Asafak Husain

IIT Roorkee

6/28/2015

2015Summer Internship Report

A

PRACTICAL INTERNSHIP REPORT On

Transmission & Distribution of Electrical Power taken at

220KV GSS RRVPNL, AJMER (Raj.)

Session 2015-16

Submitted to Submitted by

Prof. S.P. Srivastava ASAFAK HUSAIN

HOD of Electrical Deptt. B.Tech 3rd year, Electrical Science

IIT Roorkee IIT ROORKEE

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Enrollment no. 12115026

Time Period: 14 May to 28 June (45 Days)

DEPT. OF ELECTRICAL SCIENCE

IIT ROORKEE, ROORKEE (U.K.)-247667

AcknowledgementI would like to take this opportunity to express my heartfelt words for the people who were part of this training in numerous ways, people who gave me unending support right from the beginning of the training.

I am really grateful to training incharge, Mr. Tarun Issrani (JEN & executive AEN) and Mr. M. L. Jarwal (XEN) for giving proper and valuable guidance to make the project that successful.

I also would love to thank all my batch mates there in Madar. They are really helpful and curios about the subject and responsible to create a learning environment.

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AbstractThis huge 220KV Grid substation is at Madar in Ajmer (Rajasthan). This Substation is quite far from the city about 10km south. Like most of the substations in Rajasthan, here also the main source of power is Thermal Power Plant, Kota & Suratgarh, Nuclear power plant Ravatbhata and there are some private power companies as well.

The incoming supplies comes from two other substations Kishangarh and Beawar. Interestingly Madar also supplies to Kishangarh that is nothing but an example of ring type power system.

The substation has seven power transformers as its heart. Two are at incoming side of rating 160MVA 220KV/132KV/11KV (EMCO), one is of rating 40/50MVA 132KV/33KV (BBL), other two are 20/25MVA 132KV/33KV (IMP), and other two are of rating 7.5MVA 132KV/33KV/11KV (Westing House). The insulating and cooling oil flows in the transformers like blood in the veins. Rest of the equipment and accessories are either supportive or protective. In the following document I am trying to explain my learnings from my internship.

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CONTENT Introduction

1.Operation Of Substation1.1 Type of substation1.2 Operation of Substation

2.List Of Equipment And Their Ratings, General Purposes

3.Relays3.1 Mechanical Relays

3.1.1 Buckolz Relay3.1.2 Oil Surge Relay

3.2 Electrical Relay3.2.1 Overcurrent Relays3.2.2 Earth Fault Relay3.2.3 Distance Relay3.2.4 Differential Relay 3.2.5 Auxiliary Relay 3.2.6 Digital Relay3.2.7 Over Flux Relay and Flux Setting

Relay

3.3 Thermal Relay3.3.1 over current thermal relay3.3.2 CTR relay

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4.Transformers4.1 List of transformers4.2 Challenges with power

transformer4.3 Stationary parts

4.3.1 Main Tank4.3.2 Conservative Tank and Breather4.3.3 Core and Windings4.3.4 Tertiary windings4.3.5 Bushing4.3.6 Earthing

4.4 Active parts4.4.1 Transformer oil4.4.2 Cooling assembly4.4.3 Relays

4.5 Associate parts4.5.1 On Load Tap Changer (OLTC)4.5.2 Fire Protection System

5.Current Transformer 6.Potential Transformers7.Circuit Breakers8.Capacitor Bank9.Insulators10. Other Equipment

10.1Lightening Arrester10.2Bus-Main And Auxiliary Bus, Bus

Coupler10.3Isolators10.4Fuses- D.O. and HRC10.5Station Transformers10.6Concrete And Trenches10.7Earthing

11. Control Room

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12. PLCC RoomsSafety MeasuresConclusionReferencesAppendix AAppendix BAppendix C

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INTRODUCTIONThis GSS was installed in the year 1962, with the aim to supply according to load capacity of Ajmer. It is of 220KV presently upgraded from 132KV. The input supply is indirectly coming from TPP Kota and Suratgarh, NPP Ravatbhata and from some of the private sector companies.

Initially there used to be only one board in Rajasthan for everything concerned to electricity, Rajasthan State Electricity Board (RSBE) but because of its complete failure state government formed 5 new companies for each sector

1) Rajasthan Rajya Vidyut Utpadan Nigam Limited ( RRVUNL)- Generation

2) Rajasthan Rajya Vidyut Prasaran Nigam Limited ( RRVPNL)-Transmission

3) Vidyut Vitaran Nigam Limited ( VVNL) -Distribution

4) Ajmer Vidyut Vitaran Nigam Limited - Distribution in Ajmer region

5) Jodhpur Vidyut Vitaran Nigam Limited- Distribution in Jodhpur region

Since all the GSS and Substations are a part of Transmission that’s why they fall under the care of RRVPNL which has its Load Dispatch Center (LDC) in Heerapura, Jaipur. This LD takes data from all the substations and GSS of the state and dispatch an instruction to the same to keep the entire power system stable.

“Transformers are the heart of the substation and the oil used as coolant and insulator is the blood runs in its veins. Rest is either to associate or to protect transformers.”

This is all I learnt in my 45 days internship. There are many other equipment like CB, CT, PT, Insulator, Relays, LA, Wave Trap, Buses etc.

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In the following report I will try to lighten the working and construction of all important equipment and moreover I will to try fix the importance and position of each equipment under the operation of GSS.

At the end I will try to lighten importance of this GSS in the prospective of Ajmer.

About 220KV Madar Substation:

This substation has 220KV input from two feeders- Kishangarh and Beawar, is also supplies 132 KV to Kishangarh that is example of rings type power system.

This substation has two sister EMCO manufactured transformers of rating 220KV/132KV/11KV, but like all high rating power transformers, tertiary winding 11KV is not used, it has some advantages that’s why it’s provided. There is new yard had been made few years, before that it a 132KV substation.

132KV yard have three feeders- Bherunda, Kishangarh and Saradhna. This yard has 5 transformers, among them RVVPNL is planning to remove 2 Westinghouse 132//33/11KV, 7.5MVA transformer as they hardly carry any load.

33KV yard have 14 feeders and this is the one shared maximum load among all. It also have five capacitor banks to maintain the voltage level.

11KV yard has 4 feeders. And a station transformer that is like distribution transformer and an Earthing transformer.

In the substation I/C side have some lightening arresters and PTs. From there this line is comes to set of isolators, CTs, Circuit breakers and isolators.

PTs and CTs are to measure the bus voltage and feeder currents respectively and also they are integral part of switchgear.

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Circuit breaker are to cut the faulty of unhealthy section off from the power system.

1. OPERATION OF SUBSTAION

Before going to the operation, it’s necessary to get idea about the significance and location of a substation. Substations are broadly can be classified as three types according to its location

1.1 Type of Substations:

1.1.1 Generating Substation:

Generation of electrical power is not yet possible beyond 11KV in our country and to reduce the transmission losses, voltage should be much higher than this level. To have a better economy, this voltage level is raised up to 765KV in India. To increase this voltage level, A substation is made at the sending end.

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These are also called primary substations.

1.1.2 Transmission Substation:

This kind of substations are also Grid substation. There are somewhere in between the generating plant and the consumers. Although industrial class of loads are also supplied by these substations.

220KV Madar is fall under this category. These are also called secondary substations.

1.1.3 Distribution Substation:

These are at the tail of transmission, generally having a rating of 11KV/ 400V. These are meant to supply power residential, industrial loads etc.

1.2 Operation of Substation:

1.2.1 At Normal condition:

Operation of a substation unaffected until there is no fault and overcurrent. But at the normal operation as well proper and regular inspections and maintenance are essential to keep the substation healthy. Like

o Cleaning of various equipment o Testing of insulation and Earthingo Checking of transformer oilo Inspection of leakage currento Inspection of all the alarms and annunciatorso PLCC maintenanceo

1.2.2 At Faulty Condition:

If there is any fault occurs in transmission line or there is overloading, then there are protective overcurrent relays to sense that fault and these relays are connected to secondary of

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CT and secondary of PT. Once they relay senses the abnormality, it give command to Master relay or 86 relay, that is one which energizes the tripping coil of circuit breaker and cut off that faulty feeder or section.

This master relays also give command to alarms, annunciators and hooter which are placed in control room, so that concerned personnel notice it.

If there is any internal fault like insulation fault in transformers that is sensed by Buchholz relay, OSR relay or CTR relay and these relay and these relays produces the alarm and do their task to protect transformer.

2. LISTING OF EQUIPMENT

Table 2.1 Various type instruments in switchyard

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S.N.

Name of Equipment

Qnt

Remark on Location

Application

1. Transformer

7 b/w two buses

To step down the voltage

2 Relay variable Protection of various equipment by tripping and alarming

3 CT 43 b/w Isolator and Circuit Breaker

Protection and Measurement of current

4 PT 6 At main bus Protection and Measurement of voltage

5 Circuit breaker

b/w CT and Bus

Protection (to break the Circuit)

6 Station transformer

3 Under 11KV and 33KV

400v & 230v supply forControl room and panels

7 Capacitor bank

5 Near 33KV and 11KV buses

To boost up the bus voltage

8 Insulator -- Almost everywhere

To provide proper insulation

9 Lightening Arrester

11 Near buses on Insulator String

To equalize the string voltage

10 Isolator -- Between bus and CT

To isolate the circuit and transfer the load to auxiliary bus

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3. PROTECTIVE RELAYS

Relay are protective devices widely used in power system that senses any abnormality with the associated equipment and if there is any, relays gives a command to circuit breaker to break the unhealthy device.

All of the relays have three basic elements

i) Sensing element : responds to the change in the actuating quantity, the current in a protected system in case of over-current relay.

ii) Comparing element : serves to compare the action of the actuating quantity on the relay with a pre-selected relay setting.

iii) Control element : on a pick of the relay, accomplishes a sudden change in the control quantity such as closing of the operative current circuit.

Relays may be classified as to which kind of physical quantity the sensing element respond. Above concept of classification broadly results in below three types

i) Mechanical: actuated by pressure, velocity, of outflow of a liquid or gas etc.

ii) Electrical: actuated by some electrical quantity such as current, voltage, power etc. Further these are divided into two partsa)Electromagnetic: there are moving parts.b)Static: no moving parts

iii) Thermal: actuated by heating effect.

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In power system most of the relays being used are of electrical type.

In today’s scenario more advanced programmable Digital relay are being preferred in place of conventional induction type relays. In Indian power system scenario Various Relays are used in protection of Transmission lines and transformers. Protection of large transformers is governed by many relays and as the rating decreases number of relays also decreases. With larger transformer there has to be separate relays for Winding temperature and Oil temperature. Similarly transmission line also being protected by relays. These are tabulated below

Table 3.1 Relays for Transmission & Distribution Lines Protection

S.N. Line to be protected Relays to be used

1. 220 KV Transmission Line

Main-I: Non switched distance scheme (Fed from Bus PTs)Main-II: Switched distance scheme (Fed from line CVTs)With a changeover facility from bus PT to line CVT and vice-versa.

2. 132 KV Transmission Line

Main Protection: Switched distance scheme (fed from bus PT).Backup Protection: 3 Nos. directional IDMT O/L Relays and 1 No. Directional IDMT E/L relay.

3. 33 KV lines Non-directional IDMT 3 O/L and 1 E/L relays.

4. 11 KV lines Non-directional IDMT 2 O/L and 1 E/L relays.

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Most of the relays are situated in control rooms itself, from where they operates, which has incoming leads to sense some parameter like temperature, current or voltage etc., now the sensed input trip the relay that consequently awake to master 86 relay to give a command to Circuit breaker to get disconnected.. Various relays operating in the substation are tabulated below

Table 3.2 Various relays in substation

S.N.

Name of Relay Function Principle

1. Buchholz To detect internal faults in transformer

Pressure sensor

2. Oil surge relay To detect fault in OLTC

Pressure sensor

3. CTR relay To isolate main tank from conservator

Temperature sensor

4. Overcurrent Protection of transformer from excessive current

Induction type, EM attraction type

5. Earth fault Induction disc6. Distance Locate the fault7. Differential relay8. 86 relay CPU of entire

protective instruments9. Auxiliary To associate other

relaysHinged armature EM attraction type

10. Digital11. Overflux To prevent over

fluxing in transformer 12 Flus setting relay Pre over fluxing alarm13. Thermal Conjunction with other

instantaneous relaysThermal expansion of bimetallic sheet

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In a GSS most of the relays are electrical type. Digital relays are newer to the this family of relays but because of their static position they are preferred sometimes but mostly Induction disc type relays are part of protection system in Substation. Principles and working of these relays are as explained below.

3.1. Mechanical Relay

3.1.1. Buckolz relay:

It is a gas and oil operated mechanical device installed in the pipework between the top of the transformer main tank and the conservator. The function of the relay is to detect an abnormal condition within the tank and send an alarm or trip signal.  Under normal conditions the relay is completely full of oil.  Operation occurs when floats are displaced by an accumulation of gas, or a flap is moved by a surge of oil.  Almost all large oil-filled transformers are equipped with a Buchholz relay. Fault conditions within a transformer produce gases such as carbon monoxide, hydrogen and a range of hydrocarbons. 

A small fault produces a small volume of gas that is deliberately trapped in the gas collection chamber built into the relay. Typically, as the oil is displaced the upper float and a switch operates - normally to send an alarm. 

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Figure 3.1 Buchholz relay mechanism (Source: www.electrical4u.co

A large fault produces a large volume of gas which drives a surge of oil towards the conservator.  This surge moves the lower float in the relay to operate a switch and send a trip signal.  A severe reduction in the oil level will also result in a float falling.  Where two floats are available these are normally arranged in two stages, alarm followed by trip.

Figure 3.2 Buchholz relay and CTR relay these are situated between main tank and conservative tank

3.1.2. Oil Surge relay :

Oil surge relay also has same principle as the Buchholz relay but as far as the function is concerned they are differ. OLTC chamber has moving parts so there is some sparking that produces gases which further causes pressure in the chamber this excessive pressure is sensed by Oil surge relay. If the relay is tripped it

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breaks the circuit breaker and isolates the transformer from the rest of the system.

3.2. ELECTRICAL

3.2.1. Overcurrent Relays:

Whenever there is any excessive current in the lines these relays sense this abnormality and trip energizes which in return gives command to circuit breaker. Mainly electrical overcurrent relay are based on two principles electromagnetic attraction type and induction disc type. Thermal relay also applicable for overcurrent protection but it has limit when we use it in power system.

Electromagnetic Attraction type

These are simplest type of relays that work on the principle of electromagnetic attraction. They are also can be of Plunger, hinged, balanced beam, polarized moving iron type. All these relays operates on same principle.

In such relays the operation is obtained by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. The electromagnetic force exerted on the moving element is proportional to the square of flux in the air gap or the square of current flowing through the coil. It is basically single actuated relay. Such relays respond to both ac and dc, with dc they produce constant torque but with ac this torque consists of both ac as well as dc components

For dc F e=K I dc2 =constant Force

For ac F e=K (Imax sinωt)2=1

2K ¿

To get a constant torque with ac current, flux is splitted into two paths having a phase difference of 90° or by adding a shading rings on the poles of electromagnet.

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Induction type Non-directional overcurrent relay:

Electromagnetic-induction relays use the principle of the induction motor whereby torque is developed by induction in a rotor; this operating principle applies only to relays actuated by alternating current, and in dealing with those relays we shall call them simply "induction-type" relays.

Figure 3.3 Induction type relay operating principle (Source: www.elprocus.com)

An induction relay works only with alternating current. It consists of an electromagnetic system which operates on a moving conductor, generally in the form of a disc or cup, and functions through the interaction of electromagnetic fluxes with the parasitic Fault currents which are induced in the rotor by these fluxes. These two fluxes, which are mutually displaced both in angle and in position, produce a torque that can be expressed by

T= Κ1.Φ1.Φ2 .sin θ,

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Where Φ1 and Φ2 are the interacting fluxes.

θ is the phase angle between Φ1 and Φ2.

It should be noted that the torque is a maximum when the fluxes are out of phase by 90º, and zero when they are in phase. Electromagnetic forces in induction relays It can be shown that

Φ1= Φ1sin ωt, and Φ2= Φ2 sin (ωt+ θ),

Where θ is the angle by which Φ2 leads Φ1.

Thus: F= (F1 - F2) α Φ2 Φ1 sin θ α T

Induction relays can be grouped into three classes as set out below.

a) Shaded pole typeb) Wattmeter typec) Cup type

These relays are also used in limiting the power supplied to any particular feeder.

3.2.2. Earth-Fault Relay :

Earth fault relays are based on the principle of electromagnetic induction but here rather than phase current zero sequence current provides better sensitivity as ground resistance is quite so that current is small and generally does not get sensed.

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Earth current relays are used in following configuration where three overcurrent relays are connected in series with Earth fault

relay.

3.2.3. Distance relays:

The working principle of distance relay or impedance relay is very simple. There is one voltage element from potential transformer and an current element fed

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Figure 3.4 (a) Earth fault relay with Overcurrent relays

Figure 3.4(b) Connection diagram foe earth fault relay with over current relays

Source: www.eBlogBD.com

from current transformer of the system. The deflecting torque is produced by secondary current of CT and restoring torque is produced by voltage of potential transformer. In normal operating condition, restoring torque is more than deflecting torque. Hence relay will not operate. But in faulty condition, the current becomes quite large whereas voltage becomes less. Consequently, deflecting torque becomes more than restoring torque and dynamic parts of the relay starts moving which ultimately close the No contact of relay. Hence clearly operation or working principle of distance relay, depends upon the ratio of system voltage and current. As the ratio of voltage to current is nothing but impedance a distance relay is also known as impedance relay.

Impedance is thus measured as Impedance = Voltagecurrent

And the distance of fault from the substation α Impedance

Distance is relay is of three types

a) Admittance typeb) Impedance typec) Reactance relay

3.2.4. Differential relay :

Generally Differential protection is provided in the electrical power transformer rated more than 5MVA. The Differential Protection of Transformer has many advantages over other schemes of protection.

Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power

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transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.

1) The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any fault occurs in the transformer but not in oil then it cannot be detected by Buchholz relay. Any flash over at the bushings are not adequately covered by Buchholz relay. Differential relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for detecting any internal fault in the transformer but Differential Protection scheme detects the same in faster way.

2) The differential relays normally response to those faults which occur in side the differential protection zone of transformer.

General connection diagram of a differential relay is as follow

Figure 3.6 Differential protection scheme

Source: www.openelectrical.org

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3.2.5. Auxiliary Relay :

Auxiliary relays are associative relay that assist in the functioning of other protective, tripping relays. These are just supportive relays. Generally these are static type. There is a central relay named as Master relay or 86 Relay that receives signal from other relay and give command to tripping coil of circuit breaker.

Master relay is necessary in a substation as if we directly connected overcurrent relays from the tripping coil, then this relay will take around 3 seconds that is undesirable but master relay take seconds to do same. Apart from this master relay give command to alarm, annunciator and hooter.

3.2.6. Digital Relay:

A digital relay consists of the following main parts: processor, analogue input system, digital output system and independent power supply. The main difference between digital and conventional relays pertains to the method of input signal processing.

In the case of digital relays, input signals are converted into digital form within the analogue input system before being analyzed by the processor. Digital relays possess advanced programmable functionality providing high performance level, flexibility as well as additional monitoring capabilities. At present, their

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Figure 3.6 Digital relay block diagram

(Source: www.electrical-engineering-portal.com)

application is mainly in transmission system and generation unit protection.

Currently in Indian power system, there are being used instead of induction type relays.

3.2.7. Overflux relay and flux setting relay:

Over flux relay senses excessive flux in the core of the transformer. Excessive flux is a serious danger to the transformer but for their magnetic utilization operating flux is kept near to rated flux. So over fluxing may occur at any stage and to prevent it over flux relay is used. These generate an alarm if flux go pass a certain flux setting that is more than the rated.

Flux setting relay is associative relay to over flux relay and perform almost same function but here flux setting below the rated, it acts as early protective relay.

3.3. THERMAL RELAY

3.3.1. Bimetallic thermal relay

These relays operates on the principle of thermal effect of electric current. It consists of bimetallic strips which are used in small sizes and are heated by heating coils or strips supplied through a current transformer. Under normal operating condition the strip remains straight but under the action of fault the strip is heated and bent and the tension of the spring is released. Thus the relay contacts are closed which energizes the trip circuit.

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Thermal relays are not suitable for short-circuit as it will burn the element sufficiently before the strip may deflect so as to close the contacts. This type of relay is used in conjunction with instantaneous short-circuit relays of high setting or suitably

graded time limit fuses.

Figure 3.7 (b) Thermal relay at overcurrent operation (Source: www.electrical4u.com)

3.3.2. CTR Relay

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Figure 3.87(a) Bimetallic thermal relay at normal operation

This relay is situated just below the conservator tank.

It has function to isolate conservator tank from tank during fire. If seen from outside in the yard, it’s a red cubical box just below the conservator tank before the buchholz relays.

Its principle is based on thermal expansion of liquid. There are some tube filled with expandable liquid that expands after certain temperature, it consequently blast the tube that produces an alarm signal for fire.

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4. TRANSFORMERS

A transformer is the heart of a substation. This substation has four voltage levels, Incoming voltage from Beawar and Kishangarh 220KV, Step Down/Outgoing feeder Voltages 132KV, 33KV, 11KV and according to load requirement, there are some feeders from these voltage levels, which goes to distribution network.

There is mainly two type of transformers Power transformers and station transformers. Power transformers are used to much known purpose to reduce down the transmission voltage to lower value. Station transformers are used to obtain 400v, 230v voltage for local use of the substation like for control panels, battery chargers.

4.1. List of Transformers

Table 4.1 Transformers list

S.N.

Name of Equipment

type Qnt.

Ratings Location Purpose

1. Transformer EMCO

Power

2 160MVA220kv/132kv/11kv

220KV yard

To step down i/c voltage

2. TransformerBBL

power

1 40/50 MVA132kv/33kv

132 KV yard

To step down Voltage

3. TransformerIMP & EMCO

power

2 20/25 MVA132kv/33kv

132kv yard

To step down Voltage

4. Transform powe 2 7.5MVA 33kv To step

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erWesting house

r 132kv/33kv/11kv

yard down Voltage

5. Station Tr.-1

Station

1 250KVA33kv/400v

33kv yard

To yield 230 Volts

6. Station Tr-2

Station

1 100KVA33kv/400v

33kv yard

--do--

7. Station Tr-3

Station

1 100KVA11kv/400v

33kv yard

--do--

4.2. Challenges with power transformers

As the rating of electrical increases, challenges with its safe operation also increases. In this GSS there are 2 huge 160 MVA 220/132 KV transformers which provides us better opportunities to understand about the Earthing, Cooling, and Maintenance of power transformers. Main problems are listed below

Due to core losses, copper losses etc. transformer oil also heats up and this weakens insulation as its dielectric strength decrease, to overcome this problem cooling of transformer oil became a vital issue with power transformers.

This problem is relatively more challenging in case power transformers because they are designed to operate at high magnetizing flux.

These transformers are costlier, so their protection is major concern. Various protective accessories is needed in order to keep them safe if there any abnormality occurs.

Transformer oil is blood of transformer, is has to be pure (moisture free), for this purpose transformer is associated with many accessories.

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“In this report 220KV transformers will be described in detail because they are associate with most protective, cooling scheme and accessories, other are also operate almost similar to these but with lesser accessories and schemes.

Power transformers parts are classified in three categories based on their dynamic position. Power transformers are associated with many viable equipment, some of them are stationary, some are dynamic or do possesses anything that have motion inside it, and third one is other associates like fire protection and OLTC which are equally important.”

Figure 4.1: 160 MVA, 220KV power transformer in GSS, Madar

4.3. Stationary parts

4.3.1. Main tank:

Main tank of transformer keep windings, transformer oil etc. inside, are made up of stainless steel that provide a robust mechanical structure for the same. This tank has a size of our two hostel rooms and made highly mechanically robust and varnished for external protection.

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Bushing are mounted on the main tank and there is Earthing transformer as well. Apart from these this tank also contains various protective devices that helps in functioning or relays etc. There is Nitrogen gas pipes are connected to this tank for fire protection.

For transformer oil circulation there are radiators connected via two separate valve one at bottom and other at top.

Apart from these there is inspection window which is devoted for maintenance purpose.

Main tank is connected to conservative tank through buchholz relay and CTR relay and some valves for oil supply. If oil level decreases in main tank conservative tank provides oil to it and if decrease conservative tank assist it by tanking some of oil.

4.3.2. Conservative tank and Silicon breather:

Conservative tank is a cylindrical tank above the main tank. Whenever is there is expansion of oil, some oil transfers from main tank to conservative tank and if there is contraction it provide some oil to main tank, it act like reservoir of oil.

There is two type of conservative tanks are available for power transformers

Atmoseal Type Conservator

In this type conservator of transformer, an air cell made of NBR material is fitted inside the conservator reservoir. The silica gel breather is connected at the top of this air cell. The oil level in the power transformer rises and falls according to this air cell deflated and inflated. When the air cell gets deflated the air inside the air cell comes out via breather and on the other hand if the cell is inflated the outside air comes in through breather.

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This arrangement prevents direct contact of oil with air, thereby reduces ageing effect of oil.

The space available outside the cell in conservator tank is totally filled by oil. Air vents are provided on the top of the conservator for venting accumulated air outside the air cell.

The pressure inside the air cell must be maintained 1.0 PSI.

MOG:

This device is used to indicate the position of

transformer insulating oil level in conservator

of transformer. There is a PRV Relay in the

conservative tank that provides output as liquid

level. This relay is float type. Magnetic oil

level indicator of transformer consists of

mainly three parts-

1. One float,2. Bevel gear arrangement and3. An indicating dial.

Diaphragm Sealed Conservator:

Here diaphragm is used as a barrier between transformer oil and atmospheric air. In this case the conservator of transformer is

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Figure 4.2 (a) Atmoseal type of breather (source: www.electrical4u.com)

Figure 4.2 (b) Magnetic oil level gauge (PRV Relay)

(Source: electrical4u.com)

made of tow semicircular halves as shown below. The diaphragm is held between the two halves and bolted.

As oil expands it pushes up the diaphragm. The position of the diaphragm is indicated by the oil level indicator i.e. magnetic oil gauge as the rod of this MOG is connected to the diaphragm. When the oil level falls down in the conservator, the diaphragm deflects and the atmospheric air fills the vacant place. This air is sucked through silica gel breather which is connected to the top middle of conservator tank of transformer. This type of conservator has advantage over air cell conservator.

Silica gel

It is nothing but a pot of silica gel through which, air passes during breathing of transformer. The silica gel is a very good absorber of moisture. Freshly regenerated gel is very efficient, it may dry down air to a dew point of below − 40°C.

Silica gel crystal has tremendous capacity of absorbing moisture. When air passes through these crystals in the breather; the moisture of the air is absorbed by them. Therefore, the air reaches to the conservator is quite dry, the dust particles in the air get trapped by the oil in the oil seal cup.

The color of silica gel crystal is dark blue but, when it absorbs moisture; it becomes pink. When there is sufficient difference

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Figure 4.2 (c) structure of diaphragm conservative tank

(Source: electrical4u.com)

between the air inside the conservator and the outside air, the oil level in two components of the oil seal changes until the lower oil level just reaches the rim of the inverted cup, the air then moves from high pressure compartment to the low pressure compartment of the oil seal. Both of these happen when the oil acts as core filter and removes the dust from the outside air.

Figure 4.3 (a) Silica gel- blue colored is fresh and Pink is one that absorbed moisture (b) Oil cup seal filled with oil partially.

4.3.3. Core and windings

Core and windings are common to any kind of transformer. The performance of a transformer mainly depends upon the flux linkages between these windings. For efficient flux linking between these windings, one low reluctance magnetic path common to all windings should be provided in the transformer. This low reluctance magnetic path in transformer is known as core of transformer.

Core of the transformer is made up of CRGOS or Cold Rolled Grain Oriented Silicon Steel.

Here also the core is laminated to minimize eddy current and hysteresis losses.

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High rating power transformers an additional, tertiary winding is also provided because of some advantage of it.

During core manufacturing in factory some factors are taken into consideration,

1. Higher reliability.

2. Reduction in iron loss in transformer and magnetizing current.

3. Lowering material cost and labor cost.

4. Abatement of noise levels.

4.3.4. Tertiary windings

In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformer or 3 winding transformer.

1.1 Advantages of Using Tertiary Winding in Transformer

Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements-

1. It reduces the unbalancing in the primary due to unbalancing in three phase load.

2. It redistributes the flow of fault current.3. Sometime it is required to supply an auxiliary load in different

voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.

4. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

4.3.5. Bushings:

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In electric power, a bushing is an insulated device that allows an electrical conductor to pass safely through a (usually) earthed conducting barrier such as the wall of a transformer or circuit breaker.

All materials carrying an electric charge generate an electric field. In the case of DC the field remains either positive or negative, in the case of AC, the field is alternating between positive and negative.

When an energized conductor is near any material at earth potential, its can cause very high field strengths to be formed.

In this substation, porcelain bushing are used and HV side have slightly bigger bushing than LV side.

Figure 4.4 HV and LV bushings

4.3.6. Earthing:

Equipment Earthing is a connection done through a metal link between the body of any electrical appliance, or neutral point, as the case may be, to the deeper ground soil. The metal link is normally of MS flat, CI flat, GI wire which should be penetrated to the ground earth grid.

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In this GSS the Earthing mesh is damaged few year back so they earthed equipment using a grounded delta shaped wire.

4.4. Active parts 4.4.1. Transformer oil:

Generally there are two types of transformer Oil used in transformer,

1. Paraffin based transformer oil2. Naphtha based transformer oil

Naphtha oil is more easily oxidized than Paraffin oil. But oxidation product i.e. sludge in the naphtha oil is more soluble than Paraffin oil. Thus sludge of naphtha based oil is not precipitated in bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system. But in the case of Paraffin oil although oxidation rate is lower than that of Naphtha oil but the oxidation product or sludge is insoluble and precipitated at bottom of the tank and obstruct the transformer cooling system. Although Paraffin based oil has above mentioned disadvantage but still in our country it is generally used because of its easy availability. Another problem with paraffin based oil is its high pour point due to the wax content, but this does not affect its use due to warm climate condition of India.

Some specific parameters of insulating oil should be considered to determine the serviceability of that oil.

1. Electrical parameters: – Dielectric strength, specific resistance, dielectric dissipation factor.

2. Chemical parameter: - Water content, acidity, sludge content.3. Physical parameters: - Inter facial tension, viscosity, flash

point, pour point.

4.4.2. Cooling Assembly:

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The main source of heat generation in transformer is its copper loss or I 2R loss. Although there are other factors contribute heat in transformer such as hysteresis & eddy current losses but contribution of I2R loss dominate them. If this heat is not dissipated properly, the temperature of the transformer will rise continually which may cause damages in paper insulation and liquid insulation medium of transformer. So it is essential to control the temperature with in permissible limit to ensure the long life of transformer by reducing thermal degradation of its insulation system.

In electrical power transformer we use external transformer cooling system to accelerate the dissipation rate of heat of transformer.

Different Transformer Cooling Methods

ONAN Cooling of Transformer: here natural flow of using radiators and atmosphere air are the coolant.

ONAF Cooling of Transformer: Fans are used to cool the oil in the radiator.

OFAN: here pump is used. OFAF: both pump and fans are used. This type of cooling is

used for 220KV transformer whenever oil temperature exceeds normal operating temperature.

4.4.3. Relays :

Buchholz relay, OSR relay, CTR relays are main relays associated directly with transformers. “These are already explained.” Following table shows protective relays for transformer protection

Table 4.2 Transformer protection relays

S.N.

Relay on HV side

Relays on LV side

Common relays

1.2 nos O/L Relay & 1 E/L Relay

2/3 nos O/L Relays & 1 no E/L Relay

Buchholz Relay, OLTC Buchholz relay, PRV relay, OT Trip Relay,

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WT Trip Relay, Overflux Relay, Differential Relay

4.5. Associated parts Apart from these equipment and assembly of power transformers there are few more devices that are equally important in function of transformer.

4.5.1. OLTC:

In larger electrical power transformer, for proper voltage regulation of transformer, on load tap changer is required. As there is no permission of switching off the transformer during tap changing. The tapping arrangement, is placed in separate diverter tank attached to electrical power transformer main tank. Inside this tank, the tap selectors are generally arranged in a circular form. The diverter switches have contacts operating in rapid sequence with usually four separate make and break units.

4.5.2. Fire protection system:

There is nitrogen gas based fire protection system. There is a valve at the main tank body for entrance of nitrogen in case of fire. Nitrogen is stored in the underground tanks.

5. POTENTIAL TRANSFORMERS

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A voltage transformer theory or potential transformer theory is just like a theory of general purpose step down transformer. Primary of this transformer is connected across the phase and ground. Just like the transformer used for stepping down purpose, potential transformer i.e. PT has lower turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the PT.

In this yard PTs are placed near the bus bar because they have to measure the bus voltage. PTs step down bus voltage (220KV, 132KV, 33KV, 11KV) to much lower voltage 110V, 220V voltage which is fed to various relays and measuring instruments.

Table 5.1 Details of PT in substation

S.N.

Equipment

Qnt.

Ratings Location Application

1 PT-1 1 (220/√3)kv/(220/√3)v,220vNear 220kv main bus

VoltageMeasurement

2 PT-2 1 (132/√3)kv/(110/√3)v,110vNear 132kv main bus

--do--

3 PT-3 3 33/√3)kv/(110/√3)v,110v Near 33 kv main bus

--do--

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Figure 5.1 220KV PT

4 PT-4 1 11kv/110v Near 11 kv main bus

--do--

6. CURRENT TRANSFORMERS

A current transformer is used for measurement of alternating electric currents. Current transformers, together with voltage (or potential) transformers, are known as instrument transformers.

When current in a circuit is too high to apply directly 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 isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays.

There are numerous CTs in the yard those generally step down the current from several hundred ampere to 1A or 5A. Apart

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Figure 6.1 current transformer in the 220KV yard

from this these CTs are rated like 150A/1-1-1-1A. This secondary CT current is fed to various tripping relays and meters because they are all operate on this range only.

In the yard CT were placed before the circuit breaker because they are associated in the breaking of CB and before CTs, there were isolators.

7. CIRCUIT BREAKERS

Definition of circuit breaker: - Electrical circuit breaker is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, the special attention should be given during designing of circuit breaker to safe interruption of arc produced during the operation of circuit breaker.

The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of

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Figure 7.1 Circuit breaker in the yard between CTs and isolators

fault current in them which may damage the equipment and networks permanently. For saving these equipment and the power networks the fault current should be cleared from the system as quickly as possible.

Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under huge current carrying condition.

During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench these arcs in circuit breaker in safe manner. The circuit breaker is the special device which does all the required switching operations during current carrying condition. This was the basic introduction to circuit breaker.

According to their arc quenching media the circuit breaker can be divided as-

7.1 Air-Break Circuit Breakers These circuit breakers are suitable for high current interruption at low voltage, this type of circuit breaker uses air at atmospheric pressure as an quenching medium. It employs two pairs of contact main contact and the arcing contacts. They have low contact resistance. The main contact carries the current when breaker is at the closed position. When contacts are opened, the main contacts separate first, the arcing contacts remain in closed position. Therefore the current is shifted from main contacts to the arcing contacts. The arcing contacts

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separate later on the arc is drawn between them. The principle of high resistance is employed for arc interruption, the arc resistance is increased by lengthening, splitting and cooling the arc. The arc interruption is assisted by current zero in case of air break circuit breakers, high resistance is obtained near current zero. These circuit breakers are available in the voltage 400 to 12kv. They are widely used in the low and medium voltage system. The Figure (6) of air break circuit breaker is given below. Figure 6: Air break circuit breaker.

7.2 Oil Circuit Breaker Mineral oil is the best insulator than air and it has good cooling properties. So, This is employed in many electrical equipment as, well as circuit breakers. But these type of circuit breakers are not suitable for heavy current interruption at low voltages due to carbonization of oil.

7.3 Air Blast Circuit Breakers In the air blast circuit breakers, compressed air at pressure of 20-30kg/cm2 is employed as, an arc quenching medium. Air blast circuit breakers are suitable for operating voltage of 132kv and above. The main advantage of using them is their cheapness and free availability of the interrupting medium, chemical stability and inertness of air, high speed operation.

7.4 SF6 Circuit Breaker These type of circuit breakers have good dielectric strength and excellent arc quenching property. It is an inert, non-toxic, non-flammable and heavy gas. As circuit breakers are totally enclosed and sealed from atmosphere so it is very careful where explosion hazards exist. At atmospheric pressure, its dielectric strength is about 2.35 times that of air. At normal conditions it is chemically

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inert, these properties ofsf6 has made it possible to design circuit breakers with smaller overall dimensions, shorter contact gaps, which help in the constructions of outdoor breakers with fewer interrupts and evolution of metalclad. It is particularly suitable for metalclad switch-gear. It is suitable for the range 3.3kv to 765kv. They are preferred for voltages 132kv and above.

7.4 Vacuum Circuit Breaker

The dielectric strength and interrupting ability of high vacuum is superior to those of porcelain, oil, air and SF6 at atmospheric pressure. Its construction is very simple as, compared to other circuit breakers. When contacts are separated in high vacuum, an arc is drawn between them. The arc does not take place on the entire surface of the contacts but only a few spots. The contact surface is not perfectly smooth. It has certain microprojections.At the time of contact separation, these projections form the last point of separation. The current flows through these points of separation resulting in the formation of a few hot spots, these spots emit electrons and act as cathode spots. It’s enclosure is made up of insulating material such as, glass, porcelain or glass fiber reinforced plastic. The vapor condensing shield is made up of synthetic resin. Vacuum CB is now very popular for voltage rating up to 36kv.

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8. CAPACITOR BANK

Most of the domestic and industrial load are consumers of reactive power that’s why there is voltage dip in the power system. To provide the necessary reactive power capacitor bank are attached to the respective buses.

In the yard these capacitors are delta connected to provide more capacitive reactance with same number of banks.

The capacitor has following functions:

1. Voltage rise2. Energy store3. Power factor improvement

It has two components associated with it:

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8.1 SERIES REACTOR: During parallel operation of capacitor bank it is necessary to limit inrush current to a safe limit depending upon circuit breaker

Figure 8.3 Series reactor

capability, this is done by series reactor. It provides additional inductive reactance in circuit

8.2 RESIDUAL VOLATGE RANSFORMER : It provides protection to capacitor bank and for fast charging of the same. RVT is connected across the capacitor bank. It has

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Figure 8.2 RVT

dual secondary windings. One is for metering and another is for protection.

Figure 8.1 Capacitor bank

9. INSULATORS

Electrical Insulator must be used in electrical system to prevent unwanted flow of current to the earth from its supporting points. The insulator plays a vital role in electrical system. Electrical Insulator is a very high resistive path through which practically no current can flow. In transmission and distribution system, the overhead conductors are generally supported by supporting towers or poles. The towers and poles both are properly grounded. So there must be insulator between tower or pole body and current carrying conductors to prevent the flow of current from conductor to earth through the grounded supporting towers or poles.

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In high power application these type of insulators are most preferred according to the requirement

10.OTHER EQUIPMENT

10.1. Lightening Arrester: An electrical surge can be occurred in an electrical power transmission system due to various reasons. Surge in electrical system originated mainly due to lightning impulses and switching impulses. Electrical surge produces a large transient over voltage in the electrical network and system. The shape of the transient over voltage has a steeply rising front with slowly decaying tail as shown in the figure below. This steep voltage

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Figure 9.1 Pin type Insulator Figure 9.2 Post type Insulator

Figure 9.3 Strain and string type insulators in 220KV yard

wave travels through the electrical network and causes over voltage stresses on all the electrical insulators and equipment come under its travelling path.

That is why all electrical equipment and insulators of power system must be protected against electrical surges. The method of protecting system from surge is normally referred as surge protection. The main equipment commonly used for this purpose is lightning arrester or surge arrester. There are two types of surges one comes externally from atmosphere such as atmospheric lightning. Second type is originated from electrical system itself, such as switching surges.

10.2. Main bus, auxiliary bus and bus coupler

Main Bus: In power system, there is a common reservoir needed to outlet the feeder of that voltage level, so incoming supply is connected to a bus that provides supply to various feeder.

Auxiliary Bus: If there is any maintenance work is carried out at main bus then auxiliary bus take over all the feeders. Generally main bus is one that supplies power to distribution network.

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Figure 10.1 Lightening arrester

As the voltage level of bus increases strands of conductor also increases, so there are few type of conductor generally used in power system. The most common conductor in use for transmission today is aluminum conductor steel reinforced (ACSR).

220KV and above- Zebra conductor

132 KV- Panther

33KV- Dog

11KV- Weasel

Bus Coupler: This additional feature is required to charge the auxiliary bus form main bus.

10.3. Isolator: Circuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works.

Figure 10.2 Isolators

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10.4. Fuses:

In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial to provide overcurrent protection, of either the load or source circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, interrupting the circuit that it connects. Short circuits, overloading, mismatched loads, or device failure are the prime reasons for excessive current. Fuses are an alternative to circuit breakers.

10.4.1 Drop Out Fuse:

In electrical distribution, a fuse cutout or cut-out fuse is a combination of a fuse and a switch, used in primary overhead feeder lines and taps to protect distribution transformers from current surges and overloads. An overcurrent caused by a fault in the transformer or customer circuit will cause the fuse to melt, disconnecting the transformer from the line. It can also be opened manually by utility linemen standing on the ground and using a long insulating stick called a "hot stick".

10.4.2 High rupture capability fuse:

In normal working condition of electrical network, the current flows through the network is within the rated limit. If fault occurs in the network mainly phase to phase short circuit fault or phase to ground fault, the network current crosses the rated limits. This high current may have very high thermal effect which will

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Figure 10.3 Drop out Fuse (Source: www.isotechindia.tradeindia.com)

Figure 10.4 HRC fuse

Source: www.wikipedia.com

cause a permanent damage to the valuable equipment connected in the electrical network. So this high fault current should be interrupted as fast as possible. This is what an electrical fuse does. A fuse is a part of the circuit which consists of conductor which melts easily and breaks the connection when current exceeds the predetermined value. An electrical fuse is a weakest part of an electrical circuit which breaks when more than predetermined current flows through it.

10.5. Station Transformer: Station Transformers are employed for supplying power to plant auxiliary loads during the event of starting of the plant or when generating unit is not generating power. Station Transformers are connected to the switchyard bus. LV side of the station transformer is connected to the auxiliary load buses.

10.6. Concrete And Trenches: Concrete structure is used in switchyard to have lesser step voltage. Because of this step voltage, it is advised to take shorter step in yard.

Trenches are made for underground cabling from yard to control room.

10.7. Earthing: All the electrical appliances are needed to be grounded. In the same manner equipment in the switchyard also needed to be earthed. In the yard 220KV power transformer have five Earthing, two from both windings (as it’sY−Y ), two from body, one from tertiary. The Earthing is broadly divided as

a) System Earthing: Connection between parts of plant in

an operating system like LV neutral of a power transformer winding and earth. All the towers are grounded by common conductor.

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b) Equipment Earthing(safety grounding): connecting

bodies of equipment (like electric motor body, transformer tank, switchgear box, operating rods of air break switches, LV breaker body, HV breaker body, feeder breaker bodies etc.) to earth.

Equipment Earthing

1. Earth grid: A System of grounding electrodes consisting of interconnected connectors buried in the earth to provide a common ground from electrical devices and metallic structures.

2. Earth mat: A grounding system formed by a grid of horizontally buried conductors - Serves to dissipate the earth fault current to earth and also as an equipotential bonding conductor system.

11.CONTROL ROOM Although switchyard is almost the entire area of a substation that contains all the high power rating equipment but control and protection of the power system is even more important.

Almost all the relays, like overcurrent, digital, directional, Earth fault relay, master relays are located in the control room itself.

These relays take input signal from switchyard through wires and they provides tripping command to master relays which ultimately operate the circuit breaker.

Apart from relays there are various measuring devices. Most of the measuring devices are of digital type but in the old section (11KV) there is still indicating devices are used.

Ammeter, Voltmeter, pf meter, frequency meter and energy meter are most common meters in the control panels.

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Since frequency is very important parameter to check the stability of power system so there is a separate meter for frequency measurement just in the position where it can easily be seen.

There are Hooter as well that get active whenever there is any fault and once the corresponding person listen it, he shut it.

There separate panel for each bus and power transformers and they are marked accordingly. And there are various screens that shows status of various equipment. For clearance of fault there is switch that can operate the circuit breaker.

These panel also need DC supply to work so the control room is also equipped with few battery sets of 110V and 54V. These battery set are series combination of 2V cells. And all these cells are connected with metallic strips.

These batteries are charged by a charger that comprises of rectifier.

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12.PLCC ROOMS

Power line carrier communication system differs in method of calling. The Power supply or in the modulation system, each end of power is provided with identical carrier equipment consisting of transmitter, receiver, line turning unit, master oscillator,

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power amplifier etc. In this GSS carrier frequency of PLCC is 472 KHz (50-500 KHz).

Figure 12.1 PLCC Communication schematic

Brief illustration of PLCC system is given in the section

1.1 PLCC ELEMENTS

i) COUPLING CAPACITOR

The carrier equipment is connected to transmission line through coupling capacitor, whose capacitance offers low reactance to carrier frequency (472 KHz) but high reactance to power frequency (50 Hz).

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Coupling capacitors allow carrier signal to enter the equipment but does not allow 50 Hz power frequency to enter the carrier equipment.

ii) LINE TRAP UNIT

Line trap unit is also known as Wave trap. The line trap offers high impedance to the high frequency communication signal thus obstructs the flow of these signals into the substation bus bars. If these were not be there, then signal losses will be more and communication will be ineffective/probably impossible. It is inserted in between bus bar and connection of coupling capacitor to the line. It is parallel tuned circuit comprising L & C. It has a low impedance 50 HZ and high Impedance to carrier frequency. This unit prevents the high frequency signal from entering the neighboring line and carrier currents flows.

iii) LINE MACHING UNIT

This is situated near current transformer and devoted for line matching.

There is a coupling device as well that is devoted for impedance matching as mismatch in impedance will lead to high transmission losses.

Like all the substation, here also PLCC is used for following purpose

i. Data transmission: The information of load is to be transferred to a LOAD DISTAPTCH CENTER Heerapura Jaipur, from where it receives certain instructions to keep the entire power system stable.

ii. Protection: Whenever some ambiguity, faults etc. occurs anywhere in power system, that affect the nearby substation first then it spreads to others. By PLCC firstly affected substation can inform the nearby substations

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about the fault, this transmission takes 3 ms to inform other substation so they can take proper steps to protect the power system.

iii. Local communication: There is a Telephone Exchange in PLCC room that provides facility to call another substation or even to make trench call using hot line. Even if there is no power flow through transmission line, hot line still works. This get disable only when line breaks. Now a days RRVPNL has facilitate Jen with separate Mobile phones so this application is no longer a vital one. PLCC is also used for faxing in substations.

In the Urban cities like Noida, instead of conventional metering smart metering is being used and PLCC also used for this purpose. So in near future we can expect that PLCC can be used for smart metering as well.

2.1.3 ADVANTAGE OF PLCC

No separate wires are needed for communication purpose, as the power line itself carry power as well as communication signals. Hence the cost of constructing separate telephone line is saved.

1. When compared with the ordinary lines, power lines have appreciable higher mechanical strength. They would normally remain unaffected under the conditions, which might seriously damage telephone lines.

2. Power lines usually provide the shortest route between the power stations. Power lines have large cross sectional area which results in very low resistance per unit length. Consequently, the carrier signal suffer much less attenuation during they travelled on usual telephone lines of equal length.

3. Full bandwidth (300 to 3400Hz), high quality speech cum fax channels.

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4. Data transmission at 1200bps; for Computer-Networking and SCADA applications.

5. Transmission of trip signals from distance protection scheme (network) for the protection of High Voltage Transmission Lines.

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SAFETY MEASURES

Earthing of non-current part from the point of view of safety of personnel.

Yard must be layered with stone gravel layer of 100-150 mm thick to minimize the step voltage.

Shrubs, grass and trees etc. should not be allowed to develop in the yard.

Electrical checking of PRD, Buckolz relay, OLTC surge relay and replacement of the gaskets of the boxes.

IR measurement of windings. Both auxiliary and main should not be in operation at the

same time. Tightening of nuts, bolts, clamps, fixtures etc. Checking of arcing horn gap-setting of bushing. Checking of oil level. Checking of break down voltage (BDV) of transformer oil. According to BDV test, transformer oil should be regularly

sent to the laboratory and necessary steps should be taken like change of oil if BDV is below 30 KV.

Checking of alarm/indicator circuit and control and relay armlet wiring.

Checking of air/ SF6 leakage. Checking of jumpers and bus connections. Use of rubber mat in control room. Use of insulated shoes and gloves. The fences must be checked. Safety of consumers and maintenance staff from hazard of

electrical shock.

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ConclusionThe summer training at 220KV GSS RVPNL Ajmer has proven out to be good exposure to practical aspects of concepts that we learnt in past 3 years. Although the complete operation of a GSS toggle around transformers. In the theory we just learn about the concept of transformers but here in this practical arena we had learnt that how difficult it is to deal with such a huge power transformers, importance of oil for transformers was new thing to learn. This oil do not only work as a coolant but also provide necessary insulation in the transformers. The development of this transformers is an isolated engineering to me and this fascinated me a lot.

Apart from transformer Circuit breaker, CT, PT and relays plays an important role in a GSS. These are devoted for proper working of transformer, their working and maintenance is completely new to all the trainee. Principle and associate problems with relays can only be fully understand in a practical study, this GSS provide a good opportunities to achieve it.

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References1. ‘ A Course in Electrical power’ by J B Gupta2. Manuals of various equipment3. EMCO transformers manual, charts and plates on the

transformers.4. “Electrical Relays Principles and Applications” by Vladimir

Gurevich5. “Electrical Machinery” by P. S. Bhimbhra6. “ Principles of Electronic Materials and Devices” by S O

Kasap7. “ A Course in Electrical and electronic Measurements and

instrumentation” by A K Sawhney8. “Switchgear and protection” by U A Bakshi and M V Bakshi9. http://www.electrical4u.com 10. http://www.rvpnl.co.in 11. http://www.wikipedia.org 12. http://www.youtube.com 13. http://www.electricaleasy.com 14. http://www.transformerworld.co.uk 15. http://www.nptel.com 16. http://www.eblogBD.com 17. http://seminarprojects.com

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AuthorThis report is submitted by Asafak Husain. This is an internship report submitted to department of Electrical Engineering, IIT Roorkee. This report is based on the learnings and experiences from the internship taken at Grid substation, Madar.

Author is 4rd year B.Tech student, Electrical engineering, IIT Roorkee.

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Appendix A

List of device numbers and acronyms ANSI

1 – Master Element

2 – Time Delay Starting or Closing Relay

3 – Checking or Interlocking Relay

4 – Master Contactor

5 – Stopping

6 – Starting Circuit Breaker

7 – Rate of Change Relay

8 – Control Power Disconnecting Device

9 – Reversing Device

10 – Unit Sequence Switch

11 – Multi-function Device

12 – Over speed Device

13 – Synchronous-speed Device

14 – Under speed Device

15 – Speed – or Frequency, Matching Device

16 – Data Communications Device

17 – Shunting or Discharge Switch

18 – Accelerating or Decelerating Device

19 – Starting to Running Transition Contactor

20 – Electrically Operated Valve

21 – Distance Relay

22 – Equalizer Circuit Breaker

23 – Temperature Control Device

24 – Volts Per Hertz Relay

25 – Synchronizing or Synchronism-Check Device

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26 – Apparatus Thermal Device

27 – Under voltage Relay

28 – Flame detector

29 – Isolating Contactor or Switch

30 – Annunciator Relay

31 – Separate Excitation

32 – Directional Power Relay or Reverse Power Relay

33 – Position Switch

34 – Master Sequence Device

35 – Brush-Operating or Slip-Ring Short-Circuiting Device

36 – Polarity or Polarizing Voltage Devices

37 – Undercurrent or Under power Relay

38 – Bearing Protective Device

39 – Mechanical Condition Monitor

40 – Field (over/under excitation) Relay

41 – Field Circuit Breaker

42 – Running Circuit Breaker

43 – Manual Transfer or Selector Device

44 – Unit Sequence Starting Relay

45 – Abnormal Atmospheric Condition Monitor

46 – Reverse-phase or Phase-Balance Current Relay

47 – Phase-Sequence or Phase-Balance Voltage Relay

48 – Incomplete Sequence Relay

49 – Machine or Transformer, Thermal Relay

50 – Instantaneous Overcurrent Relay

50G- Instantaneous Earth Over Current Relay (Residual Method)

50N- Instantaneous Earth Over Current Relay (Neutral CT Method)

51 – AC Inverse Time Overcurrent Relay

51G- AC Inverse Time Earth Overcurrent Relay(Residual Method)

51N- AC Inverse Time Earth Overcurrent Relay(Neutral CT Method)

52 – AC Circuit Breaker

52a- AC Circuit Breaker Position (Contact Closed when Breaker Closed)

52b- AC Circuit Breaker Position (Contact Open when Breaker Closed)

53 – Exciter or DC Generator Relay

54 – Turning Gear Engaging Device

55 – Power Factor Relay

56 – Field Application Relay

57 – Short-Circuiting or Grounding Device

58 – Rectification Failure Relay

59 – Overvoltage Relay

60 – Voltage or Current Balance Relay

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61 – Density Switch or Sensor

62 – Time-Delay Stopping or Opening Relay

63 – Pressure Switch

64 – Ground Detector Relay

64R- Restricted earth fault

65 – Governor

66 – Notching or Jogging Device

67 – AC Directional Overcurrent Relay

68 – Blocking Relay

69 – Permissive Control Device

70 – Rheostat

71 – Liquid Level Switch

72 – DC Circuit Breaker

73 – Load-Resistor Contactor

74 – Alarm Relay

75 – Position Changing Mechanism

76 – DC Overcurrent Relay

77 – Telemetering Device

78 – Phase-Angle Measuring Relay or "Out-of-Step" Relay

79 – AC Reclosing Relay (Auto Reclosing)

80 – Flow Switch

81 – Frequency Relay

82 – DC Reclosing Relay

83 – Automatic Selective Control or Transfer Relay

84 – Operating Mechanism

85 – Communications, Carrier or Pilot-Wire Relay

86 – Lockout Relay/Master Trip

87 – Differential Protective Relay

88 – Auxiliary Motor or Motor Generator

89 – Line Switch

90 – Regulating Device

91 – Voltage Directional Relay

92 – Voltage and Power Directional Relay

93 – Field Changing Contactor

94 – Tripping or Trip-Free Relay

95 – For specific applications where other numbers are not suitable

96 – Bus bar Trip Lockout relay

97 – For specific applications where other numbers are not suitable

98 – For specific applications where other numbers are not suitable

99 – For specific applications where other numbers are not suitable

150 – Earth Fault Indicator

AFD – Arc Flash Detector

CLK – Clock or Timing Source

DDR – Dynamic Disturbance Recorder

DFR – Digital Fault Recorder

DME – Disturbance Monitor Equipment

ENV – Environmental Data

HIZ – High Impedance Fault Detector

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HMI – Human Machine Interface

HST – Historian

LGC – Scheme Logic

MET – Substation Metering

PDC – Phasor Data Concentrator

PMU – Phasor Measurement Unit

PQM – Power Quality Monitor

RIO – Remote Input/output Device

RTU – Remote Terminal Unit/Data Concentrator

SER – Sequence of Events Recorder

TCM – Trip Circuit Monitor

LRSS - LOCAL/REMOTE SELECTOR SWITCH

SOTF - Switch On To Fault

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APPENDIX B

Equipment earthing based on IS: 3043-1987 Standard

1. Classification of electrical equipment IS: 9409-1980

2. Important rules for safety and earthing practice is based on IE rules

1956

3. Guide on effects of current passing through human body – IS:8437-1997

4. Protection of buildings and structures from lightning – IS:2309-1969

5. Earth: The conductive mass of the earth, whose electric potential at any

point is conventionally assumed and taken as ZERO.

6. Earth electrode: A Conductor or group of conductors in intimate contact

with and providing as electrical connection to earth.

7. Earth electrode resistance: The electrical resistance of an earth

electrode to the general mass of earth.

8. Earthing Conductor: A protective conductor connecting the main

Earthing terminal to an earth electrode or other means of earthing.

9. Equipotential Bonding: Electrical connection putting various exposed

conductive parts and extraneous conductive parts at a substantially

equal potential.

10. Example: Inter connect protective conductor, earth continuity

conductors and risers of AC/HV systems if any.

11. Potential gradient: The potential difference per unit length measured

in the direction in which it is max.

12. Touch Voltage: The P.D. between a grounded metallic structure and a

point on the earth’s surface separated by a horizontal reach of one

Meter.

13. Step voltage: The P.D. between two points on the earth’s surface

separated by a distance one pace (step) assumed to be one Meter.

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14. Earth grid: A System of grounding electrodes consisting of

interconnected connectors buried in the earth to provide a common

ground from electrical devices and metallic structures.

15. Earth mat: A grounding system formed by a grid of horizontally buried

conductors - Serves to dissipate the earth fault current to earth and also

as an equipotential bonding conductor system.Transformer BDV testing

Appendix C: Relay protection of protection of transformers

1. No Buchholz relay for transformers below 500 KVA capacity.2. Transformers up to 1500 KVA shall have only Horn gap

protection.3. Transformers above 1500 KVA and up to 8000 KVA of

33/11KV ratio shall have one group control breaker on HV side and individual LV breakers if there is more than one transformer.

4. Transformers above 8000 KVA shall have individual HV and LV circuit breakers.

5. The relays indicate above shall be provided on HV and LV.6. LAs to be provided on HV & LV for transformers of all

capacities and voltage class.7. OLTC out of step protection is to be provided where master

follower scheme is in operation.8. Fans failure and pumps failure alarms to be connected.9. Alarms for O.T., W.T., Buchholz (Main tank & OLTC) should

be connected.

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