A PRACTICAL SUMMER TRAINING REPORTON

“DELHI TRANSCO LIMITED 220 KV GRID SUBSTATION AT SARITA VIHAR”

NATIONAL POWER TRAINING INSTITUTE (NR)BADARPUR, NEW DELHI

UNDER THE GUIDANCE OF SUBMITTED BY:MR. RAM SINGH (A.M.) MD. NAFIS IQBALMR. HANS KUMAR (J.E.) B.TECH. POWER ENGINEERING (5th SEM.)

ROLL NO. – 00315307511

ACKNOWLEDGEMENT

I am highly indebted to Mr. Ram Singh (Asst. Manager, 220 KV DTL S/S Sarita Vihar) for their invaluable support without which the project could have not been worked out the way it has. I am very much thankful to Mr. Hans Kumar (J.E. 220 KV DTL S/S Sarita Vihar) who helped me immensely in understanding the basics and complexities of along with additions that made the quality of report better.

I would also like to extend a note of thanks to all other employees of Delhi Transco Limited who helped me directly or indirectly in successful completion of my project.

Last but not the least, I would like to thank my parents & all my fellow trainees who have been a constant source of encouragement & inspiration during my studies & have always provided me support in every walk of life

MD. NAFIS IQBAL

CONTENT

1. Introduction

1.1 Introduction to Delhi Transco Limited 1.2 DTL Network1.3 Grid Diagram of Delhi

2. Introduction to Substation

3. 220/33/11 KV GIS Substation Trauma Centre3.1 Single Line Diagram3.2 Description of SLD3.3 Parts of Substation3.4 Equipments used in Substation

4. Description of Substation Equipments 4.1 Transmission Lines4.2 Bus Bar4.3 Isolators4.4 Transformers

4.4.1 Introduction4.4.2 Power transformer4.4.3 Parts of Power Transformer4.4.4 Transformer Cooling 4.4.5 Protection devices4.4.6 Maintenance4.4.7 Technical Specifications

4.5 Instrument Transformer4.5.1 Current Transformer (CT)4.5.2 Potential transformer (PT)4.5.3 Capacitor Voltage Transformer (CVT)

4.6 Circuit Breaker (CB)4.6.1 Types of CB4.6.2 Technical Specification

4.7 Capacitor Bank4.8 Insulator4.9 Wave Trap

5. Protection System5.1 Over Voltage Protection

5.1.1 Ground Wire5.1.2 Earth Screen5.1.3 Lightning insulator

5.2 Over Current protection5.3 Earth fault protection5.4 Primary and back up protection5.5 Relays5.6 Fuses5.7 Earthing System

6. Control Room7. Battery Room8. PLCC and SCADA System9. Operation and maintenance of Substation

9.1 Introduction9.2 Maintenance activity9.3 Maintenance Schedule9.4 Maintenance Schedule Table of

9.4.1 Oil filled power transformer9.4.2 SF6 Circuit Breaker9.4.3 Relays and protection circuit9.4.4 Arrestors9.4.5 Transmission lines

9.5 Thermo Scanning10. General Safety Precautions11. Conclusion

References

1. INTRODUCTION1.1AN INTRODUCTION TO DELHI TRANSCO LIMITED

Delhi Transco Limited, a successor company of erstwhile Delhi Vidhyut board, came into existence on 1st July 2002, as a State Transmission Utility of the National Capital. After unbundling of DVB the distribution sector has been handed over to private companies while the generation and transmission are still with the government. Over the years, DTL has evolved as a most dynamic performer, keeping pace with the manifold challenges that confront the ever increasing demand-supply power situation and achieving functional superiority on all fronts.

Being the capital of India and the hub of commercial activities in the Northern Region, coupled with the prosperity of population, the load requirement of Delhi has been growing at a much faster pace. Added to that, being the focus of socio-economic and political life of India, Delhi is assuming increasing eminence among the great cities of the world. Plus the vgision-2021, aiming to make Delhi global Metro politic and world class city demand greater infrastructure to enrich many services of infrastructure development. DTL has been responsibly playing its role in establishing. Upgrading, operating and maintaining the EHV (Extra High Voltage) network. DTL has also been assigned the responsibility of running the State load Dispatch Centre (SLDC) which is an apex body to ensure integrated operations of power system in Delhi. Delhi Transco is also committed to promote energy conservation not only in its own establishments but also in the entire Delhi. The company has done a lot to educate and sensitize the general public about the need of energy conservation. Transmission loss level has been reduced from 3.84 per cent in 200-203 to 1.38 per cent in 2009-10, which is one of the lowest transmission loss level in the country. To ensure adequate and efficient power supply. DTL has been continuously upgrading its biggest achievement has been its ability to handle the highest ever peak demand of 4720 MW in July 2010. The total availability of its transmission system stood 98.78%. The modern technologies are being implemented in DTL by way of constructing GIS sub stations and laying XLPE 220 KV cable by employing cable link techniques and would be the largest network of its kind in India.

1.2 DTL NETWORK

It contains the line diagram of the DTL network which connects all the networks of the 5 DISCOMs working in the Delhi region.

1.3 GRID DIAGRAM OF DELHI

2. INTRODUCTION TO SUBSTATION

Electrical power is generated, transmitted in the form of alternating current. The electric power produced at the power stations is delivered to the consumers through a large network of transmission & distribution. The transmission network is inevitable long and high power lines are necessary to maintain a huge block of power source of generation to the load centers to inter connected Power house for increased reliability of supply greater.An electrical substation is a part of an electricity generation, transmission and distribution system where voltage is transformed from high to low or in reverse using transformers. It also serves as a point of connection between various power system elements such as transmission lines, transformers, generators and loads. To allow for flexibility in connecting the elements, circuit breakers are used as high power switches. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps. There are different kinds of substation such asTransmission substation, distribution substation, collector substation, switching substation and some other types of substation. The general functions of a substation may include:

Voltage transformation Connection point for transmission lines Switchyard for network configuration Monitoring point for control center Protection of power lines and apparatus Communication with other substations and regional control center

Making an analogy with the human body, the role of substation in the power system to address the above mentioned issues is pivotal: the substations are the center of the “nervous, immune, musculoskeletal and cardiovascular” subsystems of the entire power system “body”. The “nervous” subsystem role of the substation is to allow the central system to sense the operating states, view status of the equipment, and make assessments of the system criticality. The “immune” subsystem role is to develop self-defense means and sustain self healing strategies. The “musculoskeletal” subsystem role is to maintain the system topology, switch the equipment state and restore the power flows. The “cardiovascular” subsystem role is to sustain normal power flow and control the synchronization.The substation includes the primary equipment (such as circuit breakers, transformers, instrument transformers, etc.) and the secondary

equipment (monitoring, control and protection devices) which are installed in control house.In the primary side, a large number of breakers and disconnectors are used in order to allow for maintenance and repair with a minimum of interruption, which occupy large space. Oil-insulated transformers are used to step-up or step-down the voltage level for purposes needed. Oil-insulated transformers usually have big size and have potential explosion problems. In addition, the maintenance is also elaborate and the noise of those transformers is also a big issue. The breakers also need an insulation media which may be oil, gas, or air.Conventional current and potential transformers (CTs and VTs) are used to convert the primary current and voltage to an operation range (0-5A and 115V) for metering and protection. The CT saturation and open secondary CT circuit safety issue are primarily of concern in such devices.All interfaces between primary and secondary equipment are connected by hard-wired cabling. Different length and types cables are bundled as shown in Fig.1.2, which makes it labor intensive for future maintenance and modification. In addition, due to the large number of wires in a highly electromechanically “polluted” substation switchyard environment, the wiring may experience significant electromagnetic interference (both conducted and radiated).

3. 220/66/11 KV GRID SUBSTATION AT SARITA VIHAR :

The 220 KV Grid Substation at Sarita Vihar is a Air Insulated Outdoor Substation. This is 220/66/11 KV Substation.There are four 220 KV incoming feeders for this substation coming from:

1. Pragati Powers2. Power Grid3. BTPS CKT. No. 14. BTPS CKT. No. 2

3.1 SINGLE LINE DIAGRAM

3.2 DESCRIPTION ABOUT SINGLE LINE DIAGRAMFigure attached shows key diagram of a typical 220/66/11 KV Sarita Vihar Grid Substation. The diagram of this grid station is explained as under:-

1) There are Four 220 KV incoming lines as one circuit from Pargati powers, one circuit from Power Grid and two circuits from BTPS. These four incoming lines are connected to the double bus bar system through a number of equipments. All these lines can be loaded simultaneously to share the grid station load. The four lines arrangement increases the reliability of the system. In case there is a breakdown of one incoming line, the continuity of supply can be maintained by the other lines.

2) As in the single line diagram the each incoming is connected to the bus bar in a sequence with a number of equipments. The equipments between the incoming lines and the line bus bar is connected in a defined sequence as following:

I. Line lightning ArrestorII. Line capacitive voltage transformer (CVT).III. Line IsolatorIV. Line Current TransformerV. Line Circuit Breaker

3) The Substation has double bus bar system, one main bus bar and the other spare bus bar. The incoming can be connected to either bus bar with the help of an arrangement of circuit breaker and isolators called Bus Coupler. The advantage of double bus bar system is that if repair is to be carried on one bus bar, the supply

need not to be interrupted as the entire load can be transferred to the other bus.

4) Each line bus bar is connected with Potential Transformer (PT) to measure the bus bar voltage.

5) There is an arrangement in to step down the incoming 220 KV supply to 66 KV by two transformer banks with capacity each of 100 MVA. The transformer bank can be connected to either of the line bus bar through the bus changer Isolator connected between the two buses. The 100 MVA Transformer is connected to line bus bar through a number of equipments in as following defined sequence:

I. Isolator arrangementII. Circuit BreakerIII. Current transformerIV. Isolator arrangementV. Lightning Arrestor (LA) and thenVI. 100 MVA transformer

6) The 100 MVA transformer steps down the 220 KV incoming to 66 KV and this output is connected to second bus bar arrangements through a sequenced equipments as follows:

I. Lightning Arrestor (LA)II. Isolator arrangementIII. Circuit breakerIV. Bus bar isolator

7) The second bus bar arrangement is also a two bus bar system each connected with Potential Transformer (PT). There is again a bus coupler between the two bus bars to couple them. Here a Capacitor Bank is provided to increase the incoming voltage if there is any voltage drop in the incoming. It can enhance the incoming voltage by 3 to 4 KV.

8) From the 66 KV bus bar there are four outgoing circuits transmitting power at 66KV to:

I. Mathura road circuit no.1II. Mathura road circuit no. 2III. DMRC circuit no. 1IV. DMRC circuit No. 2

And there are two circuits for 66/11 KV transmission. These two 66 kV incomers from the 66 KV bus bar is fed to the two Transformers of 20 MVA each.

9) The 20 MVA transformer steps down 66 KV into 11 KV and this 11 KV is supplied to a number of sub circuits from the 11 kV bus bar. The 11 KV bus bar is also connected with a capacitor bank of 5 MVAR

3.3 PARTS OF SUBSTATION

The substation can be broadly divided into two parts:

220 kV outdoor yards. 66 kV outdoor yards. 11 kV indoor yards. Control room. SCADA room Battery room.

3.3.1220 KV Outdoor Yard:

220 KV yard is an outdoor yard where 220 KV incoming is transformed into 66 KV and connected to the bus bar arrangement on which four 66 KV outgoing feeder two circuit for DMRC and two circuits for Mathura road. From the 66 KV bus bar two outgoing feeders are given as incomer to the 66/11 KV yard. There are a number of switching, protection and measuring equipments connected in the yard. These all equipments are controlled from the Control Room.

Description about the 220 KV yard:

o There are four incoming feeders of 220 KV i.e. one from Pragati Powers, one from Maharani Bagh and two from BTPS.

o Two buses named Bus-1 and Bus-2 of 220 KV each run in parallel to which all the 220 KV incoming feeders are connected and also the two 100 MVA transformers are connected to step down 220 KV to 66 KV.

o The incoming 220 KV feeder is first connected to an Oxide Film Lightening arrestor which protect all the other equipments from Lightening and ground the lightening if falls on the incoming feeders.

o After the Lightening Arrestor a Capacitor voltage transformer is provided which serves the function of measuring and protection.

o A Wave Trap is provided to trap the waves which may be dangerous to the instruments here in the substation.

o Current transformer (CT) is connected to drive the current measuring equipment and also for protection i.e. for measuring and protection.

o Line isolator with Earth switch is provided opening the circuit in no load condition and earth switch (E/S) is to ground the extra voltage which may be dangerous for any of the instrument in the substation.

o Circuit Breaker (SF6 CB) is connected in line to open or close the circuit in normal and abnormal condition.

o From circuit breaker the incoming feeder is connected to the 220 kv double bus bar system through isolator arrangements so that the connection can be changed from one bus to other bus.

o The bus is connected with Potential Transformer for measuring the line voltage and protection purpose.

o A bus coupler is provided to couple the two buses for load sharing and line protection.

o The two transformer of 100 MVA, 220/66/11 KV are connected to the 220 KV bus used to step down the voltage from 220 KV to 66 KV. This output is connected to the 66 KV bus bar.

o Circuit breaker (SF-6 CB) is connected in the transformer circuit to open and close the circuit in no load and full load condition and in normal and abnormal condition.

o Horn gap lightening arrestor is provided just before and after the 100 MVA transformers for protecting the transformer from lightening.

3.3.2 66KV OUTDOOR YARD

o There are two incomers of 66 KV coming from 220KV yard transformers. There are two buses of 66 KV in parallel and are connected to the 66 KV incomers through isolator arrangements for changing the connection from one bus to another.

o Here also a bus coupler is provided for coupling the buses for load sharing and protection.

o The bus is connected with three capacitor banks of 20 MVAR for power factor improvements and for increasing the incoming low voltage by 3 to 4 KV.

o Form the 66 KV bus bar six outgoing feeders are connected as one for Mathuara road circuit no. 1, one for Mathura road circuit no 2 and two for DMRC, and two circuits are connected to the two 20 MVA transformer.

o All the circuits are connected through deferent switching, measuring and protection equipments like Isolators, CT, Circuit Breakers, Lightening Arrestors.

o Two transformers of 20 MVA each of rating 66/11 KV are connected to 66 KV bus by bus – selection Isolator.

o Each bus is connected to PT for measurement of voltage in line.

o CT is connected at required place for measurement of current and protection of lines.

o The SF-6 CB (Circuit Breakers) is aligned in the circuit for tripping whenever any fault occurs in the circuit.

3.3.3 11 KV INDOOR YARDS (VCB ROOM)

o There are two incomers of 11 KV coming from 66 KV yard transformers of rating 20 MVA, 66/11 KV and are connected to two 11KV bus i.e. Bus no.1 and bus no.2.

o The 11 KV indoor substation is having vaccum circuit breaker (VCB) in all the outgoing feeders.

o Capacitor banks of 5 MVAR are connected in each phase of the Bus bar to increase the voltage level if there is any drop in incoming voltage.

o There are 12 outgoing feeders connected to 11 KV bus. The feeders are connected in a sequence as:On first half bus connected feeders are:

400 KVA Local Transformer. 11 KV O/G Aali Village 11 KV O/G spare 11 KV O/G Saurav Vihar-1 11 KV O/G Indian Oil Corporation 11 KV O/G S/Stn. No. 22 Sarita ViIhar

o On second half bus : 11 KV O/G S/Stn. No. 21 Sarita ViIhar 11 KV O/G Sewage Pump 11 KV O/G Jaitpur 11 KV O/G Saurav Vihar- 2 11 KV O/G American Express 11 KV O/G Spare

o A bus coupler is provided between the two buses for load sharing and line protection.

3.4 EQUIPMENTS USED IN SUBSTATION:-

S.NO.

EQUIP. CONS.FEATURE/LOCATION FUNCTION

1. BUS BAR Rigid tubular support on positions or Flexible ACSR bus bar supported from two ends of strain insulator.

Receive power from incoming and deliver power to O/G ckt.

2. SURGE ARRESTOR

Connected B/W phase conductor and ground first equip as seen from incoming O/H line and also near transformer terminal.

Discharge O/V surge to earth and protect equipment.

3. ISOLATOR Located each side of CB. Provide isolation from part for MTC.

4. EARTH SWITCH (E.S.)

Mounted on frame of isolators, generally for such I/C each bus bar.

Discharge voltage on ckt to earth for safety.

5. CURRENT TRANSFORMER (C.T.)

Protection, measuring decided by protective zone measurement requirements.

Step-down current measurement front and control.

6. VOLTAGE TRANSFORMER (V.T.)

Electro magnetic capacitive feeder side of C.B.

Step-down current measurement protection and control.

7. CIRCUIT BREAKER (C.B.)

Depend on rated voltage LV, MV, HV, EHV depend on quenching medium –SF6 MQ, AB etc.

Switching during normal abnormal and S.C. current.

8. SERIES REACTOR

Oil filled gapped core shielded, usually unswitched.

1. Control low load period voltage.2. To

compensate shunt capacitor of T.L during low load.

9. SHUNT CAPACATOR

Locate at receiving STN and DIST, substation.Banks rated -132KV, 66KV, 400KV, 11KV switched during heavy load.

1. comp. rex power.2. P.F. improves.3. VOH contran.

10. SEREIS CAPACITOR

1. Capacitor bank located at send end or receive end of line.2. Provided with bypass C.B and protect spare gaps.

Used for EHV lines to improve power transformer.

11. TRANSFORMER Oiled filed 3 Setup / down voltage.

12. MV/LT SW GR Inside swgrbling. AC power to auxiliary stnlty

13. STATION EARTHING SYTEM

Earth mat and earth electrode. For safe touch potential→ Equipment body earth.→ discharging current from SA O/H shielding and E.S.

14. INSULATORS Between the poles and conductors. Disc type shaped.

Does not allow the current to pass through it.

15. POLES It is made by joining the heavy materials with the help of nuts and bolts of requirement shape and size wherever necessary.

To provide necessary height to conductor from which current is flowing.

16. CVT Consist of two to five windings in parallel of line.

CVT are used for line voltmeters, synchronoscope, protective relays, tariff meter etc.

17. L.A. Ring type L.A. parallel in line. To drop the sky lightening effect.

18. CONDUCTORS A.C.R.S. is used wherever necessary.

Transmission current form one place to another.

19. BATTERY BANKS

Located in separate room near to control room.

To supply D.C. for controlling protection system and communication equipments.

20. CONTROL PANEL

Associate with protection relays locate in big hall.

To control all equipment of substations.

4. DESCRIPTION OF SUBSTATION EQUIPMENTS

4.1 TRANSMISSION LINES

In this category the EHV lines viz. extra high voltage lines of 400kv, 220kv, 132kv, and 66kv are considered. These high voltages are transmitted from one sub-station to other sub-station through various types of conductors.

For 400 KV line: Taran, Tulla and Marculla conductor.

For 220 KV line: Zebra conductor is used composite of Aluminum strands and steel wires.

For 66kv, 33kv lines: Panther conductors is used composite of Aluminum strands and steel wires.

The materials used in these conductors is generally Aluminum conductor steel reinforced (ASCSR).

4.2 BUS-BAR

It is a conductor to which a no. of circuit is connected. In 220kv Najafgarh there are two bus- bars running parallel to each other, one is main& other is auxiliary bus.

The purpose of using two buses is only for stand by, in each of failure of one bus we can keep the supply continue with help of other bus using isolators.

According to bus voltage the material is used. T he most commonly used material is Al, Cu. But Al. Is used because of its property & feature and also it is cheap.

Figure Typical representations of bus bars

PROPERTIES OF MATERIALS USED IN BUS BAR ARE AS FOLLOWS:-

PROPERTIES COPPER ALUMINIUM

Electrical resistively at 20 deg C

0.017241 0.0288

Temp coeff. of resistivity 0.00411 0.00403

Softening temperature 200 180

Thermal conductivity 0.923 0.503

Melting point 1083 657

When a number of lines operating at the same voltage have to be directly connected electrically bus-bar are used as the common electrical component. Bus-bar are copper or aluminium bars and operate at constant voltage. The incoming and outgoing lines in a sub-station are connected to the bus-bars. The most commonly used bus-bar arrangements in sub-station are:

I. Single bus-bar arrangement.II. Single bus-bar system with sectionalisation.III. Double bus-bar arrangementI. SINGLE BUS-BAR SYSTEM : -

It consists of a single bus-bar and all the incoming and outgoing lines are connected to it. The disadvantage of this type of system is that if repair is to be done on the bus-bar or a fault occurs on the bus. There is a complete interruption of the supply. This arrangement is not used for voltages exceeding 33KV.

II. SINGLE BUS-BAR SYSTEM WITH SECTIONALISATION : -

In this arrangement the single bus-bar is divided in to sections and load is equally distributed on all the sections. Any two sections of the bus bar connected by a circuit breakers and isolators. It has two principle advantages. Firstly, if a fault occurs on any section of the bus that section can be isolated with out affecting the supply from other sections. Secondly, repairs and maintenance of any section of the bus bar can be carried out by de-energizing that section only, eliminating the possibility of complete shutdown. This arrangement is used for voltage upto 33KV.

III. DOUBLE BUS-BARS SYSTEM : -

This system consists of two bus bars, a “main” bus bar and a “spare” bus bar. Each bus bar has the capacity to take up the entire substation load. The incoming and out going lines can be connected to either bus bar with help of bus bar coupler which consist of a circuit breaker and isolators. Ordinarily, the incoming and outgoing lines remain connected to the main bus bar of fault occurring on it, the continuity of supply to the circuit can be maintained by transferring it to spare bus bar.

4.3 ISOLATORS

An isolator is a disconnecting switch is used upon same given part circuit after circuit breaker. Thus isolators’ surge only has preventing the voltage from being applied to same given section of bus.These are essentially off load devices although they are capable of dealing with small charging currents of bus-bars and connections. The design of

isolators is closely related to the design of substations. Isolator design is considered in the following aspects:

o Space Factoro Insulation Securityo Standardizationo Ease of Maintenanceo Cost

It is required in substation to disconnect a part of the system for general maintenance and repairs. This is accomplished by isolators. An isolator is essentially a knife switch and is designed to open a circuit under no load. In other words, isolator switches are operated only when the lines in which they are connected carry no current. Isolators used in power system are generally 3 pole isolator having three identical poles each pole consist of two or three insulator posts mounted on a fabricate support. The fixed and moving conducting parts are of copper or aluminium rods. During the opening operation, conducting rods swing apart and isolation is obtained simultaneously on all 3 poles. The three poles are mechanically interlocked which operate together by operating a common operating mechanism which may be:

1. Electric motor mechanism.2. Pneumatic mechanism.

ISOLATOR WITH EARTH SWITCH: -

The earth switch is connected between the line conductor and earth. Normally, it is opened when the line is disconnected. The earth switch is closed so that the voltage trapped in line is discharge to earth. There some voltage lines due to changing current. This voltage is significant in high voltage system. Before, proceeding with the maintenance work. This voltage is discharge to earth by closing the earth switch. Normally earth switches are maintained on the frame of isolator.

4.4 TRANSFORMER

4.4.1 INTRODUCTION:

A transformer is a static device by means of which electric energy from one electrical circuit to another is transferred through the medium of magnetic field and without change in the frequency.

A high voltage is desirable for transmitting large powers in order to decrease the IR losses and reduce the amount of conductor material. A very much lower voltage, on the other hand s required for distribution , for various reasons connected with safety and convenience the transformer make this easily and economically possible.

4.4.2 POWER TRANSFORMERS

Power transformer is the main and major requirement of a sub-station to step down the supply voltage. The rating of a transformer is taken according to the load requirement.

4.4.3 Parts of power transformer:

i. Transformer coreii. Windings

iii. Tank

iv. Conservator

v. Breather

vi. Bushing

vii. Air Cell

viii. Tap Changer and O.L.T.C.

ix. Cooling Equipments

i. CORE: - It not only supports the winding also provides the low reluctance path for the magnetic circuit. It is made up of cooled rolled grain oriented (C.R.G.O.) alloy. Steel is in the form of lamination on that the iron losses could be avoided.

ii. WINDING: - Windings are arranged in concentric formation with lower voltage winding next to core. Tertiary winding is placed next to the core over L.V. winding H.V. main winding are placed.

Various types of windings are used for coils these are as follows:-

a. Low voltage winding - Spiral or helicalb. High voltage winding - Partially inverted disc / layer winding.

c. Tertiary winding - Spiral / Helical / Disc

d. Tapping winding - Inter wound spiral or helical paper covered insulated copper strips or continuously cable are used for making winding.

iii. TANK: - They are constructed from welded sheet steel, and larger ones from plain boiler plates. The lids may be of cast iron, or waterproof gasket being used at the joints. The fitting includes thermometer pockets, drain cock, rollers or wheels for moving transformer position, eye bolts for lifting, conservators and breathers, cooling tubes are welded in, but separate radiators are welded and afterwards bolted. On the outside is applied with anti corrosive primer paint and final of synthetic enamel.

iv. CONSERVATOR: - As the temp. Of oil increases or decreases there is continuous rise and fall in volume. For this an expansion vessel (conservator) is to transformer tank having the capacity of oil level equal to 75% of total oil.o Conservator is provided to tank core of the expansion and

contraction of oil, which takes place during normal operation of the transformer.

o Wherever specified flexible separators or oil cell if provided in the conservator can prevent direct contact of air with the transformer oil.

o A smaller oil expansion vassal is provided for the on load tap- changer.

o Magnetic oil level gauge is fitted on the main conservator which can give alarm / trip in the event of the oil falling below the pre-set level due to any reason.

v. BRAETHER:

Both transformer oil and celluloses’ paper are highly hygroscopic .Paper being more hygroscopic than mineral oil .The moisture, if not excluded from oil surface in conservator, this will find its way finally into paper insulation and causes reduction insulation strength of transformer to minimize this the conservator is allowed to breath only through silica gel colomin ,which absorb the moisture in air before it enters the conservator air surface

vi. BUSHING: -

Up to a voltage of 33kv, ordinary porcelain insulators can be used. Above this voltage the of conductor or oil filled terminal bushing, or a combination of two has to be considered .Of course, any type of conductors can be effectively insulated by air provided that it is at a sufficient Distance from other conducting bodies and sufficiently to prevent corona phenomena.The high voltage connections pass from the winding to terminal bushing. Thermal bushings up to 36kv class, 3150 Ampere are normally of plain Porcelain and Oil communicating type .Higher current rated bushings and bushings of 52kv class and above will be of oil impregnated paper condenser type. The oil inside the condenser bushings and will not be communicating with the oil inside the transformer oil level gauge is provided on the expansion chamber of the condenser bushings.Oil in the condenser bushing is hermetically selected and it should not be disturbed in normal operation. Oil level and oil leakage may be checked regularly.

vii. AIR CELL: -

It is a flexible rubber bag placed inside the conservator and floats on the oil surface. Air cell inflates or deflates surface of the air cell and the inner cell of air cell is provided with ozone resistant .The dry air is sucked and do not come in contact with oil, this eliminates the possibility of contamination for oil filling.

viii.TAPCANGER.

Tap changer are of two types:-a) On-load Tap changer.b) Off-load Tap changer.

ON LOAD TAP CHANGER: -

As the name implies it sets a tap for adjusting the secondary voltage in the condition of on ‘load’. It is generally connected to the primary side due to current. The tap is connected to the diverter switch of the tap changer. It may be manually operated or motor drive unit is initiated by a push button or relay. The diverter switch diverts the current. The break in the current prevented by transmission resistance tap changer.

On load tap changer is the device for changing the tapping connections of a winding, whilst the transformer is connected is on loadWhen the transformer is connected to a system it is some time necessary to vary the voltage on the secondary side to meet the load demands, as such transformer tap changer must be capable to varying the turn ratio without interruption of supply. On a double wound transformer the best position to place the tapping is at the neutral end of high voltage winding .The positioning of the tapping on the lower voltage winding is not applied on account of high current rating which would result.The tapping of the windings are brought out through a terminal board to a separate oil filled compartment, in which the on- load tap changer selector is housed. As the selector must not break current ,a further separate oil filled compartment is provided to house the diverter switch which breaks the load current by an interrupted arc forming carbon ,therefore the oil I the diverter switch compartment must be prevented from missing with the oil in the main tank.The tap changer is operated by a motor operated driving mechanism by local or remote control and a handle is fitted for manual operation in an emergency.As the changing must take place on load, the contact for the tap changer are so arranged that before one tapping is left , contact must be made with the next . This could cause a short circuited no. of turn and large current are prevented by the use of resistor or reactors.

IV.4.4 COOLING EQUIPMENT: - Transformer is having a single or mixed cooling of ONAN, ONAF,

OFAF, and OFAN by means of radiators, fans, pumps, & heat exchanger etc.In Power transformer cooling are of following Types:

1. ONAN with 50% efficiency2. ONAF with 70% efficiency3. OFAF with 100% efficiency

o For ONAN/ONAF cooling, oil flow through the winding and external cooler unit attached to the tank by themo-Syphonic effect.

o For OFAF/ODAF/OFWF cooling, the oil is directed through the winding by oil pumps provided in the external cooler unit.

o External cooler unit /units consists of passed Steel sheet radiators mounted directly on the tank or separator cooler banks for air –cooled transformer and oil to water heat exchangers for water cooled transformer.

4.4.5 PROTECTIVE DEVICES:

1. Buccholtz relay2. Pressure relief valve

3. Oil temperature indicator

4. Oil level indicator

5. Winding temperature indicator

6. Dehydrating Breather

7. Earthing Arrangements

1. BUCHHOLTZ RELAY: -It is used for protection of oil filled transformer from incipient faults below oil level. It is installed between tank and conservator. In this relay two mercury contacts are provided. The device comprises of a cast iron housing containing the hinged floats, one in upper part other in lower part. Each float is filled with the mercury switch; leads of a switch are connected to a terminal box for tripping.

APPLICATIONS:-

Bu

cch

oltz

re

lay

Double element relays can be used in detecting miner fault in a TransformerThe alarm element will operate, after a specified volume of gas has collected to give an alarm indicator.

Examples incipient faults are:-1. Shorted laminations2. Broken-down core bolt insulation3. Bad contacts4. Over heating of part of winding,

2. PRESSURE RELIEF VALVE: In case of major faults in the transformer like short circuit in the winding .The internal P.R.V. is build up to a very high level which may result in rapture of tank to avoid this P.R.V. provided.A device for avoiding high oil pressure build up inside the transformer during fault a condition is fitted on the top of the tank. The pressure relief device allows rapid release of excessive pressure that may be generated in the event of a serious fault.This device is fitted with an alarm trip switch.

3. OIL TEMPERATURE INDICATOR: -It is the distance thermometer operated on principle of liquid expansion. It indicates the top oil temp. At marshaling box. The connection between the thermometer and the dial indicator is made by steel capillary tube. The bulb is enclosed in the pocket and the pocket is situated on transformer’s hottest oil region. The pocket is to be filled with oil. It has two switches one for alarm and other for tripping.

ALARM = 95 DEG. TRIPPING = 110 DEG.

It is consist of a sensor bulb capillary tube and a dial thermometer the sensor bulb is fitted at the location of hottest oil .That sensor bulb and capillary tube are fitted with evaporation liquid. The vapor pressure varies with temperature and is transmitted to a burden tube inside the change in pressure which is proportional to the temperature.

4. WINDING TEMPERATURE INDICATOR: - It also operates on principles of liquid expansion. It indicates the top oil temp. At marshalling box hot spot temp. Of winding. The winding hot spot of top oil temp. Difference is simulative by means of CT current fed to the heater coil fitted at top senses the top oil temp. Thus, it’s temp. Reading is proportional to the load current and oil temp.

o FANS ON = 60 deg Co PUMP ON = 75 deg Co ALARM = 90 deg Co TRIP = 100 deg C

Winding temperature relay indicates the winding temperature of the transformer and operates on the principle of thermal imaging and it is not actual measurement.Winding temperature indicators consist of sensor bulb placed in oil filled pocket in the transformer tank top cover. The bulb is connected to the instrument having by means of two flexible capillary tubes. One capillary tube is connected to the measuring below of the instrument and the other to compensation below. The measuring system is filled with a liquid which changes its volume with rising temperature inside the instrument is filled with a heat resistance which is fed by a current proportionate to the current flowing through the transformer winding.The instrument is provided with maximum temperature indicator the heating resistance is fed by current transformer associated to the loaded winding of the transformer .The increase in the temperature of the resistance is proportionate to that of the winding. The sensor bulb of instrument is located in the hottest oil of the transformer the winding temperature indicates a

temperature of hottest oil plus the winding temperature rise above hot it .i.e. the hot spot temperature.

5. OIL LEVEL INDICATOR :-This indication is manufactured for considering Transformer Applications:

1. It can also be used as Content Gauge on other tanks where level of liquid inside the tank in required to be indicated continuously on a dial.

2. The position of indicator on the conservator can be selected to3. Suit site condition. Float mechanism passes through the hole in pad.4. Indicator can be mounted in titled position towards ground

(max.300degree) for easy viewing by fixing mounting pad at desired angle.

5. One mercury switch is provided for low level alarm. The Normally Open switch closes when oil level drops to 10mm above Empty land i.e. 75mm from bottom of conservator.

6. Loads from mercury switch are brought into a terminal box positioned at the bottom of indicator.

6. DEHYDRATING BREATHER: - The conservator or the air cell is connected to the outside atmosphere through the breather (silica gel) to make sure that the air in the conservator or cell is dry. When silica is saturated with moisture its color changes to pink. It can be made reusable by heating it at 100 deg C. for 48 hours.

7. EARTHING ARRANGEMENTS :-a) Core Earthing

Connecting leads from core and end frame are being terminated at the top of the cover, By connecting them to tank cover, core and frame becomes earthed .Insulation resistance between the leads from core and end frame or between leads from core and earth point can be checked by 500 volts megger. Leads from end frame have been brought out for proper earthing for end frame.

b) Tank to Tank Earthing :

Tank to tank cover earthing is done by connecting copper braid between tank rim and tank cover with the help of the bolts used to tight tank cover and tank together.

c) Earthing of Tank

For earthing of tank nut-bolts & studs are required to make perfect earthing between pads on tank and external earthing strip.

4.4.6Maintenance & Operation

In order to avoid fault and disturbance, it is important that a careful and regular supervision and control of the transformer and its components is planned and carried out.

The frequency extent supervision and control is dependent on climate and environment and service condition.

POSSIBLE LEAKAGE

After energizing of the transformer, a certain setting may appear in painting joint.Rust damage, Touch damage up painting a regular inspection of the external surface treatment of the reactor should be carried out. Possible rust damage is removed and the surface treatment restarted to original state by means of primer and finish paints that are dispatched with the transformer.THRERMO SYPHON FILTERThermo siphon filter is provided on large capacity, oil filled Power Transformer for keeping the moisture level of insulating oil at a very low level. At the time of initial erection and commissioning of transformer, most of the moisture present in the oil is removed by not oil circulation. The moisture absorption of oil is eliminated by direction the our breather in by the transformer during its operation through silica gel desiccant. Air cell in conservator avoids direct of oil with air and there by eliminating the chance of moisture absorption.It is a well known fact that water is released to the oil for the paper insulation due to ageing process. Thermo siphon filter helps in removing this moisture from oil. When the Transformer is on load, the thermos phonic action of liquid causes circulation of oil through the filter. The

absorbent filled in the Thermo siphon filter absorb moisture and keep the oil dry.

4.4.7 SPECIFICATIONS OF TRANSFORMERS

1. 100 MVA, 220/66/11KV power transformer no. 1 & no. 2 (BHEL) make.

1.Types of cooling ONAN ONAF OFAF

2.Rating HV & LV (MVA)

50 70 100

3.Rating TV (MVA) 16.67 23.33 33.33

4.No load voltage HV (kv)

220 220 220

5.No load voltage LV(kv)

66 66 66

6.Noload voltage

TV(kv)

11 11 11

7.Line current HV(Amp)

131.37 183.92 262.74

8.Line current LV (Amp) at 66kv

437.90 613.07 875.81

9.Line current LV (Amp) at 33kv

875.81 1226.13 1751.62

10.Line current TV (Amp)

875.81 1226.13 1751.62

11. Temp. rise oil ( deg C)

50 50 50

12. Temp. rise winding (degC)

55 55 55

4.5. INSTRUMENT TRANSFORMER: -

Transformer used A.C. measurement i.e. voltages current, power and energy in conjunction with the relevant instrument. Transformer small capacity transformer. There are two types:

1. Current transformer.2. Potential transformer.3. Capacitor Voltage transformer.

ADVANTAGES OF INSTRUMENT TRANSFORMER:-

1. The size of I.T. is reduced or say moderate because the secondary

Of C.T. is designed for 5A. And of P.T. for 110V.2. The replacement of damaged instrument is easy.3. Several instruments can be operated from a single I.T.4. Low consumption of metering circuit.5. Accessibility on H.T. is easy.

Instrument transformer is used to measure AC at generating station, station at transmission line in conjunction with AC measuring instruments. They are classified according to the use are referred to as current transformer (CT) & potential transformer (PT).

Functions: -

1. They serve to extent the range of AC measuring instrument.2. They serve to isolate the measuring instrument from high Voltage

4.5.1CURRENT TRANSFORMER: -

High current line can be reduced to low current to measure easily with the help of normal ammeter. To measure the very high current of the running line with out distributing it, a spilt core type current transformer is used. It is step up transformer the primary windings consist of thicker conductor having less number of turns. Some time, only a straight conductor also serves the purpose of primary winding. The secondary winding is done with thicker conductor having more number of turns.The primary winding is connected in series with the line and the M I is connected across the secondary of the current transformer. it should be clearly understood that the secondary winding of the current transformer is never opened. It should be always short circuit i.e. the secondary is

open, there is no current in the secondary winding hence, the M.M.F. of primary will not be opposed and the cares will have high flux which will cause high E.M.F. Induced or the primary and secondary winding. This E.M.F. Is dangerous and may give sever shock.The secondary of the current transformer should be earthed to avoid the danger of shock to the operator.The current transformer is kept in category of instrument transformers. The CT’s are used to reduce / stepping down A.C. from high value to lower value for measurement / protection / control.

A 'CT' has following essential parts: -

1. Magnetic core made up of continuously wound strip nickel iron alloy of CRGO material.

2. Winding having several turns wound on the insulated core.3. A bar primary passing through the winding of core and terminal.4. Insulated porcelain at primary insulator.5. Synthetic region or oil insulation.

PROPERTIES OF CT:

o The CT measures the current.o The current transformer is used with primary winding.o Connected in series with the line carrying the current to be

measured and therefore primary current is dependent upon load connected in the system.

o The primary winding of a very few turns, and therefore there is no appreciation drop across it.

o The secondary winding has large no. Of turns, exact no. Being determined by the turn ratio.

o Ammeter of wattmeter current coil is connected directly across secondary winding terminals.

o Thus CT operates at secondary terminal near by being short-circuited.

o One of the terminals of secondary winding is earth in order to protect – instrument and personal in the vicinity in event of insulation breakdown.

WORKING: - The CT has three coils different purposes.

a) Measurement: - The secondary given 5A / 1A current which operates the ammeter to note the current reading

b) Protection: - The 5A / 1A current is sent to the relay and if the current exceeds this limit then the relay operates and sends signal to the C.B. which then operates.

c) Differentiald) Spare

SPECIFICATION OF CURRENT TRANSFORMERS: -

Specification of 220 kV side CT –

a) Standard - IS 2705b) Highest system voltage (kv) - 245c) Insulation level (kv) - 460/1050d) Frequency - 50 Hze) Rated primary current - 600Af) ST current KA/ sec - 27 /1

Terminals Ratio Amp. Rating class VA Kvp / Amp1s1-1s2

1s1-1s2

300/1600/1

5P20 60 1200V/0.04A

2s1-2s22s1-2s2

300/1600/1

5P20 60 1200V/0.04A

3s1-3s23s1-3s2

300/1600/1

5P20 60 1200V/0.04A

4s1-4s24s1-4s2

600/1 .5 60 ....................

4.5.2POTENTIAL TRANSFORMER: -

Similar to CT it is another type of instrument transformer. It is also known as CVT (capacitor voltage transformer). It is used for measurement and protection. Potential transformer is used to operate voltmeter, the potential coil of wattmeter and relay from high voltage line. The primary oftransformer4 is connected across the line carrying the voltage to be measured and the voltage circuit is connected across the secondary winding to measure high voltage line.The transformer is used to measure the high voltage known as potential transformer. The primary of the potential transformer is having more number of turns of fine wire and secondary is having less number of turns. The potential transformer is step down transformer the P.W is connected across the line and S.W across the meter to measure the line voltage. The P.W when connected to line carry some current, which produces the magnetic flux. The S.W is linked with this flux causing the induction some voltage (generally 110V in case P.T.) this voltage defects the voltmeter or the secondary of the P.T.The scale is directly calibrated to obtain the actual voltage. The secondary of the P.T. is always connected to earth.

They may be of one phase or three phase. Electromagnetic P.T. In which primary and secondary are wound on magnetic core in usual transformers.

SOME TERMS RELATED TO P.T

(a) Rated Voltage: The voltage of the P.T., which it can withstand.(b) Rated Transformer Ratio: The ratio of rated primary voltage to

the rated secondary voltage.(c) Rated secondary voltage: e.g. 130/ root (3) = 63.3 VAR.

4.5.3Capacitor Voltage Transformer:-

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the secondary terminals.

CVT 220 kV rating

Type: WP-245 V

Operating voltage: 220/√3 kV

Voltage factor: 1.5 V for 30 sec.

Test voltage: 460 kV

Test impedance 1050 kv peak

Ellec cap: 4400±10% PF of 50 H ± 5%

Nominal intermediate voltage 20/√3 kv

Spark over voltage: 36 kv

Voltage divider ratio 220000/√3 /20000/√3

Total thermal burden: 1000 VA

Temperature categ: 10 to 55°C

Total weight: 900 Kg.

4.6. CIRCUIT BREAKER

A circuit breaker is equipment which can be open are closed a circuit under a normal as well as fault condition. It is so desired that it can be operated manually or by remote control under normal condition and automatically under fault condition. For the latter operation a relay is used in the circuit breaker. A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating condition, these contacts remain closed and will not open automatically until and unless the system becomes fault. The contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and moving contacts are pulled apart by some mechanism, thus opening the circuit. the basic construction of any circuit breaker requires the separation of the contacts in any insulating fluid, when serves two function:-

It extinguishes the arc drawn between the contacts when the circuit breaker open.

It provides adequate insulation between the contacts and from each contacts to earth. Many insulating fluids are used for arc extinction and the fluid chosen depend upon the rating and type of the circuit breaker.

The insulating fluids commonly used are :- Air at atmospheric pressure Compressed air Ultra high vacuum

Oil which produces hydrogen for arc extinction Sulphur hexafluoride (SF6)

Figure 7.2 Circuit breaker arrangements4.6.1 TYPES OF CIRCUIT BREAKER

I. SULPHUR HEXAFLURIOD (SF6 )In such circuit breaker sulphur hexafluoride gas is used as arc quenching medium. The SF6 is electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker an opened in a high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons in arc are rapidly captured by the gas to form relatively immobile negative ions . This loss of conduction electrons in the arc quickly builds up enough insulating strength.The SF6 circuit breaker has been found to be very effect able for high power and high voltage services.SF6 has excellent insulating strength because of its affinity for electrons i.e whenever a free electrons collides with the neutral gas molecules to form negatives ions, the electrons is absorbed by the neutral gas molecules may occur in two ways

SF6 + e -> SF6

SF6 + e -> SF5 + F

The negative ion formed are relatively heavier as compared to free electrons and therefore under a given electric field the ions do not attain sufficient energy to lead cumulative ionization in the gas.

WORKING

In closed position of the breaker, the contacts remains surrounded by SF6 gas at a pressure of about 6KG/sq.cm. When the breaker operates, the moving contact is pulled apart and arc is structure between the contacts. The movement of the moving contacts is synchronized with the opening of the valve, which permits SF6 gas at 15Kg/sq.cm pressures from the reservoir to the arc interruption chamber. The high- pressure flow of SF6 rapidly absorbs the electrons in the arc path to form immobile negative ions, which ineffective as charge carriers. Thus, medium between the contacts quickly built up high dielectric strength and cause the extinction of the arc, after the breaker operates.

II. VACCUM C IRCUIT BREAKER:

In such breakers (degree f vacuum being from 10-7 to 10-5 tore) is used as arc quenching medium. Since vacuum offers the high insulating strength, it has superior quenching properties then any other medium e.g when contacts of the breaker are opened n vacuum , the interruption occurred first current zero with dielectric strength between the contacts building at a rate of 1000th of times higher then that obtained with other circuit breaker . Thus a vacuum arc is different from the general class of low & high pressure arc . In the vacuum arc the neutral atoms, ions and electrons do not come from the medium in which the arc is drawn but they are obtained from the electrodes themselves by evaporating its surface material , because of the large mean free path for the electrons , the dielectric strength of the vacuum is a 1000 times more than when the gas is used as the interrupting medium .

III. MINIMUM OIL CIRCUIT BREAKER (MOCB)

One of the important development in the design of oil circuit breaker has been to reduce the amount of oil needed. The other advantages are reduction in tank size , reduction in total weight and reduction in cost . It used minimum amount of oil and is only used for arc extinguishing the current conducting parts are insulated by porcelain or organic insulated material. Low oil circuit breaker employees solid materials for insulations purpose and uses a small qty.of oil which is just sufficient for arc extinguishing .By using suitable arc control devices,

the arc extinguishing can be further facilitated in low circuit breaker. venting.

IV. Air blast circuit breaker Fast operations, suitability for repeated operation, auto reclosure, unit type multi break constructions, simple assembly, modest maintenance are some of the main features of air blast circuit breakers. A compressors plant necessary to maintain high air pressure in the air receiver. The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker operates repeatedly. Air blast circuit breakers is used for interconnected lines and important lines where rapid operation is desired.

Figure 7.4 Air blast circuit breakerHigh pressure air at a pressure between 20 to 30 kg/ cm2 stored in the air reservoir. Air is taken from the compressed air system. Three hollow insulator columns are mounted on the reservoir with valves at their basis. The double arc extinguished chambers are mounted on the top of the hollow insulator chambers. The current carrying parts connect the three arc extinction chambers to each other in series and the pole to the neighboring equipment. Since there exists a very high voltage between the conductor and the air reservoir, the entire arc extinction chambers assembly is mounted on insulators.Specifications of SF6 Circuit Breaker

1. Gas circuit breaker: high voltage side (220KV)Type 220-SFM-40AVoltage rating: 220kvRated lightening impulse withstand voltage: 1050 kVp

Rated short circuit breaker current: 40 kARated operating pressure: 16.5 kg/cm2-gRated Gas pressure: 6 kg/cm2-gFirst pole to clear factor 1.3Rated duration of short circuit current is 40 kA for 30 sec.Rated ling charging breaker breaking current 125 AmpRated voltage 245 kVRated frequency 50 HzRated normal current 2500 AmpRated closing voltage: 220 V dcRated opening voltage 220 V dcTotal Weight with Gas: 3900 Kg.

2. Gas circuit breaker: low voltage side (66 KV)Type 70-SFM-32BVoltage rating: 72.5 kvRated lightening impulse withstand voltage: 350 kVpRated short circuit breaker current: 31.5 kARated operating pressure: 16.5 kg/cm2-gRated Gas pressure: 5 kg/cm2-gFirst pole to clear factor 1.3Rated duration of short circuit current is 31.5 kA for 3 sec.Rated ling charging breaker breaking current 125 AmpRated voltage 72.5 kVRated frequency 50 HzRated normal current 2500 AmpRated closing voltage: 220 V dcRated opening voltage 220 V dc

4.7. CAPACITOR BANKThe capacitor bank provides reactive power at grid substation. The voltage regulation problem frequently reduces so of circulation of reactive power.Unlike the active power, reactive power can be produced, transmitted and absorbed of course with in the certain limit, which have always to be workout. At any point in the system shunt capacitor are commonly used in all voltage and in all size.Capacitors are used to control the level of the voltage supplied to the customer by reducing or eliminating the voltage drop in the system caused by inductive reactive loads.

An AC system cannot function with the highest transmission capability at minimum cost and at the highest efficiency unless the reactive compensation is carefully applied. The capacitor i.e. VAR is installed in receiving substation, load substation for fast, staples control of reactive power compensation of voltage control .

Capacitor banks are installed following purposes:

o To improve the power factor of the system & there by regulating the system voltage

o Reactive power compensation o To reduce the losso Increased voltage level at the loado Reduced system losseso Increase power factor of loading current

Fig.13.1-Capacitor Bank

Benefits of using the capacitor bank are many and the reason is that capacitor reduces the reactive current flowing in the whole system from generator to the point of installation.The insulator for the overhead lines provides insulation to the power

4.8 INSULATORThe insulators are connected to the cross arm of supporting structure and the power conductor passes through the clamp of the insulator. The insulators provide necessary insulation between line conductors and

supports and thus prevent any leakage current from conductors to earth. In general, the insulator should have the following desirable properties:

High mechanical strength in order to withstand conductor load, wind load etc.

High electrical resistance of insulator material in order to avoid leakage currents to earth.

High relative permittivity of insulator material in order that dielectric strength is high.

High ratio of puncture strength to flash over.

These insulators are generally made of glazed porcelain or toughened glass. Poly come type insulator [solid core] are also being supplied in place of hast insulators if available indigenously. The design of the insulator is such that the stress due to contraction and expansion in any part of the insulator does not lead to any defect. It is desirable not to allow porcelain to come in direct contact with a hard metal screw thread.TYPE OF INSULATORS:

1. Pin type2. Suspension type

3. Strain insulator

PIN TYPE:

Pin type insulator consist of a single or multiple shells adapted to be mounted on a spindle to be fixed to the cross arm of the supporting structure. When the upper most shell is wet due to rain the lower shells

are dry and provide sufficient leakage resistance these are used for transmission and distribution of electric power at voltage up to voltage 33 KV. Beyond operating voltage of 33 KV the pin type insulators thus become too bulky and hence uneconomical.

SUSPENSION TYPE:

Suspension type insulators consist of a number of porcelain disc connected in series by metal links in the form of a string. Its working voltage is 66KV. Each disc is designed for low voltage for 11KV.

Fig.-Suspension type insulator

STRAIN INSULATOR: The strain insulators are exactly identical in shape with the suspension insulators. These strings are placed in the horizontal plane rather than the vertical plane. These insulators are used where line is subjected to greater tension. For low voltage lines (< 11KV) shackle insulator are used as strain insulator.

Fig.4.3-Strain type insulator

Post type insulator

Post insulators have metal bolt down base as opposed to threads. Many early multipart lines are spotted with line post insulators as replacements.

Post insulators are also used in substations to insulate high voltage switching gear and transformers. There is no hobby numbering system for post insulators yet. Most insulators are used for bus bars. Post insulators consist of porcelain body, cast iron cap and flanged cast iron base. The hole in the cap is the threaded so that the bus bars can be directly to cap.

4.9 Wave Trap:Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU).It is a device used to exclude unwanted frequency components, such as noise or other interference, of a wave. Wave trap is an instrument using for tripping of the wave. The function of this trap is that it traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected to the main incoming feeder so that it can trap the waves which may be dangerous to the instruments here in the substation.This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleportation signals and in addition, voice and data communication signals.The Line trap OFFERS HIGH IMPEDANCE TO THE HIGH FREQUENCY COMMUNICATION SIGNALS thus obstructs the flow of these signals in to the substation bus bars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.

Fig. -Wave Trap

5. PROTECTION SYSTEM

INTRODUCTION: -

There are different schemes adopted for the protection of various equipment of power system against Over Voltage and heavy short circuit current.

CAUSES OF OVER-VOLTAGE: -

The over-voltage may occur in the power system due to.1. Internal causes2. External causes

INTERNAL CAUSES: -

A. Switching surgesB. Arcing groundsC. Insulation failureD. Resonance

EXTERNAL CAUSES OF OVER-VOLTAGES: -

LIGHTNING: -

An electrical discharge in our between clouds, between the separate charge in the same cloud or b/w cloud and earth is caused lightning.There are two main ways in which lightning stoke can effect a line i.e.

1. Direct stroke2. Indirect stroke

5.1 PROTECTION AGAINST OVER VOLTAGES

It has been seen that the internal causes in increase the voltages of the power system really double to that of the normal operating voltage where as the external causes may increase the voltage several times (of the order of 200 MV) to that of normal operating voltage of twice the value of normal operating voltage of the system for a reasonable length of time and to provide protective devices for the voltage having value more than this.Those devices are known as over voltage protection devices. The common device used for the protection of power system against over–voltages is:

1. Ground wires2. Earth screens3. Lightning arrestors of surge diverters

5.1.1GROUND-WIRE: -

To protect the transmission lines against direct lightning stroke, one of more bare conductors are run at the top f the tower known as ground wires. These wires are earthed at regular intervals preferably at every tower. The area of cross section of ground wires is based upon their mechanical strength rather than electrical conductivity. These should have high mechanical strength and be-non-corrosive.The ground wires not only take the burnt of the direct strokes but also provide a certain amount of electrostatic screening. This reduces the voltage induce in the line conductors by the discharge of a neighboring cloud. They also provide additional protective effect in attenuating any travelling wave that may be set up in the lines, by acting as short circuited secondary of the line conductors.

The main objections to the ground wires are; the additional cost and the possibility of the wire cracking and falling on the line conductors causing a direct short-circuit.

5.1.2EARTHING SCREEN: -

A network of copper conductors earthed at various points, and placed over and above all the substation is known as earthing screen. It provides an electrostatic shield against external fields and protects the system. It protects the system from direct lightning strokes but does not provide any protection against high voltage waves which may still reach at the terminals of equipment.

5.1.3LIGHTNING ARESSTOR OR SURGE DIVERTER: -

The lightning arrestor or surge diverters is a device which an easy conducting path or relatively low impedance path for the flow of current which the system voltage increases more than the designed value and against it is original properties of an insulator at normal voltage .A lightning arrestor voltages as on insulator at normal voltages but provides as easy path for the flow of current at abnormal voltages. A good lightning arrestors or surge diverter should have the following.

(a)It should not take any current on the working voltage of the system in other words it should act as an insulator at normal working voltages.

(b)It must provide a conducting path as and when abnormal transient voltages occur on the system.

(c) It must be capable to carry the discharge current with out getting damage it self under abnormal conditions.

TYPES OF LIGHNING ARRESTORS: -

There are many types of lightning arrestors which are used to protect the power system against over-voltage some of them are:

1. Rod gap arrestor2. Horn gap arrestor3. multi gap arrestor4. Thyrite arrestor5. Electrolytic arrestor6. Oxide film arrestor

7. Expulsion type arrestor8. Value type arrestor

Lightning arresters  are protective devices for limiting surge voltages due to lightning strikes or equipment fault or other events, to prevent damage to equipment and disruption of service. Also called surge arresters.

Lightning arresters are installed on many different pieces of equipment such as power poles and towers, power transformers, circuit breakers, bus structures, and steel superstructures in substations.

VARIOUS OTHER KINDS OF PROTECTION

5.2 DIRECTION OVER-CURRENT PROTECTION: -

The over-current protection can be given directional feature by adding directional over-current protection responds to over currents for a

particular directional flow if power flow is in the opposite directions the directional over current protection remains un-operative.Directional over current protection comprises over current relay and power directional relay in a single relay casing the power directional relay does not measure the power but is arranged to respond to the directional operation of relay is used where the selectivity can be achieved by directional relaying. The directional relay recognizes the direction in which fault occurs relative to the location of the relay. It is set such that it actuates for fault occurring in one directional only. It does not act for faults occurring in the other direction another interesting example of directional protection are that of reverse power protection of generator.

5.3 DIRECTIONAL EARTH-FAULT PROTECTION : -

In the directional over-current protection coil of relay is actuated from secondary current of line CT. where as the current coil by residual current.In directional over-current relays. The voltage coil is actuated by secondary of line VT. In directional earth-fault relay, the voltage coil is actuated by the residual voltage. Direction earth fault relay sense the direction which earth fault occurs with respect to the relay location; and it operates for fault in a particular direction. The directional earth fault relay (single phase unit) has two coils. The polarizing quantity is obtained either from residual current (IRS = Ia + Ib+ Ic) orResidual voltage (VRS = Vae + Vbe + Vce), where Vae Vbe Vce are phase voltage.One of the coils is connected in residual current circuits. This coil gets current during earth faults. The other coil gets residual voltage. The coil connected in potential transform secondary circuit gives a polarizing field.

5.4 PRIMARY AND BACK UP PROTECTION: -

There are times when the primary protection may fail. This could be due to failure of CT/VT or relays, pr failure of circuit breaker one of the possible causes of the circuit breaker failure is the failure of the trip-battery due to inadequate maintenance.

5.5 RELAYS

A relay is a low-powered device used to activate a high-powered device. Relays are used to trigger circuit breakers and other switches in substations and transmission and distribution systems.

The electrical quantities which may change under fault condition are:1. Voltage2. Current3. Frequency4. Phase angle

Through the change in one or more of these quantities, the fault signals there presence type and location to the protective relay is obtained. Moving detect the fault, the relay operates close the trip circuit of the breaker. This result in the opening of the breaker and disconnect the fault section.

TYPES OF RELAY

Basically relay are based on two principal:-

o Electromagnetic attractiono Electromagnetic induction

But different relay based on this are used in this S/S such as:

Differential relay

O/F protection +FFR

Group A trip relay

CB trouble relay

Group B trip relay

DR earth switch relay

O/C protection relays

Breaker failure relay

RES E/F +O/LProtection relay

1. Over Current Relay: - It is used in over current scheme. Over current protection is the name given to protected relay scheme devised to rise in current in a protected circuit.

2. Differential Relay: - A differential relay is one that operates when the vector difference of two or more quantities exceeds pre determined value.

3. Oil Surged Relay4. Buccholtz relay5. Gas operated relay

RELAYS OF 100 MVA AND 20 MVA TRANSFORMERS

o OLTC Buccholtz relayo Main Buccholtz relayo Differential relayo Restrict earth fault relayo Over current relay

FEEDER RELAYS:

o Out of step blocking relayo Directional current relayo Directional earth fault relayo Fuse failure relayo Auxiliary relay type

o Tripping relayo Instantaneous Earth Fault relay

Protection Relays

1. DIFFERENTIAL RELAY: -

A differential relay is “the relay that operates when the vector difference of two or more similar electrical quantities exceeds a pre determined amount.” Almost every type of relay when connected in a certain way can be made to operate as differential relay, mast of the differential relays are of the “current differential type.” Fig.1 shows the over current relay used as “differential relay” and operates when the currents at two points of the system are unusual. For example of the current on at two ends of alternator, windings are unusual. There is either a fault to earth or b/w phases. When there is continuous over current and the current over current and the current on both sides are equal, than the relay will not sense the fault. It will sense fault only if there is a difference of current on two sides of circuit.A very important disadvantage in simple balance system is due to inequalities of current transformers. Hence the differential CT’s should not be erroneous or should be identical.

This disadvantage can also be overcome by using a based beam relay.Fig : Differential Relay

2. DISTANCE RELAYS: -

Distance or impedance relays should have the least position spread in value of operating impedance or reactance. Any deviation of Z from the impedance setting canal bring about variation in the operation zone length of the relay it effects the reliability of the relay operation and venders the co-ordination of the protection on then adjoining circuit much more difficult. Hence for this reason the fictitious operating impedance should not exceed impedance setting.

3. DIRECTIONAL (OVER CURRENT OR EARTH FAULT ) RELAY :-

4.The non directional relay can operate for fault flow in either direction. In order to achieve operation for the fault flowing in a specific direction, it is necessary to add a directional element to the non directional element. Such a relay which responds to fault flow in a particular directional is called a directional relay

5. IDMT RELAY:The IDMT relay work on the induction principle, where an aluminum or copper disc rotates between the poles of electromagnet and damping magnet. The fluxes induce eddy current in the disc which interact and produce rotational torque. The disc rotates to a point where it operates a pair of contact that breaks the circuit and removes the fault condition.

6. RESTRICTED EARTH FAULT PROTECTION RELAY:

The REF protection method is a type of "unit protection" applied to transformers or generators and is more sensitive than the method known as differential protection.

An REF relay works by measuring the actual current flowing to earth from the frame of the unit. If that current exceeds a certain preset maximum value of milliamps (mA) then the relay will trip to cut off the power supply to the unit.

Differential protection can also be used to protect the windings of a transformer by comparing the current in the power supply's neutral wire with the current in the phase wire. If the currents are equal then the differential protection relay will not operate. If there is a current imbalance then the differential protection relay operates.REF protection is applied on transformers in order to detect ground faults on a given winding more sensitively than differential protection.

7. TRIPPING RELAY:

Figure Tripping RelayThis type of relay is in the conjunction with main relay. When main relay sense any fault in the system, it immediately operates the trip relay to disconnect the faulty section

AUXILIARY RELAY:

Auxiliary RelayAn auxiliary relay is used to indicate the fault by glowing bulb alert the employee.

5.6 FUSES :Fuse is a essentially a short piece of metal ( or a fusible material ) inserted in a circuit which melts when a predetermined value of current flows through it and thus breaks the circuits .The protective element of the fuse is a fuse-link inserted in series with the circuit being protected . The most generally material used for fuse element is a low melting point material such as tin, lead or zinc .Fuses may be low voltage type or high voltage type : low voltage can be further divided into two classes namely semi- enclosed rewire able fuse and the cartridge type fuse.

5.7 EARTHING SYSTEM The provision of an earthing system for an electric system is necessary by the following reason.

In the event of over voltage on the system due to lightening discharge or other system fault. These parts of equipment, which are normally dead, as for as voltage, are concerned do not attain dangerously high potential.

In a three phase, circuit the neutral of the system is earthed in order to stabilize the potential of circuit with respect to earth.

The resistance of earthing system is depending on: Shape and material of earth electrode used.

Depth in the soil.

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

PROCEDURE OF EARTHING:Technical consideration the current carrying path should have enough capacity to deal with more faults current. The resistance of earth and current path should be low enough to prevent voltage rise between earth and neutral. The earth electrode must be driven in to the ground to a sufficient depth to as to obtain lower value of earth resistance. To sufficient lowered earth resistance a number of electrodes are inserted in the earth to a depth, they are connected together to form a mesh. The resistance of earth should be for the mesh in generally inserted in the earth at 0.5m depth the several point of mesh then connected to earth electrode or ground conduction. The earth electrode is metal plate copper is used for earth plate.

NEUTRAL EARTHING:Neutral earthing of power transformer all power system operates with grounded neutral. Grounding of neutral offers several advantages the neutral point of generator transformer is connected to earth directly or through a reactance in some cases the neutral point is earthed through an adjustable reactor of reactance matched with the line.

The earth fault protection is based on the method of neutral earthing.

The neutral earthing is associated switchgear.

The neutral earthing is provided for the purpose of protection arcing grounds unbalanced voltages with respect to protection from lightening and for improvement of the system.

An earthed neutral system has the following advantages :-A ) It provides a better protection against earth faults.B ) It ensures nearly constant voltage of healthy phases because

neutral point is not shifted.C ) This system provides a better reliability of service .D ) It is safer for personal and equipment.E ) It requires lesser maintenance expense as compared to the

unearthed neural (isolated system.F ) In the system, transient voltages produced are very small.G ) Ground fault relaying is simple . The earth may be utilized to

operate protective relays to isolate the fault.H ) Persistent arcing ground can be eliminated by employing protective gear.

SAFETY EARTHING :-It is required to provide protection to the operating staff working in the yard and sub station from any injury during fault condition by keeping the voltage gradient within safe limits. The above two parts have common earth mat from which flat iron risers are taken out to connect all the non-current carrying metal parts of the equipment. At the sametime the earth mat conductor rise to voltage, which is equal to the resistance of the earth mat multiplied by ground fault current. This difference of potential results in voltage gradients.

6. CONTROL ROOM

Control panel contain meters, control switches and recorders located in the control building, also called the dog house. These are used to control the substation equipment to send power from one circuit to another or to open or to shut down circuits when needed.

Fig. -Control RoomMEASURING INSTRUMENT USED:

ENERGY METER: To measure the energy transmitted energy meters are fitted to the panel to different feeders the energy transmitted is recorded after one hour regularly for it MWHr, meter is provided.

WATTMETERS: It is attached to each feeder to record the power exported from GSS.

FREQUENCY METER: To measure the frequency at each feeder there is the provision of analog or digital frequency meter.

VOLTMETER: It is provided to measure the phase to phase voltage .It is also available in both the analog and digital frequency meter.

AMETER: It is provided to measure the line current. It is also available in both the forms analog as well as digital.

MAXIMUM DEMAND INDICATOR: There are also mounted the control panel to record the average power over successive predetermined period.

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

7.BATTERY ROOM

Battery is the heart of power system control and protection as all the power system protection equipment and the communication equipments works on D.C supply. In the event of failure of station supply if standby D.C supply is not available then it will be dangerous for the breaker and other protective equipment so also the communication system will be great hampered and during such emergency there will be no communication for help or to transmit information to the concerned authorities and the fault would be attended very late. Thus battery installation, its commissioning and subsequent maintenance plays very important role.

Batteries are to be installed in a room in close vicinity of control room. This room should be constructed in a such a way that it is well ventilated and the dimension of the room should be such that it can easily accommodate the stands provided for supporting desired no. of cells.

There should be adequate provision for artificial lightning and the windows should be located in such a way that direct sunlight on the cells be avoided. Exhaust fan for ventilation of gases, when on quick charge at high rate possible.

Room temperature should be maintained b/w 20 C to 35C for getting best results. Higher temperature reduces the capacity. Battery cells should be arranged on the stands in such a way that each cell can be easily accessed for any maintenance purpose viz., inspection, topping up etc.Battery room should always be kept dry as damp room is dangerous due to possible leakages from the battery.Storage of the battery is the most dependable source of supply of DC power required for closing and tripping of CB , RELAY, signaling equipment, remote control apparatus, telephone service, SCADA, emergency light etc.Battery room is the heart line of D.C. system. In case of failure of the A.C. system the control system should remain operative so we use D.C. control system through DC set.

Maintenance of Battery

For effective and trouble free services of station batteries following maintenance activities are suggested:-

o Battery Room and Ventilationo Herein battery rooms door are kept closed, exhaust fan checked for

air circulation, metal structures checked for corrosion and painted if necessary.

o Base or Rackso Wooden racks checked for cracks and deterioration, base pads for

deterioration. o Cells and Jarso Leaky jars checked for cracks replaced if necessary, clean jars-wash

covers are wiped out. Plates inspected for signs of deterioration.o Intercell Connectors and Terminalso Terminals cleaned for corrosion and sulphation.o Chargeo Output of charging equipment is adjusted for normal conditioning of

battery, ampere meter should show as fraction of ampere.o Annual Maintenanceo Voltage of each cell which should be b/w 2.15 to 2.2 V per cell

during trickle charge is checked.

o Electrolyteo Electrolyte level and add distilled water as it is necessary, specific

gravity and electrolyte is checked. Keep the distilled water container and keep some storage of distilled water always ready for topping.

INITIAL SP. GRAVITY

FINAL SP. GRAVITY

ACID QUANTITY WATER QUANITY

1· 840 1· 190 18 871· 825 1· 400 40 661· 825 1· 190 18 861· 400 1· 190 45 56

TECHNICAL PARTICULARS:

1. A.C. input 415 v +10% three phase 50 c/s2. No. of cells 1103. DC output 110 cells while supplying a load of 18 amp.

(a) float charger capable of floating cells of 2.65v per cell(b) Boost charge 220 v load at a max. Of charging current of 20 amp.

8.Power line communication & SCADA system

Delhi Transco Limited (DTL) has a very large network of high voltage transmission lines in whole Delhi. Transmission lines transfer power from power houses to substations and from one substation to many other substations or vice versa. Power is generated at low Voltage (of the order of 3.3KV to 25KV) and is stepped-up to high voltage (765KV, 400KV, 220KV & 132KV) for evacuating power into the grid network through transmission lines.

Transmission of Data

Below in Figure 1, main equipment from substation/power house to its subLDC has been shown in a very simple form.

Figure 1: Transmission of Data from substation/Power house to subLDC

Current Transformers (CTs) and Potential Transformers (PTs), installed on transmission lines, provide inputs to transducers of SIC (Supervisory Interface & Control) & RTU (Remote Terminal Unit) panel. Circuit breakers & isolators' status are extended up to SIC panel. If for such extension extra potential free contacts are not available in the Control Panels, Contact Multiplying Relays (CMRs) are used to provide potential free contacts. The output of RTU is connected to the communication equipment, through Modem. In between substation & subLDC, a communication link has been shown. Telephone exchanges are connected with the communication equipment. Such communication links can be of any type. DTL has got its own three different type of communication systems, i.e. PLCC (Power Line Carrier Communication), microwave and fibre-optic. Modem output at receive side is connected with the CFE (Communication End Frame). Its output is connected with data takes over. Each RTU is automatically polled by Server of Sub LDC to obtain each data of repeats at least once in 10 sec and is stored in the database of sub LDC. This data is processed in database formats and is retrieved for different applications. These formats or graphics are displayed or printed as per requirement. At sub LDC, System Control Officers use this data to monitor and analyze position of the grid.

Communication for Power System

Following are mainly three inter-related areas of functions in DTL for management of power system:

A) Telecommunication

B) SCADA- Supervisory Control and Data Acquisition System.

C) EMS- Energy Management System

A) TELECOMMUNICATION

There are three different types of telecommunication systems in UPPTCL i.e.

i. Microwave Communication System, ii. Fibre-optic Communication System,

iii. PLCC-Power Line Carrier Communication.

Voice Frequency (VF) channels of all these systems have been integrated/interconnected to make a hybrid communication system. Microwave & Fibre Optic are multi-channels communication systems and are also called 'Wideband communication system'. PLCC is single channel communication system.

SCADA SYSTEM

In SCADA system measured values, i.e. analogue (measured value) data (MW, MVAR, V, Hz Transformer tap position), and Open/Closed status information, i.e. digital data (Circuit Breakers/Isolators position i.e. on/off status), are transmitted through telecommunication channels to respective sub-LDCs. For this purpose Remote Terminal Units (RTUs) at 400KV, 220KV sub-stations have been installed. System values & status information below 66 KV have not been picked up for data transmission, except for 33KV Bus isolator position and LV side of generators. Secondary side of Current Transformers (CT) and Potential Transformer (PT) are connected with 'Transducers'. The output of transducers is available in dc current form (in the range of 4mA to 20mA). Analogue to digital converter converts this current into binary pulses. Different inputs are interleaved in a sequential form and are fed into the CPU of the RTU. The output of RTU, containing information in the form of digital pulses, is sent to subLDC  through communication links. Depending upon the type of communication link, the output of RTU is connected, directly or through Modem, with the communication equipment. At subLDC end, data received from RTU is fed into the data servers. In general, a SCADA system consists of a database, displays and supporting programmes. In DTL, subLDCs use all major functional areas of SCADA except the 'Supervisory Control/Command' function. The brief overview of major 'functional areas' of SCADA system is as below:

1. Communications - Sub-LDC's computer communicates with all RTU stations under its control, through a communication system. RTU polling, message formatting, polynomial checking and message retransmission on failure are the activities of 'Communications' functional area.

2. Data Processing - After receipt of data through communication system it is processed. Data process function has three sub-functions i.e. (i) Measurements, (ii) Counters and (iii) Indications.

'Measurements' retrieved from a RTU are converted to engineering units and linearised, if necessary. The measurement are then placed in database and are checked against various limits which if exceeded generate high or low limit alarms.

The system has been set-up to collect 'Counters' at regular intervals: typically 5 or 10 minutes. At the end of the hour the units is transferred into appropriate hour slot in a 24-hour archive/history.

'Indications' are associated with status changes and protection. For those statuses that are not classified as 'alarms', logs the change on the appropriate printer and also enter it into a cyclic event list. For those statuses, which are defined as an 'alarms' and the indication goes into alarm, an entry is made into the appropriate alarm list, as well as in the event list and an audible alarm is generated in the sub-LDC.

3. Alarm/Event Logging - The alarm and event logging facilities are used by SCADA data processing system. Alarms are grouped into different categories and are given different priorities. Quality codes are assigned to the recently received data for any 'limit violation' and 'status changes'. Alarms are acknowledged from single line diagram (or alarm lists) on display terminal in LDCs.

4. Manual Entry - There is a  provision of manual entry of measured values, counters and indications for the important sub-station/powerhouse, which are uncovered by an RTU or some problem is going on in its RTU, equipment, communication path, etc.

5. Averaging of Measured Values - As an option, the SCADA system supports averaging of all analogue measurements. Typically, the averaging of measured values over a period of 15 minutes is stored to provide 24 hours trend.

6. Historical Data Recording (HDR) - The HDR, i.e. 'archive', subsystem maintains a history of selected system parameters over a period of time. These are sampled at a pre-selected interval and are placed in historical database. At the end of the day, the data is saved for later analysis and for report generation.

7. Interactive Database Generation - Facilities have been provided in such a way that an off-line copy of the SCADA database can be modified allowing the addition of new RTUs, pickup points and communication channels.

8. Supervisory Control/Remote Command - This function enables the issue of 'remote control' commands to the sub-station/powerhouse equipment e.g. circuit breaker trip command. As such, related/associated equipment have not been ordered.

9. Fail-over - A 'Fail-over' subsystem is also provided to secure and maintain a database of devices and their backups. The state of the device is maintained indicating whether it is 'on-line' or 'failed'. There is a 'backup' system, which maintains database on a backup computer and the system is duplicated.

SLDC Minto road has a large and active 'Mimic Board' in its Control room. This mimic board displays single line diagram of intra State transmission system i.e. grid network of 400KV, 220KV and important 66 KV sub-stations, transmission lines, thermal powerhouses. Outgoing feeders, shown in the mimic board, have 'achieve' (LED display) colored indications, of three different colors, to show the range of power flow at any moment i.e. 'Normal', 'Nominal' or 'Maximum' of its line capacity. For new substations and lines, displays in active and passive forms are required to be made in the Mimic diagram. But, Mimic Board has a limitation that it cannot incorporate/add large volume of displays for substations/power houses/transmission lines in 'active' form due to space constraint and congestion. Due to this Mimic Board is going to be supplemented with a Video Projection System (VPS) at SLDC, Minto Road in near future. Also in SLDC & subLDCs, displays of single line diagrams of RTU sub-stations/power house are viewed on VDUs of large size (21").

9.Operation and Maintenance of 220KV Substation

9.1. INTRODUCTION

Maintenance is a key activity for utilities in order to assure the proper operation of the networks. And it implies a huge amount of human and economic resources. Saving Maintenance costs means that it is needed to proper operation of substation equipments. The availability of reliable and quality power has made the job of substation more important. This can be achieved by establishing the new substation, with most efficient and reliable equipments and taking more care in their operation and maintenance. Maintenance may be defined as the upkeep of the substation electrical equipment in proper working and efficient condition to derive the Reliable and efficient operation, Optimum utilization Availability of quality power, reduced down time, Detection of premature faults, Minimizing revenue losses etc. To meet the above requirement, the equipment has to be checked, attended to, trouble shoots and operated under specified conditions. A large percentage of failure of electrical equipment are due to deterioration of insulation, loose contact, abnormal operating condition etc. many of these failure can be anticipated by regular application of simple tests and timely maintenance . If the fault condition leading to failure is detected in the early stage itself, the extent of damage can be reduced and the equipment can be reconditioned and put back in to service. Any abnormality will be followed by warming signal like variation in sound, excess temperature, vibration, sparks, blown out fuses, frequent trappings, tripping before full load. The detection of incipient faults in electrical equipment depends up on use of proper diagnostic tools, its effective use, correlation and proper interpretation of test results and observation based on experience, manufacturers guidance etc.

9.2. MAINTENANCE ACTIVITY

1. Corrective or breakdown maintenance: Corrective or breakdown maintenance is carried out as and when necessary. This applies only to low value and auxiliary equipments, breakdown of which does not affect power supply continuity.

2. Preventive maintenance: Preventive maintenance calls for advance plan is made to carry out preventive maintenance. The advantage lies in uninterrupted power supply, increased availability of the equipment and reduction in maintenance cost.

3. Condition based maintenance: Condition base maintenance is based on condition assessment of the equipment by tests ON or OFF the line. This is ideal for prevention of equipment failure and other associated consequent damages.

4. Reliability centered maintenance: Reliability centered maintenance is generally carried out on old equipment by conducting ‘ remaining life assessment studies’ and based on economics, life extension techniques are adopted without sacrificing reliability and availability. The maintenance costs are also reduced.

5. Equipment failure analyses: Equipment failure analysis is the major responsibility of maintenance personnel to prevent repeated failure of equipment and provide inputs foe necessary change in design parameters, new equipment design, quality control plan, erection and subsequent maintenance technique.

6. Techniques for reducing down time: Techniques of reducing down time play a vital role in continuity of power supply. Hot line maintenance of one line of double circuit\ line with other circuit in live condition, deployment of emergency restoration system etc, is few examples.7. Spare management: Spares management ensures availability of right spares most frequently required and at the right location and thereby help immediate restoration of power supply.

8. Documentation/ computerization on maintenance: The documentation is a record of the type of maintenance activity carried out, any abnormalities noticed during checking etc, chronologically documented and computerized for further analysis and action.

9.3. MAINTENANCE SCHEDULE

Maintenance schedule is categorized into daily, weekly, monthly, quarterly and yearly maintenance schedules.

Power transformer (100 MVA, 20MVA) Check and re-condition of silica jelly. Check the working cooling fans, pumps Release gas from BH relay Clean the bushing, radiator, body etc., Check earthing connection Check jump connections

Check OLTC motor drive and control panela) Lubricate bearing and cleaningb) Check the gear box oil levelc) Check operation of limit switch, sequence switch with

transformer in off conditionsd) Check gasket joints for oil leakage

HT Circuit breakers Clean the porcelains Check the connections for loose contact Check tripping through relays Check the wiring for loose contact Vermin Proofing of control box Check annunciation scheme Lubricate moving/link mechanisms wherever recommended

by manufacturer

HT: CT’s, PT’s & Lightning arresters. Clean the porcelain and metal body Check connection both primary and secondary for tightness Check oil level Take IR values Check earth connection for proper contact

11 kV Switchgears Clean the breakers, panels and bus bars thoroughly, clean

insulators with CTC or Petrol Check II values of the bus bars and individuals, breakers

between phases and earth Check operation of breakers on local remote through relay

and corresponding annunciation Check and lubricate operating mechanism wherever

necessary Tighten the terminal connection of all auxiliary circuit and

wiring Check all earth connections between the panel and electrodes

for tightness and check the contact resistance of earth connection.

Check contact travel, contact erosion in VCB Check 11 kV CT’s and PT’s connections

Isolator Check jump connection and replace PG clamps, if necessary

Check the alignment of isolator Cleaning and applying petroleum jelly to contacts

GOS-HR fuses –Station yard Earthing

Check clean and grease the GOS and check contacts for erosion, clean insulator

Check operation for proper closing of the insulator Check the fuses and renew the same wherever HR fuses are

provided Check the earth resistance of earthing mat and all individual

earthing if any, the resistance should be within the prescribed limit, otherwise action should be taken to Bering the same to within limit immediately as it is very important aspect for the safety of the equipment in any station.

TABLE 9.4.1: Maintenance Schedule for Oil-Filled Power Transformers

Maintenance or Test Recommended Interval

Review equipment ratings 5 years

Preventive maintenance As Per manufacturer’srecommendations

Transformer physical inspection Annually

Bushings – visual inspection Quarterly and 3-5 years

Bushings - check oil level Weekly

Bushings – cleaning 3-5 years

Transformer and bushings -Doble test

3-5 years (6 months to 1 year forsuspect bushings)

Transformer and bushings–infrared scan

Annually

Insulating oil - DGA, physical, Annually after first year of

and chemical tests operation

Leakage reactance, Turns Ratiotests, SFRA test

If problems are indicated by othertests

Cooling fans – inspect and test Annually

Oil pumps and motors - inspectand test

Annually

Heat exchangers – inspect Annually

Conservator and bladder - inspect 3-5 years

Top oil and windingthermometers

Annually inspect and infrared scan3-5 years calibrate

Oil level indicator operation 3-5 years

Pressure relief device Annually inspect and performfunction test 3-5 years check oilleaks

Sudden pressure relay Annually inspect and performfunction test 3-5 years test permanufacturer’s recommendations

Buchholz relayInspect foundation, rails, trucksInspect foundation, rails, trucks

Annually inspect and performfunction test3-5 years

TABLE 9.4.2: Maintenance Schedule of SF6 Breaker

Maintenance or Test Recommended Interval

Review equipment rating 5 years

Preventive maintenance Per manufacturer’s instructionmanuals

Record gas pressure and Monthly

temperature, compare withtolerances

Record operations counter Monthly

Visual inspection Monthly, annually,5 years

Check foundation, grounds, paint 5 years

Check external screws, bolts,electrical terminals tight

Annually

Contact resistance test, powerfactor insulation test, motionanalyzer, trip test, moisture test ongas

5 years, if required bymanufacturer

Verify operation and calibration oftemperature and pressure switchesand gauges

5 years

Check lube points, heateroperation, tightness of terminals,linkages screws, bolts; latch,linkage, operating mechanismadjustments

5 years

Overhaul breaker with new seals,contacts, nozzles

5 years10 to 15 years or 4,000 to 10,000operations (more frequent if highcurrent operation)

Overhaul disconnect, grounding,and breaking switches

15 years or 5,000 to 10,000operations

Gas cart maintenance Per manufacturer’s instructionmanuals

TABLE 9.4.3: Maintenance Schedule for Relays and Protection Circuits

Maintenance or Test Recommended Interval

Fault/load study and recalculatesettings

5 years

Electro-mechanical relaysCalibration and functional testing

Upon commissioningyears

Solid-state relays Calibration andfunctional testing

Upon commissioning 1 year aftercommissioning and every 3 years

Microprocessor relays calibrationand functional testing

Upon commissioning 1 year aftercommissioning and every 8-10years

Protection circuit functional test,including lockout relays

Immediately uponand/or upon any changes and every 3-6 years

Check red light lit for lockoutrelay and circuit breaker coilcontinuity

Daily

Lockout relays Cleaning andlubrication

5Years

TABLE 9.4.4: Maintenance Schedule for Arresters

Maintenance or Test RecommendedInterval

Review equipment rating 5 years

Visual inspection with binoculars Quarterly to semiannually

Clean insulator and checkconnections3-6 years Ambient dependent

3-6 years Ambient dependent

Doble test (power frequency 3-6 years Ambient dependent

dielectric loss, direct current [DC]insulation resistance, power factor)

Replace all silicon carbide arresterswith metal oxide varistor type

As soon as possible

Infrared scan Annually

TABLE.9.4.5: Maintenance Schedule for Transmission LinesMaintenance or Test Recommended Interval

Review equipment ratings 5 years

Visual inspection with binoculars Semi -annually

Infrared scan Annually

9.5 THERMO SCANNING

   A sub station having worth crores of rupees can be scanned in two days time for which charges for scanning comes around Rs. 30,000. By thermo scanning any incipient fault can be identified in its initial stages if thermo scanning is done on regular interval.  Thus damage of equipment worth of crores of rupees can be avoided and also this technique prevents disruptions of power to Customers in case of damage of equipment.

This is done with thermo vision camera based on FLIR system.Thermo graphic Inspection:During the thermo-visual inspection of sub-station equipment, several hot spots are noticed and these spots are due to loose joints. The temperature difference between the hot spots and normal spot is reported and this aspect indicates the severity of the fault. Four types of fault are graded from zero to three indicating normalcy to sever fault.

GENERAL SAFETY PRECAUTION REQUIRED

(a)Don’t wear loose garments; they get caught leading to

accidents.

(b)Long and unruly hairs are dangerous particularly when

working near revolving part.

(c) Do not smoke near prohibited area.

(d)Keep the work area clean, dry and free of obstructions.

(e)Do not touch or operate equipment unless are authorized

so.

(f) Lubricate the M/C part with both hands. Use cotton waste

brush etc.

(g)Ensure all guards in position before M/C working on job.

(h)Ensure all machines control of the machine is in your

access.

(i) Ensure all tools are in good conditions. Look and report any

accident hazard.

(j) For any injury whether small or big get first aid first.

CONCLUSION

It has been really a knowledgeable experience pursuing training at DTL, 220 KV Sarita Vihar sub-station. It is beyond doubt; DTL is not only an industry in itself but also offers vocational training to engineering graduates as well as professionals.

This phase of practical training has proved to be quiet fruitful, beneficial in every respect. It provided an opportunity to encounter big and sophisticated equipments of the Sub-Station.

The architecture of the Sub-Station and the way various equipments are linked together to work as a unit and methodological approach in working of whole s/s is controlled renders the impression that engineering is not just learning the structured description and working of various equipments, but greater part is of planning proper management.It was definitely a knowledgeable experience and pride to be a part of 220 kv Sarita Vihar s/s for such a short period of time.No doubt it showed that mere theoretical and bookish knowledge need to be supplemented with able practice knowledge. And this opportunity to gain practical knowledge, imparted by very able personals of DTL at Srita Vihar, New Delhi was a learning experience.