with the available close and trip time of 6.6KV breakers, closing of the healthy side breaker shall be delayed accordingly.This is achieved by Timers UT-T1 and SUT-T1.

Synchronism between the faulty supply and incoming supply (representedby the 6.6 KV bus voltages) is checked by synchronism check relay (25) andif it is permissive, the fast transfer shall take place. The fast transfer shall becompleted within a set time; otherwise it will be blocked. This is achievedthrough timer UT-T2 and SUT-T2.

In case fast bus transfer fails, the change over shall be achieved by slow bustransfer scheme provided the voltage of affected bus has fallen below 20%

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of rated voltage This is achieved by energizing relays C41-27-UTX, C43-27-UTX, C42-27-UTX, C44-27-UTX (UT to SUT transfer initiation) andC41-27-SUTX, C43-27 SYTX, C42-27-SUTX, C44-27-SUTX (SUT to UTinitiation) through 2/27-3 of respective 6.6 KV bus and 52X & 86 contactsof supply breaker.

Subsequent to fast transfer, if both healthy and faulty source breakers re-main closed simultaneously, both breakers will be tripped instantaneously.This is achieved through relays UT-C41X, UT-C43X, UT-C42X, UT-C44C(UT to SUT transfer) and SUT-C41X, SU-C43X, SUT-C42X and SUT-C44X(SUT to UT transfer).

Advantages of Fast TransferThe fast transfer schemes proposed in above will have the following advan-tages:

• All motors will reaccelerate quickly and consequently all essential ser-vices will be available immediately.

• The voltage dips during change over will be of momentary nature only.

• High inrush currents in individual motors as well as in auxiliary systemare reduced.

• There will be no perceptible flicker in the lighting systems, and hencefluorescent and HPMV lamps, which are susceptible to voltage dips,will remain unaffected.

• Since the residual bus voltage does not go down perceptibly, the 415V motor contactors do not drop off during the change over sequence.Thus restarting of 415V motors after the changeover can be avoided.

4. 7 Emergency Transfer Scheme

EMTR scheme is initiated for any of the following conditions:

• Loss of normal class-IV supplies to any one or more number of 6.6 KVclass-III buses.

• Loss of supply to 415 class-III buses due to 6.6 KV/433 V auxiliarytransformer failures.

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• Failure of ups in class-II system/ups static bypass beyond preset dura-tion.

• Loss of class-II supply.

• Sensing of the above conditions is done either by detecting under-voltage on the bus when there is no bus fault or directly by checkingequipment failure at EMTR logic diagrams.

4. 7.1 6.6 KV Class-III, 415 Class-III Bus Supply AndFeeder Restoration

On initiation of EMTR, all motor feeder breaker and other predefined loadsare tripped and reclosing is blocked by under-voltage lockout relay. The clos-ing of each feeder is blocked until the blocking feature is reset by the breakerhand switch manually or by restoration of sequence contact of emergencytransfer panel. This sequence is required after permanent supply is availableto class-III buses.

The sequence is initiated by EMTR logic. This energizes number of soft-ware timers, each of which is set at a time step of 4 seconds. This contactof each timer will give permission to close the corresponding class-iii feeder.After the last restoration of loads is done by timer it automatically resetEMTR scheme.

4. 7.2 EMTR Initiation For 415 V Class-III Buses

Loss of voltage on any of the 415 V class-III buses initiates EMTR. EMTRis also initiated on tripping of any one of the 6.6 KV/433 V auxiliary trans-formers normally supplying to a 415 V class-III bus. EMTR restores powersupply to the affected 415 class-III bus by closing the standby transformersecondary side circuit breaker after checking for conditions such as healthi-ness of the bus, availability of breaker etc. After power supply is restored tothe affected bus, EMTR restores the loads in a predetermined sequence.

4. 7.3 EMTR Initiation for 415 V Class-II Buses

Loss of voltage on class-II bus initiates EMTR. EMTR closes the class-III-class-II tie breaker and restores supply to the affected class-ii bus. Priorto closing the class-III-class-II tie breaker, EMTR will check for healthinessof the bus, availability of breaker etc. EMTR will also start DGs of that

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division but the dg will be manually connected to the relevant class-III busby operator action.

4. 7.4 Emergency Transfer Panels

Two emergency transfer panels are provided for each unit of TAPP-3 andTAPP-4. One emergency transfer panel is dedicated for each division. Emer-gency transfer scheme for one division is completely independent from emer-gency transfer scheme for the other division. Functions of any division emer-gency transfer scheme are independent of other division scheme with no com-munication between them. Each EMTR has two redundant PLCs runningin parallel all the time.

The output issued to the field is generated by combining the output of boththe PLCs in such a manner that n case of failure of any one plc, requiredfunction is met by the other healthy plc. For this purpose, normally openoutput contacts of the two PLCs are connected in parallel and normallyclosed contacts are connected in series.

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Electrical Protection Relays

5. 1 Introduction

In generating stations, all electrical circuits and machines are subject tofaults. A fault is generally caused by the breakdown of insulation between aconductor and ground or between conductors. The result is a flow of excesscurrent through a relatively low resistance resulting in a severe damage un-less cleared quickly.

The types of fault that can occur in an electrical system:

Shunt Faults

• Phase to Ground

• Phase to Phase

• Three Phase Fault

Series Faults or Open Conductor Fault

5. 2 Shunt Faults

Those faults, which involve only one of the phase conductors and ground, arecalled ground faults. Faults involving two or more phase conductors, with orwithout ground, are called phase faults.

Power systems have been in operation for over a hundred years now. Ac-cumulated experience shows that all faults are not equally likely. Single lineto ground faults (L-G) are the most likely whereas the fault due to simul-taneous short circuit between all the three lines, known as the three-phasefault (L-L-L), is the least likely.

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Further, the probability of faults on different elements of the power systemare different. The transmission lines which are exposed to the vagaries of theatmosphere are the most; likely to be subjected to faults. Indoor equipmentis least likely to be subject to faults.

Table 5.1: FAULT STATISTICS WRT TYPE OF FAULTFault Probability of Occurence Severity

L-G 85% Least SevereL-L 8%

L-L-G 6%L-L-L 2% Most Severe

Table 5.2: FAULT STATISTICS WRT REFERENCE TO POWER SYS-TEM ELEMENT

Power System Element Probability of Occurence

Overhead Lines 50%Underground Cables 9%

Transformer 10%Generator 7%Switchgear 12%.

CT, PT, Relay 12%

5. 3 Purpose of Electrical Protection

The function of protective relaying is to ensure the removal of a faulty elec-trical system component from the rest of the electrical system, thereby pro-tecting the remaining system from damage and instability.

5. 4 Basic Relay Terminology

RelayA relay is an automatic device by means of which an electrical circuit is indi-rectly controlled and is governed by a change in the same or another elctricalcircuit. Protective Relay

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A protective relay is an automatic device which detects an abnormal condi-tion in an electrical circuit and causes a circuit breaker to isolate the faultyelement of the system. In some cases it may give an alarm or visible indica-tion to alert operator.

5. 5 Types of Relays

• Auxiliary RelayAuxiliary Relays assist protective relays. They repeat the operationof protective relays, control switches etc. They relieve the protectiverelays of the duties like tripping, time lag, sounding the alarm etc.They may be instantaneous or may have a time lag.

• Electromagnetic RelaysSuch relays operate on electromagnetic principle i.e. an electromagnetattracts magnetic moving part or a force is exerted on a current carryingconductor when placed in a magnetic field or a force is produced bythe principle of induction etc. These relays are either of attractedarmature or induction cup or induction disc versions. They possesmechanical inertia and thus take longer time to operate as compare tostatic relays. These relays are provided as backup relays for stationelectrical auxiliary systems.

• Static Relaysthese are solid state relays and employ semiconductor diodes, transis-tors, thyristors, logic gates etc. The measuring circuit is a static circuitand there are no moving parts.

• Numerical Relays These are programmable version of solid state re-lays based on digital signal processing by microprocessor. The mainadvantage is -In its modular architecture allowing the same unit to beprogrammed in to different types of relays, compact in nature, featureof displaying the online parameters, event reports can be displayed anddownloaded from such relays.

5. 6 Essential Features of Protective Relay

SensitvityThe protective system must be alive to the presence of the smallest faultcurrent. The smaller the fault current it can detect, the more sensitive it is.

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SelectivityIn detecting the fault and isolating the faulty element, the protective systemmust be very selective. Ideally, the protective system should zero-in on thefaulty element and isolate it, thus causing minimum disruption to the system.

SpeedThe longer the fault persists on the system, the larger is the damage to thesystem and higher is the possibility that the system will lose stability. Thus,it helps a lot if the entire process of fault detection and removal of the faultypart is accomplished in as short a time as feasible. Therefore, the speed ofthe protection is very important. It must, however, be mentioned that speedand accuracy bear an inverse relationship! The high-speed systems tend tobe less accurate. This is for the simple reason that the high speed system haslesser amount of information at its disposal than a slow-speed system. Theprotection engineer has to strike a balance between these two incompatiblerequirements.

Reliability and DependabilityA protective system is of no use if it is not reliable. There are many ways inwhich reliability can be built into the system. Good engineering judgementplays a great part in enhancing the reliability of the protective system. Ingeneral, it is found that simple systems are more reliable. Systems whichdepend upon locally available information, tend to be more reliable and de-pendable than those that depend upon the information at the remote end.However, in spite of best, efforts to make the system reliable, we cannot ruleout the possibility of failure of the (primary) protection system. Therefore,we add features like back-up protection to enhance the reliability and de-pendability of the protective system.

StabilityIt is defined as the quality of the protection system by virtue of which the pro-tective system remains inoperative and stable under certain specified condi-tion such as system disturbances, through fault, transient etc. Design aspectslike biased differential scheme and harmonic restraint relay add to stabilityof transformer protection system.

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5. 7 Description of M.S. English Electric Re-

lays

Majority of Relays in the system are English Electric make and the type isdescribes in letters such as CDG, CCG, DTH etc. There can be maximumof 4 nos. of such letters.

The first letter signifies the operating quantity, for eg:A - Phase angle comparison,C - Current,D - Differential,F - Frequency,V - Potential, etc.

The second letter signifies movement, for eg:A - Attracted armature,B - Buchholz,C - Induction cup,D - induction Disc,T - Transistor, etc.

The third letter signifies application, for eg:A - Auxiliary,E - Earth,G - general or Generator,J - Tripping,V - Voltage, etc.

Fourth letter:M - Special variation.First Figure - Indicates the number of units in the relay essential to its op-eration not including seal - in auxiliary units.Second Figure - Indicates a particular characteristic on a group of similarrelays, eg: CDG 11, CDG 12, CDG 13, CDG 14 are all inverse time overcur-rent relays but with different characteristic curves.

For eg:

CDGC - Current is the operating quantity.

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D - Movement of Disc.G - General Usage.

DTHD - Differential QuantityT - TransistorizedH - Harmonic Restraint

5. 8 Working of Trip Circuit

Figure 5.1: Trip Circuit

When a fault occurs in a protected circuit, the relay connected to CT andPT as shown in figure 1 actuates and closes its contacts. Current flows fromthe battery in the trip circuit. As the trip coil of the CB is energized, theCb operating mechanism is actuated and it operates for opening operation.

5. 9 MICOM P220

Micom P220 is a numerical relay used for the protection of motors. Theprotection features provided by this relay:

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Figure 5.2: Micom P220

• Thermal Overload Protection

• Short Circuit Protection

• Excess long start Protection

• Stalling Protection (Locked Rotor)

• Negative Phase protection

• Earth Fault Protection

• Loss of Load

5. 9.1 Thermal Overload

Overload can result in excessive stator temperature rise in excess of the ther-mal limit of winding insulation. This may not cause the motor to burn outimmediately, it has been shown that the life of the motor can be shortenedif these overload persists. The life of the motor is not purely dependent onthe temperature of the winding but on the time that it is exposed to thesetemperature. Due to relatively high thermal storage capability of Induc-tion Motor, in frequent overloads of short duration may be tolerated withoutdamage.

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The P220 Relay has features of direct measurement through the use of tem-perature sensors and indirect measurement by means of current measure-ment. This relay has a thermal alarm to inform the user if the motor is likelyto become overload before a trip occurs. Remedial actions can be taken be-fore the motor is tripped. Also, an adjustable Cooling Time constant (Tr) inorder to take into account the various modes of cooling.

A typical setting of thermal protection:

• Thermal overload current threshold between 105% - 108% of motorrated current.

• Negative sequence current recognition factor (Ke)=3

• Heating time constant (Te1) during startup time(Te2) and cooling timeconstant(Tr). Te1 = Te2 = 14min and Tr = 28min.

• Alarm Threshold Its setting is related to motor operation mode andthe concept of protection. A typical adjustment consists of setting thealarm threshold to slightly higher than the ratio (I/ IRated Motor) , whichgenerally corresponds to a value of 90%.

5. 9.2 Short Circuit Protection

The P220 relay is provided with an over current element operating at funda-mental component, with a settable Definite time delay. The current thresholdmust be set as low as possible without tripping due to the start up currentof the motor, the contribution of the motor to an external fault as well asthe reacceleration current due to voltage drops.The typical settings are:

Imax = 130% * KSTART * INMOTOR & [t Imax]=100ms

Imax = 180% * KSTART * INMOTOR & [t Imax]=0ms

5. 9.3 Earth Fault protection

The P220 is provided with two independent earth fault over current elementswith suitable Definite Time Delays. This function react only to the funda-mental component of the earth fault current. The earth fault protection maybe provided either by Residual connection of the 3 phase CTs or by use of acore balance CT. However, it is preferable to use a core balance CT as this

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is more stable and is more sensitive.

Incorrect tripping can result from the saturation of one or more CT dur-ing motor starting. Increased Stability can be achieved by either increasingthe current threshold or with the help of a stabilising resistance in series withP220 relay.

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Switchgear

6. 1 Generator Circuit Breaker(GCB)

The GCB is located indoor at turbine building between generator and gener-ator transformer in the run of main isolated phase bus duct (IPBD). Duringunit trip, the GCB will isolate the generator from the system and the auxil-iary loads connected to UT will continue to receive power supply from 400KVsystem through GT.

Figure 6.1: Generator Circuit Breaker

The GCB is suitable for following functions and conditions:

• Breaking any load current up to rated current as well as break andmake magnetising current of GT and UT.

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• Connecting the generator with the system.

• Making , carrying for 1sec and interrupting the fault current.

• Interrupting of current during out of phase switching.

• Interruption of current during generator pole slipping.

GCB is of air blast type supplied by M/S ALSTON, France. It uses pres-surised air for quenching as well as media for operating mechanism. Doubleair blasting is generated for its arc quenching. GCB is having 3 main polesand a spare pole. Spare pole is provided for the replacement of any one ofthe main pole, when trouble comes in the main pole, so that down time canbe reduced. The 3 separate has the following advantages - it is safe, sincehigh ampere current flows in generator. Phase to phase clearance is reducedand chances of phase to phase fault are reduced.

The complete assembly of GCB includes:

• The CB comprising of 3 poles with coupling system, pneumatic cubicleand pneumatic relay and electrical cubicle.

• The compressor air plant comprising of compressor and compressorcontrol panel ; high pressure air receiver , pressure reducing panel andautomatic bleeding cabinet and connecting pipes.

The main pole consists of following main parts:

• Enclosure: it is a conducting housing with insulated paint on it. Itconnects the busbar from generator on one side and GT from the otherside.

• Main Chamber : it consists of main contact, moving contact and blast-ing valves, enclosed in a chamber with muffler (conducting part) atboth the ends where the busbar termination will get terminated. Italso encloses pressurized air which helps in operating mechanism.

• Auxiliary Chamber : it consist of moving contact, fixed contact, au-tomatic blasting valves, time delay valve arrangement which controlinsertion time of resistance for arc quenching and a resistor assembly,consist of number of resistors in parallel and further connected to muf-fler. While closing the circuit breaker the main chamber contact closesfirst then after 10msec delay, the auxiliary chamber contact closes. Andwhile opening the circuit breaker, main contact will open first and then

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after 10msec delay, auxiliary contact will open. The arc interruptiontakes place in the auxiliary chamber only. During running condition,main chamber carries 90% of the current whereas auxiliary chambercarries only 10% of the current.

• Heat Exchanger : It is located on top of the enclosure. It helps incooling of hot pressurized air of main chamber and auxiliary chamber.It consists of two radiator units cooled by fans. 6 fans are provided foreach pole.

• View Ports are provided on the top right side of the enclosure. Thereare two view ports, one for main chamber and other for auxiliary cham-ber contacts. Through view ports, we can see the actual position ofmoving contact and fixed contact.

• Pyrometer is located on top of view port, it gives the temperature ofpressurized air inside the chamber.

The auxiliary circuit of GCB consist of - Compressed Air System: it consist oftwo compressors, one high pressure receiver, pressure reducing valve, pressureswitches each of which are described as follows:

• HP Receiver :It stores the air at high pressure of 250 bars.

• Pressure Reducing valve: From HP receiver the air comes at 250 bars,which is reduced to 33.4 bars through these pressure reducing valves.

• Pressure Switches : It is provided for pole air pressure control anddraining of compressor and HP receiver.

• LP Receiver : It is a low pressure receiver. It stores the air at 33.4bars. It provides air in air inlets of opening and closing channels of thebreaker.

• Pilot Valve: These are solenoid valve provided for drain of compressorsand HP receiver.

• Pneumatic Cubicle: It is provided for air pressure control on pole andalso for closing and opening operation of breaker.

6. 1.1 Operating Mechanism

The air blast circuit breaker design uses a blast of compressed air serving asthe arc-quenching medium for the GCB. The design of the breaker is such as

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not to require conditioning of air in main receiver during service. Dampingresistors are thermally suitable for the specified duty without deteriorationof either resistor or resistor supports. Pressure switches are provided whichshall give an alarm and then lock out the breaker operation in case of low airpressure. The exhaust gases from both pneumatic operating mechanism andair blast breaker pole are effectively cooled and demonized before releasing tothe atmosphere. The loss of air pressure in the breaker interruption chamberwhile in operation does not cause any reduction of current carrying capacityor cause damage to the insulation.

The GCB can be closed and opened electrically from both local as well asremote control points. Provision is made for emergency tripping of the GCBmanually without the requirement of an electrical control supply. Provisionis there to provide locking of this means of tripping. Independently ad-justable pressure switches with potential free contacts are furnished on eachcircuit breaker mechanism for purpose of low and high air pressure alarmand lock out in case of insufficient air pressure to perform a closing /openingoperation. However, if the breaker is already performing opening operation,operation shall be completed before locking out on insufficient pressure.

It is possible to operate the breaker manually allowing for slow closing/openingof the breaker contacts for purposes of testing and maintenance. While suchprovision is in operation, movements of the contacts shall remain fully underthe control of the operator at which time the operation of the trip/ closemechanism shall have no effect. Two trip coils are provided for greater reli-ability. Trip coil supervision relays suitable for monitoring of the trip coilsboth in the open and close positions of the breaker are provided for each tripcoil. The trip coils have sufficient continuous rating to cater to the trip coilsupervision relay current.

The circuit breaker control scheme includes all necessary electrical and prefer-ably mechanical interlocks to preclude the single phase operating of the cir-cuit breaker. Separate auxiliary relays are provided for tripping and alarm.

A mechanical indicator is provided on each pole of the breaker to show openand closed positions where it will be visible through a glass window to aman standing on the ground. An operation counter is provided on the localcontrol panel to indicate the number of close-open operation.

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Table 6.1: GCB Specifications

NOMINAL SYSTEM VOLTAGE 21 KVRATED MAXIMUM VOLTAGE 24 KVRATED CONTINUOUS CURRENT 19.1 KARATED FREQUENCY 50 HzRATED SHORT-CIRCUITCURRENT(SYMMETRICAL) 120 kA - 1secRATED INTERRUPTING TIME 5 cyclesMINIMUM OPENING TIME 5 cyclesNORMAL OPERATING PRESSURE 33.4 barMINIMUM OPERATING PRESSURE 30.0 bar (tripping)DUTY CYCLE CO - 30 min CO

6. 2 Air Circuit Breaker (ACB)

ACBs have been used for breaking currents in 415V switchgear panels. Theair at atmospheric pressure is used as an arc extinguishing medium in ACB.These CBs employ high resistance interruption principle. The arc core, sodeveloped by current breaking, is a conducting path of plasma. The sur-rounding medium contains ionized air. The arc is extinguished by length-ening the arc, cooling it & splitting the arc. The arc is rapidly lengthed bymeans of arc chutes and arc runners. The arc resistance increases to suchan extent that the system voltage cannot maintain the arc & the arc getsextinguished at current zero of AC wave. The ACBs have anti-pumping andtrip free operational features.

6. 2.1 Design Features and Working

The L&T ACBs are designed to handle all the switching duties that occur in415V auxiliary supply distribution system. These breakers are meant for nor-mal industrial environment. It is important that these CBs are subjected tohostile environment e.g. dust, vapour, corrosive gases are appropriately en-closed. The breakers are extremely reliable in service, withstand wide voltagefluctuation, accommodate aluminium termination, require only a minimumof maintenance and have a long life expectancy

The three breaker poles are mounted to common mechanism housing. Theenergy storing spring mechanism can be supplied for actuation by motor orby hand. The ACB operates in following sequence:

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Figure 6.2: Air Circuit Breaker

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Motor changes the closing mechanism spring. The supply to the motor getsdisconnected by a limit switch at the end of charging operation. The closingelectromagnet, when actuated, discharges the spring and closes the circuitbreaker. After the ACB closes, the spring gets automatically charged forthe next operation. Spring condition (charged/discharged) is indicated by amechanism indicator visible through transparent window on the front.

If the indicator displays yellow, the spring is charged or else if it displaysblack, the spring is discharged.In case of control supply failure to motor, theoperating handle can be used to charge the spring. With the spring charged,the breaker can be locally closed by actuating the push button.

6. 3 Vacuum Circuit Breakers (VCB)

Vacuum CBs have been used for breaking currents in 6.6 kV switchgear pan-els, i.e. for all loads above 200 kW. Vacuum CB have very low maintenance,and very high reliability.

The vacuum is used as an arc extinguishing medium in VCB. The vacuum assuch is a dielectric medium and arc cannot persist in ideal vacuum. However,the separation of current carrying contacts causes the vapour to be releasedfrom contact giving rise to plasma. Thus, as the contacts separate the contactspace is filled with vapours of positive ions liberated from contact materials.The vapour density depends on the current in the arc. During the deceasingmode of current wave the rate of release of the vapour reduces and after thecurrent zero, the medium regains dielectric strength provided vapour densityaround contacts has substantially reduced.

The arc generally has several parallel paths originating and sinking in ahot spot of current. Thus the total current is divided into several arcs. Theparallel arc repel each other so that the arc tends to spread over the con-tact surface. Such an arc is called diffused arc. The diffused arc can getinterrupted easily. These CBs employ zero current interruption principle.Very small amount of arc is developed by circuit breaking because there isno ionized medium inside the vacuum bottle. Arc is extinguished during zerocurrent of AC wave by natural cooling. The VCBs have anti-pumping andtrip free operational feature.

ANTI-PUMPING: After a close-open cycle, it is not possible to reclose the

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breaker as long as the closing command is maintained. This feature known asanti-pumping, is assured mechanically, both in local and remote control op-erations. For reclosing the breaker, the permanent closing command shouldbe momentarily interrupted.

TRIP-FREE: The moving contacts of the breaker return & remain in openposition, when the opening operation is initiated after initiation of closingcommand. In this case, the closing command is overruled by the trippingcommand.

Table 6.2: VCB Specifications

Circuit Breaker Type VacuumMaximum Rated Voltage 7.2 KVOne Minute Withstand Voltage 28 KV (RMS)Current Rating At TAPP 3&4 2500A, For Incomer Of Class IV 6.6 KV.

2000A, For PCP Motor Feeder.1250A, For All Other Feeders.

Rated Breaking Capacity 44 KA (RMS)Rated Making Capacity 110 A (PEAK)Total Opening Time ¡ 65 msArcing Time ¡15 msTotal closing Time ¡ 80 msTrip Free & Anti Pumping Feature ProvidedOperating duty Cycle O-3min-CO-3min-COAuxiliary Control Voltage For closing/tripping coil - 220V DC

For spring charging motor-220 V DC

6. 4 SF6 Gas Circuit Breaker

200 kV GIS circuit breaker (HB9 type) and 400 kV GIS circuit breaker (HB10type)

Each CB comprises of:

• 3 metal clad breaker poles, each pole being actuated by its operatingmechanism, one supporting frame for the three poles.

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• Each pole is provided with one single break interrupt of the single pres-sure puffer type with separate contact system for carrying continuouscurrent and for arching whereby control erosion is reduced to a negli-gible level ensuring long life.

• Simplicity of interrupt operation: the moving contact with a compres-sion cylinder, which, during tripping operation generates the pressur-ized SF6 gas, required for arc quenching

• Only minor over voltage of switching of small inductive currents, owingto optimized interruption process which prevents current chopping.

6. 4.1 Properties of SF6

• In pure form it is inert, exhibits exceptional thermal stability and hasexcellent arc quenching properties as well as exceptional high insulatingproperties, one of the most stable component, non-flammable, non-toxic and odorless.

• Its density s more than that of air and heat dissipation in it is alsomuch more than that in air. At the atmospheric pressure the dielectricstrength is about 2.4 times that of air at about 3 kg/cm2 it is same asthat of oil.

• There is some decomposition of gas after the long periods of arcing.However such decomposition is very little and has no effect upon di-electric strength and interrupting capability. The solid arc productformed by arcing is metallic fluoride which appears in the form of finegray powder. This powder has high dielectric strength under dry con-dition as existing in the breaker. A good quality absorbent is used inthe apparatus to remove most of the gaseous decomposed by-productsso the level of this gaseous by-product is kept very low.

6. 4.2 Operating Principle

A circuit breaker in which the current carrying contacts operate in SulphurHexafluoride or SF6 gas is known as an SF6 Circuit Breaker.SF6 has ex-cellent insulating property. SF6 has high electro-negativity. That means ithas high affinity of absorbing free electron. Whenever a free electron collideswith the SF6 gas molecule, it is absorbed by that gas molecule and forms anegative ion.

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The attachment of electron with SF6 gas molecules may occur in tow differentways,

• SF6 + e = SF6-

• SF6 = e = SF5- + F

These negative ions obviously much heavier than a free electron and there-fore over all mobility of the charged particle in the SF6 gas is much less ascompared other common gases. We know that mobility of charged particle ismajorly responsible for conducting current through a gas. Hence, for heavierand less mobile charged particles in SF6 gas, it acquires very high dielectricstrength. Not only the gas has a good dielectric strength but also it has theunique property of fast recombination after the source energizing the sparkis removed. The gas has also very good heat transfer property. Due to itslow gaseous viscosity (because of less molecular mobility) SF6 gas can ef-ficiently transfer heat by convection. So due to its high dielectric strengthand high cooling effect SF6 gas is approximately 100 times more effectivearc quenching media than air. Due to these unique properties of this gasSF6 Circuit Breaker is used in complete range of medium voltage and highvoltage electrical power system. These circuit breakers are available for thevoltage ranges from 33KV to 800KV and even more.

6. 4.3 Types of SF6 Circuit Breaker

There are mainly three types of SF6 circuit breaker depending upon thevoltage level of application:

1. Single Interrupter SF6 Circuit Breaker applied for up to 245KV(220KV)system

2. Two Interrupter SF6 Circuit Breaker applied for up to 420KV(400KV)system

3. Four Interrupter SF6 Circuit Breaker applied for up to 800KV(715KV)system

6. 4.4 Working of SF6 Circuit Breaker

The working of SF6 circuit Breaker of first generation was quite simple itis some extent similar to air blast circuit breaker. Here SF6 gas was com-pressed and stored in a high pressure reservoir. During operation of SF6circuit breaker this highly compressed gas is released through the arc and

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collected to relatively low pressure reservoir and then it pumped back to thehigh pressure reservoir for reutilize.

The working of SF6 circuit breaker is little bit different in modern time. In-novation of puffer type design makes operation of SF6 circuit breaker mucheasier. In buffer type design, the arc energy is utilized to develop pressure inthe arcing chamber for arc quenching.

Here the breaker is filled with SF6 gas at rated pressure. There are twofixed contact fitted with a specific contact gap. A sliding cylinder bridgesthese to fixed contacts. The cylinder can axially slide upward and downwardalong the contacts. There is one stationary piston inside the cylinder whichis fixed with other stationary parts of the breaker, in such a way that it cannot change its position during the movement of the cylinder. As the piston isfixed and cylinder is movable or sliding, the internal volume of the cylinderchanges when the cylinder slides. During opening of the breaker the cylin-der moves downwards against position of the fixed piston hence the volumeinside the cylinder is reduced which produces compressed SF6 gas inside thecylinder. The cylinder has numbers of side vents which were blocked by up-per fixed contact body during closed position. As the cylinder move furtherdownwards, these vent openings cross the upper fixed contact, and becomeunblocked and then compressed SF6 gas inside the cylinder will come outthrough this vents in high speed towards the arc and passes through the ax-ial hole of the both fixed contacts. The arc is quenched during this flow ofSF6 gas.

During closing of the breaker, the sliding cylinder moves upwards and asthe position of piston remains at fixed height, the volume of the cylinderincreases which introduces low pressure inside the cylinder compared to thesurrounding. Due to this pressure difference SF6 gas from surrounding willtry to enter in the cylinder. The higher pressure gas will come through theaxial hole of both fixed contact and enters into cylinder via vent and duringthis flow; the gas will quench the arc.

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Figure 6.3: Vacuum Circuit Breaker

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Figure 6.4: Working of SF6 Breaker

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Generator

7. 1 Introduction

The generator is a two pole, cylindrical rotor provided with hydrogen cool-ing of rotor windings and water cooling of stator windings, including phaseconnecting busbar, terminal bushings. The losses in other parts of gener-ator such as stator iron losses, frictional & winding losses are removed bycirculating through air gap. The generator stator frame is of pressure resis-tant and gas tight connection with horizontal coolers. The frame itself formspart of ventilation and closed loop cooling system. The generator consists offollowing components:

• Stator : Stator body, stator core, stator winding & gas coolers.

• Rotor : Rotor shaft, rotor winding, rotor retaining rings & field connec-tions.

• Bearings and Brush Gear assembly

• Shaft Seals :

For the operation of Generator following auxiliary systems are required:

• Seal Oil System.

• Gas System (hydrogen cooling system).

• Stator Water Cooling System.

• Excitation System.

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7. 2 Constructional Detals

7. 2.1 Stator

Stator body is totally enclosed gas tight fabricated structure made up of highquality mild steel & austenitic steel. It is suitable ribbed with annular ringscalled inner walls to ensure high rigidity and strength. The entire stator coreis made up of thin laminations, in order to minimize the magnetising & eddycurrent losses. The stator has a 3-phase double layer, short pitched & bartype of winding having two parallel patches. Each slot accommodates twobars. The slot lower bar & the slot upper bar are displaced from each otherby one winding pitch and connected at their ends to form a coil group. Thecoil group are connected together by busbar inside the stator frame.

7. 2.2 Rotor

The rotor shaft is forged, measuring more than 9 meters in length and slightlymore than 1 meter in diameter. The main constituents of the steel arechromium molybdenum, nickel and vanadium. The rotor body is providedwith longitudinal slots to accommodate field winding. The field windingconsists of several coils inserted into the longitudinal slots of the rotor body.The rotor winding is solidly connected to slip rings by means of field leadbars, current carrying bolts, field lead core bar and flexible leads. The fieldcurrent to the rotor winding is provided through the brush gear. The cur-rent carrying brush gear assembly is rigidly supported to an independentpedestal (base) on the exciter side. Brush holders are fitted and cooled by 4gas coolers mounted longitudinally inside the stator body. Gas coolers con-sists of cooling tubes made up of cupro-nickel with coiled copper wire woundon them to increase the surface area of cooling. Cooling water flows throughthe tubes while hydrogen flowing across the cooler cones in contact with theexternal surface of the cooling tubes.

7. 2.3 Retaining Rings

The overhanging portion of field winding is held by non-magnetic steel forgingof retaining ring against centrifugal forces. They are shrunk fitted to the endsof rotor body barrier at one end, while the other side of retaining ring does notmake contact with the shaft thus ensuring an unobstructed shaft deflectionat the end winding and eliminating the chances of corrosion. The counteringrings are shrunk fitted at the free end of the retaining rings, sealing endwinding in axial direction at the same time. To reduce stray losses, the

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retaining rings are made of non-magnetic austenitic steel and cold worked ,resulting in high mechanical strength.

7. 2.4 Fan

The generator cooling gas is circulated by two single stage axial flow propellertype fans. The fans are shrunk fitted on either sides of rotor body. Fan hubsare made up of alloy steel forging with 3 peripheral grooves milled on it. Tocheck the fan blades from coming out of the hub, ground tapered pins areused.

7. 2.5 Stroboscope

This instrument has been specially developed and constructed for monitoringthe Brushless excitation system of turbo generator. It involves synchronizingthe flashing rate with the speed of the generator so that a completely sta-tionary image is produced. The built in periodicity adjustment (forward &reverse motion) enables the timing of the operation to be rearranged.

This controller was specially developed for stroboscope monitoring of brush-less exciter fuses. In order to be able to synchronize the flashing rate withthe speed of the generator, the supply frequency is employed as trigger.

With the help of phase locked loop (PLL) auxiliary frequency is divertedfrom the supply frequency using adjustable dividers. Depending on the pre-selection it is possible to produce a slightly different frequency (lower orhigher), resulting in a slowly rotating image. In case of image advance, thenumbering of the fuses increases during observation, with image reverse itdecreases. The continous flashing rate can be interrupted any time by de-activating the switch. This results in a stationary image which ensures theaccurate observation of just one fuse.

After about 90 sec (approximately 4 rotation), the instrument automaticallyswitches itself OFF. If this does not produce sufficient observation time, theinstrument can be switched ON again without incurring any delay for further90 sec by simply pressing the ON switch again. Special consideration wastaken during the design of this instrument into its serviceability. In orderto achieve minimum Down Time in the event of a fault, the entire circuitis mounted onto 4 plug in PCBs. Each of the board has monitoring pointswhere the function of the board can be checked with a measuring instrument.If a fault develops, the whole circuit board can be replaced.

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7. 3 Generator Excitation and AVR

Excitation system used in taps 3 & 4 is brushless excitation supplied byBHEL. The automatic voltage regulator (AVR) regulates the voltage (and /or the flow of reactive power during parallel operation) from the synchronousmachine (generator) by the direct control of main exciter field current using(static) thyristor converters.It consists of pilot exciter, main exciter, rotating diodes assembly and rotormain field winding on one hand and regulation circuit, thyristor circuit andfield suppressor circuit on the other. Combined together they perform thefollowing functions:

• Control generator voltage in isolated mode.

• Control reactive power when synchronised with the grid.

• To limit the generator MW & MVAR capabilities to the extent allow-able to its safe operation

• To provide important measured values in MCR panel and exhaustiveSCADA.

• To give alarm in MCR in case of abnormal condition with an exhaustivelist on local DCPS (Digital Processor System).

• Through fully automatic redundant channels for smooth and effectivecontrol of static excitation system.

• Immediate tripping of excitation in case of malfunctioning of controlcircuit.

7. 3.1 Power Circuit

The source is PMG (Permanent Moving Generator ) moving on the extremeend of the rotor. It consists of rotating magnet as a rotor and a fixed statorcontaining three phase distributed windings. Whenever the rotor is rotating,induced EMF is generated and if the circuit is complete, power will flow(hence is called a generator). This is also called as pilot exciter. Next in thepower circuit comes the thyristors. The power from PMG is converted toDC by thyristor. The firing angle of these thyristor is adjusted by separatecircuit and is explained in the control circuit part which follows later. Therectified output from the thyristors is now fed to the field winding of the mainexciter. The main exciter is basically an amplifier consisting of stationary

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field winding and a rotating armature winding. By regulating the DC input tothe field winding corresponding changes can be brought about in the rotatingarmature windings. To the output of this revolving armature windings areconnected diode assembly. These converts the three phase AC output fromthe main exciter winding to a DC and ultimately feeds the main field windingof the generator rotor.

7. 3.2 Control Circuit

The main function of the control circuit is to control the power circuit so asto bring about the required final effect of generator voltage / reactive power.The inputs to the control circuit is from PTs and CTs connected in the out-put of the main generator. The voltage and current inputs are processed inthe AVR which is the heart of the control circuit. The converted values arecompared to the reference values available as Pref digital information. Anydeviation is corrected by changing the firing angle of the thyristors of powercircuit. The control in normal operation is done by one of automatic CH-1 &CH-2. The voltage is intended for excitation and control of generator equip-ment with alternator exciter employing rotating non controlled rectifier.Theexcitation equipment of the generator and its interconnections with voltageregulators.

7. 3.3 Description and general Information with im-portant operating parameter

There are three main part of voltage regulator equipment. They are reg-ulator, thyristor and field suppressor. Regulation is having two automaticchannels for controlling generator voltage. These two auto channels haveindependent gate control unitExcitation of generator is started by closing the field circuit breaker Q2 andby switching on excitation which leads to releasing of pulses to thyristorgates. The converter TY has been broken down for this purpose into twoautonomous converter blocks TY1 & TY2. Each converter block can cater100% of the system requirement. Thus when both the converter are in oper-ation 2 X 100% of the system requirement is available. Both blocks has itsown Final Pulse Stage and a monitoring of the equipment.Redundancy in regulator section is ensured by means of fully separate Auto-matic channels with independent measuring inputs and extensive monitoring.

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7. 3.4 Power Supply System

In brushless excitation system here, the PMG provides the power supply forthe converter. It also supplies the synchronising supply to both channels andthe gate control. Unit need these voltage to issue the pulses at a given firingangle relative to input voltage of the converter.Two station CLASS-I supply is provided for tripping control of the fieldcircuit breaker. In addition to this, one CLASS-I 220 V DC & one CLASS-IV-415 V three phase 50 Hz supply converted to 220 V DC and paralleledwith station DC supply is used for all controls of AVR and closing supply ofFCB.

7. 3.5 Field Circuit Breaker

The CB in the field circuit is used to isolate the field circuit from the con-verter. It is capable of switching off the synchronous machine from full loadunder the maximum condition of three phase short circuit. In addition to itsmain contacts, the field CB also has a de-excitation contacts with its fieldenergy stored in the field can be dissipated across the de-excitation resistor.The de-excitation contact closes shortly before the main contact open so asto ensure proper commutation of the field current from the main contact tothe de-excitation contact when the breaker is switched off. The field CB isswitched on by the electromagnetic forces and is kept switched on by a me-chanical latch. When the latch is released by a trip coil, the CB is thrown.The CB also has auxiliary contacts that reports its status.

7. 3.6 De-excitation

When malfunction occurs, the stored field energy must be dissipated asquickly and safely as possible to protect the generator. This is done by theconverter, the field breaker and the de-excitation resistor. The de-excitationtakes place in following stages:

• The converter, driver to its inverter limit position (negative ceilingvoltage), recovers a portion of field energy into the network. A tripcommand is given to the field CB.

• The de-excitation contact closes, diverting the field voltage to the de-excitation resistor.

• Then immediately, the main contact open, building voltage. The fieldvoltage commutates to the de-excitation resistor.

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• The current diminishes at a given time constant TE: Lf * (Rf + Re)Due to reversal of field voltage by the converter, the filed current com-mutates from the main contacts of the field CB to the de-excitationresistor in a very early phase. This reversal of field voltage preventsburn off on the main contacts and provide effective protection for thefield CB.

Depending on the operating policy, an operational shutdown of the excitationcan also be affected with field CB closed. This method is useful mainly whenthe excitation is switched on and off frequently. In this case, the converter ismerely driven into the inverter limit position so that field energy is recoveredinto the network. The converter then blocks since it is supplying positivecurrent only.

7. 3.7 Cooling System

The heat losses arising in the generator interior are dissipated to the sec-ondary coolant through hydrogen and primary water. Direct cooling es-sentially eliminates hot spots and differential temperature between adjacentcomponents which could result in mechanical stress, particularly to the cop-per conductor, insulation, rotor body and stator core.

1. Hydrogen Cooling SystemThe hydrogen is circulated in the generator interior in a closed circuitby one multistage axial fan arranged on the rotor at the turbine end.Hot gases is drawn by the fan from the air gap and delivered to thecoolers where it is re- cooled and then divided into three flow pathsafter each cooler.

Flow-path 1 is directed into the rotor at the turbine end below thefan hub for cooling of the turbine end and half of the rotor.

Flow path-II is directed from the coolers to the individual frame com-partments to cool the core.

Flow path-III is directed to the exciter-side stator-end winding viacross-over ducts in the casing to cool the exciter-side rotor half andthe end sections of the core.

Having fulfilled their cooling function, the three flows are directed tothe air gap where they are mixed and returned to the fan and thus tothe cooler.

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The hydrogen temperatures are controlled automatically by varying thesecondary cooling water flow through H2 coolers to maintain a uniformgenerator temperature level at various load and secondary cold-watertemperature.

2. Cooling of RotorFor direct cooling of rotor winding, cold gas is directed to the rotorand windings at the turbine and exciter ends. The rotor winding issymmetrical relative to the generator central line and pole axis. Eachcoil quarter is divided into two cooling zones. The first cooling zoneconsists of the rotor end winding and the second one of the windingportion between the rotor body end and the midpoint of the rotor. Coldgas is directed to each cooling zone through separate opening directlybefore the rotor body end. The hydrogen flows through each conductorin close cooling ducts. The heat removal capacity is selected such thatapproximately identical temperatures are obtained for all conductors.The gas of the first cooling zone is discharged from the coils at the polecentre into a collecting compartment within the pole area below theend winding. From there the hot gases passes in the air gap throughpole face slots at the end of rotor body. The hot gas of the secondcooling zone is discharged into the air gap at the midpoint of the rotorbody through radial openings in the hollow conductors and wedges.

3. Cooling of Stator CoreFor cooling of stator core cold gas is admitted to the individual framecompartments via separate cooling gas ducts. From these frame com-partments the gas then flows into the air gap through slots in the corewhere it absorbs the heat from the core. To dissipate the higher lossesin the core ends, the cooling gas slots are closely spaced in the coreend sections to ensure effective cooling. These ventilating ducts aresupplied with cooling gas directly from the end winding space. An-other flow path is directed from the stator end winding space past thedamping fingers between the pressure plate and core end sections intothe air gap. A further flow path passes into the air gap along eitherside of flux shield.

All the flow mix in the air gap and cool the rotor body and statorcore surface. The gas is then returned to the coolers via axial flow fan.To ensure that the cold gas directed to the stator end cant be directlydischarged into the air gap, an air gap choke is arranged with the range

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of the stator end winding cover and the rotor retaining at the exciterend.

4. Primary Cooling Water CircuitThe trearted water used for cooling of the stator winding phase connec-tors and bushings is dissipated as primary water in oirder to distinguishit from the secondary coolant ()raw water, condensate etc). the pri-mary water is circulated in a closed circuit and dissipates the absorbedheat to the secondary cooling water in the primary water cooler. thepump is supplied with hot primary water from the primary water tankand delivers the water to the generator via the coolers. The cooledwater flow is divided into two flow paths:

Flow path 1 cools the stator windings. The flow path first passes to awater manifold on the exciter end of the generator and from there tothe stator bars via insulated hoses. Each individual bar is connectedto the manifold by a separate hose. Inside the bars the cooling waterflows through hollow strands. At the turbine end, the water is passedthrough similar hoses to another water manifold and then returned tothe primary water tank. Since a single pass water flow through stator isused, only a minimum temperature rise is obtained for both the coolantand the bars. Relative movement due to different thermal between thetop and bottom bars are thus minimized.

Flow path 2: it cools the phase connectors and the bushings. Thebushings and the phase connectors consist of thick walled copper tubesthrough which the cooling water circulates. The six bushings and thephase connectors arranged in a circle around the stator end windingare hydraulically inter connected so that three parallel flow paths areobtained. The primary water enters the three bushings and exits fromthe three remaining bushings. The secondary water flow through pri-mary water cooler should be controlled automatically to maintain auniform generator temperature level for various loads and cold watertemperature.

7. 4 Technical Data and nameplate Ratings

7. 4.1 Generator

• Rated output- 659 MVA

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• Power factor- 0.85

• Rated terminal voltage- 21 kV 5 %

• Rated phase current- 18.118 KA

• Rated speed- 50s-1

• Rated Frequency- 50 Hz

7. 4.2 Cooling Media

• Cooling of stator winding- H2

• Cooling of rotor winding- H2 direct

• Cooling of stator core- H2

7. 4.3 Peak Short Circuit Current

• 3 Phase- 27.2 kA

• 2 Phase- 43.1 kA

7. 4.4 No load short circuit ratio (saturated)= 0.46

7. 4.5 Permissible unbalanced load= 8%.

7. 4.6 Efficiency at p.f.-0.8, full load = 98.58%

7. 4.7 Stator winding: 6 terminal, double star wind-ing

7. 4.8 Rated Field Current- 4463 A

7. 4.9 Rated Field Voltage- 370 V

7. 4.10 Reactances

• Xd” (saturated)- 19.3%

• Xd’ (saturated)- 27.1%

• Xd’ (unsaturated)-259%

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7. 5 Protections Of Generator

The most important factors, which make protection necessary for a gener-ator, are the electrical stresses imposed on the insulation, the mechanicalforces acting on the various parts of the machine, and temperature rises.

Since every generator is connected to a power system, its protection sys-tem must contain elements which, should a fault occur in the system, willdisconnect the generator in a manner compatible with the protection systemof the power system.

The number and variety of faults to which a generator may be subjected be-ing great, several protective systems are employed, both of the discriminativeand non-discriminative type. Great care has to be exercised in coordinatingthe systems used and the settings adopted, so that a sensitive, selective anddiscriminative protection scheme is available.

As both the main and the unit auxiliary transformers form a part of thegenerator, their protection is inevitably associated with that of the genera-tor and its associated main and unit auxiliary transformers against insulationfailure and the other hazards.

The various forms of protection applied to generator units fall into one ofthe following two categories:

1. Protective relay or device to detect faults occurring outside the gener-ator unit.

2. Protective relay or device to detect faults occurring within the genera-tor unit and the associated connections.

7. 6 Nature of Faults in Generator and their

protection

7. 6.1 Stator Winding Faults and protection

Failure of the stator windings or connection insulation can result in severedamage to the windings and stator core. The extent of the damage will de-pend on the magnitude and duration of the fault current. An earth faultinvolving the stator core results in burning of the iron at the point of fault

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and welds laminations together.Relay application for the stator earth fault is mainly influenced by the methodof stator earthing. When the stator neutral is earthed through a resistor, aC.T. is mounted in the neutral to earth connection. Either an inverse timerelay or an instantaneous relay is used across the C.T. secondary, dependingon whether the generator is directly connected to the station busbars or via adelta/star transformer. In the former case, the inverse time relay will requiregrading with other fault relays in the system; but in the latter case, becausethe earth fault loop is restricted to the stator and transformer primary wind-ing, on discrimination with other earth fault relays is necessary.With resistor earthing, it is impossible to protected being dependent on thevalue of the neutral earthing resistor and the relay setting. When the neutralis earthed through the primary winding of a distribution transformer, earthfault protection is provided by connecting an overvoltage relay across itssecondary, as shown in figure below. The maximum earth fault current is de-termined by the size of the transformer and the loading resistor R. Optimumloading is when the power dissipated in the resistor equals the capacitive lossin the generator system. At this point the transient overvoltages possible areat a practical minimum. Increasing the power dissipation in the resistor be-yond this point increases the energy in the fault arc and therefore the degreeof fault increases.The amount of stator winding protected using distribution or voltage trans-former earthing, depends upon the relay setting, which is expressed as apercentage of the rated secondary output voltage of the transformer. Thusa 10% setting would protect 90% of the winding. The time setting shouldbe chosen to avoid operation due to interwinding capacitance as mentionedabove, a setting of 1.5 sec. at 10x voltage setting being adequate for mostapplications.

7. 6.2 Overcurrent Protection

Overcurrent protection of generators may take two forms. Plain overcur-rent protection may be used as the principle form of protection for smallgenerators, and back-up protection for larger ones where differential protec-tion is used as the primary method of generator stator winding protection.Voltage dependent overcurrent protection may be applied where differentialprotection is not justified on larger generators, or where problems are met inapplying plain overcurrent protection.

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7. 6.3 Overvoltage Protection

Overvoltages on a generator may occur due to transient surges on the net-work, or prolonged power frequency overvoltages may arise from a variety ofconditions.

A sustained overvoltage condition should not occur for a machine with ahealthy voltage regulator, but it may be caused by the following contingen-cies:

• Defective operation of the automatic voltage regulator when the ma-chine is in isolated operation.

• Operation under manual control with the voltage regulator out of ser-vice.

• Sudden loss of load (due to tripping of outgoing feeders, leaving theset isolated or feeding a very small load) may cause a sudden rise interminal voltage due to the trapped field flux and/or overspeed.

7. 6.4 Undervoltage Protection

Undervoltage protection is rarely fitted to generators. It is sometimes usedas an interlock element for another protection function or scheme, such asfield failure protection or inadvertent energization protection, where the ab-normality to be detected leads directly or indirectly to an undervoltage con-dition.Where undervoltage protection is required, it should comprise an undervolt-age element and an associated time delay. Settings must be chosen to avoidmaloperation during the inevitable voltage dips during power system faultclearance or associated with motor starting.

7. 6.5 Rotor Earth Fault Protection

A single earthfault on the field winding or in the exciter circuit of a generatoris not in itself a danger to the machine. Should a second earth fault develop,however, part of the field winding will become short circuited resulting inmagnetic unbalance of a field system with subsequent mechanical damage tothe machine bearings.

Three Methods are available to detect this type of fault. They are:

• Potentiometer Method

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• A.C. Injection Method

• D.C. Injection Method

Each scheme relies upon the rotor earth fault closing an electrical circuit, theprotection relay forming one branch of the circuit.

7. 6.6 Loss of Excitation

The main effects are:

• Causes Asynchronous running (¿ Sync speed).

• Main flux provided by reactive (Stator) current supplied from system,i.e. Operation at leading power factor.

• AC induced in Rotor, which causes heating.

• Stator current limit may be exceeding

Failure of the field system results in a generator operating at above syn-chronous speed as an induction generator, drawing magnetizing current fromthe system. There is no immediate danger to a set operating in this manner.However, overloading of the stator and overheating of the rotor result fromcontinued operation and therefore the machine should be disconnected andshut down if the field cannot be restored. It is often to protect in the eventof a field circuit failure. This is often done with a DC relay set to operatewhen field circuit current falls to around 5% of nominal.

A more sophisticated scheme, recommended for all large, important sets,allows for tripping the machine in the presence of swing conditions resultingfrom loss of field. It initiates load shedding with subsequent tripping of themachine if the field is not restored, and initiates load shedding with subse-quent tripping of the machine if the field is not restored within a prescribedtime. The scheme comprises an offset Mho relay, type YCGF and an instan-taneous under voltage relay, type VAG.

Figure 8 shows typical machine terminal impedance characteristic on lossof excitation plotted on an R-X diagram together with the offset Mho fieldfailure relay characteristic. Relay operation occurs immediately the terminalimpedance locus enters the relay characteristic, in this example approxi-mately 5 sec. after the machine loss its field supply.

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Figure 7.1: Relay Characteristic for loss of excitation

It is usual to offset the relay characteristic by an amount OA along the Xaxis, equal to half the direct axis transient reactance of the machine and makethe diameter of the characteristic AB equal to the direct axis synchronousreactance. In this way operation on power swings and loss of synchronismnot accompanied by loss of field are prevented.

As previously stated, it is not always necessary to shut the set down im-mediately the loss of field relay operates unless there is a danger of systeminstability. The best indication of a systems ability to maintain stability isthe system voltage. Thus the offset mho relay is arranged to shut the machinedown instantaneously only when its operation is accompanied by a collapsein system voltage, this condition being detected by an instantaneous under-voltage relay, set to approx. 70% of normal volts. Should the offset mhorelay operate alone, it is arranged to initiate load shedding of the set downto a safe value and to initiate the master tripping relay after a prescribedtime delay.

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7. 6.7 Unbalance Loading

The main effects are:

• Gives rise to Negative Phase sequence currents in the stator, whichcauses contra rotating magnetic field.

• Stator flux cuts rotor at twice sync speed inducing double frequencycurrent in field system and rotor body.

• Resulting eddy currents causes overheating.

Negative phase sequence currents in the stator, resulting from unbalancedloading, produce a location field rotating at twice synchronous speed withrespect to the rotor and hence induce double frequency currents in the ro-tor. These currents are very large and result in severe over heating of therotor, especially in steam generators with cylindrical rotors. The ability of agenerator to withstand negative phase sequence currents depends upon themachine construction. Cooling is also important and large power stationgenerators use hydrogen as a coolant. The direct cooling method involvesthe coolant being directed down ducts adjacent to the rotor conductors.

It is necessary to limit the time for which negative phase sequence currentscan flow in a steam generator. The time for which a generator may beallowed to operate with unbalanced stator currents with-out danger of per-manent damage is obtained from expression I2T=K where K is a constantdepending on the type of machine and the form of cooling and I2 is the av-erage negative phase sequence current over time T seconds.

Internal stator faults are cleared instantaneously by the differential protectionbut external faults or unbalance resulting from an open circuit may remainundetected or persist for a significant period depending on the protection co-ordination of the system. It is therefore necessary to install a negative phasesequence relay with a characteristic to match the withstand curve of the ma-chine, arranged to trip the main circuit breaker and give an alarm should thecontinuous withstand be exceeded. The generator protection scheme suitablefor this application employs a type CTN relay, at TAPS-3&4.

7. 6.8 Overspeed Protection

While it is the general practice to provide mechanical overspeed device onboth steam and hydro turbines, which operate directly on the steam throttlevalve or main stop valves (stop valves refer to hydro-electric sets only), it is

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not usual to back-up these devices by an over speed relay on steam-drivensets. It is, however, considered good practice on hydro-electric units, as theresponse of the governor is comparatively slow and the set is more prone tooverspeed. The relay when fitted is usually supplied from the permanentmagnet generator used for the control of governor.

7. 6.9 Overfluxing

Overfluxing occurs when the ratio of voltage to frequency is too high. Theiron saturates owing to the high flux density and results in stray flux occur-ring in components not designed to carry it. Overheating can then occur,resulting in damage. It is usual to provide a definite time-delayed alarm set-ting and an instantaneous or inverse time-delayed trip setting, to match thewithstand characteristics of the protected generator.

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Transformers

The types of transformers for the power generation include:

• Generator Transformer(GT)

• Unit Transformer (UT)

• Start-Up Transformer (SUT)

8. 1 Generator Transformer (GT)

The power from the unit will be evacuated over 400kv lines into interstategrid through a bank of three single phase, 210 MVA , 242.49 KV with offload tap changer generator transformer.

The core of GT is made up of high grade non ageing cold rolled grain orientedsilicon steel laminations having high permeability and low hysteresis loss. Athree legged, mitred and interleaved type core construction is adopted. Thetransformer has two windings i.e. HV and LV windings . HV winding iscontinuous disc type. LV winding is helical type. The two windings are con-centric and one winding is placed over the other. They are symmetrical withrespect to the center line of the height of the windings. The HV winding alsohouses the taps of the offload tap changer.

8. 1.1 Bushings

The transformer is provided with oil impregnated paper condenser type HVand HV neutral bushings. LV bushings are high current bushings. LV bush-ings are of oil less type mounted on non magnetic turrets suitable for connect-ing to IPBD. Liquid/oil filled for 420 KV side are equipped with liquid levelindicator and means for sampling and draining the liquid. Oil in oil filledbushings have same specification as main transformer oil. Tank is providedwith a resealing type pressure release device, which operates at a pressure

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below the test pressure of the tank and radiators. It shall be mounted directon the tank. The device shall be rain proof after blowing and is providedwith a device visible from ground to indicate operation. The pressure releasedevice is equipped with remote monitoring / alarm contact.

8. 1.2 Transformer Tank

The tank is of bell shaped construction and has an oil tight flanged joinedat the bottom. The particular part of the tank above the joint can be liftedoff to provide access to the core and coil. The tank is shielded against stayfluxes to avoid any temperature rise. Non magnetic materials are used for LVbushing flanges to avoid heating by high current. The tank can withstandfull vacuum. The top of the tank is suitably stepped for rain water to trickledown. Provision for oil inlet and outlet on either side of the tank is alsoprovided.

8. 1.3 Conservator

It is mounted on a support extending from the transformer tank and is con-nected to tank by a pipeline with a shutoff valve. It has a drain valve at thebottom, a dial type magnetic oil level gauge. The conservator is of sufficientvolume to maintain the oil seal from the minimum ambient temperature of300C up to an oil temperature of 1050C, with oil level varying with the min-imum and maximum visible level.

The conservator is provided with following accessories:

1. Two number of weather proof dehydrating breather with silica gel andoil seal to eliminate constant contact with the atmosphere mounted ona level of about 1400 mm above ground level.

2. A flexible oil resistant air bag is provided in the conservator to preventthe air from coming into contact with the oil. The airbag is designedto withstand repeated expansion and contraction due to changes in theoil level.

8. 1.4 Gas Sealed Conservator

In this, method the contact between transformer oil and atmospheric air iseliminated by providing cushion of an inert gas over oil surface in the con-servator vessel. The gas pressure is always higher than atmospheric pressure

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to avoid ingress of gas. The gas normally used for this purpose is nitrogenhaving high purity and dryness.

8. 1.5 Construction and Operation

High pressure nitrogen at 15MPa flows out of the cylinder and is admitted inthe conservator after passing through multi stage pressure reducing valves.The pressure reducing valve automatically cuts of the N2 gas supply whenthe pressure in the conservator reaches 3-5KPa. Due to increase in ambienttemperature and load, the gas pressure builds up.

The system is designed to release any excess pressure through a pressurerelief valve. When the pressure drops below 3-5KPa, the valve open to admitnitrogen from cylinder and this cycle continues until cylinder becomes empty.The system may be designed to operate alarm under abnormal conditions.

8. 1.6 Bellow and Diaphragm Sealed Conservator

The contact between atmospheric air and transformer oil is prevented by abarrier which is made from a synthetic rubber compound.

In case of bellow type barrier, as oil level in the conservator vessel falls,air is sucked from the atmosphere through a silica gel breather, inflating thebellow. The bellow deflates as oil level goes up. The conservator is also fittedwith pressure vacuum bleeder to pass either oil or such in air in the event ofover filling of conservator.

When diaphragm is used as a barrier between oil and atmospheric air, theconservator vessel is made into two semicircular halves. The diaphragm isheld between the two halves and bolted. As the oil expands and moves up,it pushes the diaphragm upwards. The position of diaphragm is indicated byoil level indicator as the oil level connecting rod is connected to diaphragm.When oil level falls down in the conservator, the diaphragm deflates creatinga vacuum, which is filled by air getting sucked through a silica gel breather.

These types of sealing systems have one advantage over the gas sealed con-servator. If the gas is pressurized to a high level, it gets dissolved in oil. Overa period of time, the amount of gas in oil reaches the saturation point. Ifat this stage, the load on transformer is suddenly dropped, or the ambienttemperature falls severely, the pressure falls, oil becomes supersaturated andgas bubble will be evolved. If there is a pump connected in the cooling cir-

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cuit, it will help generation of bubbles. These bubbles may cause insulationfailures in the region of strong electric field.

8. 1.7 Thermosyphon Filter

Thermosyphon filters are intended for prolonging transformer oil life by ex-tracting harmful constituents like water, acid, etc from oil. Such filters arenormally installed on ONAN or ONAF cooled transformers. Normally thefilter is mounted directly on transformer tank. The filter, generally of cylin-drical shape, has a number of perforated steel trays filled with an absorbentmaterial. The absorbents generally employed are silica gel and active alu-mina, the layer one being slightly more effective. As a result of difference inupper and lower layers of oil in tank of transformer in operation, the oil cir-culates through filter by convection currents. The water absorption capacityof alumina at relative humidity of 60% is about 15% by weight.

8. 1.8 Cooling System

The GT will have the following type of cooling:

1. Oil Natural, Air Natural (ONAN)

2. Oil Natural, Air Forced (ONAF)

3. Oil Forced, Air Forced (OFAF)

For this type of cooling, the transformer is fitted with radiators. 2*100% oilpumps and cooling fans mounted on the radiators. An ONAN rating of 60%OFAF can also be released with this type of cooling arrangement.

8. 1.9 Manholes

They are provided access to HV bushing connection and tap changer con-nection. Hand holes are available for access for LV load connections.

8. 1.10 Marshaling Box

One outdoor type marshalling box housing oil and winding temperature in-dicator is mounted on the side of the transformer tank. All ct leads, contactsof magnetic oil level gauge and Buchholz relay are terminated in the mar-shaling box. Also, relays for remote tap position indication are mounted onthe same.

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8. 1.11 Technical Data

• Name of Manufacturer - TELK, Angamally.

• Type of construction 1 phase core type, two winding, outdoor trans-former.

• Full load rating- 210 MVA.

• Rated current - HV-865 ampere(HV), LV-10000 ampere(LV).

• Rated noload voltage - Hv-420/3 KV, LV 21 KV.

• Type of cooling - ONAN/ ONAF/ OFAF.

• Rated % impedance- 15%.Rated frequency- 50 HZ.

• Winding connections HV star, LV delta,

• Vector group - YND11, tapping on winding-Off load. Ranges - +2.5%to -7.5% in steps of 2.5% in HV winding.

8. 2 Start-Up Transformer

During startup of the plant, SUT will obtain full power from the availableelectric grid and supply it to station electrical auxiliary loads at requiredvoltage levels. Once the plant will be in operation SUT will share 50% loads.However, SUT is rated to meet the entire auxiliary loads of one unit. But dueto provision of GCB, both UTS can be charged from 400 KV grid throughGT. Thus during startup as well as unit operating condition ut1 and ut2 canshare 50% of station load and remaining 50% will be shared by SUT.

This SUT interfaces 220 KV switchyard and 6.6 KV systems for supplyingplant auxiliary loads. The basic aim of getting higher redundancy is achievedby adopting SUT with two secondary windings, which are connected to fourdifferent buses of CLASS-IV, 6.6 KV in div-I and div-II. The capacity ofSUT has been selected on the basis of total maximum running load undernormal operation of the unit with 10% margin. SUT is selected as 220 KVon primary winding and two number of secondary winding, each having 6.9KV on terminal voltage, which falls to 6.6 KV under fully loaded condition.To compensate for small changes in primary supply or variation of secondaryvoltage with progressive loading, onload tap changing arrangement has beenprovide on primary side. Since both the LV windings have approximate equal

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loading, OLTC on HV winding maintains 6.6 KV voltage on both secondarybuses. Onload tap changer will be kept on remote end and with manualcontrol.

8. 2.1 Technical Data

• Manufacturer - Crompton Greaves

• Rated voltage - HV-220 KV, LV1 & LV2-6.9 KV.

• Rated frequency - 50 HZ.

• Rated full load current - HV-184 Amperes, LV1 & LV2 -2930 Amperes.

• Cooling - ONAN, ONAF

• Vector Group - YnynOynd1

• Maximum noload losses - 38 KW.

• Maximum load losses - 204 KW.

8. 3 Unit Transformer

Two UTS of 35 MVA rating are provided for each unit. This UTS will be inservice till generators or GT will be in service, if generator will trip then GTwill start importing and UTS will remain in service continuously. The UTinterfaces generator and 6.6 KV system through generator circuit breakerfor supplying plant auxiliary loads. During normal operating conditions,UT will feed the power to auxiliaries, sharing the loads with SUTS . But ifSUT is required to be shut down or if it trips, the entire load of the systemwill be catered by UT. UT is designed in such a way that it can withstandsystem variation without any system damage. The capacity of the UTS havebeen selected on the basis of the total maximum running load under normaloperation of the unit with 10% margin. UT is selected as 21kv on primarywinding and secondary winding having 6.9 KV as terminal voltage on opencircuit which falls to 6.6 KV under fully loaded condition to compensatefor the small changes in the primary supply or variation of secondary voltagewith progressive loading. Onload tap changing has been provided on primaryside. OLTC on HV winding maintains 6.6 KV voltage.

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8. 3.1 Technical Data

• Rated voltage - HV-21KV, LV-6.9 KV.

• Rated frequency - 50 HZ.

• Full load current - HV-963 Amperes , LV-2932 Amperes.

• Vector Group - Dyn1

8. 4 Transformer Faults

• Winding and terminal faults.

• Core faults.

• Tank and transformer accessory faults.

• Onload tap changer faults.

• Abnormal operating conditions.

• External faults.

8. 4.1 Winding Faults

A fault on a transformer winding is controlled in magnitude by the followingfactors:

• source impedance.

• neutral earthing impedance.

• transformer leakage reactance.

• fault voltage.

• winding connection.

8. 4.2 Core Faults

A conducting bridge across the laminated structures of the core can permitsufficient eddy-current to flow to cause serious overheating. The bolts thatclamp the core together are always insulated to avoid this trouble. If anyportion of the core insulation becomes defective, the resultant heating mayreach a magnitude sufficient to damage the winding.

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Figure 8.1: Differential Protection

8. 4.3 Tank Faults

Loss of oil through tank leaks will ultimately produce a dangerous condition,either because of a reduction in winding insulation or because of overheat-ing on load due to the loss of cooling. Overheating may also occur due toprolonged overloading, blocked cooling ducts due to oil sludging or failure ofthe forced cooling system, if fitted.

8. 5 Transformer Protection

8. 5.1 Transformer Differential Protection

Principle of Differential ProtectionSimilar to bus protections, transformers are protected by differential relays.Inter-winding faults (short circuits) and ground faults within power trans-formers can be detected by this protection scheme. Failure to detect thesefaults and quickly isolate the transformer may cause serious damage to thedevice.

A differential relay is basically an instantaneous over current relay that op-erates on the difference of current flowing into and out of the protected zone.For transformers the differential protection is basically the same as that for abus but there are certain differences that we will look more closely at. Thesedifferences are a direct result of three characteristics or a transformer.

• A transformer has a turns ratio so the current in is not really equalto the current out. The current transformers are not likely exactly

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matched to the transformer turns ratio so there will always be an un-balance current in the operating coil of a transformer differential relay.

• Transformers require magnetizing current. There will be a small cur-rent flow in the transformer primary even if the secondary is opencircuited.

• A transformer has an inrush current. There is a time period after atransformer is energized until the magnetic field in the core in alternat-ing symmetrically. The size and the length of this inrush depend on theresidual field in the core and the point in the ac cycle the transformer isre-energized. In large transformers in might be ten or twenty times thefull-load current initially and it might take several minutes to reduceto negligible values.

Transformer differential relays have restraint coils as indicated in Figure be-low. The value of the operate current has to be a certain set percentagehigher than the current flowing in the restraint coils.

The current is very high. The restraint coils also prevent relay operationdue to tap changes, where the ratio of transformer input to output currentcan continuously vary. One other item included in transformer differentialrelays but not shown in the diagram is second harmonic restraint.

When transformers are first energized there is over-fluxing (saturation) ofthe core and the large inrush energizing current has a distorted waveform.This waveform is described as having high second harmonic content. Thetransformer differential relays make use of this known fact and add in extrarestraint when it detects this second harmonic. This extra feature preventsthe transformer from tripping due to magnetizing current when being ener-gized, but does not add any time delay.

Because the differential relay will not operate with load current or faultsoutside the protected zones (through faults), it can be set to operate ata low value of current thereby giving rapid operation when a fault occurs.There is no need to time delay the operation of the relay and therefore afast acting type of relay can be used. Basic Consisderation for TransformerDifferential Protection Relay Settings

• Line current transformer primary ratingsThe rated currents of the primary and the secondary sides of a twowinding transformer will depend on the MVA rating of the transformer

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and will be in inverse ratio to the corresponding voltages. For threewinding transformers the rated current will depend on the MVA rat-ing of the relevant winding. Line current transformers should thereforehave primary ratings equal to or above the rated currents of the trans-former windings to which they are applied.

• Current transformer connectionsThe CT connections should be arranged, where necessary to compen-sate for phase difference between line currents on each side of the powertransformer. If the transformer is connected in delta/star as shown infigure, balanced three phases through current suffers a phase angle of30 degree which must be corrected in the CT secondary leads by ap-propriate connection of the CT secondary windings.

When CTs are connected in delta, their secondary ratings must bereduced to 1/ 3 times the secondary rating of star connected CTs, inorder that the currents outside the delta may balance with the sec-ondary currents of the star connected CTs.

• Inter posing CTs (ICTs) to compensate for mismatch of Line CTsBesides their use for phase compensation, interposing CTs may be usedto match up currents supplied to the differential protection from theline CTs for each winding. The amount of CT mismatch which a re-lay can tolerate with out mal-operation under through fault conditionswill depend on its bias characteristic and the range over which the tapchanger can operate. If the combined mismatch due to CTs and tapchanger is above the accepted level, then interposing CTs may be usedto achieve current matching at the mid point of the tap changer range.

For the protection of two winding transformers interposing CTs shouldideally match the relay currents under through load conditions corre-sponding to the maximum MVA rating of the transformer.

8. 5.2 Gas actuated relay

APSE make buchholz relay of new version is provided. This relay avoidserroneous action of transformer protection caused by self vibration underearthquake condition. The relay is mounted on the pipe between the trans-former and the conservator. In case of fault inside the transformer, violentmovement of gas and oil will be produced from the oil. The velocity of theoil surge and the other actuates one of the float by gas movement. Both

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Figure 8.2: Buchholz relay

the float actuates a set of magnetic reed switches. The gas operated float isused for giving an alarm in the control room and the velocity operated floatgives a trip signal to the transformer. On the buchholz relay two valves areprovided, one is oil drain valve and the other gas release valve. Under nor-mal condition, the gas release valve is kept in open condition and the drainvalve in close condition. Gas release valves are connected to a gas connectionchamber. In this chamber, these valves are gas sampling valve, inlet valveto the chamber, chamber drain valve. One sight glass is also provided on thesampling chamber. During normal operating condition, the gas inlet valve tosampling chamber will be kept in open condition; pipes are completely filledwith oil.

8. 5.3 Pressure Relief Device (PRD)

Two numbers of PRDs are provided on top of transformer main tank. Theseare of spring loaded type. When the pressure inside the transformer tankexceeds the set pressure of PRD, it operates and discharges the gas/oil intothe atmosphere. The same gets reset once the pressure inside falls. Theoperated status of PRD is indicated by the red flag mounted on the device andvisible from outside. This high pressure is generated because of short circuitof the HV bushings and if not rectified can cause damage to transformer tankand it may burst.

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8. 5.4 Magnetic Oil Level Gauge (MOLG)

The oil level inside the main conservator for transformer main tank is mea-sured with magnetic oil level indicator. This also gives an alarm when thelevel falls below the minimum mark (indicated by red mark on the levelgauge). The indicator has a float, which moves up and down depending onthe level of oil inside the conservator .the movement of float is transmittedto the pointer by using a magnetic coupling. One set of mercury switchesare provided for alarm purpose. Level measurement in OLTC conservator isdone with help of level gauge.

8. 5.5 Oil temperature indicator (OTI) and Windingtemperature indicator (WTI)

Heat is generated in a power transformer by current flow in the primary andthe secondary windings as well as internal connections due to I2R losses. Atlow loads, the quantity of heat produced will be small. But, as the loadincreases, the amount of heat becomes significant. At full load, the windingswill be operating at or near their design temperature. The nameplate on atransformer will provide information on the maximum allowable in-servicetemperature rise for its windings and connections and will indicate whatmethod of cooling is employed to remove the heat generated under load. Atemperature of about 105C is considered to be the normal maximum workingvalue for large power transformers, based on an assumed maximum ambienttemperature of 40C. The winding temperature is sensed and indicated by awinding temperature gauge/alarm assembly. A 150 mm dial type thermome-ter measures the Oil temperature. The indicator has two sets of adjustablecontacts. They are connected to give Oil temperature high (alarm) and oiltemperature very high (trip). WTI is a device for measuring the hot spottemperature of the winding. It comprises the following-

• Current transformer.

• Image coil.

• Temperature sensing element.

• Calibration device.

• 150 mm diameter local indicating instrument with four adjustable elec-trical independent ungrounded contacts, two four control of coolingequipment and two for winding temperature alarm and trip.

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• A pointer to register the highest temperature reached and capable ofbeing reset without tools.

• Automatic ambient temperature compensation.

8. 5.6 Cooler System Failure

Local and remote alarms are provided for various conditions of the coolingfailure. Further flow indicators for oil are provided in the cooler controlcabinet for indicating the flow quantities.

8. 5.7 Overfluxing Protection

Increase in power frequency voltage causes increase in working magnetic flux,thereby increases the iron loss and magnetizing current. The core and corebolt get heated and the lamination insulation is affected. Over-fluxing pro-tection is provided for generator transformer and feeder transformer where apossibility of over fluxing due to sustained over-voltages exists. The reduc-tion in frequency also increases the flux density and consequently, it has thesimilar effects as those due to over-voltage.

The expression for flux in the transformer is given as:φ∝V/fwhere, = flux, V = applied voltage, f = frequency and all are p.u. values.When V/f exceeds unity, it has to be detected. Usually 10% of Over fluxingcan be allowed without damage. If V/f exceeds 1.1, over fluxing protec-tion operates. Over fluxing does not require high speed tripping and henceinstantaneous tripping is undesirable when momentary disturbances occur.But the transformer should be isolated in 1-2 min if over fluxing persists. Thealarm is definite time delayed whilst the trip characteristic may be selectedas either definite time, or an IDMT curve.

8. 5.8 Earth Fault Protection

Earth fault protection of transformer can be in one or more types such as:

• Residually connected earth fault protectionDelta windings and ungrounded star windings are best protected byzero-sequence overcurrent relays (Earth fault relays) supplied by CTssituated at the terminals of power transformer.

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Such relay can only operate for a ground fault in the transformer wind-ing since it does not have an earth connection through which it cansupply an external fault.

The relay is usually instantaneous but must be of high impedance typeif supplied with residually connected CTs in the three phases. The highimpedance relay is required to prevent wrong operation of the relay onfalse residual currents during heavy external fault between phases dueto transient differences in the CT outputs. An ordinary overcurrentrelay is acceptable if it is supplied by a core-balance type CT becausein this case, the magnetic conditions of the CTs are the same for allthe three phases.

• Neutrally connected earth fault protectionThe relay is connected across the secondary of a CT whose primary isconnected in the neutral to earth connection of a star connected trans-former. The fault current finds a path through the earth and earth toneutral connection of the transformer.

The magnitude of the earth fault current is dependent on the typeof earthing and the location of the fault.

In both the above types of protection the zone of protection cant beaccurately defined. The protected area is not restricted to the trans-former winding alone. The relay may sense an earth fault beyond thetransformer winding depending upon position of the source.

Hence, such protection is called unrestricted earth fault protection.In residually connected earth fault relays where the zone of protectionis not restricted to transformer winding only and in neutral connectedearth fault relays, IDMT earth fault protection co-ordinated with down-stream is to be provided.

• Restricted earth fault protectionWhen the primary winding is delta connected or star connected withoutneutral earthing, earth faults on secondary side are not reflected on theprimary side as the zero sequence impedance between the primary andsecondary is infinite (i.e., open). In such cases an earth fault relay con-nected in the residual circuit of 3 CTs on primary side will operate oninternal faults in primary winding only. During external ground faultthe sensitivity of low impedance relay is limited by the fact that the

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magnetizing current of the neutral CT is three times that of each of the

If this resistance balance does not exist, it can theoretically be remediedby adding resistance on the neutral CT side, but this is not the practicebecause the balance would not hold during transient conditions or if theneutral CT saturates. The proper solution is to use a stabilizing resis-tance in series with low impedance relay or to use high impedance relay.

This protection is based on high impedance differential principle, offer-ing stability for any type of fault occurring outside the protected zoneand satisfactory operation for faults within the zone.

8. 5.9 Backup Protection

IDMT overcurrent protection is provided in the the HV circuits ground faultsare covered by a numerical relay which is high speed, high impedance, circu-lating current type HV phases and neutral as back up protection for trans-former internal and external faults. The CTSs for back up phase overcurrent,ground overcurrent are provided in the transformer bushings. The underimpedance protection provided in the generator will also serve as back upprotection for fault inside the transformer.

8. 5.10 Lightning Arrestors

Three station type lightning arrestors and of zinc Oxide type are providednear HV terminal. The arrestors are mounted on the separate structureinside the transformer fence. The lightning arrestor is located as close to thetransformer as possible.

8. 5.11 Dissolved gas analysis (DGA)

Insulating oil in the transformer may break down to produce hydrocarbongases, due to arching inside the transformer. Thus DGA technique is a pow-erful diagnostic tool to find out the incipient faults in the transformer.

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Table 8.1: Characteristic Transformer Electrical FaultsType offaults

Causes Effects actuationof Buch-holzRelay

Majorcompo-nents ofGasesevolved

Highenergydis-charges(arcing)

Short circuits inthe windings.External short cir-cuit from parts atpotential to earth.Breakdown be-tween the windings.conductors

Pyrolytic decompo-sition of insulatingoil.Formation of oilcarbon.Decreases in theflash point of oil.

Suddenlyactua-tions

Methane,Hydro-gen &Acetylene(Carbonmonoxideif solid in-sulation isinvolved.)

Highenergypartialdis-chargeswithtracking

Poor impregnationPresence of cavitiesin the insulation.Electrical over-stressing of theinsulation.

Ionization pro-cesses. (Excitationand dissociationof hydrocarbonmolecules bycollision withhigh-energy elec-trons, ions, atomichydrogen etc.)

Lowenergypartialdis-chargeswithouttracking

Poor impregnationor cavities in the in-sulation

Ionization pro-cesses

Methane,Hydrogen.

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Table 8.2: Characteristic Transformer Thermal FaultsType offaults

Causes Effects actuationof Buch-holz Relay

Majorcompo-nents ofGasesevolved

Overhaulingto temper-aturebetween150C &300C (Hotspot).

Excessive magneticlosses and Insufficientcooling.

Slight decompo-sition of oil.

Aftera longperiod

Hydrogen,Methane,Ethylene,(Ethylenepredomi-nates), NoAcetylene.

Local over-heating(300C to1000C)

High circulating cur-rents in the core

Decompositionof oil with for-mation of oilcarbon

After sometime

Hydrogen,Methane,Ethylene,(Ethylenepredomi-nates), NoAcetylene.

Local over-heatingbeyond1000C

Shorting links be-tween core laminates

Decompositionof oil with for-mation of oilcarbon. De-struction oforganic insula-tion. Meltingspots, Coreburn, meltedconductors andscorching points.

Actuationof relayafter accu-mulationof littlequantitiesof gases.

Hydrogen,Methane,Ethylene,(Ethylenepredom-inates),Consid-erableamount ofAcetylene.

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Online Oil Purification Method

For the first time at NPCIL (Nuclear Power Corporation of India Limited),online purification of transformer oil was done at TAPS 3 & 4. Online impliesthat the Generator Transformer was not shut-down to purify the transformeroil. GT contains 36 kL of oil and we require five cycles to completely purifythe oil.

9. 1 Importance of Transformer Oil

• Oil as as insulation between the windings.

• It acts as a cooling agent.

• Oil sampling also lets us detect internal faults inside the transformer,for extreme case, if acetylene is present inside the oil, than the paperwindings of the transformer is damaged.

9. 2 Why Oil Purification is Required?

According to IEEE, there has to be certain proportion of gases mixed withthe oil above which the oil purification needs to be done. Some of the gaseswhich are present in oil are:

• CO2

• H2

• CO2

• O2

• C2H2

• C2H4

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• CH4

• H2O

9. 3 How is Online Purification Different from

Offline Mode?

Since the transformer is in generating state, we need to be extra cautiouswhile doing online purification. Oil is taken directly from the transformertank, purified in the de-gasing chamber and then transferred to the trans-former tank directly in offline mode but in online mode, the oil is transferredthrough the conservator tank. This is done because the oil gushes in thetransformer tank with a high pressure which can damage the windings, sincethe windings are charged in online mode. Also, flow rate of the oil is around22 L/min in online mode which is much less as compared to offline mode(3000 L/hour).

9. 4 Process for Oil Filtration

The components used for purification of oil are listed below:

1. Pre-filterOil taken from transformer tank first passes through the Pre-filter tank,which is used for absorbtion of magnetic particles present in the oil.

2. Inlet PumpInlet pump with a power rating of 1.5 KW/1405 rpm, is used to pumpthe oil to the heater tank.

3. heater TankThe solubility of gases decreases as the temperature increases. There-fore, the temperature of the oil is maintained at 600C, so that the gasesbecome insoluble in the oil.

4. Coarse Filter and Fine FilterThese are used to remove filter the impurities in the oil.

5. De-gasing ChamberThe main chamber for purification is de-gasing chamber. Hot oil flowsthrough the chamber. Here the gases present in the oil are removed

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through a vacuum pump and the purified oil is transferred to the con-servator using the outlet pump.

6. NRVNon-Return Valves allow the flow of oil in a particular direction onlyand prevents the back flow of the oil.

7. TrapTrap is used to absorb moisture present in the gases released in thedegasing chamber

8. Vacuum PumpVacuum pump is used to create vacuum in the de-gasing chamber.

9. 5 Conclusion

Though it takes much more time and regular monitoring for purifying the oilin online mode, the only advantage for doing this is that we do not need toshut down the GT for oil purification, which implies we can generate powerand at the same time purify the oil.

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Switchyard

Switchyard in the nuclear power station is the link between the power stationand the electrical grid for evacuation of power generated and also for drawingstartup power. The sophisticated state of gas insulated switchyard(GIS) atTAPS 3 & 4 is situated indoor to prevent saline deposition and corrosioneffect. Based on techno-economic study 220 KV GIS having 220 KV tie lineto the existing TAPS 1 & 2 switchyard to provide startup power and 400 KVGIS have been provide for power evacuation at TAPS 3 & 4.

10. 1 400 kV GIS

The 400KV switchyard consist of GIS housed inside a civil building andoutdoor conventional equipments for takeoff overhead conductors. Copperconductors connects 400 KV switchyard to GT 3 and 4 and its control arelaid through underground cable trenches in cable trays. GIS has in totalprovision for 9 bays, 4 for line feeders, 1 for bus coupler and bus VTs, 2 forGT feeders and 2 for future purpose.

The switchyard has been divided into 9 no. of Bays as follows:Each bay consist of all the devices attached to the wiring diagram bus-

bar components, circuit breakers and various disconnectors, the total controlmonitoring cubicle for the bay devices and local bay control panel. Out of9 bays, 7 bays are in present day utilization. The 4 number of feeder baysconsists of two double circuit lines, one for Phadge and another for Boisar.However one circuit line is enough to evacuate 1050 MWe of power generatedfrom the two turbo generators.

The major aspects connecting the 400 KV system are as follows:

• Any bus can be taken out for maintenance by transferring all the feedersto the other bus without affecting or initiating any load shedding.

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Figure 10.1: Gas Insulated Switchgear

• CB of both the transformer feeders can be taken out of service formaintenance one at a time by substituting it with bus coupler breaker,without interrupting the concerned circuit from the service

Limitations of AIS

• Large dimensions due to statutory clearances and poor dielectric strengthof air.

• Wastage of space .

• Life of steel structures.

• Seismic instability.

• Large planning & execution time.

• Grounding-mat is essential for containing touch and step potentials

• Hot line washing and regular maintenance of the substation is essential,requires spares inventory.

• man-power.

• Insulation deterioration with ambient conditions and susceptibility topollutants.

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Table 10.1: 400 kV GIS bays

Bay no. Circuit Breaker No. Alloted For

1 513-CB-1 Feeder for line 12 513-CB-2 Feeder for GT-33 513-CB-3 Feeder for line 24 513-CB-4 Feeder for bus coupler

& VTs5 513-CB-5 Feeder for line 36 513-CB-6 Feeder for GT-47 513-CB-7 Feeder for line 48 513-CB-8 Feeder for future line9 513-CB-9 Feeder for future line

The need for GIS

• Expansion / up-rating of existing substation.

• Non availability of sufficient space for substation.

• Difficult climatic and seismic conditions at site.

• Urban site (high rise bldg).

• High altitudes.

• Limitations of AIS.

10. 2 Local Bay Controller

A dedicated micro processor based local bay controller provided for each bayin GIS building. It can measure electrical parameters of feeders, control ofall local bay equipments, monitoring healthiness and abnormal condition ofbay equipments and flashing an alarm on LED screen. By using soft buttonsvarious measurement values and monitoring values can be assessed.

10. 3 Components of GIS

10. 3.1 Power Line Carrier Communication

To provide independent communication between tarapur project and othersubstations, PLCC has been used. Here, audio signal is superimposed on

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carrier signal and is transmitted by overhead transmission lines. The samecarrier signal is also used for telemetry and tele protection. The carriercommunication system is connected to 400 KV systems through CVTs andwave traps. Wave traps are provided at the sending and receiving ends oftransmission lines and are in series with them. Wave traps works like lowpass filter offering high impedance to the PLCC signal and low impedanceto low frequency power wave and allows it to enter the plant substation.PLCC equipments are located in switchyard control room and are poweredby dedicated 48 V battery charger and battery bank.

10. 3.2 Current transformer(CT)

Constructional DetailsThe CT is of ring type. The straight conductor passing through the coresact as a single turn primary winding. The phase conductor of GIS formsthe primary winding. The secondary winding on the core are braced insidea retaining frame and are so inserted with the screening cylinder in the castaluminium enclose. The number of cores to be accommodated in an encloserdepends upon the primary current, the accuracy class and the required corespecification. The ends of secondary winding are brought into the terminalbox through a tight bushing plate. The CT terminal box consists of a barrierinsulator with several outlet. CT consists of one or more magnetic cores onwhich the secondary turns are wound. Each winding may offer several CTratios. The interlayer insulation is made up of a synthetic film. The magneticcore are mounted on a sheath like metallic armature, which ensure gooddistribution of electric field. The phase bar passes through this shield. Thecores are carefully steadied in order to withstand the mechanical vibrationoccurring during transportation and when in use in GIS. For metering andprotection purpose pressurized SF6 filled, five core CTs are provided. Theallocation of cores are as follows:

• Core 1-main protection 1.

• Core 2- main protection 2.

• Core 3- busbar differential protection.

• Core 4- metering .

• Core 5- check zone

The ratings for the current transformer are as follows:

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• Core 1- 500-1000-2000/1 ampere.

• Core 2- 500-1000-2000/1 ampere.

• Core 3 -2000/1 ampere.

• Core 4- 500-1000-2000/1 ampere.

• Core 5-2000/1 ampere.

The underline CT core are being used in TAPS 3&4.

10. 3.3 Voltage Transformer

Electromagnetic potential transformer(EPMT) are used to transform hightension line voltage to low voltage in order to supply appropriate voltageto measuring instruments, meters and relays. They can be used with volt-meter for voltage measurement or they can be used in combination with CTfor wattmeter or watt hour meter measurement. They are used to operateprotective relays. Three cores are provided for EMPT in 220 KV GIS andtwo cores are provided for EMPT in 440 KV GIS. The interface insulation ofthe primary winding is made up of a synthetic film, selected for its dielectricproperties, thermal stability and low moisture absorbency. There are twoVTs active part of each is formed rectangular core and series of deflectorsdistributes the electric field optimally. Pressured SF6 gas insulates the highvoltage conducting parts. VT always forms a separate compartment fromrest of GIS. The signal from these PTs is used for:

• Metering

• Synchronizing

• Protection

Ratings for PT:400/√

3kV/110/√

3V/110/√

3V

10. 3.4 Capacitive Voltage Transformer (CVT)

The CVT consist of a capacitive potential divider and an inductive mediumvoltage circuit. The inductive part is immersed in mineral oil and hermiticallysealed with an N2 cushion inside a steel tank. One, two or three capacitorunit are mounted on steel tank and are used as capacitor potential divider.They consist of condenser stacks with paper foil as dielectric under mineral

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oil with hydrogen gas cushion and are hermetically sealed.

In line with the requirement Maharashtra state electrical board(MSEB) forcommunication purpose 4400 PF capacitance CVTs have been used. TheseCVTs serve dual function- provide auxiliary supply to protection relays andas coupling capacitor for carrier communication. These CVTs are providedon three phase, each outgoing line from switchyard has its own voltage trans-former. Line voltage transformer are of capacitor type. Capacitive voltagetransformer are used with power line carrier communication system(PLCC)and are suitable for a PLCC system frequency range of 40 KHz and 500 KHz.

Types of CVTsThis line CVTs are of single phase type and each unit is connected betweenrespective phase and earth. Each CVT has three secondary winding andwinding connection will of phase to ground.

Functions of CVTs

• Core-1 : metering and synchronising.

• Core-2 : back, overcurrent protection and CVT fuse fail protection.

• Core-3 : earth fault directional protection.

• CVT is used with PLCC.

• CVT is used for carrier inter tripping with PLCC

10. 3.5 Lightning Arrestor

Lightning arrestors are provide in 220 KV and 400 KV transmission lines inswitchyard for suppression of lightning stroke in transmission line.

ELPRO international chinchwad, pune of lightning arrestors have been pro-vided in each bay in all the three phases except in bus coupler bay. Thearrestor include arrestor unit insulating base surge monitor. Aluminiumcasting on either end, a line terminal bracket, a ground terminal bracket andline assembly is provide to have uniform voltage gradient. It also consist acorona ring. Lightning arrestor are gapless type and are simple in construc-tion. A series of zinc oxide blocks are stacked, each set of block is placedin asset of stack plates. Arrestor incorporates a pressure release system tovent high pressure caused by the flow of system fault current in the event

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of arrestor failure. It is essential to direct the exhaust vent ports away fromthe protected equipments and otherarrestors poles to prevent damage by hotunused gases.

Parts of Lightning Arrestors

• Surge MonitorSurge monitor is provided on all the three phases and is located onthe mounting structure of lightning arrestor. The monitor monitorsare connected to the arrestors by copper strips. The surge monitormaintains the count of operation undergone by the arrestor. Surgemonitor has full scale deflection of 1 MA. It has three colour bands-Yellow - PrecautionGreen - HealthyRed - First check if counter healthy, then clean insulator and if still inred then remove arrestors.

• Grading RingLightning arrestors are provided with grading rings on the top. Thelightning arrestors are supplied in three stacks per phase. These stacksare assembled one over the other and are bolted. The top unit consistof necessary arrangement of connecting power conductors. The bottomunit are bolted to the supporting structure.

10. 3.6 Earthing Switch and Disconnector Switch

Isolators are connecting switches which can be used for disconnecting a cir-cuit under no current condition. They are generally installed along with thecircuit breaker. An isolator can be operated after the CB. After opening theisolator, the earthing switch can be closed to discharge the trapped electri-cal charges to the ground. Earthing switches are generally installed on theframes of the isolators.

The disconnector switch and earthing switch are motor operated and arecapable of remote operation from control room as well as local control panel.In the event of requirement if not operable from remote they can also beoperated by manual operation with operating handle. Fast earth switch andmaintenance earth switch are the two type of earth switch used for GIS.The maintenance earth switch is a slow device used to ground the high volt-age conductors during maintenance schedules, inorder to ensure the safetyof the maintenance staff. The fast earth switch is used to protect the circuitconnected instrument voltage transformer from core saturation caused byDC current flowing through its primary as a consequence of remnant charge

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(stored online during switching of the line). In such a situation, the use offast earth switch provides a parallel (low resistance) path to drain the resid-ual static charge quickly, thereby protecting the voltage transformer fromdamage. Earthing switch are provided on the line, transformer and bus elec-tromagnetic potential transformer(EMPT) isolators. These are electricallyinterlocked with the main isolators so that they are not closed to earth whenthe system is charged. All operating mechanism are sufficiently earthed asrequired by standards to prevent any electrical shock for operator duringnormal operation.

The disconnector switch is capable of making and breaking

• Magnetising current of EMPT

• Capacitive current of the buses and short connection

Disconnectors are placed in series with circuit breaker to provide additionalprotection and physical isolation. In a circuit, two isolators are generallyused, one on the line side and other on the feeder side. Isolators are designedfor the interruption of small currents, induced or capacitively coupled. Theycan be motorized or driven manually. The switchyard has 24 numbers ofisolators and 22 numbers of earthing switches.

10. 3.7 Hot Line Washing

The hot line insulator washing system is specifically designed to meet theparticular requirement for 400/220 KV switchyard for taps 3 & 4. Eachswitchyard has its own special features which include a variety of insulatorsperforming different tasks. These have varying shapes, sizes and power rat-ings and therefore demand different design of insulator washing. Sprayingcan take the form of circular, square and rectangular rings having a numberof specially designed nozzles mounted on them. The number of nozzle de-pends upon the size and rating of the insulator. Each nozzle is accuratelydesigned to achieve the required spray pattern and directed on to the insu-lator. Each spray ring with its associated nozzles and supporting clamp istailor made to each type of insulator.

The spraying are arranged in washing zones to enable economic pump setand pipe work sizing. It also provide identification of a particular area inwhich insulator are washed together. Zoning obviously takes into accountvarious electrical consideration and is arranged to minimize the possibility offlashover due to water over spray.

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The water supply system takes the form of pipe work distribution mainsaround the station which supply various zone feeder. Pipes are isolated fromthe ring main by a number of electrically operated fast closing valves. It isimportant to have fast acting valves so that nozzle form correct spray patterninstantly.

The pumping system incorporates the duty and standby wash pumps andauxiliary equipments connected to the demineralised water storage tank onthe suction side and the ring main on the delivery side via interconnectingpipe work and vice versa. Prior to the washing sequence the primary valveopens, if the system requires priming and using the wash pumps the ringmain is primed ton preset levels. Once primed, the system automaticallyinitiates the main wash sequence which provides the necessary quantity andhead of water for washing the insulator in each wash zone. On completion ofwashing sequence the system is left with full of water, although a drain valveis provided to allow partial or complete drainage of the system if requiredfor maintenance reason. All operations are carried out from the wash controlpanel located within the switchyard control and relay panel building.

The use of low conductivity water is imperative in hot line insulator washingsystem. There design includes for a monitoring system, which constantlymeasures the conductivity of water between water storage tanks and washpumps. The maximum conductivity permissible is preset and if exceededstops the washing sequence instantly.

Operation of the HLW system is carried out manually from wash controlpanel. The frequency of wash is determined by rate of pollution build up,which in turn depends upon the location of site and time of the year. Theinitial wash frequency is selected to be on safe side and can be reduced orincreased to suit the prevailing conditions.

The water storage tank will have a storage of 60 M3. This will provide suf-ficient water for one complete wash of switchyard insulators assuming thatthe system is fully primed prior to washing.

Washing sequenceThe washing sequences are determined by wind direction and relative levelsof insulators. To prevent over spraying of unwashed insulator washing musttake place into the wind such that any over spaying is blown into insulatorthat have already been washed. Similarly low level insulator is to be washed

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before spraying high level insulators so that any water from washing of highlevel insulator falls on to insulator already washed. This is to prevent pol-luted washed water falling onto already polluted insulators causing a furtherbuild up of pollution on the insulator to a level which may cause flash over.The wash control panel will allow the operator to select washing of 400 KVequipment only or washing of the 200 KV equipment only or both.

10. 3.8 Station Billing and Metering System

The billing and metering system comprises of energy billing metering, 220KV line metering , 400 KV line metering and station metering panels. En-ergy billing metering records net energy exported through GT; and importedthrough GT, UT and SUT. Line metering records energy exported or im-ported through 400 KV and 200 KV transmission line. And station meteringrecords station auxiliary consumption. Billing and metering system also pro-vides transducer outputs required for indicating meters provided on variouscontrol room panels and displays in SCADA system for operator informationand monitoring. Energy billing metering, 220 KV metering, 400 KV meter-ing panels are located in switchyard control room. Station metering panelsare located in respective control equipment room. Special energy meteringpanel is also provided in switchyard control room.

Energy billing and metering system comprises of main billing and checkbilling metering panels. Energy billing metering is done at 400 KV and 220KV level. The main billing and check billing panels are located in switch-yard control room. The main and check billing metering record net energyexported through GT and net energy imported through GT and UT (whengenerator is not generating) and SUT. Net energy exported through both GTis summed up and net energy is imported by both the startup transformerand GT, unit auxiliary transformer are summed up. Difference between netenergy exported and net energy imported by transformer gives the net energyimport or export. The net energy export/import is used for the purpose oftariff calculation. High precision solid state active and reactive power energymeters( 14 in nos.) are provided for this purpose in both main and checkbilling metering panels. The input signals for this meters are taken fromCT and PT located at 220 KV side of SUT and 400 KV side of GT. Theimpulse from the energy meters are fed to microprocessor based telecountinginstrument, which calculates net active energy export or import, maximumdemand, net reactive energy export or import.

The potential supply for the SUT energy meters are taken from one of the

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220 KV bus PTs (either bus1 PT or bus2 PT) as bus coupler is normallyclosed. In the event of that PT fuse failure or undervoltage in any of thebuses, automatic change over scheme is provided to change over the PT sup-ply to other healthy 220 KV bus PT. similarly, PT supply for GT energymeters are taken from 400 KV bus PTs with automatic change over scheme.

In a particular time period differential reading of main and check billingmeters should be same. However, corrective actions can be taken if largedeviations between reading of two is noticed. In addition to energy meters,transducers are also provided in energy billing panels. The output from thesetransducers are used for various indicating meters of main control room andswitchyard panels.

10. 3.9 Bay Control Mode

The control mode determine the control possibilities of the devices in eachbay. However opening of the circuit breaker by tripping of the protectivedevices continues to be possible in all the control models. There are threepossible control modes, selected from bay cubicles by a multi-position keyselector switch.

Remote mode: This is the normal control mode in which local control ofdevices are remotely controlled from a control station.

Local mode: This mode is used when orders are sent from bay cubicle. Localmode is mainly used for maintenance.

Manual mode: This control mode is used for disconnector and earthing switchmaintenance and emergency operations, particularly in event of breakdown.Each disconnector and earthing switch can be manually operated if interlock-ing conditions so permit. As soon as manual operation of device is activated,all the other devices are electrically locked. The operation is performed usinga crank handle from the device control box.

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Table 10.2: ratings of 220 KV and 400 KV GIS

Sr. No. Description 220 KV GIS 400 KV GIS

1 rated Voltage 220 KV 400 KV2 Rated Frequency 50 Hz 50 Hz3 Rated Current(in Feeders &

Busbars)1250 A 1620A

4 Rated Breaking Capacity5 Rated Short time withstand

Current40 KA 40 KA

6 Rated Operating Sequence O-0.3s-CO-3min-CO O-0.3s-CO-3min-CO7 Lightning Impulse (Uw) 1050 KV 1425 KV8 Power Frequency (Us) 460KV- MIN 520KV-MIN9 SF6 Pressure setting (mPa):a Filling Pressure (Pn) 0.65 - 0.35 0.65 - 0.35b First Stage Alarm 0.62 - 0.32 0.62 - 0.32c Second Stage Alarm 0.60 - 0.30 0.60 - 0.3010 Auxiliary Voltage Services

Rated Valuesa Control Devices 110 V DC 110 V DCb Motor(CB Control) 415V AC-50Hz(3-φ) 415V AC-50Hz(3-φ)c Tripping & closing

Coils(CB Control)110 V DC 110 V DC

d Heating & Lighting 220 V AC-50 Hz 220 V AC-50 Hz

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DGs, CUPS, PUPS and Batter-ies

11. 1 Diesel Generator Unit

Each unit of TAPP 3 & 4 is provided with four standby diesel generator of2720 KW rating. DG 1,2,3 and 4 are connected each to 6.6 KV CLASS-IIIbus C-31, C-32, C-33, C-34 respectively. These buses are normally suppliedfrom 6.6 KV CLASS-IV supply system through C-41, C-42, C-43 and C-44respectively. The neutral of DG is connected through 12.7 ohm resistor toearth to limit the earth fault current to about 300 ampere. Neutral ground-ing resistor (NGR) is capable of carrying this current for 30 sec.

These Dgs are capable of operating in following modes:

• Whenever CLASS-III 6.6 KV power supply is lost, DG set starts au-tomatically through EMTR on sensing under voltage in CLASS-III 6.6KV bus to re-establish CLASS-III power supply within 10 sec.

• DGs are capable of parallel operation with the station CLASS-IV 6.6KV system.

• DGs are capable of parallel operation with other DGs.

11. 1.1 Design Criteria

DGs are designed, manufactured, inspected, tested and installed conformingto the IEEE-387 and US NRC regulatory guide 1.9. following are importantdesign features:

• As per IEEE-387, DG sets should be required to operate for about 4000hours and make 4000 starts during the life time. These DG sets arecapable to start adequate number of times and operate continuously atrated load.

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• Designed for operation during and after safe shut down earthquake.

• Designed to start and take load as per EMTR scheme such that atno point in the loading cycle the voltage droop is more than 20% andfrequency droop is 3%. Voltage and frequency recovers to about 90%and 98% respectively with 2.4 sec i.e. 60% of loading sequence timeinterval.

• Designed to start and accelerate to rated speed and build up ratedvoltage within 10 sec from start signal.

• Designed to supply continuously required station safety related loadsunder one DG condition in each division by load shedding throughEMTR scheme.

• Designed to start and accept load without external auxiliaries and cool-ing water for 2 minutes. Only 24 V DC control supply is required tostart the DG set and 220 V DC control supply for DG protection.

• Designed to supply 2720 KW at 0.85 p.f. and 6.6 KV continuously and2992 kW at 0.8 p.f. at 6.6 KV for 2 hours in every 24 hours.

11. 2 Diesel Engine and Auxiliary System

11. 2.1 Low Temperature Water System

The LT water circuit ensures the cooling of lube oil and of super charging air.These circuit is a close loop. The heat power transferred from lube oil andsuper charging air to water is evacuated by means of the LT heat exchangeritself linked to non-active process water (NAPW).

Heat exchanger- the lube oil flow collects the heat dissipated by frictionat the bearing and engine mechanical parts during running operation. TheHX ensures evacuation by LT water of the collected calorific power

Specifications

• Heat power exchange = 431 KW.

• HX oil inlet temperature = 630C.

• HX oil outlet temperature = 610C.

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11. 2.2 High Temperature Water System

The HT water circuit ensures two functions depending on DG unit operationstages; running stage or standby waiting stage. When DG unit is in runningstage, the circuit is used for the cooling of the engine, mechanical parts suchas cylinders, cylinder heads, exhaust valves, injector nozzles. The heat col-lected by fluid is evacuated by means of HT heat exchanger itself linked to afresh water cooler NAPW. When the DG unit is in standby waiting stage, inorder to keep it at warm condition so as to ensure engine emergency startingin good conditions, the HT water circuit operates continuously as a pre-heating circuit. HT water is pre-heated by means of electric pre-heaters andis used to preheat the lube oil in the lube oil/HT water heat exchanger.

Heat Exchanger: The HT water circuit is used Normal operation data tocollect the heat dissipated by engine mechanical parts and turbocharger dur-ing running operation. The HX ensure evacuation of the heat collected byHT water circuit.

Specifications

• Heat power exchange = 568 KW.

• HX oil inlet temperature = 820C.

• HX oil outlet temperature = 750C.

11. 2.3 Lube Oil

The lube oil circuit ensure two functions depending on DG unit operation;running stage or standby waiting stage. When the DG unit is in runningstage, the circuit is used for:

• Lubrication of engine(bearing, piston liners ) and its auxiliaries(supwecharger bearings, pumps, etc)

• Cooling down of piston heads

• Engine and circuit oil cleaning and protection against corrosion andoxidation.

When the DG set is in standby stage the pre-lubrication circuit allows tokeep warm oil flow thus ensuring good condition for engine fast start andloading.

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11. 2.4 Fuel Oil System

The fuel oil (FO) circuit ensures feeding of the diesel engine with fuel oil.The fuel oil day tank is located on an outdoor platform at a level of +104.00meters to assure a slight positive pressure (minimum self feeding capacity)at the engine driven fuel pump. The fuel oil day tank is fed from the fuelstorage main tanks used to ensure rated diesel engine service. The fuel oilday tank of 9m3 capacity constitutes an intermediate storage to the mainfuel oil storage tank and the engine. This capacity allows limited autonomyand therefore the tank is equipped with permanent link to the main storagetank allowing refuelling of service tank.

The waste oil tank is used to collect the diesel engine leakages. Due to pol-lution (water, dirt, etc), these leakage cant be reused for diesel engine supply.

External Fuel oil system- Each DG set is provided with day oil tank of stor-age capacity around 90m3. Each day oil tank storage is adequate to operateDG unit at full load for 8 hours.

11. 2.5 Compressed Air System

The compressed air system is constituted of three circuits; one very highpressure (40.8 to 30.6 kg/cm2), one high pressure (20.4 kg/cm2) and one lowpressure circuit(7.14 kg/cm2).

1. Very high pressure air circuitThe very high pressure air circuit ensures the following functions:

• Starting of diesel engine by means of inlet air on cylinder heat(40.8 kg/cm2).

• Starting of the pressure in governor and also ensure the feeding ofthe governor (30.6 kg/m2).

• Overspeed stop by lifting the fuel oil injection pumps (30.6 kg/cm2).

• Supply to the high pressure air circuit (20.4 kg/cm2).

• Supply to the low pressure air circuit (7.14 kg/cm2).

The compressed air is produced by two independent electric motordriven air compressors. The very high pressure is obtained from thepressure air by reducing the pressure from 40.8 to 30.6 kg/cm2 .

2. High pressure air circuitThe HP air circuit ensures the starting of diesel engine by means of air

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starter. The high pressure is obtained from the very high pressure airby reducing the pressure from 40.8 to 20.4 kg/cm2.

3. Low pressure air circuit The LP circuit ensures the normal stop andsafety stop operation. The LP air is obtained from very high pressureair by reducing the pressure from 40.8 to 7.14 kg/cm2.

Compressor and receiver operationThe purpose of air compressor is to keep a pressure between 34.7 and 40.8kg/cm2 in the air receiver. When the pressure in the HP air receiver fallsbelow 34.7 kg/cm2 , the corresponding air compressor is started. The com-pressor is stopped when this pressure is over 40.8 kg/cm2.

Table 11.1: DG Specifications

Quantity Description

Make of Engine S.E.M.T. PielstickMake of Alternator ALSTOM

Continous Output Rating 2720 KWRated frequency & Phase 50 Hz & 3 phase

Rated Phase Voltage 6.6 KVRated Current per Phase 297 A

Type of Cooling Air CooledSpeed 1000 rpm

Field Current at Rated Output Voltage 398 AFull Load losses 90.3 KW

Armature Copper Loss 20.4 KWRotor Copper Loss 23 KW

Core Loss 21 KWStray Losses 6.5 KW

Efficiency at Full Load 96.79%PMG(Pilot Exciter) output voltage(Vac) 80-275 Vrms

PMG Frequency Range 40- 400 HzOutput DC Voltage 0.8 * Vac

11. 3 UPS

UPS System are used to feed loads which

• Cant tolerate any interruption in supply whatsoever.

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• Impose very strict demands as regards the constancy of the voltage.

• Are sensitive to main transients.

CLASS-II power supply is having two buses- bus D-21 in division-1 and busD-22 in division-2. In each division one power UPS set is provided to haveuninterrupted power to the loads connected to respective CLASS-II bus.Power UPS system broadly consist of following components:

• Rectifier (charger)Converts AC supply to DC which acts input to inverter. It also chargesthe power battery (float as well as boost).

• InverterTakes input from rectifier or battery and converts it to AC to cater theload demands.

• Inverter static switch (EA)Whenever UPS goes on bypass, rectifier inverter section is isolated byblocking the inverter static switch.

• Bypass static switch (EN)Whenever main power UPS system develops some trouble or gets over-loaded, static switch fires and take the entire load smoothly. Q028 isan isolator for bypass switch.

• Manual bypass switch (Q050)Whenever whole of the UPS system is to be taken under maintenanceauto / bypass switch is put on bypass, thereby rectifier-inverter sectionplus static bypass section become potential free. Q050 is overlapping(make before break) switch i.e. no interruptions at the load whenswitching over. Under normal condition Q050 is to be placed on auto

• Input mains breaker (Q001)as protection to rectifier- inverter module.

• Output breaker (Q100)

• Battery breaker (Q004)

• Bypass isolator (Q028)

• Battery (BY)Battery forms the backup to CLASS-III supply and comes into picturewhenever CLASS-III 415 V supply fails. Battery is connected to mainUPS system through Q004 breaker.

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11. 4 Modes of Operations

The UPS can be operated in different operating modes, depending on theavailability of the mains, battery voltage and actual load. The differentoperating modes and priorities for an UPS with static switch EA & EN areas follows:

11. 4.1 Highest priority

Manual bypass switch at position AUTO.

• Normal operation.

• Battery operation.

• Bypass operation.

• Charger only.

• Standby only.

11. 4.2 Lowest priority

Manual bypass switch at position AUTO.

11. 4.3 Test/Maintenance

Manual bypass switch at position BYPASS.

1. Normal operationAt normal operation, the manual bypass switch is always at positionAUTO. The AC input (mains) is fed to the phase angle controlledrectifier via matching transformer. The rectifier compensates voltagefluctuations as well as load deviations, and maintains the DC voltageconstant. The rectifier supplies the inverter with energy and securesthat the connected battery is kept on standby (float charge or boostcharge depend on the charging condition and on the type of battery).The downstream inverter converts the DC voltage by means of opti-mised sine wave pulse width control (PWN) into AC voltage and sup-plies the connected load via static switch EA.

Note: if the supply of the load from the inverter is not secured, dueto E.G. overload (¿ 150% 1min, 125% 10min) or a fault in the inverterthe UPS will change over to bypass operation.

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2. Battery operationAt battery operation the manual bypass switches is always at position¡¡AUTO¿¿. In the event of power sags or failure the inverter is nolonger supplied from the rectifier. The battery is connected to the dcintermediate circuit is called up automatically and without interrup-tion to supply the current. The discharging of the battery is signalled.The battery voltage drop is a function of duration and magnitude ofdischarging current. The voltage drop is controlled by the inverter andthus the UPS output voltage is kept constant

If the limit of the battery is approached an alarm is activated. Onmains recovery own emergency power generation by means of a dieselgenerator before the limit of discharge is reached. The system changesautomatically back to normal operation, not available or outside thetolerance range the system will shut down automatically. On main re-covery or on emergency power generation by means of a diesel generatorthe rectifier start-up after 60sec and will charge the battery, if the UPSsystem is programmed for ¡¡AUTOSTART¿¿ it mains recovery. If thesystem is not programmed for auto start, it has to be restart manually.

3. Bypass operationAt bypass operation the manual switch is always at position AUTO.This system part enables the consumer to be changed over without in-terruption to direct mains power supply (via bypass) under adherenceto the specified tolerances. The changeover can be initiated automati-cally by a control signal, or manually. Every interrupt free change over,whether automatic or manual is only possible if voltage, frequency andphase relation of the UPS system are synchronized to the bypass mains.Deviation in the main frequency outside the specified tolerance rangeinhibits a changeover. Automatic changeover of the load to the bypasstakes place when the power supply within the specified tolerance rangeis not assured by the inverter. If a bypass fault occurs the system willin this situation automatically change over to normal operation if themains is available, otherwise a changeover to battery operation takesplace if the battery is available.

4. Standby operationIn this mode the load is not supplied but the UPS is ready for switchingON.

5. Manual BypassThe position ¡¡BYPASS¿¿ can be used for test purpose e.g. for the

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synchronisation of the inverter to the bypass, or for switching attemptsbetween Inverter-Bypass-Inverter. Therefore an external artificial loadcan be connected at the TEST purpose which may not exceed the ratedoutput.

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Appendix

1. SUT Start up Transformer.2. GT/UT Generator Transformer

Unit Transformer or Unit Auxiliary Transformer (UAT).3. SAPSS Station Auxiliary Power Supply System.4. Div 1 Electrical systems and plant auxiliaries fed by UT dur-

ing normal condition.Div -2 Electrical systems and plant auxiliaries fed by SUT dur-

ing normal condition, but fed by UT during abnormalcondition.

5. DG Diesel Generators.6. SWRD Switchyard.7. CWPH Cooling Water Pump House.8. DM De-Mineralizing water plant.9. GIS Gas Insulated Switchgear.10. CT / PT Current Transformer / Potential Transformer.11. CB Circuit Breaker.12. SF6 Sulphur Hexafluoride gas.13. AVR Automatic Voltage Regulator.14. Backup protection A protection system intended to supplement the main

protection in case the latter should be ineffective, or todeal with faults in those parts of the power system thatare not readily included in the operating zones of themain protection.

15. HV High Voltage16. IGBT Insulated gas bipolar transistor.

A special design of transistor that is suitable for han-dling high voltages and currents . Frequently used instatic power control equipment due to the flexibility ofcontrol of the output.

17. LV Low Voltage18. IDMT Inverse time relay with definite minimum Time.

An inverse time relay having an operating time thattends towards a minimum value with increasing valuesof the electrical characteristic quantity.

19. PLCC Power Line Carrier Communication20. UPS Uninterruptible Power Supply

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Bibliography

• SWITCHGEAR AND PROTECTIONS, SUNIL .S. RAO.

• ELECTRICAL TRAINING MANUAL, NTC, NPCIL.

• TRAINING ON POWER SYSTEM PROTECTION, ALSTOM.

• FUNDAMENTALS OF POWER SYSTEM PROTECTION BY Y.G.PAITHANKAR,S.R.BHIDE.

• 5-482-630EN-C, GIS MANUAL, MERLIN GERIN.

• IS 9001- 2008 CURRENT TRANSFORMER.

• & M MANUAL, FOR 400KV SWITCHYARD.

• O & M MANUAL, UNIT TRANSFORMER.

• RELAY MANUALS, ALSTOM.

• & M MANUAL, FOR START UP TRANSFORMER.

• ELECTRICAL TRAINING MANUAL, NTC, NPCIL.

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