CHAPTER 1

INTRODUCTION

1.1. NATIONAL TRANSMISSION & DESPATCH COMPANY:

National Transmission & Despatch Company (NTDC) Limited was incorporated on 6th November, 1998 and commenced commercial operation on 24th December, 1998. It was organized to take over all the properties, rights and assets obligations and liabilities of 220 KV and 500KV Grid Stations and Transmission Lines/Network owned by Pakistan Water and Power Development Authority (WAPDA). NTDC operates and maintains twelve 500 KV and twenty nine 220 KV Grid Stations, 5077 km of 500 KV transmission line and 7359 km of 220 KV transmission line in Pakistan.

NTDC was granted Transmission License No.TL/01//2002 on 31st December 2002 by National Electric Power Regularity Authority (NEPRA) to engage in the exclusive transmission business for a term of thirty (30) years, pursuant to Section 17 of the Regulation of Generation, Transmission and Distribution of Electric Power Act, 1997.

1.2. MAIN FUNCTIONS Central Power Purchasing Agency System Operator Transmission Network Operator Contract Registrar and Power Exchange Administrator

i) Central Power Purchasing Agency (CPPA): As the Central Power Purchasing Agency (CPPA), for procurement of power from GENCOs, Hydel & IPPs on behalf of Distribution Companies (DISCOS), for delivery through 500 kV, 220 kV & 132kV Network.

ii) System Operator: For secure, safe and reliable operation, control and despatch of generation facilities.

iii) Transmission Network Operator: For Operation & Maintenance, Planning, Design and expansion of the 500 kV and 220 kV transmission network.

iv) Contract Registrar and Power Exchange Administrator (CRPEA): As CRPEA, to record and monitor contracts relating to bilateral trading system.

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1.3. MISSION STATEMENT NTDCTo contribute in the development of prosperous Pakistan by managing smooth and economical transmission and despatch system through excellence of professional work.

1.4. ORGANOGRAM

To view organogram in a bigger view, please click on the following image:

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CHAPTER 2

500 KV GRID STATION JAMSHORO

2.1. 500 KV GRID STATION JAMSHORO:

500KV Jamshoro Grid Station. with 2x450MVA, 500/220KV Auto T/F Bank is commissioned during 1987-88. Four banks of 500KV shunt reactors each of capacity 22x3MVAR is installed at the Grid Station. The Grid Station. is being fed through two 500KV Circuits from 500 KV Dadu Grid Station and connected with HUBCO (IPP) through one 500KV S/Circuit and In & Out arrangement at NKI Grid Station. 500KV NKI Grid Station is being fed from 500KV HUBCO Power House through 500KV single circuit and connected with 500KV Jamshoro Grid Station through 500KV single circuit. This Grid Station. is also feeding 2x160MVA, 220/132 Auto T/Fs installed at the Grid Station.

For 500KV as well as for 220KV bays double bus bar with one & half breaker scheme is used. While single scheme is used on the 132 KV bay.500KV Dadu Grid Station is being fed from Guddu Power House through two 500 KV S/C and connected with 500 KV Jamshoro & 500 KV Guddu G/Stations through two 500 KV Single Circuits.

2.2. RESPONSIBILITIES OF STAFF WORKING:

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• Carrying out the maintenance, operation, service, troubleshooting of Substation Components and Transmission lines.

• Fault Troubleshooting in switch yard, control panels, relay panels, dc battery banks

• Design and Implementation of Power System Protection Schemes of Power Transformers(10 to 600 MVA), Transmission lines (500/220/132KV) and Bus Bar (500/220/132 KV)

• Testing and commissioning of switch yard equipments, Power Transformers, Current Transformers, Potential Transformers, Lightening Arrestors, circuit breakers, isolators, Power cables, Control cables.

• Carrying out schedulling and planning for all preventive maintenance of Substation.

• Power Dispatch ,Control Room Operation.

• Tests of Power Transformer :

Bushing high Voltage testing, Tertiary Winding Test, TTR Test, Insulation Test(Meger) , Oil insulation testing, C & DF Test , Pick up Test, Timing Test , checking of vector groups

• Tests of C.Ts :

Ratio Test with Primary Injection Test Set (1600-1200-1000-600/1 A), checking wiring polarity and continuity, checking secondary wiring insulation and resistance, CT insulation , (HVLV,HV-Ground,LV-Ground), Saturation testing.

• Test of High Voltage C.Bs :

Opening/Closing Timming Test, Contact Resistance Test, Pole discrepancy relay testing,

SF6 Leakage Test.

• Protective Relays Testing :

Power Transformer Relay Testing Performed

Main Differential Relay Testing- Pick up Test, Slope Test, Restraining Tests, Rough Balance and Connection Point Differential Relay Testing, HV and LV over current relay Testing, Restricted Earth Fault (REF) relay Testing, Over Fluxing (V/F) Relay Testing, Tertiary Winding Over current and Earth fault relay Testing, Over loading relay Testing.

• Tests of P.Ts :

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Ratio Test with AC-Hipot (500-220-132KV/110V) , Wiring Polarity and Continuity test, Checking of Phases rotation sequence, Measuring Wiring Insulation, Measuring Wiring Resistance,Capacitive Coupled Voltage transformer Polarity Testing.

2.3. SINGLE LINE DIAGRAM:

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CHAPTER 3

BUS BAR SCHEMES

3.1.BUS BAR SCHEMES:

There are many different electrical bus system schemes available but selection of a particular scheme depends upon the system voltage, position of substation in electrical power system, flexibility needed in system and cost to be expensed.

The Main Criteria to be Considered During Selection of one Particular Bus – Bar Arrangement Scheme Among others are

Simplicity of system. Easy maintenance of different equipments. Minimizing the outage during maintenance. Future provision of extension with growth of demand. Optimizing the selection of bus bar arrangement scheme so that it gives

maximum return from the system.

Some very commonly used bus bar arrangement are discussed below-

3.2. SINGLE BUS SCHEME SYSTEM:

Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as show.

Advantages of Single Bus Scheme System:

1) This is very simple in design.2) This is very cost effective scheme.3) This is very convenient to operate.

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Disadvantages of Single Bus System1) One but major difficulty of these type of arrangement is that, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.2) The indoor 11KV switchboards have quite often single bus bar arrangement.

3.3. SINGLE BUS SYSTEM WITH BUS SECTIONALIZER:

Some advantages are realized if a single bus bar is sectionalized with circuit breaker. If there are more than one incoming and the incoming sources and outgoing feeders are evenly distributed on the sections as shown in the figure, interruption of system can be reduced to a good extent.

Advantages of Single Bus System with Bus Sectionalizer:

If any of the sources is out of system, still all loads can be fed by switching on the sectional circuit breaker or bus coupler breaker. If one section of the bus bar system is under maintenance, part load of the substation can be fed by energizing the other section of bus bar.

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Disadvantages of Single Bus System with Bus Sectionalizer

1) As in the case of single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.2) The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated ‘off circuit’ and which is not possible without total interruption of bus – bar. So investment for bus-coupler breaker is required.

3.4. DOUBLE BUS SCHEME SYSTEM:

It is also called Duplicate Bus (Double Bus Single Breaker).

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1) In double bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus.

2)Actually every feeder is connected to both of the buses in parallel through individual isolator as shown in the figure.

By closing any of the isolators one can put the feeder to associated bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where feeder is transferred. Lastly after this transfer operation he or she should open the bus coupler breaker.

Advantages of Double Bus System

Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System

The arrangement does not permit breaker maintenance with out interruption.

3.5. DOUBLE BREAKER BUS SCHEME:

In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. Only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown in the figure. By closing any of the breakers and its associated isolators, one can put the feeder to respective bus. Both of the buses are energized and total feeders are divided

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into two groups, one group is fed from one bus and other from other bus similar to previous case. But any feeder at any time can be transferred from one bus to other. There is no need of bus coupler as because the operation is done by breakers instead of isolator. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred and then he or she opens the breaker and then isolators 

3.6. DOUBLE BUS SYSTEM WITH BYPASS ISOLATORS:

This is combination of the double bus system and main and transfer bus system. In Double Bus System with Bypass Isolators either bus can act as main bus and second bus as transfer bus. It permits breaker maintenance without interruption of power which is not possible in double bus system but it provides all the advantages of double bus system. It however requires one additional isolator (bypass isolator) for each feeder circuit and introduces slight complication in system layout. Still this scheme is best for optimum economy of system and it is best optimum choice for 220KV system.

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3.7. ONE AND A HALF BREAKER BUS SYSTEM:

This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers and these two feeders are coupled by a third breaker which is called tie breaker. Normally all the three breakers are closed and power is fed to both the circuits from two buses which are operated in parallel. The tie breaker acts as coupler for the two feeder circuits.

During failure of any feeder breaker, the power is fed through the breaker of the second feeder and tie breaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by tie breaker.

Advantages of One and A Half Breaker Bus System

During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.

Disadvantages of One and A Half Breaker Bus System

This scheme is much expensive due to investment for third breaker.

3.8. MAIN AND TRANSFER BUS SYSTEM

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This is an alternative of double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of circuit breaker and associated isolators at both side of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with feeder is switched off or isolated from system, the feeder can still be fed in this way by transferring it to transfer bus.

Switching Operation for Transferring a Feeder to Transfer Bus from Main Bus without Interruption of Power

(i) First close the isolators at both side of the bus coupler breaker.

(ii) Then close the bypass isolator of the feeder which is to be transferred to transfer bus.

(iii) Now energized the transfer bus by closing the bus coupler circuit breaker from remote.

(iv) After bus coupler breaker is closed, now the power from main bus flows to the feeder line through its mainbreaker as well as bus coupler breaker via transfer bus.

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(v) Now if main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker and hence this breaker will serve the purpose of protection for the feeder.

(vi) At last the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system.So it can be concluded that in Main & Transfer Bus System the maintenance of circuit breaker is possible without any interruption of power. Because of this advantage the scheme is very popular for 33KV and 13KV system.

3.9. RING BUS SYSTEM:

The schematic diagram of the system is given in the figure. It provides a double feed to each feeder circuit, opening one breaker under maintenance or otherwise does not affect supply to any feeder. But this system has two major disadvantages. One as it is closed circuit system it is next to impossible to extend in future and hence it is unsuitable for developing system. Secondly, during maintenance or any other reason if any one of the circuit breaker in ring loop is switch of reliability of system becomes very poor as because closed loop becomes opened. Since, at that moment for any tripping of any breaker in the open loop causes interruption in all the feeders between tripped breaker and open end of the loop.

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CHAPTER 4

BUS BAR SCHEMES USED IN 500/220/132 KV GRID STATION:

4.1. SCHEMES USED IN 500/220/132 KV GRID STATION:

One and half Scheme is used in the 500KV and 220 KV yard while Double Bus Scheme is used in the 132 KV yard..

One and half Scheme Double Bus Scheme

One and half breaker bus scheme is also called two lines and three breakers scheme. This sort of scheme avoids any kind of interruption during the maintenance and during the emergency repairs or under any fault condition. Therefore the supply continues without any disturbance.

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Double bus Scheme is used in the 132 KV yard, but interruption in one line due to any fault condition and maintenance cannot be avoided in this type of scheme.

One and half Scheme (220 KV)

Double Bus Scheme (132 KV)

CHAPTER 5

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PROTECTION DEVICES (RELAYS BREAKERS) & WAVE TRAP

5.1. 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 the designing of circuit breaker to safe interruption of arc produced during the operation of circuit breaker.

Circuit breakers can be classified by considering either of the following two media.

Operating Media Arc Quenching Media

5.2. TYPES OF CIRCUIT BREAKER:

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

 Oil Circuit Breaker

 Air Circuit Breaker

 SF6 Circuit Breaker

 Vacuum Circuit Breaker

Oil Circuit Breaker: Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and moving contact are immerged inside the insulating oil. Whenever there is a separation of current carrying contacts in the oil, the arc is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas and ultimately creates a hydrogen bubble around the arc. This highly compressed gas bubble around the arc prevents re-striking of the arc after current reaches zero crossing of the cycle. The Oil Circuit Breaker is the one of the oldest type of circuit breakers.

Working Principle of Oil Circuit Breaker:

When the current carrying contacts in the oil are separated an arc is established in between the separated contacts.

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Actually, when separation of contacts has just started, distance between the current contacts is small as a result the voltage gradient between contacts becomes high. This high voltage gradient between the contacts ionized the oil and consequently initiates arcing between the contacts. This arc will produce a large amount of heat in surrounding oil and vaporizes the oil and decomposes the oil in mostly hydrogen and a small amount of methane, ethylene and acetylene. The hydrogen gas can not remain in molecular form and its is broken into its atomic form releasing lot of heat. The arc temperature may reach up to 50000K. Due to this high temperature the gas is liberated surround the arc very rapidly and forms an excessively fast growing gas bubble around the arc. It is found that the mixture of gases occupies a volume about one thousand times that of the oil decomposed. From this figure we can assume how fast the gas bubble around the arc will grow in size. If this growing gas bubble around the arc is compressed by any means then rate of de – ionization process of ionized gaseous media in between the contacts will accelerate which rapidly increase the dielectric strength between the contacts and consequently the arc will be quenched at zero crossing of the current cycle. This is the basic operation of oil circuit breaker. In addition to that cooling effect of hydrogen gas surround the arc path also helps, the quick arc quenching in oil circuit breaker.

Air Circuit Breaker:  This type of circuit breakers, is those kind of circuit breaker which operates in air at atmospheric pressure. After development of oil breaker, the medium voltage air circuit breaker (ACB) is replaced completely by oil circuit breaker in different countries. But in countries like France and Italy, ACBs are still preferable choice up to voltage 15 KV. It is also good choice to avoid the risk of oil fire, in case of oil circuit breaker. In America ACBs were exclusively used for the system up to 15 KV until the development of new vacuum and SF6 circuit breakers.

Working principle of Air Circuit Breaker:

The working principle of this breaker is rather different from those in any other types of circuit breakers. The main aim of all kind of circuit breaker is to prevent the reestablishment of arcing after current zero by creating a situation where in the contact gap will withstand the system recovery voltage. The air circuit breaker does the same but in different manner. For interrupting arc it creates an arc voltage in excess of the supply voltage. Arc voltage is defined as the minimum voltage required maintaining the arc. This circuit breaker increases the arc voltage by mainly three different ways,

It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma is decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is required to maintain the arc.

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It may increase the arc voltage by lengthening the arc path. As the length of arc path is increased, the resistance of the path is increased, and hence to maintain the same arc current more voltage is required to be applied across the arc path. That means arc voltage is increased.

Splitting up the arc into a number of series arcs also increases the arc voltage.

SF6 Circuit Breaker:

A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker.

SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has high affinity of absorbing free electron. Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.

These negative ions obviously much heavier than a free electron and therefore over all mobility of the charged particle in the SF6 gas is much less as compared other common gases. We know that mobility of charged particle is majorly responsible for conducting current through a gas.

Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength. Not only the gas has a good dielectric strength but also it has the unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property. Due to its low gaseous viscosity (because of less molecular mobility) SF6 gas can efficiently transfer heat by convection. So due to its high dielectric strength and high cooling effect SF6 gas is approximately 100 times more effective arc quenching media than air. Due to these unique properties of this gas SF6 Circuit Breaker is used in complete range of medium

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voltage and high voltage electrical power system. These circuit breakers are available for the voltage ranges from 33KV to 800KV and even more.

Types of SF6 Circuit Breaker:

There are mainly three types of SF6 CB depending upon the voltage level of application

1) Single Interrupter SF6 CB applied for up to 245KV(220KV) system

2) Two Interrupter SF6 CB applied for up to 420KV(400KV) system

3) Four Interrupter SF6 CB applied for up to 800KV(715KV) system

Working Principle of SF6 Circuit Breaker:

The working of SF6 CB of first generation was quite simple it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for reutilize.

Vacuum Circuit Breaker:

A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage Vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulator. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 -6 barThe material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. Vacuum interrupter technology was first introduced in the year of 1960. But still it is a developing technology. As time goes on, the size of the vacuum interrupter is being reducing from its early 1960’s size due to different technical developments in this field of engineering. The contact geometry is also improving with time, from butt contact of early days it gradually changes to spiral shape, cup shape and axial magnetic field contact. Thevacuum circuit breaker is today recognized as most reliable current interruption technology for medium voltage system. It requires minimum maintenance compared to other circuit breaker technologies.

Working Principle of Vacuum Circuit Breaker:

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The main aim of any circuit breaker is to quench arc during current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next current zero. At current zero this vacuum arc is extinguished and the conducting metal vapour is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of contact surface, for next cycle of current. That means, the arc cannot be reestablished again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero.

2. According to the operating mechanism of circuit breaker they can be divided as

Spring operated Circuit Breaker

Pneumatic Circuit Breaker

Hydraulic Circuit Breaker

5.3. RELAYS:

A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.

5.4. TYPES OF RELAYS USED AT 500 KV GRID STATION:

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Distance Relay: A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.

Buchholz Relay:

Construction of Buchholz Relay

Buchholz Relay in transformer is an oil container housed the connecting pipe from main tank to conservator tank. It has mainly two elements. The upper element consists of a float. The float is attached to a hinge in such a way that it can move up and down depending upon the oil level in the Buchholz Relay Container. One mercury switch is fixed on the float. The alignment of mercury switch hence depends upon the position of the float.

The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just in front of the inlet (main tank side ) of Buchholz Relay in transformer in such a way that when oil enters in the relay from that inlet in high pressure the alignment of the baffle plate along with the mercury switch attached to it, will change.

In addition to these main elements a Buchholz Relay has gas release pockets on top. The electrical leads from both mercury switches are taken out through a molded terminal block.

Buchholz Relay Principle

The Buchholz Relay working principle of is very simple.Buchholz Relay function is based on very simple mechanical phenomenon. It is mechanically actuated. Whenever

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there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to decomposition of transformer insulating oil will accumulate in the upper part the Buchholz Container which causes fall of oil level in it.

Trip Circuit Supervision Relay:

There are different contacts connected in series along a trip circuit of a electrical circuit breaker. There must be some situation when the circuit breaker should not trip even a faulty current passes through its power contacts. Such situations are low gas pressure in SF6 Circuit Breaker, low air pressure in pneumatic operated circuit breaker etc. In this situation the trip coil of the CB must not be energized to trip the CB. So there must be NO contacts associated with gas pressure and air pressure relays, connected in series with breaker trip coil. Another scheme of trip coil is that it should not be reenergized once the circuit breaker is open. That is done by providing one NO contact of breaker auxiliary switch in series with trip coil. In addition to that the trip circuit of a CB has to pass through considerable numbers of intermediate terminal contacts in relay, control panel and circuit breaker kiosk. So if any of the intermediate contacts is detached, the circuit breaker fails to trip. Not only that, if dc supply to the trip circuit fails, the CB will not trip.

To overcome this abnormal situation, trip circuit supervision becomes very necessary. The figure below shows the simplest form of trip circuit healthy scheme. Here one series combination of one lamp, one push bottom and one resistor is connected across the protective relay contact as shown. In healthy situation all the contacts except protective relay contact are in close position. Now if push bottom (PB) is pressed, the trip circuit supervision network is completed and lamp glows indicating that the breaker is ready for tripping.

Overcurrent and earth-fault relay:

Design and principle

The combined overcurrent and earth-fault relay is a secondary relay to be connected to the current transformers of the protected object. The three-phase overcurrent unit and the earth-fault unit continuously measure the phase currents and the neutral current ofthe protected object. On detection of a fault the relaystarts, trips the circuit breaker, initiates auto-reclosing, provides alarm, records fault data etc. in accordance with the application and the configured relay functions.

When the phase current exceeds the set start currentof the low-set stage I>, the overcurrent unit starts delivering a start signal after a preset ~60 ms start time. When the set operate time at definite time operation or the calculated operate time at inverse time operation elapses, the overcurrent unit operates. In the same way the high-set

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stage I>> of the overcurrent unit starts delivering a start signal after a preset ~40 ms start time, when the set start current is exceeded. When the set operate time elapses, the overcurrent unit operates. When the earth-fault current exceeds the set start current of the low-set stage Io>, the earth-fault unit starts delivering a start signal after a preset ~60 ms start time. When the set operate time at definite time operation or the calculated operate time at inverse time operation elapses, the earth-fault unit operates. In the same way the high-set stage Io>> of the earth-fault unit starts delivering a start signal after a preset ~40 ms start time, when the set start current is exceeded. When the set operate time elapses, the earth-fault unit operates.

Auto Recloser:

In electric power distribution, a recloser, or autorecloser, is a circuit breaker equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault.[1][2] Reclosers are used on overhead distribution systems to detect and interrupt momentary faults. Since many short-circuits on overhead lines clear themselves, a recloser improves service continuity by automatically restoring power to the line after a momentary fault.

Reclosers are predominantly located on the distribution feeder, though as the continuous and interrupting current ratings increase, they are seen in substations, where traditionally a circuit breaker would be located. Reclosers have two basic functions on the distribution system: reliability and overcurrent protection. Reclosers are frequently applied to increase reliability, mainly due to three of their benefits: reclosing capability, single phase reclosing, and automated loop capabilities

5.5. WAVE TRAP:Wave trap, as its name indicates that it is used to trap some waves.Wave trap is used for communication purpose in substations.

20 years back our country was not developed in telecommunication sector,then our electrical engineers developed a system that communication can be done through our power lines, it is called power line carrier communication (PLCC).

Another advantage is it has separate communication like,which will not depend on Telecommunication so that have less interruption

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we can transmit electrical power and communication at the same time with single conductor but with different frequencies ,so they can be separated easily by wave trap at entrance of substation.

Generally electrical power waves have frequency of 50 Hz, and we have tele- communication waves with high frequencies like 150kHz,200kHz etc..,

Wave trap consists of filter circuit (combination of capacitor and inductor) which is used to allow only power frequency waves to electrical equipment's and trap the tele-communication waves and send them to PLCC panel.

Why wave trap is located at entrance of substation As communication waves are high frequency (and not power frequency) they will act as harmonics towards electrical equipment's like transformer, breaker etc...,so in order to protect them we should connect at entrance so that wave trap will trap the communication waves.

If any fault in any substation, it should intimate to all connected substations to it. each telephone is connected to only one particular substation only.

CONCLUSION

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The Internship at 500 KV grid station was very informative theoretically as well as practically as I was able to see the functioning of different electrical equipments.

I had a sound knowledge of the incoming and outgoing Transmission lines from this grid station to different cities and industrial Sectors.

I became abreast of the Protective devices being installed to undo any heavy damage to the Yards in case of fault currents and other reasons. Breaker Schemes used enhanced my knowledge greatly.

So, indeed it was a great experience learning under the supervision of the 500 KV Grid Station Staff.

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