THDC Training Report

2012

Faraz Ahmad

Zakir Husain College of Engineering and

Technology, Aligarh Muslim University

Tehri Hydro Power Plant

Tehri Hydro Development Corporation

Tehri Hydro Power Plant | Summer Training Report

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Table of Contents

Serial Number

Topics Page Number

i Submission Details 2

ii Acknowledgement 3

1 Introduction 4

2 Salient Features 6

3 Components of a HPP

Rock Filled Earthen dam

Head Race Tunnel

Power House

9

4 Power Generation

Excitation System

Braking System

Governor System

Oil Pressure Unit (OPU)

Unit Control Board (UCB)

17

5 Gas Insulated Switchyard 32

6 Epilogue 39

7 Photograph Gallery 41


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Submission Details

Name of the Organization Tehri Hydro Development Corporation

Location of Powerhouse Tehri-Garwal, Uttarakhand, India

Duration July 1st, 2012 to July 30th, 2012

Title of Report Tehri Hydro Power Plant

Name Faraz Ahmad

College Zakir Husain College of Engineering & Technology

University Aligarh Muslim University

Discipline Electrical Engineering

Guides:

Er. Sanjeev R

Er. C Pradeep Raj

Er. Ashish Mamgain

Er. J S S Kaithat

Er. Shrikant Pant

Er. Sumit Tamta

Er. Anoop

Er. Sachin Rathor


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Acknowledgement

I would like to sincerely thank Mr. D K Govil, General

Manager THDC, for his immense support and guidance

that has made this training possible. I would also like to

thank Mr. C P Raj (Manager O&M), Mr. Ashish Mamgain

(O&M) for their supervision, Mr. J S S Kaithat (JE

Protection) and Mr. Shrikant Pant were a real guide and I

would like to thank them whole heartedly for the efforts

they put in to make us understand the working and

principal behind every operation.

Faraz Ahmad

Z H College of Engg and Tech

Aligarh Muslim University


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1. Introduction


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Introduction

About THDC

TEHRI HYDRO DEVELOPMENT CORPORATION, or THDC, is a venture of the Central

Government and Uttarakhand State Government.

About Tehri Dam

The Tehri Dam is a multi-purpose, rock and earth-fill embankment dam on the Bhagirathi

River near Tehri in Uttarakhand, India. It is the primary dam of the Tehri Hydro

Development Corporation India Ltd. and the Tehri power complex. Phase 1 was completed

in 2006; the Tehri Dam withholds a reservoir for irrigation, municipal water supply and the

generation of 1,000 MW of hydroelectricity. Two more phases with an additional 400 MW

Koteshwar and 1,000MW pumped storage hydroelectricity are under construction. This

dam is Asias largest & worlds 3rd largest rock fill dam. The area of the reservoir is around

44 km2 and the catchment area expands to over 7511 km2. The complex will afford

irrigation to an area of 270,000 hectares (670,000 acres), irrigation stabilization to an area

of 600,000 hectares (1,500,000 acres), and a supply of 270 million gallons of drinking water

per day to the industrialized areas of Delhi, Uttar Pradesh and Uttarakhand, thereby

supplying drinking water to over ten million people.


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2. Salient Features


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Salient Features

I) LOCATION

State: Uttarakhand District: Tehri Vicinity: Diversion dam on the River Bhagirathi. Powerhouse on the left bank.

II) HYDROLOGY

Catchment Area at Dam: 7511 Km2

Reservoir Area: 44 Km2

III) RESERVOIR

Full Reservoir Level (FRL): 830.0m Max Reservoir Level: 835.0m Min Draw Down Level (MDDL): 740.0m

IV) DIVERSION TUNNEL

Location: 2 Numbers on the left bank of the River & 2 on river Bhagirathi. Number: Four Shape: Horse Shoe Shaped

V) DAM

Type: Rock Filled Earthen Dam Maximum height about deepest foundation: 260.5m Elevation of Dam top: 842.0m Length at top: 592.0m Top Width: 25.50m

VI) POWER TUNNEL (HRT)

Size & Cross Section: 7m Horse Shoe Shaped Number: Four Length: 779m, 855m, 997m & 1033m


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VII) POWER HOUSE Type: Underground Installed Capacity of HPP: 1000MW (250x4=1000 MW) Installed Capacity of PSP*: 1000MW (250x4=1000 MW) Size: 197x24x65.2 m3 Type of Turbine: Francis Vertical Axis Gross Head: 230.1 m Design Head: 188.0 m

*Under Construction

VIII) TAIL RACE TUNNEL (TRT)

Number & Type: 2 Numbers, 9.0m Horse shoe Length: 748.0m & 862.0m

IX) GAS INSULATED SWITCHYARD (GIS)

Type: Indoor SF6 Insulated package switchyard Capacity: 400KV

X) POWER GENERATION

Peak Capacity during the lean period: 1000MW Annual energy generation in a 90% dependable: 4300million units Annual energy generation at average water availability:

5300million units


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3. Components of a

Hydro Power Plant


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TECHNICAL PARAMETERS & WORKING

The technical details of the few of the major components/equipment are as under: Earth & Rock Fill Dam: The dam is of Earth & Rock Fill type and height of the dam is 260.5 m. The length of the dam at top is 592.0 m & the width is 25.5m. The elevation of top of the dam is 842.0 m.

Figure 1: Tehri Dam

Headrace Tunnel: There are 04 Numbers of HRTs on the Right Bank of the reservoir having circular shape, 8.5 m diameter. The lengths of HRTs are 779 m, 855 m, 997 m &1033 m. It takes water from reservoir and supplies it to power house machines for generation. HRT-1 and HRT-2 carry the later to Hydro power plant (HPP). HRT-3 and HRT-4 carry the water to the pump storage plant (PSP) which is still an ongoing project and is not functional right now.

Figure 2: Head Race Tunnel (HRT)


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Power House: It is an underground powerhouse having four generating units. Its different constituents are as follows:-

1. Penstock: There are four numbers of penstocks attached with butterfly valve. Water enters through the different penstocks to the different generating units. Each penstock comprises upper horizontal, vertical & lower horizontal reach including upper & lower bend. The diameter of penstock is 5.75 m. 2. Inlet Valves: One Butterfly Inlet Valve (BIV) at mouth of the each penstock and one machine inlet valve (MIV) before the spiral casing are installed. The valve remains close in case of unit shutdown. When the unit is started, then after sensing the starting command, MIV opens first to build-up pressure in spiral casing and roll the turbine. The BIV is usually open and is shut during the maintenance period. MIV is a spherical valve. BIV is shown in the following diagram.

Figure 3: Section of BIV(left) and Butterfly Inlet Valve-BIV (right)


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Figure 4: Spherical Valve- MIV

3. Turbine: It is vertical Francis type turbine having net head of 188 m and generator has rated output of 255 MW at a discharge of 150cum/ sec at a rated speed of 214.3 rpm. Water from the spiral casing enters through the 28 guide vanes and strikes the 14 blades of the runner, which is coupled to the main shaft. The shaft is coupled to the rotor of the generator at the other end. The opening of guide vanes depends upon the generation requirement. Water strikes the blades of the runner and fall axially in the draft tube. For keeping shaft vertically, Turbine guide bearing is installed which is of rotating sump self-cooled type. Runner removal from the bottom of the unit is possible through the runner removal gallery.

Rated Output 255 MW

Head Maximum 230.1m

Head Rated 188.0m

Head Minimum 122.6m


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Figure 5: Diagram showing Turbine Pit and Spiral Cage


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4. Generator: It is 278 MVA, 0.9pf, 50HZ, 3 Phase generator which generates power at a voltage of 15.75 KV. A generator has two parts, Stator and Rotor. Power generated is transmitted through isolated phase bus duct to transformer gallery from where it is stepped up to 400 KV by GSU transformers. Two bearings named as Thrust bearing and upper guide bearing are installed for sustaining the outward thrust of rotor and keeping shaft verticality. Both the bearings are of water cooled type. It has 10% overload capacity for short duration. Carbon dust collection system has been installed which works during application of brakes.

Rated power 278MVA

Figure 6: Animated Diagram of a Salient Pole Hydro-Alternator

Rated Voltage 15.75 KV Power factor 0.9 Current frequency 50Hz Rated rotational speed 214.3rpm Runaway speed 410 rpm


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5. Transformer Gallery: 4 in numbers, 306 MVA, 15.75/ 400KV GSU 3 Phase transformers are installed in transformer gallery (TG) for the four units. 15.75 KV is generated from the each unit and this power is stepped up to 400 KV here and sent to switchyard through oil filled cables. These generating transformers are provided with makeup valve which is filled with water that cools the oil that is being used to cool the transformer. These transformers are also equipped with micro wave detector which detects the microwave that generated In case of some spark and raises the alarm. Transformer oil conditioner is used to purify the oil which is used to cool the transformer and eliminates different gases from the oil that got mixed during the cooling process.

Figure 7: Generating Transformer for HPP housed inside the Transformer Hall

Total Power 306 MVA Primary Voltage 15.75 KV Secondary Voltage 420KV Number 04


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6. Ventilation System: Circulation of air inside the powerhouse is done through a ducting network and blowers/ fans of different capacity and is kept in operations 24hours a day.

Figure 8: Ventilation Chamber

7. Tailrace Tunnel (TRT): There is TRT-1, TRT-2 for the 4 Units. The dia. of the TRT is 9.0 m and the lengths of TRTs are 748 m & 862 m. Tail race tunnels basically carry the water from the draft of Hydro Power plant (HPP) to the main stream river. TRT-3 and TRT-4 carry the water from PSP (Pump storage plant) to the downstream river.

Figure 9: Tail Race Tunnel (TRT)


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4. Power Generation


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Power Generation at Tehri Hydro Power Plant

Figure 10: Plan of Tehri Project

a. Water from the reservoir enters the HRT-1 and HRT-2. b. Each HRT further divides into 2 penstocks, which is

equipped with butterfly valve, which controls the water flow in the penstock.

c. Thus four penstocks lead to 4 turbines. d. There, water rotates the turbine blades which rotates

the excited rotor, thus induces changing flux in AC the stator winding.

e. Thus is how power is generated from 4 units. f. Water from the turbine enters the draft tube and there

after Tailrace tunnel (TRTs). TRT-1 and TRT-2 takes the water from HPP.

g. These TRTs take the water to the mainstream river.


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Figure 11: Water Flow Outline Diagram


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Excitation System Introduction The basic principle of power generation is when a magnetic field is moved across a stationary conductor, voltage is induced in the conductor. Voltage will be induced even if conductor is rotated and magnetic field is kept stationary, Generators consist of two circuits an electric circuit and a magnetic circuit; one is rotating with respect to other. The magnetic circuit of a generator is called exciter. In modern generators magnetic field is produced by an electromagnet. The intensity of magnetic field can be varied by varying the amount of DC current applied to electromagnet. Generator output voltage is affected by the following factors: 1) Intensity of the flux in the rotating magnetic excitation field. This can be varied by varying the DC current applied to the electromagnets. 2) Rate at which flux lines cut by the conductor. This is not variable since the generators operator at the rated constant speed. 3) Length of the conductor (Not variable).

The function of the generator exciter is to provide variable magnetizing power to the generator magnetizing flux field. When the intensity of exciter field is increased the generator output voltage is increased. In actual usage, the exciter is used to vary the generator output voltage to match the system demands.

Types of Exciters

1) Commutator type DC generator: These DC generators are either driven from the shaft of the main generator or from a separate motor. In some large exciters, a speed reducing gear train is used between main generator shaft and exciter shaft. 2) AC generators used in connection with rectifiers: In this case can alternator is driven by the main generator to produce AC power which is then converted to DC by rectifiers. 3) A static excitation system which uses the AC power generated by the main generators instead of a separate generating unit. In this type, the power generated by the main generator is stepped down to required level using rectifiers. For initial excitation, DC from station battery of DC from station auxiliary power converted by rectifiers is used. Static Exciters With the development of electronics, the control systems became very simple. Electronics control circuits has the advantages of fast response, easy maintenance and long life. The control circuits can be assembled in modules called printed circuits boards. Each module


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can be easily replaced during a fault. With the development of electronics the excitation system of AC generators are also considerably and today almost all the manufacturers are using electronic exciter called static exciters. a) This has practically no moving parts. b) It draws AC power from the output of the main generator. It is rectified and controlled using electronic rectifiers and fed back to the field. c) No separate AC generator or DC generator is required for a static exciter. d) The excitation transformer is a special three phase transformer used to step down the generator output AC voltage (usually 11kv). The secondary is designed as per the requirements. e) The auxiliary power is derived from the excitation transformer secondary.

The Thyristor

1) This device is basically a controlled rectifier. It belongs to a family of called Silicon Controlled Rectifier (SCR). 2) Its advantage is that it can carry large currents and it can be controlled using a very small current of the order of a few milli-amperes. 3) The conduction of Thyristor can be controlled using the control input called gate. It has 3 terminals called anode, cathode and Gate. For starting conduction, anode is to be positive w.r.t cathode and there should be gate current flowing through gate. 4) Even if the anode is positive w.r.t cathode the conduction will not start until a gate current is supplied. So we can delay the conduction by delaying the gate signal. Once the conduction is started, the gate will not have any control (even if it is removed). 5) The conduction is will continue until the current through the device is reduced below a particular value called the holding current. It is the minimum current required to maintain the conduction and the value is specified by the manufacturer.

Working of the static exciter

11KV generator output voltage is stepped down to a low voltage using the exciter transformer. This AC voltage is rectified using a thyristor bridge. The rectified output DC voltage is given to the field winding of the generator through a field breaker. The exciter draws power from the output of the generator and converts to the controlled DC and feeds to the generator field. During starting, voltage will not be available at the output. So the exciter cannot function and excite the generator. For initial excitation external supply is required for 5 to 10 sec. usually the supply is given from the station battery and this process is called Field Flashing. At about 95% of the rated speed of generator, the station battery supply is given to the field using a set of field flashing contractors. A current limiting resistor is also used in series with the battery current. The blocking diode in the circuit prevents any current flowing to the battery from the exciter.


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Figure 12: Static Excitation Scheme

When the battery supply is given to the field, the generator starts to generate output voltage. Then the exciter starts functioning and the field current is shared by battery and exciter. When the generator output reaches around 7KV, the exciter can independently supply the field current. Then the battery supply is isolated automatically by switching off the field flashing contractor. During every starting field flashing is required. Also when the machine trips on non-lockout, generator keeps on running, excitation is switched off and machine is isolated from the system. A timer relay is also started which isolates the battery by de-energizing the field flashing contractors in case the required voltage is not achieved. Re-exciting is required. At that time also field flashing is required. Continuous field flashing can reduce the battery life. Modern generators use field flashing from station auxiliary supply which is stepped down to the required level and then rectified during diode bridge rectifiers and for field flashing.


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For large generators the excitation power required is more and more Thyristor bridges are connected in parallel to share the load.

The Field Breaker

The field breaker connects the DC from the exciter to the field. In addition to the two main contacts, the field breaker is usually provided with a power rated auxiliary contact. This auxiliary contact is used to provide a discharge path for the voltage induced when the field is isolated using the field breaker. During opening of the field breaker, the field is connected to a field discharge resistor (FDR) through the closed contact of field breaker.

The Automatic Voltage Regulator (AVR)

1) It is basically a comparator which compares the output voltage of the generator with the set value. Any error between the set value and the output voltage is reflected as an output of the comparator. 2) This signal is amplified and given to the gate pulse generating electronic circuit. The position of the pulse is shifted and the conduction of the thyristor is controlled and output voltage is brought to the set value. 3) The set value given to the comparator can be varied by the operator using a motorized potentiometer. The feedback to the comparator is taken from the generator output using potential transformers.

Auto Regulation Cubicle:

1) CH-1 is used for everything required for the machine to operate. 2) In case CH-1 is fails, then CH-2 is used and the parameters of CH-1 will be used by auto channel CH-2. 3) In case CH-2 also fails then CH-3 is used and operated manually. 4) Ch-4 is for braking.

Unit Auxiliary Transformer:

Unit auxiliary transformer basically taps the generating 15.75KV to 415V secondary. This secondary voltage is used to run the auxiliaries such as air oil system etc. Total Power Total Power- 400 kVA Primary Voltage- 15.75 kV Secondary voltage- 400 V

Self-Excitation Transformer:

This transformer is used to feed the required voltage to the rotor windings for the generator excitation. This is called self-excitation because the voltage generated is back fed


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to the rotor windings after being tapped down through this transformer. Here the primary voltage is 15.75KV (generated voltage) and secondary voltage is 712V. The Total Power is 1912 kVA.

Figure 13: Self Excitation Transformer

Excitation Process:-

1) Incomer 11KV line is used for the initial excitation. For this 11KV is stepped down 415V followed

by stepping down to 30V DC which is used for the initial excitation.

2) When 30MW is generated then 11KV line is isolated & the self-excitation is carried out. This is

done automatically.

3) For this 15.75KV, being generated is tapped down to 712V through the self-excitation

transformer.

4) This 712V AC is fed to thyristor bridges which get converted to DC and the gate pulse to thyristor

bridges are provided by AVR through pulse transformer to the gate of thyristor.


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5) These Pulse transformer smoothen the gate pulses.

6) DC from thyristor after passing through field breaker goes to the rotor windings.

7) This is how the excitation process continues till the machine operates.

Braking

1) To stop the machine, first wicket gates are closed.

2) It is then left in the idle condition, so that the speed reduces to 50%.

3) Dynamic brakes: When the speed is reduced to 50% dynamic brakes are applied. For this, first the

11KV incomer line is stepped down to 230V which after passing through circuit breakers followed by

field breakers is supplied to the rotor in opposite direction (opposite to the direction of excitation)

and the 3 phase AC is shorted, which applies the force in opposite direction and helps in reducing

the speed.

4) Mechanical brakes: - Mechanical brakes are applied when the speed reduces to 4% after

application of Dynamic brakes.

Figure 14: Mechanical Brake

Governor System:

1) Main Function of a governor is to maintain the rotational speed of the turbine within certain

acceptable limits by controlling the water input despite variations in the load.


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2) Hydraulic power is used to operate the vanes and the valves.

3) It consists of two parts:

i) The sensing and signal processing part

ii) The operational part.

4) For controlling the speed of a turbine a signal proportional to the speed is to be fed to the control

system.

5) Oil pressure system is a separate unit which supplies pressure oil at an appropriately constant

pressure.

6) For achieving automatic control, a portion of the system output is fed back to the system and this

signal is called feedback. The governor then automatically adjusts the flow to control the prime

movers power.

Oil Pressure System:

Application of oil pressure system:

1) The heat generated by the moving parts can be carried away by the oil which can be transmitted

to exchanger.

2) Various devices (big or small) like operating server motors for the wicket gates and opening and

closing of the spherical valve of MIV (Main inlet valve) can be operated using a simple energy

source.

3) The oil acts as a lubricant which can increase the component life.

4) Hydraulic actuators have a higher response speed.

Working of Oil Pressure System:

1) The governor oil sump tank is filled with the required quantity of governor oil. The air-oil vessel is

filled with 35% oil and 65% compressed air.

2) Oil form the sump tank is pumped to the system using induction motors. The isolation valve is

also kept opened so that the accumulator is also connected to the system.

3) Air from air vessel goes into the air-oil vessel and increases the pressure in that vessel which

pushes the oil. This is oil is released at the required places i.e. for opening and closing of wicket

gates and spherical valve of the MIV (Main inlet valve).


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Figure 15: Oil Pressure Unit. Oil and Air-Oil Pressure Vessel (left), Oil Pumping Unit (right)

Figure 16: Compressor Room

1. In the compressor room the air is compressed and stored in the cylinders. This compressed air is

then needed for various processes like in air vessel to carry on various hydraulic operations.

Effective Pressure 6.3MPa Tank Pressure 9.45MPa Capacity 3.2 m3 Temperature 10-40oC Effective Pressure 6.3 MPa

Tank Pressure 9.45 MPa Capacity 3.2 m3 Temperature 10-40oC

Air

Ves

sel

Air-O

il Ve

ssel


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2. There are two compressor cubicles. Cubicle-1 is on Main mode and Cubicle-2 is on Standby

mode. In case cubicle-1 fails, cubicle-2 takes over the operation.

3. When the pressure increases to 6MPa it switches on and at 6.3MPa it gets switched off. In case

the line pressure reduces to 4.4MPa, then both of them starts operating.

4. There are 4 stages of air compression.

5. After the air is compressed, it is stored in various cylinders and stored at various pressures to

carry out different operations. There are total of 9 cylinders.

Stage 1

Pressure: 196-235 KPa

Temperature: Less Than 160o

Stage 2

Pressure: 0.77-0.84 MPa


Stage 3

Pressure: 2.27-2.98 MPa


Stage 4

Pressure: 6.28MPa


Cylinder No: 3, 4, 5, 6

Pressure: 40KgF/cm2

Operates in Condenser

mode

Cylinder No: 1, 2, 3

Pressure: 63KgF/cm2

For Unit OPU & MIV/Pressure Reducing line

Cylinder No: 8

Pressure: 8KgF/cm2

For Braking System

Cylinder No: 9

Pressure: 8KgF/cm2

For Braking system/ service air / condenser

mode


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UNIT CONTROL BOARD (UCB):

Unit control boards are kept at EL 600. Several panels are there in the UCB in order to control the

operation of the turbine (unit). There are 4 unit control boards, one for each turbine.

(1#MEB30) Unit # MIV OPU Control cubicle:-

This cubicle is used to open and close the spherical valve of the Main inlet valve (MIV). Counter

weights have been provided in the MIV which are operated through the hydraulic mechanism.

When the spherical valve is to be opened, the oil from the air-oil vessel lifts the counter weights; as

a result the spherical valve is opened.

(1#MEX30) Unit # Turbine. Control cubicle:-

From this cubicle the oil transfer pump is controlled. Oil pump cooler and the wicket gates of the

turbine are also controlled through this cubicle.

(1#MEX10) Unit # Governor electronic cubicle:-

This cubicle basically maintains constant rpm of the turbine with the help of actuators. It is hydro

electrical. It also shows the opening of the wicket gates, speed of turbine and power generated. It is

being controlled from the control room with the help of computers.

(1#CJAO1) Unit # Manual Control Cubicle:-

The operations of the turbine are controlled from this cubicle. Turbine can be manually started from

this cubicle In case there is some problem in the control room. Brakes can also be applied from this

cubicle. It also shows the power generated, phase L1 current, phase L2 current, phase L3 current,

frequency, stator voltage, reactive power, phase, rotor voltage and rotor current.

(1#CJAO2) Unit # Alarm & synchronization cubicle:-

In this cubicle various component error are listed. In case of any fault or any warning the alarm

corresponding to that component blows and the corresponding LEDs start blinking. Alarms 1 to 49

are always high priority alarms. Not giving attention to these alarms may result in machine

shutdown. Temperatures of various components of the turbine can also be monitored here. For this

Resistance Temperature detector (RTDs) are installed at various unit components. The RTDs used

here is Pt-100 which gives 100 ohm at 0oC. Temperatures can also be monitored with the help of

Dial type thermometer (DTTs) installed in case RTDs fails. Synchronization of the power generated is

also done in this cubicle. When the phase sequence, phase voltage, frequency matches with that of


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power grid, it gets synchronized. For that an electronic device called synchro-tact is installed in that

cubicle which automatically synchronizes power house with the power grid.

Operating a unit from Manual Control Cubicle

1) First the unit cooling water is switched on. The function is to cool the stator windings.

2) Now the command to open the Main inlet valve (MIV) is given the help of electronic switches.

Water is allowed to flow into the spiral cage and rotates the turbine blades. The flow is controlled

through the MIV. In case of maintenance MIV is closed.

3) Now governor is started. This starts the machine. Wicket gate opens and water flows through the

spiral casing, which forces the turbine blades to rotate.

4) Now the field breaker is switched on.

5) Now self-excitation is switched on. In this the generated 15.75KV is tapped and is stepped down

and after converting to DC is fed back to the rotor for the excitation. 11KV line is isolated.

6) The generated voltage is 15.75KV. It is then stepped up to 420KV from the Generating

transformers.

7) The powerhouse is now synchronized with the power grid. For this phase sequence, phase

voltage and frequency are matched with that of power grid. It is done with the help of synchro-tact.

Figure 17: Steps in Synchronization

Step

1

Unit Water Cooling turned ON

Step

2

MIV Opened

Step

3

Governer Started and Field Breaker ON

Step

4

Self Excitation ON

Step

5

Stepped upto 420 kV

Step

6

Synchronized to the Grid


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Machine Operating Modes

1) Turbine Mode: In this mode, the units are operated as a generator. Water strikes the turbine

blades and produce electricity. This is mode usually followed for the machines. Voltage generated is

15.75KV with a frequency of 50Hz.

2) Condenser Mode: The loads are generally of inductive nature. Under heavy inductive load, grid

may collapse. To avoid this capacitive load is needed. This machine can also be used as a motor. First

wicket gates are closed. Then water in the draft tube is compressed with the help of compressed air

at 40KgF pressure which renders the turbine free to move without water friction. Now the machine

is connected to the grid and is run in over excited mode which acts as a capacitor and thus

compensates the power factor due to inductive load. In short, it is used for the power factor

correction.

3) Back to Back mode: This back to back mode will be used in Pump storage plant (PSP). In this the

water will be pumped from the TRT back to HRT and is allowed to fall back on the turbines. Since,

the PSP is an ongoing project so this mode is not used now.

4) Standstill mode: This keeps the machine idle.

Shutdown Modes:

1) Normal Shutdown: This is the machine shutdown mode usually followed. First the machine is

unloaded. When the power becomes 10 to 20 MW, circuit breaker opens. Now the machine is in

spinning mode. Excitation is now switched off i.e. no more DC supply is given to the machine rotor

windings. Now Main Inlet Valve is closed. Field Breaker is then switched off. No more Cooling water

is given to the machine now.

2) Emergency shutdown: It is automatic shutdown in case there some big fault in the machine.

Machine gets switched off automatically. MIV is also automatically closed.

3) Rapid Shutdown: It is similar to Emergency shutdown where Butterfly valve closure is the

additional step.


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5. Gas Insulated Switchyard


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GAS INSULATED SWITCHGEAR (GIS)

After the power is generated and stepped up to 420KV through generating transformers, it is passed

through GIS before sending it to power grid.

a) 3 phase AC wave is first passed through the surge arrestors which arrests the high peaks in

the wave. Basically it is the capacitor which charges and discharges in order to remove those

high peaks.

b) Current transformers are there for the protection. Their rating is 500A/1A.

c) Isolators give option to switch between the bus bars.

d) Circuit breakers are also there, which contains Nitrogen gas to pressurize the oil. Nitrogen

pushes the piston as a result the circuit is completed and the conduction starts. Circuit

breakers here are hydraulic type. The pressure of SF6 in circuit breaker is 6.9MPa.

e) Bus Coupler: - It connects the power generated to the transfer line. B11 is connected to B12

& B21 is connected to B22.

f) Insulation: - For insulation SF6 is used. AS soon as the line charges, spark is generated, SF6

quenches the spark. SF6 is used because Fluorine ions have high electro negativity which

quickly recombines to form SF6.

g) L1 line trap is provided for the hotline communication.

h) Here capacitive voltage transformers are used because it works well on high voltage and are

economical.

i) In GIS red cylinder contains Nitrogen whereas blue cylinders contain SF6.

j) PD Monitoring System: - It monitors and gives visual indication of any faults in GIS bus bar

due to partial discharge.

k) Buses from here carry power to Koteshwar and from there to Meerut.

Figure 18: Gas Insulated Switchyard (GIS)


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6. Maintenance,

Production & Protection


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Maintenance

Maintenance of turbines is done periodically. Major maintenance is carried out after every 4 years. Minor maintenance is carried out at short intervals. Maintenance sequence is given below:

1) Butterfly valve and Main inlet Valve for the unit undergoing maintenance are closed to stop the water coming to the turbine. 2) Water from the penstock is drained with the help of valves. 3) Draft tube is emptied. Valves present on the elbow draft tube are opened and the water is drained. 4) Spiral casing is then opened. 5) Chemical line assembly is set up in order to clean the pipe to reduce the moisture. For this casting soda is used. 6) Wicket gates are opened for maintenance. 7) Filler gas is used to check the gaps. 8) Then runner in the spiral casing is checked. 9) All the 28 rotor coils are removed and cleaned. Oil cooling system is also removed and cleaned. 10) Servo motors are opened and oiling is done of various components of the servo motors. 11) Oil pressure unit is now opened and checked. 12) Unloading and loading valves are then opened. 13) Resistance temperature detector (RTDs) and Dial type thermometers (DTTs) are then checked. 14) All pads of upper and lower bracket are removed and cleaned. 15) Thrust pads are then removed. 16) Meanwhile, all the UCBs are also opened and all the electronic circuits are checked. 17) Panels in the excitation system are also opened and checked. 18) All the pads are fitted again. And then undergo Megger test. Through Megger test insulation of the pads is checked. 19) Machine centering is done then with the help of pads. 20) Rotor is now jacked on the thrust pads, machine is centered and the pads are locked. 0.20 to 0.25 mm clearance is left between shaft and pads. 21) Machine then undergoes High voltage test (HV test). 22) After the maintenance is done, machine is started again.


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Figure 19: Rotor Pole during maintenance

Figure 20: Stator Windings Exposed during maintenance


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Figure 21: Upper Bracket of Generator during maintenance

Figure 22: Oil Cooling System during maintenance


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Production and Protection

a) Tehri Hydro Project has the capacity to produce 2400MW, 1000MW from the Hydro Power

Plant, 400MW from the Koteshwar Project and 1000MW from the Pump Storage Plant

(PSP). Currently the PSP is an ongoing project. Koteshwar was last year submerged, due to

heavy rainfall that resulted in high water current. Koteshwar plant is being restored these

days, so only 2 units are currently functional there. Annual energy generation at average

water head is 5300 million units. The total energy generation from hydropower plant in

India is around 35,000MW, so we can see that contribution of Tehri Project is significant.

b) In case the 11KV line needed for the initial excitation fails, then there are various other

modes through which machine and its auxiliaries can be run. There are 2 sets of Diesel

Generators which come into play in case 11KV fails. In case Diesel generator-1 also fails then

Diesel Generator-2 starts working.

c) If both the diesel generator fails then there are two sets of Battery banks. One of them is

kept in standby if battery bank 1 fails. Each battery bank can provide power for 5 hours

which implies that generator auxiliaries can be operated on batteries for 10 hours on a

whole. In each battery bank there are 23 cells each of 2.2 V making total of 48 volts. These

batteries are always kept charged in case of emergency. Earth leakage of the battery should

be zero.

d) Neutral Grounding Transformer grounds the neutral point of the 3 phase AC generated. In

case there is some current leakage, it gets grounded with the help of this transformer.

Various protection systems have been installed in the powerhouse.

a) Smoke sensors have been installed in the machine hall, GIS and control room. Firefighting

system has also been installed.

b) Red pipes in the powerhouse contain the water for firefighting. This water is taken from the

draft tube.

c) In case of big fault, the units automatically shut down.

d) If there is some fault in UCB and everything gets uncontrolled then emergency shutdown is

done in which the butterfly valve closes automatically. It is closed through the

electromagnetic induction.

e) Dam is earth and rock fill which makes it withstand an earthquake of 8.3 on the Richter

scale.


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7. Epilogue


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Epilogue

The Tehri Hydro Development Corporation, a joint venture of Central and Uttarakhand state

Government, is a landmark in the development of the Nation. With a huge structure and the state-

of-the-art facilities, it is indeed an engineering marvel that stands firm on the landmass of India. This

project caters to electricity and irrigation requirements of several states. There are both pros and

cons for building the Tehri dam. Many people suffered the loss during its construction. More than 50

villages drowned. But they were rehabilitated in the newly constructed city called New Tehri.

It is a zone of high security as it is a sensitive point for the terrorist attacks as destruction of the dam

can result in submergence of the cities downstream such as Rishikesh, Haridwar, Dehradun and

many other places. It also rests on the earthquake prone region but the structure is designed so as

to withstand an earthquake of Richter Scale 8.3.

The project has generation capacity of 2400 MW, 1000 MW contribution by Hydro Power Plant in

Tehri, 400 MW contributions by Hydro Power Plant in Koteshwar and 1000MW contribution by

Pumped Storage Power Plant in Tehri, which is yet to be commissioned. A gas insulated switchyard

(GIS) is an important part of the power plant. The electricity is supplied to the grid in Meerut and is

responsible for fulfilling the demand in peak hours.

Though, it is Engineering Marvel but also a great tourist spot. There are various areas of

development of tourism in Tehri that can be an additional source of income for the people here.


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Images Courtesy: Google, THDC Journals.

Documents

THDC Training Report