Ricky(1)

A

Seminar Report

On

“Practical Training”

Taken at

220 kV G.S.S. DausaAnd

Submitted in the partial fulfillment for award the degree ofBachelor of Technology

In

Electrical Engineering

From

Rajasthan Technical University, Kota

Session 2012-2013

Submitted To Submitted by

Mr. Arun Sharma Rinku Kumar Meena

Head Of Department B.Tech. 4th year (7th Sem.)

Electrical Engineering Roll no. 10EDSEE099

S.D.I.T. Dausa Enroll. no.

Submitted to

Department of Electrical Engineering

SHREE DIGAMBER INSTITUTE OF TECHNOLOGY,

BHANDAREJ MODE, DAUSA

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ACKNOWLEDGEMENT

I feel immense pleasure in conveying my heartiest thanks and deep sense of

gratitude to Head of the Electrical Engineering Department of SHREE

DIGAMBAR INSTITUTE OF TECHNOLOGY, Dausa for his efforts and for

technical as well as moral support.

Engineers and other technical and non technical staff, for helping in understanding

the various aspects and constructional detail of work and site in 220kV Grid-Sub

Station, Dausa.

It may not be possible for me to acknowledge the support of all my friends, but I

am thankful to all my colleagues and other trainees for their valuable ideas and

support during training period.

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PREFACE

A rapid rise in the use of electricity is placing a very heavy responsibility on

electrical undertaking to maintain their electrical network in perfect condition,

young engineers is called upon to do design, system planning and construction and

maintenance of electric system before he had much experience and practice soon

may be responsible for specialize operation in an ever expending industry.

Theoretical knowledge gained in their college courses need to be supplemented

with practical know-how to face this professional challenge, so……..

As a part of our practical training we have to attempt the rule of Rajasthan

Technical University, Kota. I look my practical training at 220 kV G.S.S. Dausa.

Since my training centre was of Grid Sub-station hence I have included all updated

information, to the extent possible, including general introduction and brief

description of starting sub-station of 220 kV G.S.S. in this study report.

During my 30 Working days practical training, I had undertaken my training at 220

kV G.S.S. at DAUSA.

I had taken my first practical training at 220 kV G.S.S. Dausa.

The period of training was from 17/05/2012 TO 15/06/2012.

This report dealt with the practical knowledge of general theory and technical

data/detail of equipments, which I have gained during the training period at 220 kV

GSS, Dausa.

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CONTENTS

1. Introduction……………………………………………………………..02-04 220 kV G.S.S. Dausa………………………………………………..03 Incoming feeders…………………………………………………….03 Outgoing feeders…………………………………………………….04 Radial feeders………………………………………………………..04

2. Bus bars…………………………………………………………………05-07 Bus bar arrangement………………………………………………...05

3. Isolators…………………………………………………………………….084. Insulators………………………………………………………………..08-11

Type of insulators……………………………………………………09o Pin type……………………………………………………….10o Suspension type………………………………………………10o Strain type…………………………………………………….11

5. Protective relays………………………………………………………...12-16 Differential relays…………………………………………………...14 Buchholz relay………………………………………………………14

o Construction…………………………………………………..14o Operation……………………………………………………..15o Advantages…………………………………………………...15o Disadvantages………………………………………………...16

6. Circuit breakers………………………………………………………....16-24 Operating principle………………………………………………….16 Arc phenomenon…………………………………………………….17 Classification of circuit breakers…………………………………….18 SF6 C.B……………………………………………………………...18

o Construction…………………………………………………..19o Working………………………………………………………20o 220 kV SF6 C.B. ratings……………………………………...21o Advantages of SF6 C.B………………………………………22o Demerits of SF6 C.B………………………………………….23o Applications…………………………………………………..24

7. Power Transformers…………………………………………………….24-29 Basic parts of transformer…………………………………………...25 Transformer ratings………………………………………………….26

8. Current transformer………………………………………………………...309. Potential transformer……………………………………………………….31

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10.Capacitive voltage transformer (CVT)…………………………………31-33 Description…………………………………………………………..31 Applications…………………………………………………………32 Ratings of cvt………………………………………………………..33

11.Transformer oil & its testing……………………………………………34-35 Breakdown voltage………………………………………………….34 Flash point…………………………………………………………...34

12.Lightening arrestor……………………………………………………...36-38 Thyrite type………………………………………………………….36 220 kV lightening arrestor…………………………………………..38

13.Control panel……………………………………………………………39-41 Reactor………………………………………………………………39 C.B…………………………………………………………………..39 Bus couplers…………………………………………………………40 Disturbance reactor………………………………………………….40 Event logger…………………………………………………………40 On load tap changer (OLTC)………………………………………..40 No load tap changer (NLTC)………………………………………..41 Synchronoscope……………………………………………………..41

14.Earthing of the system………………………………………………….42-43 Procedure of earthing………………………………………………..42 Neutral earthing……………………………………………………...42

15.Power line carrier communication…………………………………………44 PLCC system in Rajasthan…………………………………………..44

16.Corona effect……………………………………………………………45-47 Factors affecting corona……………………………………………..46 Advantages and disadvantages of corona…………………………...47

17.Conclusion………………………………………………………………….4818.Reference…………………………………………………………………...49

INTRODUCTION

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Electrical power is generated, transmitted in the form of alternating

current. The electric power produced at the power stations is delivered to the

consumers through a large network of transmission & distribution. The

transmission network is inevitable long and high power lines are necessary to

maintain a huge block of power source of generation to the load centers to inter

connected. Power house for increased reliability of supply greater.

The assembly of apparatus used to change some characteristics (e.g. voltage, ac to

dc, frequency, power factor etc.) of electric supply keeping the power constant is

called a substation.

Depending on the constructional feature, the high voltage substations may be

further subdivided:

(a) Outdoor substation

(b) Indoor substation

(c) Base or Underground substation

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Incoming feeders

Outgoing feeders

220 kV G.S.S. DAUSA

1. It is an outdoor type substation.

2. It is primary as well as distribution substation.

3. One and half breaker scheme is applied.

Incoming feeders:

The power mainly comes from:

220 kV:-

1. ANTA-1 & ANTA-2

2. PGCIL BASSI-1 & BASSI-2.

220 kV G.S.S.Dausa

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Out going feeders:

220 kV 132 kV 32 kV 11 kV

1) Alwar 1) Lalsot 1) Paprada 1) Hodaily

2) Bharatpur 2) Bassi 2) Bhandarej 2) Nangal

3) Hindaun 3) Toonga 3) Bapi 3) Dausa-1

4) Padasoli 4) Dausa-2

5) Dausa-3

6) Dausa-4

As this substation following feeders are established:

1. Radial Feeders.

2. Tie Feeders

RADIAL FEEDERS:

1. 220 kV DAUSA - ALWAR.

2. 220 kV DAUSA – HINDAUN.

3. 220 kV DAUSA – BHARATPUR

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BUS BARS

Bus Bars are the common electrical component through which a large no of feeders

operating at same voltage have to be connected.

If the bus bars are of rigid type (Aluminum types) the structure height are low

and minimum clearance is required. While in case of strain type of bus bars

suitable ACSR conductor are strung/tensioned by tension insulators discs

according to system voltages. In the widely used strain type bus bars stringing

tension is about 500-900 Kg depending upon the size of conductor used.

Here proper clearance would be achieved only if require tension is achieved. Loose

bus bars would effect the clearances when it swings while over tensioning may

damage insulators. Clamps or even affect the supporting structures in low

temperature conditions.

The clamping should be proper, as loose clamp would spark under in full load

condition damaging the bus bars itself.

3.1) BUS BAR ARRENGEMENT MAY BE OF FOLLOWING TYPE

WHICH IS BEING ADOPTED BY R.R.V.P.N.L.:-

3.1.1) Single bus bar arrangement

3.1.2) Double bus bar arrangement

a) Main bus with transformer bus

b) Main bus-I with main bus-II

3.1.3) Double bus bar arrangement with auxiliary bus.

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3.1.1) DOUBLE BUS BAR ARRANGEMENT :

1. Each load may be fed from either bus.

2. The load circuit may be divided in to two separate groups if needed

from operational consideration. Two supplies from different

sources can be put on each bus separately.

3. Either bus bar may be taken out from maintenance of insulators.

The normal bus selection insulators can not be used for breaking load

currents. The arrangement does not permit breaker maintenance without

causing stoppage of supply.

3.1.2) DOUBLE BUS BAR ARRANGEMENTS CONTAINS MAIN BUS

WITH AUXILARY BUS :

The double bus bar arrangement provides facility to change over to either bus

to carry out maintenance on the other but provide no facility to carry over

breaker maintenance. The main and transfer bus works the other way round. It

provides facility for carrying out breaker maintenance but does not permit bus

maintenance. Whenever maintenance is required on any breaker the circuit is

changed over to the transfer bus and is controlled through bus coupler

breaker.

Fig.:- Bus Bars

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ISOLATORS

“Isolator" is one, which can break and make an electric circuit in no load

condition. These are normally used in various circuits for the purposes of Isolation

of a certain portion when required for maintenance etc. Isolation of a certain

portion when required for maintenance etc. "Switching Isolators" are capable of

Interrupting transformer magnetized currents

Interrupting line charging current

Load transfer switching

Its main application is in connection with transformer feeder as this unit makes it

possible to switch out one transformer, while the other is still on load. The most

common type of isolators is the rotating centre pots type in which each phase has

three insulator post, with the outer posts carrying fixed contacts and connections

while the centre post having contact arm which is arranged to move through 90` on

its axis.

The following interlocks are provided with isolator:

a) Bus 1 and2 isolators cannot be closed simultaneously.

b) Isolator cannot operate unless the breaker is open.

c) Only one bay can be taken on bypass bus.

d) No isolator can operate when corresponding earth switch is on breaker.

INSULATOR

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The insulator for the overhead lines provides insulation to the power conductors

from the ground so that currents from conductors do not flow to earth through

supports. The insulators are connected to the cross arm of supporting structure and

the power conductor passes through the clamp of the insulator. The insulators

provide necessary insulation between line conductors and supports and thus

prevent any leakage current from conductors to earth. In general, the insulator

should have the following desirable properties:

High mechanical strength in order to withstand conductor load, wind

load etc.

High electrical resistance of insulator material in order to avoid

leakage currents to earth.

High relative permittivity of insulator material in order that dielectric

strength is high.

High ratio of puncture strength to flash over.

These insulators are generally made of glazed porcelain or toughened glass. Poly

come type insulator [solid core] are also being supplied in place of hast insulators

if available indigenously. The design of the insulator is such that the stress due to

contraction and expansion in any part of the insulator does not lead to any defect. It

is desirable not to allow porcelain to come in direct contact with a hard metal

screw thread.

4.1) TYPE OF INSULATORS:

4.1.1: Pin type

4.1.2: Suspension type

4.1.3: Strain insulator

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4.1.1) PIN TYPE: Pin type insulator consist of a single or multiple shells

adapted to be mounted on a spindle to be fixed to the cross arm of the

supporting structure. When the upper most shell is wet due to rain the

lower shells are dry and provide sufficient leakage resistance these are

used for transmission and distribution of electric power at voltage up

to voltage 33 KV. Beyond operating voltage of 33 KV the pin type

insulators thus become too bulky and hence uneconomical.

Fig.:- Pin Type Insulator

4.1.2) SUSPENSION TYPE: Suspension type insulators consist of a

number of porcelain disc connected in series by metal links in the

form of a string. Its working voltage is 66KV. Each disc is designed

for low voltage for 11KV.

Fig.:- Suspension type insulators

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4.1.3) STRAIN TYPE INSULATOR: The strain insulators are exactly

identical in shape with the suspension insulators. These strings are

placed in the horizontal plane rather than the vertical plane. These

insulators are used where line is subjected to greater tension. For low

voltage lines (< 11KV) shackle insulator are used as strain insulator.

Fig.:- Strain Insulators

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PROTECTIVE RELAYS

Relays must be able to evaluate a wide variety of parameters to establish that

corrective action is required. Obviously, a relay cannot prevent the fault. Its

primary purpose is to detect the fault and take the necessary action to minimize the

damage to the equipment or to the system. The most common parameters which

reflect the presence of a fault are the voltages and currents at the terminals of the

protected apparatus or at the appropriate zone boundaries. The fundamental

problem in power system protection is to define the quantities that can differentiate

between normal and abnormal conditions. This problem is compounded by the fact

that “normal” in the present sense means outside the zone of protection. This

aspect, which is of the greatest significance in designing a secure relaying system,

dominates the design of all protection systems.

Fig.: -Relays

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7.1) Distance Relays:

Distance relays respond to the voltage and current, i.e., the impedance, at the

relay location. The impedance per mile is fairly constant so these relays respond to

the distance between the relay location and the fault location. As the power

systems become more complex and the fault current varies with changes in

generation and system configuration, directional over current relays become

difficult to apply and to set for all contingencies, whereas the distance relay setting

is constant for a wide variety of changes external to the protected line.

7.2) Types of Distance relay:-

7.2.1) Impedance Relay:

The impedance relay has a circular characteristic centred. It is non

directional and is used primarily as a fault detector.

7.2.2) Admittance Relay:

The admittance relay is the most commonly used distance relay. It is the

tripping relay in pilot schemes and as the backup relay in step distance

schemes. In the electromechanical design it is circular, and in the solid state

design, it can be shaped to correspond to the transmission line impedance.

7.2.3) Reactance Relay:

The reactance relay is a straight-line characteristic that responds only to the

reactance of the protected line. It is non directional and is used to

supplement the admittance relay as a tripping relay to make the overall

protection independent of resistance. It is particularly useful on short lines

where the fault arc resistance is the same order of magnitude as the line

length.

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Buchholz Relay:

This has two Floats, one of them with surge catching baffle and gas

collecting space at top. This is mounted in the connecting pipe line between

conservator and main tank. This is the most dependable protection for a

given transformer.

Gas evolution at a slow rate that is associated with minor faults inside the

transformers gives rise to the operation or top float whose contacts are wired

for alarm. There is a glass window with marking to read the volume of gas

collected in the relay. Any major fault in transformer creates a surge and the

surge element in the relay trips the transformer. Size of the relay varies with

oil volume in the transformer and the mounting angle also is specified for

proper operation of the relay.

Fig.:-Buchholz Relay

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CIRCUIT BREAKER

The function of relays and circuit breakers in the operation of a power system is to

prevent or limit damage during faults or overloads, and to minimize their effect on

the remainder of the system. This is accomplished by dividing the system into

protective zones separated by circuit breakers. During a fault, the zone which

includes the faulted apparatus is de-energized and disconnected from the system.

In addition to its protective function, a circuit breaker is also used for circuit

switching under normal conditions.

Each having its protective relays for determining the existence of a fault in that

zone and having circuit breakers for disconnecting that zone from the system. It is

desirable to restrict the amount of system disconnected by a given fault; as for

example to a single transformer, line section, machine, or bus section. However,

economic considerations frequently limit the number of circuit breakers to those

required for normal operation and some compromises result in the relay protection.

Some of the manufacturers are ABB, AREVA, Cutler-Hammer (Eaton), Mitsubishi

Electric, Pennsylvania Breaker, Schneider Electric, Siemens, Toshiba, Končar

HVS and others.

Circuit breaker can be classified as "live tank", where the enclosure that contains

the breaking mechanism is at line potential, or dead tank with the enclosure at

earth potential. High-voltage AC circuit breakers are routinely available with

ratings up to 765,000 volts.

6.1) Various types of circuit breakers:-

6.1.1) SF6 Circuit Breaker

6.1.2) Air Blast Circuit Breaker

6.1.3) Oil Circuit Breaker

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6.1.4) Bulk Oil Circuit Breaker (MOCB)

6.1.5) Minimum Oil Circuit Breaker

6.1.1) SF6 CIRCUIT BREAKER:-

Sulphur hexafluoride has proved its-self as an excellent insulating and arc

quenching medium. It has been extensively used during the last 30 years in

circuit breakers, gas-insulated switchgear (GIS), high voltage capacitors,

bushings, and gas insulated transmission lines. In SF6 breakers the contacts

are surrounded by low pressure SF6 gas. At the moment the contacts are

opened, a small amount of gas is compressed and forced through the arc to

extinguish it.

Fig. 8-SF6 Circuit Breaker

6.1.2) AIR BLAST CIRCUIT BREAKER:

The principle of arc interruption in air blast circuit breakers is to direct a

blast of air, at high pressure and velocity, to the arc. Fresh and dry air of the

air blast will replace the ionized hot gases within the arc zone and the arc

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length is considerably increased. Consequently the arc may be interrupted at

the first natural current zero. In this type of breaker, the contacts are

surrounded by compressed air. When the contacts are opened the

compressed air is released in forced blast through the arc to the atmosphere

extinguishing the arc in the process.

Fig. 9-Air Blast Circuit Breaker

Advantages:

An air blast circuit breaker has the following advantages over an oil circuit

breaker:

The risk of fire is eliminated

The arcing products are completely removed by the blast whereas the oil

deteriorates with successive operations; the expense of regular oil is

replacement is avoided

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The growth of dielectric strength is so rapid that final contact gap needed for

arc extinction is very small. this reduces the size of device

The arcing time is very small due to the rapid build up of dielectric strength

between contacts. Therefore, the arc energy is only a fraction that in oil

circuit breakers, thus resulting in less burning of contacts

Due to lesser arc energy, air blast circuit breakers are very suitable for

conditions where frequent operation is required

The energy supplied for arc extinction is obtained from high pressure air and

is independent of the current to be interrupted.

Disadvantages:

Air has relatively inferior arc extinguishing properties.

Air blast circuit breakers are very sensitive to the variations in the rate of

restricting voltage.

Considerable maintenance is required for the compressor plant which

supplies the air blast

Air blast circuit breakers are finding wide applications in high voltage

installations. Majority of circuit breakers for voltages beyond 110 kV are of

this type.

6.1.3) OIL CIRCUIT BREAKER:

Circuit breaking in oil has been adopted since the early stages of circuit

breakers manufacture. The oil in oil-filled breakers serves the purpose of

insulating the live parts from the earthed ones and provides an excellent medium

for arc interruption. Oil circuit breakers of the various types are used in almost all

voltage ranges and ratings. However, they are commonly used at voltages below

115KV leaving the higher voltages for air blast and SF6 breakers. The contacts of

an oil breaker are submerged in insulating oil, which helps to cool and extinguish

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the arc that forms when the contacts are opened. Oil circuit breakers are classified

into two main types namely: bulk oil circuit breakers and minimum oil circuit

breakers.

The advantages of using oil as an arc quenching medium are:

1. It absorbs the arc energy to decompose the oil into gases, which have

excellent cooling properties.

2. It acts as an insulator and permits smaller clearance between live

conductors and earthed components.

The disadvantages of oil as an arc quenching medium are:

1. Its inflammable and there is risk of fire

2. It may form an explosive mixture with air.

3. The arcing products remain in the oil and it reduces the quality of oil after

several operations.

4. This necessitates periodic checking and replacement of oil.

6.1.4) BULK OIL CIRCUIT BREAKER:

Bulk oil circuit breakers are widely used in power systems from the lowest

voltages up to 115KV. However, they are still used in the systems having voltages

up to 230KV. The contacts of bulk oil breakers may be of the plain-break type,

where the arc is freely interrupted in the oil, or enclose within the arc controllers.

Plain-break circuit breakers consist mainly of a large volume of oil contained in a

metallic tank. Arc interruption depends on the head of oil above the contacts and

the speed of contact separation. The head of oil above the arc should be sufficient

to cool the gases, mainly hydrogen, produced by oil decomposition. A small air

cushion at the top of the oil together with the produced gases will increase the

pressure with a subsequent decrease of the arcing time.

6.1.5) MINIMUM OIL CIRCUIT BREAKER:

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Bulk oil circuit breakers have the disadvantage of using large quantity of oil. With

frequent breaking and making heavy currents the oil will deteriorate and may lead

to circuit breaker failure. This has led to the design of minimum oil circuit breakers

working on the same principles of arc control as those used in bulk oil breakers. In

this type of breakers the interrupter chamber is separated from the other parts and

arcing is confined to a small volume of oil. The lower chamber contains the

operating mechanism and the upper one contains the moving and fixed contacts

together with the control device. Both chambers are made of an insulating material

such as porcelain. The oil in both chambers is completely separated from each

other. By this arrangement the amount of oil needed for arc interruption and the

clearances to earth are roused. However, conditioning or changing the oil in the

interrupter chamber is more frequent than in the bulk oil breakers. This is due to

carbonization and slugging from arcs interrupted chamber is equipped with a

discharge vent and silica gel breather to permit a small gas cushion on top of the

oil. Single break minimum oil breakers are available in the voltage range 13.8 to

34.5 KV.

220 kV SF6 C.B. RATINGS:-

Manufacture: BHEL Hyderabad.

Type: DCVF (220-245 kV)

Rated voltage: 245 kV

Rated Frequency: 50 Hz

Rated power Frequency voltage: 460 kV

Rated Impulse withstands voltage:

Lightning: 1450 kV

Switching: 1050 kV

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Normal current Rating:

At 50 c ambient: 1120 Amp

At 40 c Ambient: 1250 Amp

Short time current rating: 20 kV for 1 sec.

Rated operating duty: 0 to o.3 sec. c-0-3min-mb

Rated short circuit duration: 1 sec.

BREAKING CAPACITY [BASED ON SPECIFIED DUTY CYCLE]:

a. Capacity at rated voltage: 14400 MVA [220 kV]

b. Symmetry current: 20 kV

c. Asymmetry current: 25 kV

Making capacity: 100kV

Rated pressure of hydraulic operating (gauge): 250-350 bars.

Rated pressure of SF6 gas at degree: 7.5 bars.

Weight of circuit breaker: 1500 Kg.

Weight of SF6 gas: 76.5 Kg.

Rated trip coil voltage: 220 V AC

Rated closing voltage: 220 V DC

ADVANTAGES OF SF6 CIRCUIT BREAKER:

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1. Due to the superior arc quenching property of SF6, such circuit breakers

have very short arching time.

2. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such

breakers can interrupt much larger currents.

3. The SF6 circuit breaker gives noiseless operation due to its closed gas

circuit and no exhaust to the atmosphere unlike the air blast circuit

breaker.

POWER TRANSFORMER

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Distribution transformers reduce the voltage of the primary circuit to the voltage

required by customers. This voltage varies and is usually:

120/240 volts single phase for residential customers,

480Y/277 or 208Y/120 for commercial or light industry customers.

Three-phase pad mounted transformers are used with an underground primary

circuit and three single-phase pole type transformers for overhead service.

Network service can be provided for areas with large concentrations of businesses.

These are usually transformers installed in an underground vault. Power is then

sent via underground cables to the separate customers.

Parts of Transformer:-

8.1) Windings:

Winding shall be of electrolytic grade copper free from scales & burrs. Windings

shall be made in dust proof and conditioned atmosphere. Coils shall be insulated

that impulse and power frequency voltage stresses are minimum. Coils assembly

shall be suitably supported between adjacent sections by insulating spacers and

barriers. Bracing and other insulation used in assembly of the winding shall be

arranged to ensure a free circulation of the oil and to reduce the hot spot of the

winding. All windings of the transformers having voltage less than 66 kV shall be

fully insulated. Tapping shall be so arranged as to preserve the magnetic balance of

the transformer at all voltage ratio. All leads from the windings to the terminal

board and bushing shall be rigidly supported to prevent injury from vibration short

circuit stresses.

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Fig. 11-Power Transformer

8.2) Tanks and fittings:

Tank shall be of welded construction & fabricated from tested quality low carbon

steel of adequate thickness. After completion of welding, all joints shall be

subjected to dye penetration testing.

At least two adequately sized inspection openings one at each end of the tank shall

be provided for easy access to bushing & earth connections. Turrets & other parts

surrounding the conductor of individual phase shall be non-magnetic. The main

tank body including tap changing compartment, radiators shall be capable of

withstanding full vacuum.

8.3) Cooling Equipments:

Cooling equipment shall conform to the requirement stipulated below:

(a.) Each radiator bank shall have its own cooling fans, shut off valves at the top

and bottom (80mm size) lifting lugs, top and bottom oil filling valves, air release

plug at the top, a drain and sampling valve and thermometer pocket fitted with

captive screw cap on the inlet and outlet.

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(b.) Cooling fans shall not be directly mounted on radiator bank which may cause

undue vibration. These shall be located so as to prevent ingress of rain water. Each

fan shall be suitably protected by galvanized wire guard.

Fig. 12-Radiator with fan

8.4.2) Temperature Indicators:

Most of the transformer (small transformers have only OTI) are provided with

indicators that displace oil temperature and winding temperature. There are

thermometers pockets provided in the tank top cover which hold the sensing

bulls in them. Oil temperature measured is that of the top oil, where as the

winding temperature measurement is indirect.

Fig. 14-Winding and oil temperature indicator

8.4.3) Silica Gel Breather:

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Both transformer oil and cellulosic paper are highly hygroscopic. Paper

being more hygroscopic than the mineral oil The moisture, if not excluded

from the oil surface in conservator, thus will find its way finally into the

paper insulation and causes reduction insulation strength of transformer. To

minimize this conservator is allowed to breathe only through the silica gel

column, which absorbs the moisture in air before it enters the conservator air

surface.

Fig.:-Silica gel Breather

8.4.4) Conservator:

With the variation of temperature there is corresponding variation in the oil

volume. To account for this, an expansion vessel called conservator is added

to the transformer with a connecting pipe to the main tank. In smaller

transformers this vessel is open to atmosphere through dehydrating breathers

(to keep the air dry). In larger transformers, an air bag is mounted inside the

conservator with the inside of bag open to atmosphere through the breathers

and the outside surface of the bag in contact with the oil surface.

CURRENT TRANSFORMER

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As you all know this is the device which provides the pre-decoded fraction of the

primary current passing through the line/bus main circuit. Such as primary current

60A, 75A, 150A, 240A, 300A, 400A, to the secondary output of 1A to 5A.

When connecting the jumpers, mostly secondary connections is taken to three

unction boxes where star delta formation is connected for three phase and final

leads taken to protection /metering scheme.

Fig.:-Current Transformers

It can be used to supply information for measuring power flows and the electrical

inputs for the operation of protective relays associated with the transmission and

distribution circuit or for power transformer. These current transformers have the

primary winding connected in series with the conductor carrying the current to be

measured or controlled. The secondary winding is thus insulated from the high

voltage and can then be connected to low voltage metering circuits.

POTENTIAL TRANSFORMER

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A potential transformer (PT) is used to transform the high voltage of a power line

to a lower value, which is in the range of an ac voltmeter or the potential coil of an

ac voltmeter.

The voltage transformers are classified as under:

Capacitive voltage transformer or capacitive type

Electromagnetic type.

Capacitive voltage transformer is being used more and more for voltage

measurement in high

voltage transmission network, particularly for systems voltage of 132KV and

above where it becomes increasingly more economical. It enables measurement of

the line to earth voltage to be made with simultaneous provision for carrier

frequency coupling, which has reached wide application in modern high voltage

network for tele-metering remote control and telephone

communication purpose.

CAPACITIVE VOLTAGE TRANSFORMERS (CVT)

A capacitor voltage transformer (CVT) is a transformer used in power systems to

step-down extra high voltage signals and provide low voltage signals either for

measurement or to operate a protective relay. In its most basic form the device

consists of three parts: two capacitors across which the voltage signal is split, an

inductive element used to tune the device to the supply frequency and a

transformer used to isolate and further step-down the voltage for the

instrumentation or protective relay. The device has at least four terminals, a high-

voltage terminal for connection to the high voltage signal, a ground terminal and at

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least one set of secondary terminals for connection to the instrumentation or

protective relay. CVTs are typically single-phase devices used for measuring

voltages in excess of one hundred kilovolts where the use of voltage transformers

would be uneconomical. In practice the first capacitor, C1, is often replaced by a

stack of capacitors connected in series. This results in a large voltage drop across

the stack of capacitors that replaced the first capacitor and a comparatively small

voltage drop across the second capacitor, C2, and hence the secondary terminals.

Fig.:- CVT connection

The porcelain in multi unit stack, all the potentials points are electrically tied and

suitably shielded to overcome the effect of corona RIV etc. Capacitive voltage

transformers are available for system voltage.

CVT is affected by the supply frequency switching transient and magnitude of

connected Burdon. The CVT is more economical than an electromagnetic voltage

transformer when the nominal supply voltage increases above 66KV.

The carrier current equipment can be connected via the capacitor of the CVT.

There by there is no need of separate coupling capacitor. The capacitor connected

in series act like potential dividers, provided, the current taken by burden is

negligible compared with current passing through the series connected capacitor.

Capacitive voltage transformer is being used more and more for voltage

measurement in high voltage transmission network, particularly for systems

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http://en.wikipedia.org/wiki/File:Cvt.png

voltage of 132KV and above where it becomes increasingly more economical. It

enables measurement of the line to earth voltage to be made with simultaneous

provision for carrier frequency coupling, which has reached wide application in

modern high voltage network for tele-metering remote control and telephone

communication purpose.

The capacitance type voltage transformers are of twp type:

Coupling Capacitor type

Pushing Type

TRANSFORMER OIL & ITS TESTING

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The insulation oil of voltage- and current-transformers fulfills the purpose of

insulating as well as cooling. Thus, the dielectric quality of transformer is a matter

of secure operation of a transformer.

Since transformer oil deteriorates in its isolation and cooling behavior due to

ageing and pollution by dust particles or humidity, and due to its vital role,

transformer oil must be subject to oil tests on a regular basis.

In most countries such tests are even mandatory. Transformer oil testing sequences

and procedures are defined by various international standards.

Periodic execution of transformer oil testing is as well in the very interest of

energy supplying companies, as potential damage to the transformer insulation can

be avoided by well timed substitution of the transformer oil. Lifetime of plant can

be substantially increased and the requirement for new investment may be delayed.

Transformer oil testing procedure

To assess the insulating property of dielectric transformer oil, a sample of the

transformer oil is taken and its breakdown voltage is measured.

The transformer oil is filled in the vessel of the testing device. Two

standard-compliant test electrodes with a typical clearance of 2.5 mm are

surrounded by the dielectric oil.

A test voltage is applied to the electrodes and is continuously increased up to

the breakdown voltage with a constant, standard-compliant slew rate of e.g. 2

kV/s.

At a certain voltage level breakdown occurs in an electric arc, leading to a

collapse of the test voltage.

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An instant after ignition of the arc, the test voltage is switched off

automatically by the testing device. Ultra fast switch off is highly desirable, as

the carbonization due to the electric arc must be limited to keep the additional

pollution as low as possible.

The transformer oil testing device measures and reports the root mean

square value of the breakdown voltage.

After the transformer oil test is completed, the insultaion oil is stirred

automatically and the test sequence is performed repeatedly. (Typically 5

Repetitions, depending on the standard)

As a result the breakdown voltage is calculated as mean value of the

individual measurements.

LIGHTNING ARRESTOR

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A lightning arrester (in Europe: surge arrester) is a device used on power

systems and telecommunications systems to protect the insulation and conductors

of the system from the damaging effects of lightning. The typical lightning arrester

has a high-voltage terminal and a ground terminal. When a lightning surge (or

switching surge, which is very similar) travels along the power line to the arrester,

the current from the surge is diverted through the arrestor, in most cases to earth.

In telegraphy and telephony, a lightning arrestor is placed where wires enter a

structure, preventing damage to electronic instruments within and ensuring the

safety of individuals near them. Smaller versions of lightning arresters, also

called surge protectors, are devices that are connected between each electrical

conductor in power and communications systems and the Earth. These prevent the

flow of the normal power or signal currents to ground, but provide a path over

which high-voltage lightning current flows, bypassing the connected equipment.

Their purpose is to limit the rise in voltage when a communications or power line

is struck by lightning or is near to a lightning strike.

If protection fails or is absent, lightning that strikes the electrical system introduces

thousands of kilovolts that may damage the transmission lines, and can also cause

severe damage to transformers and other electrical or electronic devices.

Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.

Potential target for a lightning strike, such as a television antenna, is attached to

the terminal labeled A in the photograph. Terminal E is attached to a long rod

buried in the ground. Ordinarily no current will flow between the antenna and the

ground because there is extremely high resistance between B and C, and also

between C and D. The voltage of a lightning strike, however, is many times higher

than that needed to move electrons through the two air gaps. The result is that

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electrons go through the lightning arrester rather than traveling on to the television

set and destroying it.

A lightning arrester may be a spark gap or may have a block of a semi

conducting material such as silicon carbide or zinc oxide. Some spark gaps are

open to the air, but most modern varieties are filled with a precision gas mixture,

and have a small amount of radioactive material to encourage the gas

to ionize when the voltage across the gap reaches a specified level. Other designs

of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp)

connected between the protected conductor and ground, or voltage-activated solid-

state switches called varistors or MOVs.

Lightning arresters built for power substation use are impressive devices,

consisting of a porcelain tube several feet long and several inches in diameter,

typically filled with disks of zinc oxide. A safety port on the side of the device

vents the occasional internal explosion without shattering the porcelain cylinder.

Lightning arresters are rated by the peak current they can withstand, the amount of

energy they can absorb, and the break over voltage that they require to begin

conduction. They are applied as part of a lightning protection system, in

combination with air terminals and bonding.

220 kV LIGHTNENING ARRESTOR:

Manufacture: English electric company

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No. of phase: One

Rated voltage: 360 kV

Nominal discharge current: (8×20µs) 10 kA

High current impulse: (4× 100µs) 100 kA

Long distribution rating: (200µs) 500 kA

CONTROL PANEL

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Control panel contain meters, control switches and recorders located in the control

building, also called the dog house. These are used to control the substation

equipment to send power from one circuit to another or to open or to shut down

circuits when needed.

Fig.:-Control Room in GSS Dausa

12.1) MEASURING INSTRUMENT USED:

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12.1.1) ENERGY METER: To measure the energy transmitted energy meters

are fitted to the panel to different feeders the energy transmitted is

recorded after one hour regularly for it MWHr, meter is provided.

12.1.2) WATTMETERS: It is attached to each feeder to record the power

exported from GSS.

12.1.3) FREQUENCY METER: To measure the frequency at each feeder

there is the provision of analog or digital frequency meter.

12.1.4) VOLTMETER: It is provided to measure the phase to phase

voltage .It is also available in both the analog and digital frequency

meter.

12.1.5) AMETER: It is provided to measure the line current. It is also

available in both the forms analogue as well as digital.

12.1.6) MAXIMUM DEMAND INDICATOR: There are also mounted the

control panel to record the average power over successive predetermined

period.

12.1.7) MVAR METER: It is to measure the reactive power of the circuit.

Capacitor Bank:-

The capacitor bank provides reactive power at grid substation. The voltage

regulation problem frequently reduces so of circulation of reactive power.

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Unlike the active power, reactive power can be produced, transmitted and absorbed

of course with in the certain limit, which have always to be workout. At any point

in the system shunt capacitor are commonly used in all voltage and in all size.

Fig. 20-Capacitor Bank

Benefits of using the capacitor bank are many and the reason is that capacitor

reduces the reactive current flowing in the whole system from generator to the

point of installation.

1 .Increased voltage level at the load

2. Reduced system losses

3. Increase power factor of loading current

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EARTHING OF THE SYSTEM

The provision of an earthing system for an electric system is necessary by the

following reason.

In the event of over voltage on the system due to lightening discharge or

other system fault. These parts of equipment, which are normally dead, as

for as voltage, are concerned do not attain dangerously high potential.

In a three phase, circuit the neutral of the system is earthed in order to

stabilize the potential of circuit with respect to earth.

The resistance of earthing system is depending on:

Shape and material of earth electrode used.

Depth in the soil.

Specific resistance of soil surrounding in the neighbourhood of system electrodes.

15.1) PROCEDURE OF EARTHING:

Technical consideration the current carrying path should have enough capacity to

deal with more faults current. The resistance of earth and current path should be

low enough to prevent voltage rise between earth and neutral. The earth electrode

must be driven in to the ground to a sufficient depth to as to obtain lower value of

earth resistance. To sufficient lowered earth resistance a number of electrodes are

inserted in the earth to a depth, they are connected together to form a mesh. The

resistance of earth should be for the mesh in generally inserted in the earth at 0.5m

depth the several point of mesh then connected to earth electrode or ground

conduction. The earth electrode is metal plate copper is used for earth plate.

15.2) NEUTRAL EARTHING:

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Neutral earthing of power transformer all power system operates with grounded

neutral. Grounding of neutral offers several advantages the neutral point of

generator transformer is connected to earth directly or through a reactance in some

cases the neutral point is earthed through an adjustable reactor of reactance

matched with the line.

The earth fault protection is based on the method of neutral

earthing.

RATINGS

17.1) TRANSFORMER:

Total No. of transformers = 6 No. of

transformers

220/132 KV------------------------------------ 100MVA 2

132/33 KV--------------------------------------20/25MVA 2

132/33KV---------------------------------------40/50MVA 1

132/11 KV---------------------------------------10/12.5 MVA 1

MAKE Company

220/133 KV, 100MVA X-Mer 1----------------------------------- TELK

220/133KV, 100 MVA X-Mer 2---------------------------------- ALSTOM

132/33 KV, 20/25 MVA X-Mer 1---------------------------------- TELK

132/33 KV, 20/25 MVA X-Mer 2-----------------------------------BBL

132/33 KV, 40/50 MVA X-Mer 3-----------------------------------T&R

132/33 KV, 10/12.5 MVA X-Mer 1---------------------------------EMCO

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17.2) CIRCUIT BREAKER:

No. of 220KV breaker - 6



No. of Capacitor Bank (33kv) - 4


SF6 CB

BREAKER SERIAL NO. 030228

RATED VOLTAGE 145KV

NORMAL CURRENT 1250A

FREQUENCY 5OHz

LIGHTNING IMPULSE WITHSTAND 650KV (Peak)

FIRST POLE TO CLEAR TO CLEAR FACTOR 1-2

SHORT TIME WITHSTAND CURRENT 31.5KA

DURATION OF SHORT CIRCUIT 3 Sec.

(SHORT CIRCUIT SYM. 31.5KA

BREAKING CURRENT) ASYM. 37.5KA

SHORT TIME MAKING CURRENT 8.0KA

OUT OF PHASE BREAKING CURRENT 7.9KA

OPERATING SEQUENCE 0-0.3-CO-3min-CO

SF6 GAS PRESSURE AT 20C 6.3 Bar

TOTAL MASS OF CB 1300Kg

MASS OF SF6 GAS 8.7Kg

17.3) BATTERY CHARGER:

Battery Charger – 220AH VDC HBL NIFE LTD.

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440AH VDC HBL NIFE LTD.

Capacitor BankNo.-1 BHEL 38KV 6.6MVAR

Capacitor BankNo.-2 BHEL 38KV 7.2MVAR

Capacitor BankNo.-1 ABB 38KV 7.2MVAR

Capacitor BankNo.-1 WS 38KV 7.2MVAR

17.4) CURRENT TRANSFORMER:

FREQUENCY 50Hz

HIGHEST SYSTEM VOLTAGE 245KV

SHORT TIME CURRENT 40KA/15

RATED CURRENT 600A

CURRENT RATIO 600-300-150/1

MIN. KNEE POTENTIAL VOLTAGE 850V at 150/1

MAX. EXCITING CURRENT 100MA at 150/1

MAX. SEC. WINDING RESISTANCE 2.5OHM at 150/1

17.5) CAPACITIVE VOLTAGE TRANSFORMER:

SERIAL NO. 0173537

INSULATION LEVEL 460KV

RATED VOLTAGE FACTOR 1.2/cont

TIME 1.5/30sec.

HIGHEST SYSTEM VOLTAGE 245KV

PRIMARY VOLTAGE 22OKV/1.732

TYPE OUTDOOR Wgt. 850Kg

PHASE SINGLE TBONP.CAT 50C

SECONDARY VOLTAGE 110/1.732 110/1.732

RATED BURDON 220Va 110Va

FREQUENCY 49.5-50.5Hz

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CONCLUSION

A technician needs to have not just theoretical but practical as well and so

every student is supposed to undergo practical training session after 2nd year where

I have imbibed the knowledge about transmission, distribution, generation and

maintenance with economical issues related to it.

During our 30 days training session we were acquainted with the repairing of

the transformers and also the testing of oil which is a major component of

transformer.

At last I would like to say that practical training taken at 220 kV GSS has

broadened my knowledge and widened my thinking as a professional.

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REFERENCE:-

Principles of Power System-by V.K. MEHTA

Electrical Power System-by C.L. WADHWA

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