Project- Design of Ehv Substation at FACT UDL

DESIGN OF EHV SUBSTATION AT FACT UDL

A Project Report

Submitted by

Arunkumar M.P

Bijesh K.B

Jijo Francis

Lipin A.K

In partial fulfillment of the requirements for award of the degree of Bachelor of

Technology in Electrical & Electronics Engineering

FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT) Angamaly-683577, Ernakulam

MAHATMA GANDHI UNIVERSITY

Kottayam-686560

May 2011

DESIGN OF EHV SUBSTATION AT

FACT UDL

A Project Report

Submitted by

Arunkumar M.P

Bijesh K.B

Jijo Francis

Lipin A.K

In partial fulfillment of the requirements for award of the degree of Bachelor of Technology

in Electrical & Electronics Engineering

FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT) Angamaly-683577, Ernakulam Affiliated to

MAHATMA GANDHI UNIVERSITY

Kottayam-686560

May 2011

FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT)

Mookkannoor (p.o), Angamaly-683577

CERTIFICATE

This is to certify that the project report titled DESIGN OF EHV SUBSTATION AT

FACT UDC submitted by Jijo Francis, towards partial fulfillment of the

requirements for the award of the degree of Bachelor of Technology in Electronics

and Instrumentation Engineering is a record of bonafide work carried out by them

during the academic year 2007 –2011.

Internal Guide Head of the Department

Name Ms. Sreeja Dr. Pailo Paul

Designation Lecturer Asst. professor

Department of EEE Department of EEE

FISAT FISAT

Place: Mookkannoor Date:

ACKNOWLEDGEMENT

First we would like to express our sincere thanks to God Almighty for giving us

enough strength to complete our project successfully

We have unique pleasure in thanking our teacher Ms. Sreeja for her unflinching

devotion and help, which led us to completion of this project.

We place on record our sincere thanks to our external guide Mr. Gopala Krishnan,

Assistant Engineer, FEDO Department, FACT in providing necessary guidance and

back up throughout the whole course of our project.

The project that we did at FACT (UDL DIVISION) was quiet inspiring and we

gained a lot of knowledge. We do hope that this project would be of help to other

electrical students.

ABSTRACT

The major requirement of a power system to an industry is a well designed

electrical system with stability and minimum interruption. This project deals with

the design of electrical system of FACT Udyogamandal division which lies under

continuous process category. The contract demand is 16MVA. Hence in this project

the design of substation and its auxiliaries, earthing, etc are carried out in detail.

This project deals with the design of an 110KV, 20 MVA Industrial type outdoor

substation. The main source of power in this substation is from a 16MW turbo

generator and KSEB supply. The KSEB supply is taken from the nearby parent

station through over head XPLE cables. Since all connections are made using over

head cables, lighting arresters are used.

The existing 110KV substation is installed in 1947. The installed capacity of the

circuit breakers is 250MVA. But, now the fault level of 110KV systems is 5445MVA.

So the circuit breakers used cannot withstand fault level. Two 12.5/20 MVA

transformers are used.

CONTENTS

Chapter 1 INTRODUCTION……………………………….……1 Chapter 2 SINGLE LINE DIAGRAM…………………….…….2

Chapter 3 FAULT LEVEL CALCULATION……….………….4

Chapter 4 EARTHING RESISTANCE MEASUREMENT…...7

Chapter 5 SUBSTATION…………………………………….….13

Chapter 6 SUBSTATION COMPONENTS……………………17 Chapter 7 CONCLUSION………………………………..………40 Chapter 8 REFERENCES…………………………..……………41

INTRODUCTION

Federal Institute of Science And Technology (FISAT)

1. INTRODUCTION

A substation is a part of an electrical generation, transmission, and distribution system,

where voltage is transformed from high to low, or the reverse, or many other important functions.

Electric power may flow through several substations between generating plant and consumer, and

may be changed in voltage in several steps.

A substation that has a step-up transformer increases the voltage while decreasing the current,

while a step-down transformer decreases the voltage while increasing the current for domestic and

commercial distribution. The word substation comes from the days before the distribution system

became a grid. The first substations were connected to only one power station where the

generators were housed, and were subsidiaries of that power station.

For economical transmission and distribution, higher voltage should be achieved as follows. But

the consumers do not use very high voltages such as, 220KV, 110 KV etc. So they must be

transformed into low voltages by means of transformers in sub-stations. Thus, a substation may

be called as link between the generating stations and consumers.

Electrical & Electronics Engineering 1

SINGLE LINE DIAGRAM


Electrical & Electronics Engineering 2Federal Institute of Science And Technology (FISAT)


FAULT LEVEL CALCULATION


3. FAULT LEVEL CALCULATION

Kalamassery fault level = 3600 MVA

25% future expansion = 4500 MVA

10% voltage Regulation = 5445 MVA

Base MVA = 100 MVA

Source Impedance =Base MVA × 100Fault level MVA

=

100× 100

5445 = 1.8365

Length of ACSR = 5.1 Km

Ohmic impedance of the conductor = √0.292+0.5772

= √0.0841+0.332929

= 0.6458/km

= 0.6458 × 5.1=3.29Ω

% Z = KVA × Z

10× KV 2 =3.29× 100 ×1000

10× 1102 = 2.72%

% impedance of 110 KV bus at UDL = 1.8365 +2.72 = 1.8365+2.72

= 4.5565

Fault level =100× 100

4.5565 = 2194.67 MVA



Fault current = 2194.6100√3

= 11.53KA

% Impedance of 20 MVA Transformer = 15.6%

% Impedance of 100 MVA base = 100× 15.6

20 = 78

% Impedance of 11 KV side of Transformer = 78+4.5565 = 82.5565

Fault level of 11 KV side of Transformer = 21.13 / √3 ×11= 6.365 KA

3.1 DESIGN OF SUBSTATION

Earth Resistivity 4 spike method

Earth Resistance 10mm distance - 0.03 Ω

5m distance - 0.45 Ω

Earth pit - 0.31 Ω

P, C, are shorted and is connected to the earth pit. P2 & C2 are connected to the two terminals

which are connected to the 2 electrodes which are at 20m distances.

3.2 SITE SELECTION

Main points to be considered while selecting the site for EHV Sub-Station are as follows:

i) The site chosen should be as near to the load center as possible.

ii) It should be easily approachable by road or rail for transportation of equipments.

iii) Land should be fairly leveled to minimize development cost.


iv) Source of water should be as near to the site as possible. This is because water is

required for various construction activities; (especially civil works, earthing and for

drinking purposes etc.)

v) The sub-station site should be as near to the town/city but should be clear of public

places, aerodromes, and Military/police installations.

vi) The land should be have sufficient ground area to accommodate substation

equipments, buildings, staff quarters, space for storage of material, such as store yards

and store sheds. With roads and space for future expansion.

vii) Set back distances from various roads such as National Highways, State Highways

should be observed as per the regulations in force.

viii) While selecting the land for the substation preference to be given to the Govt. land

over private land.

ix) The land should not have water logging problem.

x) The site should permit easy and safe approach to outlets for EHV lines.


EARTH RESISTANCE MEASUREMENT


4. EARTH RESISTANCE MEASUREMENT

4.1 RESISTIVITY

r = 40 ×3.14× 0.03

1.89−0.714 = 40 ×3.14× 0.03

0.18 = 20.93

4.2 EARTH GRID AREA CALCULATIONS

(Ac) 2 = If (tc.λr.rr.× 104/TCAP) / ln (1 + (Tm-Ta) (K0 + Ta))

Where, If = Fault current = 25980 A

Tc = Fault current duration = 1 Sec

lr = Efficient of thermal expansion = 0.00423

rr = Soil resistivity = 20.93Ωm

TCAP = Thermal capacity / unit volume in J/cm3 0c = SH×SW×4.184

= 4.184 ×0.114 × 07.8 = 3.749

Where, SH = Specific heat

SW = Specific weight

TM = Maximum allowable temp in 0C

= 6200C

Ta = Ambient temp in 0C

= 500C

K0 = 1λ r

−Tr = 216.64


Tr = reference temperature for material constant = 200C

Earth grid area, Ac2

= 25.980 (1 × 0.00423 x104 × 20.93/3.749) / ln (1 + (620-50) / (216 + 50))

= 533501.28m2

Ac = 730.41m2

By including earth satellite earth mat the area of the earth grid can be increased to

: 875 m2 (35 × 25)

4.3 DESIGN

4.3.1 Conductor size:

Ondor clonk’s formula provides a reasonable method to compute the area of cross section of

electrode for earthing.

A = I√ t x √¿¿

Where, A = Area of iron section in m2

I = rms current in amps

Tm = Max allowable temp in 0C

Ta = Ambient Temperature in 0C

Max allowable temp for steel in 6200C for welded joint and 3100C for bolted joints.

The formula is simplified as: A = KI √ t


4.3.2 Initial design

Assume a preliminary layout of 21m x 18m grid with equally spied conductors and shown in

figure with spacing D = 15 m, grid burial depth h = 0.75m

Ground rod Grid conductor

4.4 Grounding mat design:

Considering further expansion, fault current for the design procedure as 259804.

For grounding mat and welded joint are to be provided.

Area of cross section = A = KI√ t = 0.0122 × 25980√1 = 317m2

Area of 40 mm diameter Rod = 40 × 317 = 1257 m2

Hence area of cross section is sufficient.

Earth mat is designed at a normal spacing of 15m between conductors. As per initial design

length of earth grid including down rods = 36220m

No: of towers is assumed as 3 and lower foot resistance as 10Ω

Resistance of three towers = 10/3 = 3.33Ω

Total resistance of line = Resistance of tower + Resistance of ground wire


Resistance of ground wire is assumed as 3Ω

Total resistance of one line = 3.33 + 3 = 6.33 Ω

31

23

Total resistance of two lines together = 6.33/2 = 3.165 Ω

The resistance of the earth grid may be calculated as R = (r/4r) +(r/L)

r = 20.93 Ωm

R = Radius of the equivalent surface of the grid

= √area of switch yard /3.14¿¿ = √875/3.14 = 16.68m

R = (20.93/4× 16.68 + 20.93/36220) = 0.31Ω

Value of electrode resistance = 1/ [(1/0.31) + (1/3.165)] = 0.28

For fault current of 25980 A in 110Kv bas grid ground return current = 0.357 × 0.28 × 11530 =

1152.54A

Minimum length of ground conductor required:

L = Km × Ki × r× Ig × √ t / (116 + 0.174 Cs rs )

Where,

K = 1/2π [ln [[D2 / 16 h.d] + [(D+2. h)/8 Dd ]2 – [h / 4d]] + [Kit / Kh] ×∈¿(2n – 1)]

D = spacing of conductor = 15m

d = diameter of conductor = 0.04m

h = depth of buried conductor = 0.75m

n = √14 ×1 9¿

¿ = 16

Ki = 0.656 + 0.172 × 16 = 3.4

Kit = 1

Kh = √1+(0.75 /1)= 1.322

Km = 0.677


Mini grid length L = 0.677 ×3.4 × 20.93 ×1152.54

116+o .174 ×1× 3000 = 87.03m

4.5 EARTHING CONDUCTOR SIZE REQUIRED FOR LV SIDE

Soil resistivity = 20.93Ωm

Permeable current density (for 3sec) for copper = 118A/mm2

Fault current = 6.365 KA

Earthing conductor size required = 6.365 X 103

118 = 53.94mm2

4.6 Tolerable step voltage

E step actual = Ks × r × Ki× Ig/L

Ks = (1/π) ×[(1/2h) ÷ (1/Dh) + (1/D)] = 0.255

E step actual = 0.255 × 3.4 × 20.93 × 1152.54/36220

= 0.58v

E touch actual = Km x Ki x r x Ki x Ig/ L

= 0.677 × 3.4 × 20.938 × 1152.54 / 36220 =1.53V


4.6.1 Safe let go touch voltage

E touch let go = (R3 + R2Fp)/ Ib

Rbe Human body, R = 1000Ω (assumed)

R2Fp = Contact resistance of two feet in parallel = 1.5 Cs x rs

Ib = Safe let go current = 0.009 A (assumed)

E touch let go = (1000 + (1.5 × 1 ×3000) 0.009) = 49.5V

To establish a value of sustained current within the limit of let go value of body current.

49.5 V ≥ Km x Ki x Cs x rx Tsust/ L

Isust = 49.5 ×36220 /0.677 ×3.4 × 1× 3000=260 A

Hence ground relays clearing ground fault current must be set for a mini pick up value less than

260A.


SUBSTATION


5. SUBSTATION

The substation may be defined us “on assembly of apparatus which transforms the characteristics

of electrical energy from one form to another, say for example from alternating current to direct

current or from one voltage to another.” For economical transmission, higher and higher voltages

should be achieved. At present normal voltages are 66KV, 110K.V and 220 KV; however 440KV

will be used for the national grid system in future.

The consumers do not use such high voltages and so it must be transformed to low

voltages by means of substation. Transformation may take place in several stages in sequence

starting at the generating plant where the voltage is increased for transmission purposes and is

then progressively reduced to the voltage required for household or industrial use. Thus a

substation may be called as link between the generating stations and the consumer. The

distribution voltages generally used in practice are 6.6 KV, 11KV and 33KV.

Substations or switching stations arc integral part of transmission system, and function as

a connection or switching point for transmission lines, substation feeders, generating circuits and

step up and step down transformers. Substations of voltages 66KV to 400 KV are termed as EHV

substations. Above 500 KV, they come under the terminology of UHV system.

Substations generally contain one or more transformers and have switching, protection

and control equipment. In a large substation, circuit breakers are used to interrupt ay short circuits

or overload currents that may occur on the network.


In a large substation, circuit breakers arc used to interrupt any short-circuits or overload

currents that may occur on the network. Smaller distribution stations may use autorecloser circuit

breakers or fuses for protection of branch circuits.

A typical substation will contain line termination structures, high-voltage switchgear, one or

more power transformers, low voltage switchgear, surge protection, controls, grounding

(earthing) system, and metering. Other devices such as power factor correction capacitors and

voltage regulators may also be located at a substation. Substations may be on the surface in

fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may

have indoor substations. Indoor substations are usually found in urban areas to reduce the noise

from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or

pollution conditions. Where a substation has a fence, it must be properly grounded to protect

people from high voltages that may occur during a fault in the transmission system. Earth faults at

a substation can cause ground potential rise at the fault location.

An important function performed by a substation is switching, which is the connecting and

disconnecting of transmission lines or other components to and from the system. Switching events

may be "planned" or "unplanned". A transmission line or other component may need to be de-

energized for maintenance or for new construction: for example, adding or removing a

transmission line or a transformer. To maintain reliability of supply, no company ever brings

down its whole system for maintenance. All work to be performed, from routine testing to adding

entirely new substations, must be done while keeping the whole system running. Perhaps more

importantly, a fault may develop in a transmission line or any other component. Some examples

of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind.

The function of the substation is to isolate the faulted portion of the system in the shortest

possible time.


5.1 VARIOUS PHASE OF SUBSTATION DESIGN

5.1.1 Planning

Planning includes the determination of the needed capacity, evaluation of alternative methods of

service, selection of the service voltage and required facilities and the related financial

requirements. This stage concludes with a contract between the utility and the plant.

5.1.2 Design

This stage includes the developments of detailed engineering drawings, finalizing of facility

requirements, building documentation and specifications for the facilities and then changes

required. This stage concludes with the lifting of construction contracts. This stage is the detailed

following up to all the works in the planning by using the preliminary single line diagram

developed and agreed upon in the planning stage. From this and other related design parameters

developed in the planning stage, detailed design and engineering drawings and specifications

along with construction cost estimates are developed.

5.1.3 Construction

This includes the construction and energizing the substation facilities.

5.1.4 Operation

This includes the development implementation and the documentation of the procedures for

operating and maintaining the substation. These procedures are typically developed during the

design and construction stages and completed prior to energizing the substation.

5.1.5 Site testing

Site testing should be done to determine the load-bearing strength of the ground. While

preliminary testing can be performed at an early stage, it is more useful if a plot plan layout is

available and equipment weights are known.

Electrical & Electronics Engineering 15 Federal Institute of Science And Technology (FISAT)

The testing is generally performed by specialized soil boring and test companies.

Soil boring companies can sufficient test boring to determine the deign parameters for foundation

needed for the utility's incoming towers, circuit breakers, transformers and the plant's primary

switch gear building. Land that has been filled even 15 or 20 years ago may not have developed

sufficient load bearing capabilities, installing caissons or piles to support the foundation may be

necessary, especially for a large substation.


SUBSTATION COMPONENTS


6. SUBSTATION COMPONENTS

6.1 POWER TRANSFORMERS

A transformer has been defined by ANSI/IEEE as a static electrical device, involving no

continuously moving parts, used in electric power systems to transfer power between circuits

through the use of electromagnetic induction. The term power transformer is used to refer to those

transformers used between the generator and the distribution circuits and are usually rated at 500

kVA and above. Power systems typically consist of a large number of generation locations,

distribution points, and interconnections within the system or with nearby systems, such as a

neighboring utility.

Figure 1.1: POWER TRANSFORMER



The complexity of the system leads to a variety of transmission and distribution voltages. Power

transformers must be used at each of these points where there is a transition between voltage

levels. Power transformers are selected based on the application, with the emphasis towards

custom design being more apparent the larger the unit. Power transformers are available for step-

up operation, primarily used at the generator and referred to as generator step- up (GSU)

transformers, and for step-down operation, mainly used to feed distribution circuits.

Power transformers are available as a single phase or three phase apparatus. The construction of a

transformer depends upon the application, with transformers intended for indoor use primarily

dry-type but also as liquid immersed and for outdoor use usually liquid immersed.

Two number power transformers each of 12.5 /20 MVA ratings at the 110 KV substations shall

be provided. The transformer shall be ONAN / ONAF type. On load tap changer shall be

provided for voltage variation up to -10% to 10%. Necessary primary protection shall be

provided for the protection of the transformer. The transformer shall be provided with the

forced air cooling system with suitable enhanced 'MVA' capacity. Adequate provision shall be

there for connecting 1000A, XLPE cable to secondary terminals.

Power transformers are usually the largest single item in a substation. For economy of service

roads, transformers are located on one side of a substation, and the connection to switchgear is by

bare conductors. Because of the large quantity of oil, it is essential to take precaution against the

spread of fire. Hence, the transformer is usually located around a sump used to collect the excess

oil. Transformer that are located and a cell should be enclosed in a blast room.


6.1.1. Selections of transformers

Load = 15MW

Full load current = 15×103

0.8×0.8×11×√3 = 1231.61A

Starting current = 7 × full load current

The Transformers should provide the starting current also ;

The transformers should provide this much current primary side

Transformer voltage on secondary side is 11 KV

Transformer Rating = 11 × √3 ×1231.61 = 23.465MVA

30% future expansion = 30.509 MVA

Two 20MVA 110/11KV Transformer is selected.


Maker TELK-TRANSFORMER WITH ON-LOAD TAP-CHANGER

Transformer Specification Ref. No. IS 2026-1977Maker’s Sl. No. 120270-2

Type SALOCRForm 3NYCPYear of Manufacture 1988Rated KVA HV 12500/20000

LV 12500/20000Volts – no load HV 110000 V

LV 11000 VAmperes HV 65.7/105.1

LV 657 / 1051Type of cooling ONAN / ONAFImpedance Voltage (20MVA base) HV/LV 15.60%Phases HV 3

LV 3Frequency C/S 50 HzVector Symbol Y N y n OTransportation Mass 24000 KgUn-tanking Mass 20000 KgMass of oil 9495 KgTotal Mass 41000 KgOil liters 10550 LitersAir Circulation 8 x 90 m3 / min


POSITION HV CONNECTION HV VOLTS HV CURRENT 1 1N–15 116600 99.2 2 IN-14 114950 100.6 3 IN-13 113300 102 4 IN-12 111650 103.5 5 IN-11 110000 105.1 6 IN-10 108350 106.1 7 IN-9 106700 108.4 8 IN-8 105050 10.1 9 IN-7 103400 111.8 10 IN-6 101750 113.6 11 IN-5 100100 115.5 12 IN-4 98450 117.4 13 IN-3 96800 119.4

6.2 SWITCHING EQUIPMENTS

The design of the high-voltage substation must include consideration for the safe operation and

maintenance of the equipment. Switching equipment is used to provide isolation, no load

switching, load switching, and/or interruption of fault currents. The magnitude and duration of the

load and fault currents will be significant in the selection of the equipment used. System

operations and maintenance must also be considered when equipment is selected. One significant

choice is the decision of single-phase or three-phase operation. High-voltage power systems are

generally operated as a three- phase system, and the imbalance that will occur when operating

equipment in a single- phase mode must be considered.

The main classes of equipment are as follows:

Isolators

Switches

Fuse-switch combinations

Circuit breakers

Earthing switches



6.2.1 CIRCUIT BREAKER

Circuit Breaker is equipment which can make or break a circuit at all conditions i.e. no load, full

load and short-circuits conditions.

6.2.1.1 Principles of Operation

A circuit breaker consists of a fixed and moving contact called electrodes. These contacts remain

closed under normal operating conditions. It will open automatically when the system becomes

faulty. When the fault occurs on any part of the system, the moving contacts are pulled apart by

some mechanism, contacts of the circuit breaker are separated under fault condition, and an arc is

formed between them. The current in this circuit unable to continue until the discharge stops. The

production of are not only delays the current-interruption process but it also generates a large

amount of heat which may cause damage to the system or to the breaker, itself. Therefore the

main problem in a circuit breaker is to extinguish the arc within the shorted possible time. So that

heat generated by it may not reach a dangerous value.

6.2.1.2 Circuit Breaker Rating

1. Breaking capacity

2. Making capacity

3. Short time rating.

6.2.1.3 Breaking Capacity

It is the current that a CB is capable of breakable at a given recovery voltage and under specified

conditions, (e.g. power factor, rate of re-striking voltage)

Breaking capacity in MVA = √3 × breaking current × rated voltage.



6.2.1.4 Making capacity

The peak value of the current during the first cycle of current wave after closure of Circuit

Breaker is known as making capacity.

Making capacity : 2.56 × Symmetrical breaking capacity.

6.2.1.4 Short time rating

It is the period for which the CB is able to carry fault current while remaining closed.

Figure 1.2: CIRCUIT BREAKER



6.2.1.5 Classification of Circuit Breakers

The circuit breaker can be classified according to the voltage below 100V are called Low voltage

Circuit Breaker, and above 1000V are called High Voltage Circuit Breakers.

Circuit Breaker’s are classified according to the medium of are extinction. It is shown below.

(a) Air break CB

(b) Oil CB

(c) Air blast CB

(d) Sulphur hexa fluoride CB (SF6)

(e) Vacuum CB.

6.2.1.6 Sulphur hexa fluoride (SF6) Circuit Breaker

The SF6 CB’s are used for high voltage service. The SF6 gas is used as the arc quenching

medium. The SF6 is an electronegative gas and gas a strong tendency to absorb free electronics.

The contacts of the breaker are opened in a high pressure flow of SF6 gas and arc is struck

between them. The conducting free electronics in the arc are rapidly captured by the gas to form

relatively immobile negative ions. The decreasing conducting electronics in the are quickly

increase ions. The decreasing conducting electronics in the arc quickly increase the insulation

strength to extinguish the arc.


6.2.1.7 Construction of SF6 Breaker

SF6 circuit breaker consists of two parts namely:

(i) The interrupt unit (ii) the gas system.

i) The Interrupt Unit:

This consists of fixed contacts which comprise a set of current carrying fingers and an arising

probe. When the breaker is in the closed position, the fingers make contact round the

circumstance of moving contact which has the arising probe enclosed within its hollow end. The

contacts are surrounded by interrupting nozzles and a blast shield which controls are displacement

and movement of the hot gas. The moving contact is in the form of hollow nozzle sliding in a

second set of spring loaded fingers. Side vents in the moving contact allow the high pressure gas

into the main tank.

As soon as the moving contact is withdrawn from the fixed contacts, an arc is drawn between the

moving nozzle and the arcing probe. As the contacts move further apart, the arc is extended and

attenuated. It is finally extinguished by the gas flow from high pressure to the low pressure

system.

ii) The gas-system:

The closed circuit gas system is used in SF6 circuit breakers. Since the gas pressure is very high,

lot of care is to be taken to prevent gas leakages at joints by providing perfect sealing. The low

and high pressure system are filled with low pressure alarm and a set of lock-out switched which

gives a warning the moment the gas pressure drops below a certain value, because otherwise there

will be reductions in the dielectric strength and are generating ability of the breaker is endangered.

If the damaged limit is reached, the safety devices immobile the Circuit Breaker, the over-riding

safety devices see to it that a fault in the control circuit does not permit compressor to build up

excessive pressure in the high pressure reservoir or continue to pump gas into the atmosphere in

the event of a major leak. The gas is stored in the high pressure chamber at 16 atmosphere.



Whereas the gas pressure on the low pressure side is 3 atmospheres. There temperature is 20oC. In

order to prevent liquidification of the gas in the high pressure chamber at low temp, heater is

fitted in the high pressure chamber. A thermostat is set to switch on when the ambient temp falls

below 16oC.

6.2.1.8 SHORT CIRCUIT CURRENT RATING

The short circuit current rating determines the minimium size of cable

conductor to be used:

Fault level for 11KV = 121.13 MVA

Isc = 121.13 MVA

11√3 KV = 6.365 KA



As per table

Mini cable size for secondary side is 70mm2.

Current rating of 11KV XLPE (cable)

As per tables:

For secondary side 3 runs of 3 core – 70mm2 (cable)

Primary Circuit breaker for 110/11KV 20MVA Transformer:

The full load secondary current is = 20× 103 11√3

= 1049.73 KA

Primary current is 20× 103

110√3= 104.97 A

Thus, SF6 CB is selected (3150A)

FEEDER CIRCUIT BREAKERMake Crompton Greaves Everyday SolutionsType Gas circuit breaker 120-SFM-328Year of manufacture 2006Rated voltage 145 kVRated Frequency 50 HzRated Normal Current 3150 ARated making Capacity 100 kApRated Short circuit breaking capacity 40 kARated short time current 40 kARated lighting impulse withstand voltage 650 kVpFirst pole to clear factor 1.5Rated opr. Seq 0--.3Sec – Co – 3 Min – CORated Gas Pressure 7 Kg /cm2 –g (at 200C)Gas Wt 8 KgTotal weight 1550 Kg



TRANSFORMER CIRCUIT BREAKERSMake Crompton Greaves Nasik, INDIAType GAS CIRCUIT BREAKER 120 – SFM – 32AYear of manufacture 1990Rated Voltage 145 kVRated Frequency 50 HzRated Normal Current 3150 ARated Making CapacityRated Short Circuit Breaking Capacity 31.50kARated short time current 31.5 kA for 3 secondsRated lighting impulse withstand voltage 650 kVpFirst pole to clear factor 1.5Rated opr. Pressure (Air) 15 Kg/Sq CMRated Gas Pressure 5 Kg / cm2 – g (at 200C)Gas Wt 9 KgTotal Weight 2000 Kg

6.2.2 ISOLATORS

Isolators are disconnecting switches which operate under no load conditions. It doesn’t have any

specified current breaking capacity making capacity. Isolators are sued in addition to circuit

breakers, while opening a circuit, the circuit breaker is opened first, then Isolator. While closing a

circuit the Isolator is closed first then the circuit breakers. Isolators are necessary on supply side

of circuit breakers in order to ensure isolation of the circuit breaker from live parts for the purpose

of maintenance.


Isolator design is considered in the following aspects:

• Space Factor

• Insulation Security

• Standardization

• Ease of Maintenance

• Cost

Some types of isolators include:

• Horizontal Isolation types

• Vertical Isolation types

• Moving Bushing types

6.2.2.1 Working

Isolators used in power system are generally 3 pole Isolator. The 3 pole Insulators have 3 identical

poles. Each pole consists of two or three Isolator posts mounted on a fabricated support. The

conducting parts are supported on the Insulator posts. The conducting part consists of conducting

Cu or AI rod, fixed and moving contacts. During the opening operation the conducting rod swings

apart and Isolation is obtained. The simultaneous operation of the 3 poles is obtained by

mechanical interlocking of the 3 poles.

The operating mechanism is manual plus one of the following.

a. Electrical motor mechanism

b. Pneumatic mechanism.



6.2.2.2 TYPES OF ISOLATORS

According to the constructional details the Isolators are classified as:

1. HORIZON BREAK CENTRE ROTATING DOUBLE BREAK ISOLATOR

In this type, there are 3 Insulator stacks per pole, the two on each side are fixed and one of the

centers is rotating type. The central Insulator stack cans wing about its vertical axis through about

90o. The fixed contacts are provided on the top of each of the Insulator stacks on the side. The

contact bar is fixed horizontally on the central Insulator stack. In closed position, the contact shaft

connects the two fixed contacts. While opening, the central stack rotates through 90o and the

contacts shaft swings horizontally giving a double break. The Insulators are mounted on a

galvanized, rolled steel frame. The 3 poles are interlocked by means of steel shaft.

2. PANTOGRAPH ISOLATION

These types of Isolators are preferred for rotted voltages 420KV and above. While closing the

linkages of pantograph are brought nearer by rotating the insulator column, in closed position two

arms of the pantograph close on the overhead station bus bar giving a grip. The current carried by

the upper bus bar to the lower bus bar through the conducting arms of the pantograph.

While opening the rotating Insulator column is rotated about its axis. Thereby the pantograph

blades collapse in vertical plane and vertical Isolator is obtained between the line terminal and

pantograph upper terminal. The Insulator covers less floor area. Each pole can be located at a

suitable point in a plane at desired angle with the bus axis.

3. VERTICAL BREAK TYPE

These are outdoor air break disconnecting switches and are designed for all out door applications

including isolation of circuit breakers, transformer banks, lighting arrestors and line sections.

Horizontal upright mounted switched can be equipped with crossing horns for interruptions of

small currents such as line charging or transformer magnetizing currents.



4. EARTHING SWITCH

Earthing switch is connected between the line conductor and normally it is open when the line is

disconnected, the earthing switch is closed so as to discharge the energy trapped in capacitance or

inductance of the line. Though the line is disconnected, there is some voltage on the line to which

the capacitance between the line and earth is charged. This voltage is significant to high voltage

system. Normally, the earthing switches are mounted on the frame of Isolator.

These are essentially off load devices although they are capable of dealing with small charging

currents of Bus bars and connections. The design of isolators is closely related to the design of

substations.

Figure 1.3: ISOLATOR



To carry out maintenance on particular items of plant, it is necessary to provide isolating

switches on each of the circuits to which the plant is connected. Except for outdoor installations,

these normally operate under no load conditions ensured by suitable interlocks. Outdoor isolators

for the higher voltages may however have to break the significant transformers magnetizing and

line charging currents. Earthing switches are also needed commonly in preference to temporary

earth connections. With indoor cubicle mounted or metal clad switchgear the isolators may be

simple knife switches that are incorporated in the structure and interlocked with the circuit

breaker; alternately, with a clad gear they make the form of plugs through which the circuit

breaker is connected to the bus bars and feeder, the circuit breaker being drawn out horizontally

or lowered vertically to effect isolation.

FEEDER ISOLATORS WITH EARTH SWITCHMake GR POWER SWITCH GEAR LTD,

JEEDIMETLA, HYDERABAD, 500055Type DBKV Rating 123Current Rating 1600AYear of manufacture 2005Impulse Voltage 550 kV peakShort time current for 1 sec 26.24 kA rmsOperating Mechanism MotorControl voltage 110V DCDrive 3f, 415V, AC, 1500 RPM, Induction Motor


6.3 CURRENT TRANSFORMERS

Current transformer (CT) is used for measurement of electric currents. Current transformers are

also known as instrument transformers. When current in a circuit is too high to directly apply to

measuring instruments, a current transformer produces a reduced current accurately proportional

to the current in the circuit, which can be conveniently connected to measuring and recording

instruments. A current transformer also isolates the measuring instruments from what may be

very high voltage in the primary circuit.

Figure 1.4: CURRENT TRANSFORMER

Current transformers are commonly used in metering and protective relays in the electrical power industry. Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. The alternating current flowing in the primary produces a magnetic field in the core, which then induces current flow in the secondary winding circuit. A primary objective of current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current. The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured.


The CTs primary circuit therefore consists of a single turn of conductor, with a secondary of

many hundreds of turns. The primary winding may be a permanent part of the current

transformer, with a heavy copper bar to carry current through the magnetic core. Window-type

current transformers are also common, which can have circuit cables run through the middle of an

opening in the core to provide a single-turn primary winding. When conductors passing through a

CT are not centered in the circular (or oval) opening, slight inaccuracies may occur.

Current transformers are basically used to take the readings of the currents entering the

substation. This transformer steps down the current from 800 amps to 1 amp. This is done

because we have no instrument for measuring of such a large current. The main use of this

transformer is (a) distance protection; (b) backup protection; (c) measurement.

Current transformer LV side

Type 132 kV CT

Core 1 core 2 core 3

Ratio (A/A) 800/1 400/1 800/1 400/1 800/1 400/1

Sec. Conn: 1S1-1S2 2S1-2S3 3S1-3S3

Accuracy class: 0.2 5P 10 PS

Burden (VA): 30 15 NA

Highest system

Voltage: 145 kV insulation burn 275 kV/ 65014 Vp

FEEDER CURRENT TRANSFORMER -1 (FCT -1)Ring Winding TransformersMake Brown Boveri – StromwandlerType TMRg 110Year of Manufacture 1957Voltage 110/220kVRatio 300 -1 50 – 75 - /1/1/1Power 60VAClass 0.5 & S201 Therm 23/12/61 Dyn 60/30/15


TRANSFORMER CT s (TCT -4)Make Transformer and Electrical Kerala

Ltd – TELKManufacturing No. 220104Sl No. 1R, 2R, 3RPO No. 1910211 Dt: 21/1/95Type NPOULVZ – RBasic Ins Level 230/550kVRated Voltage 126kVFrequency 50Hz

6.4 VOLTAGE TRANSFORMER

There is a step down transformer, which step down the high voltage to a value that can be measured using the measuring instruments in the control room.

This has an additional core for the carrier communication.

The CVT are connected between phase and ground in parallel to the circuit

Figure 1.5: CVT


6.5 BUS BAR

Used to interconnect the loads and sources of electrical power.

It connects generator and main transformer in power plant

Material used: Copper or Aluminum

Size of bus bar is used to determine maximum amount of current passed.

6.5.1 Bus bar sizing calculations

Short time rating of ACSR Zebra conductor

I = A √[ TCAP×10Tc . Xr . Pr

]−4

x ln [ Ko−Tm

Ko−Ta ]

Where,

I = RMS Current in KA = 25.98 KA

(ACSR Zebra conductor)

r20 = R20 × a/1

R20 = 0.06079Ω/ Km

a = 4.275 cm2

l = 1Km

r20 = 2.598 micro Ω cm

Ko = 230

I = 37.38 KA, which is greater than 25.98 KA specified.

Hence section of ACSR zebra conductor is safe.



6.6 LIGHTNING ARRESTOR

A lightning arrester is a device used on electrical power systems to protect the insulation on the

system from the damaging effect of lightning. Metal oxide varistors (MOVs) have been used for

power system protection since the mid 1970s. The typical lightning arrester also known as surge

arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching

surge travels down the power system to the arrester, the current from the surge is diverted around

the protected insulation in most cases to earth.

Figure 1.6: LIGHTNING ARRESTOR

These lightening arrestors are used to prevent the lightening from damaging the instruments in

the substation. Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used.


The lightening arrestors do not let the lightening to fall on the station.

If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any

substation the main important is of protection which is firstly done by these lightening arrestors.

The lightening arrestors are grounded to the earth so that it can pull the lightening.

6.6.1 WHAT EXACTLY DOES A LIGHTNING ARRESTOR DO?

It does not absorb the lightning

It does not stop the lightning

It does divert the lightning to ground

It does clamp (limit) the voltage produced by the lightning

It only protects equipment electrically in parallel with it.

6.6.2 Selection of lighting arrestor

When an arrestor connected between phase and earth it is a 3 system, the rated voltage must be

equal to or greater than the highest RMS value of power frequency voltage which can be applied

to it under normal and abnormal condition of operation including fault. Conduction of risk of

damage is to be avoided.

System Voltage = 110KV

Rated arrestor voltage = 1.1 x 110 x 0.8 (coefficient of earthing)

= 96.8 KV

Standard rating available = 96KV


Rated frequency = 50 Hz

Power frequency spark voltage = 1.5x rated arrest voltage

= 1.5 x 96 KV = 144 KV


CONCLUSIONFederal Institute of Science And Technology (FISAT)

7. CONCLUSION

The 66kV substation that is designed in this project is strictly based on the design procedures

accepted by the authorities. The intense care was taken through the whole procedure to ensure

protection for the equipment as well as the personnel in case of any failure or over current which

are possible to appear in the power system. The circuit breakers and the protective relays are

assigned for these purposes with proper design. The protective signs and symbols are also

employed for the workers to take precaution against failures. First of all a proper site to the

substation has to be identified such that some basic requirements have to be satisfied. From the

purely economic point of view, a major EHV substation would necessarily be of the outdoor type.

There should be enough space for the further developments of the plant because in future even

higher capacity may be needed to fulfill the power requirements. In order to protect the whole

plant from lightning, the lightning arresters and the earthing screens are provided above the

substation with suitable design criteria. These devices are capable to withstand the high current

due to lightning and also to pass this current safely to the earth without altering the present active

supply system. The other important equipments in the substation such as transformers, bus bars,

switch gear, potential transformers and current transformers, fuses, cables earth wires, batteries

etc are designed as per the requirements of the power system. The main objective of earthing is

protection from short circuits and over voltages that may occur in the power system due to many

reasons. A properly designed earthing system can protect the entire equipments and person that

comes under that specific power network. For proper designing of a power system, a deep

knowledge about the various faults that occur in the power system is necessary. Earthing is

considered as the one of the most important step in the design of a typical substation because the

proper earthing ensures the reliability of power supply- without many faults also ensures the

safety of the equipments as well as the personnel, So intense care has to be taken while designing

an earthing system for an extra high voltage substation.


REFERENCES


8 REFERENCES

[1] IEEE RED DATA BOOK, 1993

[2] NEWNESS ELECTRICAL ENGINEER’S HAND BOOK, D.F.WAENE, SECOND EDITION.

[3] ISO MANUAL ELECTRICAL SYSTEM STUDY AND FAULT LEVEL

CALCULATION BY FEDO.

[4] POWER GENERATION OPERATION ANDCONTROL BY ALLEN JWOOD AND BRUDE T W WOLLENBERG.

[5] ISO MANUAL FOR EARTHING AND SYSTEM STABILITY CALCULATION BY FEDO.

[6] STD 80-2000 IEEE GUIDE FOR SAFETY


Documents

Project- Design of Ehv Substation at FACT UDL