ACKNOWLEDGEMENT

In the prescribed syllabus of Electrical Engineering there is a

provision of practical training of six weeks after completion of

sixth semester in summer.

I ROHIT KUMAR have undertaken practical training at Najafgarh

220KV Substation, New Delhi, under the supervision and

guidance of Er. Ram Singh (Assistant Manager Technical,

Najafgarh, New Delhi), and Er. Sri Krishan Kumar (JE Najafgarh

New Delhi). I am thankful to all the employees of this sub-

station who helped me to gain the practical knowledge, and

answered my queries to me to best my satisfaction. I feel

obliged by the gaining knowledge under the esteemed

guidance of able personals at DTL.

During the training from 04/06/2009 to 17/07/2009 I have

prepared this report of practical training, which gives insight

information about instruments and apparatus used in this sub

station and their working in brief.

I can say that this report is a summary of what I observed and

learnt there.

Signature (Er. Ram Singh) Signature (Er. Krishna Kumar)

BRIEF HISTORY OF POWER SECTOR IN DELHI

 

    1. Introduction: - Electricity plays a vital role in our day-to-day life.  It powers our houses, industries, hospitals and in fact our entire economy.  Historically speaking the modern electricity industry utility system was first introduced to the world on the opening of Thomas Edison’s Pearl Street Electricity Generating Station on September 4th, 1882 at New York (United States of America).  Insofar as Delhi is concerned, the position is that as per available records, the first diesel Power Station was established in Delhi in the year 1905 when private English Company by name M/s.  John Fleming was given permission to generate electricity under the provisions of the Indian Electricity Act 1903.  The above mentioned Company was given the responsibility both of generation and distribution of power in a limited manner.  That Company after obtaining license under the provisions of Electricity Act 1903 had set up a small 2 MW Diesel set at Lahori Gate in Old Delhi.  Later on, this very Company was converted as Delhi Electricity Supply and Traction Company.   In the Year 1911, the power generation was augmented by Steam Generation Station.  In the year 1932, the management of Central Power House was handed over to New Delhi Municipal Committee (NDMC).  In the field of power generation and distribution, a major break through was achieved in 1939 when Delhi Central Electricity Power Authority (DCEPA) was established.  This Company was responsible for the supply of power to the areas covered by Local Bodies, namely, the Municipal Committees of Delhi, West Delhi and South Delhi, the Notified Area Committees of Red fort, Civil Lines, Mehrauli, Najaf Garh, and the District Board of Delhi.  The supply of electricity to the Municipal Committees of Delhi-Shahdara and the Notified Area of Narela was done by different private agencies.  In 1947 DCEPA took over a Private Limited Company by name Delhi electric Supply & traction Company Limited.       

     2.  Promulgation of Electricity (Supply)Act 1948:-  In the year 1948,  electricity (Supply) Act 1948 came into force, which inter-alia provided  for the constitution of an electricity Board in the States that was to function as a vertically integrated electricity utility in the entire

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State, undertaking all the functions of activities related to electricity, which included electricity generation, transmission, distribution, supply,  planning coordination and also was to act as regulatory authority for carrying out other functions incidental and ancillary thereto.  In other words, the Electricity (Supply) Act 1948 was entitled to become a monopolistic undertaking in the field of electricity control by an instrument of the state and not by private sector.  The principal objective behind the above policy decision of the Government of India in providing for the constitution of State electricity to all, particularly in semi-urban and rural areas because till then the availability of electricity was confined to urban areas and was mainly served by private electricity distribution licenses issued under the Indian electricity Act 1910.

      3.   Formation of Delhi State Electricity Board: - In pursuance of the provisions of the Electricity (Supply) Act, 1948, in Delhi, in the year 1951 the Delhi State Electricity Board (DSEB) came into existence and the responsibility of generation and distribution of electricity was taken over by DSEB from DCEPA.  The entire staff of DCEPA and other agencies was absorbed by DSEB under the existing terms & conditions of service.

      4.   Notification of Industrial Policy Resolution:-   In the year 1952 the Government of India notified the Industrial Policy Resolution under the Industries Development and Regulation Act 1951 where under the electricity industry, which included all aspects of generation, transmission, distribution, and supply of electricity, came to be reserved for State sector.  In other words, the private sector was not entitled to commence any business of generation, transmission, distribution, and (or) supply of electricity.

      5.   Formation of Delhi Electric Supply Undertaking by promulgation of DMC Act 1957:-   After the promulgation of the Delhi Municipal Corporation Act 1957, the DSEB was dissolved and the functions of DSEB were taken over by Delhi Electric Supply Undertaking (DESU), which came into existence in 1958.  After the formation DESU, the generation and distribution of electricity to all the areas of Delhi came under DESU and the employees of erstwhile DSEB were also absorbed by DESU.

      6.   Constitution of Delhi Vidyut Board: -   The Government of the National Capital Territory of Delhi vides notification no.  F.11 (10)/92-LSG /PF (II) dated 24.02.1997, issued under the Electricity (Supply) Act, 1948, constituted a separate Electricity Board, i.e. the Delhi Vidyut Board (DVB) for the NCT of Delhi w.e.f. 24.02.1997 for the purpose of generation and distribution of power to the entire area of NCT of Delhi

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except the areas falling within the jurisdiction of NDMC and Delhi Cantonment Board.

      7.   Practical difficulties in the working of Delhi Vidyut Board:-   The activities of Delhi Vidyut Board from its inception, and as a matter of fact even prior thereto when the activities were being undertaken by DESU, were not financially viable on account of several factors affecting the electricity industry including the high level of losses in the system and the revenues being not able to meet the cost with result that like other State electricity Boards, Delhi vidyut Board suffered operating deficit in aggregate to the tune of Rs.2,386.72 crore during the period from 1995-96 to 2000-01.   In addition the Delhi Vidyut Board was required to make adequate provision for bad and doubtful debts.  The cumulative effect of all these factors was that the Delhi Vidyut Board was not in a position to meet its financial obligations and commitments including the payment for power purchased from generation companies  and suppliers, such NTPC Limited,  Nuclear Power Corporation Limited, national Hydroelectric Corporation Limited, etc., etc.

     8.   Unbundling of Delhi Vidyut Board in six entities: -   In the recent for alleviating the concerns of consumers in the power sector, some reforms started gaining momentum.  In that very direction with a view to safeguard the overall interests of the consumers GNCTD took some policy initiatives as as a result of which DVB was split into six Companies, viz.,  BSES Rajdhani Power Limited, BSES Yamuna Power Limited, North Delhi Power Limited, Delhi  Transco Limited,  Indraprastha Power Generation Company Limited, and Delhi Power Company Limited, as per the provisions contained in Delhi electricity reform Act 2000 read with Delhi Electricity Reform (Transfer Scheme) Rules 2001.

9.   Growth in demand of electricity: -   Thus, starting the humble origin, i.e., Private Limited Company having a few employees with primitive generation process, the generation, transmission, and distribution of power to the citizens of Delhi has now come in the hands of above mentioned six Companies with an employee strength which has grown over the years from a meager figure of few hundred to about 20,000.  Prior to 1951, the demand of power in Delhi was about 27 MW which now has grown to about 4,000 MW. Availability of reliable and cheap power is absolutely essential for economic development of any developing society and consumption of electricity is an important indicator of the stage of development of agriculture, industry and commerce.  With the growth of population, industries, importance of Delhi being the national Capital and with the advancement of technology, life style and increased use of new

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electrical & electronic gadgets, the demand of power has gone up enormously.

     10. Present Scenario: -    The role of Delhi Transco Limited is confined to arrange and provide transmission network of 400 KV and 220 KV source from Northern Grid.  The present infrastructure for this purpose under 400 KV systems is 4,725 MVA (2520 MVA with DTL and 2205 with Power Grid Corporation).  As against this, 220 KV sub Stations have the capacity of 6,300 MVA is available for Delhi.

     11. Future Plans: -     In the 11th Plan ending 2011-12 the transmission capacity is proposed to be augmented to meet the future requirements.  Under 400 KV systems, it is proposed to establish new Sub Stations at Mundka, South-East Delhi near Mandi village and East Loni Road with a capacity of 630 MVA each by DTL and also increase the capacity of existing sub-Station at Maharani Bagh by 630 MVA b Power Grid Corporation of India Limited. Similarly, under 220 KV systems, augmentation and new addition in capacity to the tune of 1660 MVA under the existing Sub Stations is proposed.  Further, new Sub Station at DSIDC Bawana-II (320MVA), Chandrawal (200 MVA), Jhatikara More (320 MVA),.  Ridge Valley  (320 MVA),   Rohini-II  (480 MVA),  Sultanpuri  (320 MVA), Electric lane (200 MVA),  Trauma Centre (200 MVA),  Wazirpur Industrial Area  (320  MVA) and IGI Airport  (320 MVA ) are proposed to be established.  Thus, the capacity of 2520 MVA and 5940 MVA will be added in the 400 KV system and 220 KV system, respectively.

 

      To sum up, by 2011-12 transformation capacity of 8460 MVA will be added and a total capacity of 19485 MVA will be available to Delhi

 

 

 

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CONTENTS

Chapter's Name Page NumberDTL an Overview 7Substation 10Classification of S/S 11Equipments relating to S/S 14Single line diagram of 220 KV Najafgarh

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220/66/33/11 Kv. Najafgarh S/S at glance

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Transmission Line 25Bus Bar 26Transformers 28Instrument Transformer 43Isolator 50Circuit Breaker 51Protection 54Relay 60Control Panel 62Battery Room 63Clearances in the line 65Capacitor Bank 66SCADA 67Thermal Scanning 69Safety Precaution 71Conclusion 72

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DELHI TRANSCO LIMITED AN OVERVIEW

Delhi Transco Limited is responsible for 400/220 KV Transmission System in Delhi besides providing bulk power supply to the Distribution licenses. After unwinding of DVB, DTL’s responsibilities have changed radically. Now it is endeavoring to deliver quality bulk Power as well maintain transmission system in a reliable & efficient manner to the expectation of the stake holder in an optimal way.The peak demand in Delhi in the past has been increasing by 6 to 8 % annually. As a result the peak demand met in Delhi by DTL has already touched 3626 MW during June 2006.Delhi Transco Limited, a successor company of erstwhile Delhi Vidyut Board, came into existence on 1st July 2002, as a state transmission utility of the national capital of Delhi. The objective behind the creation was to upgrade the transmission management in order to maintain an efficient and effective grid network for transmission of power in Delhi in a cost effective manner with due social concern. Apart from this DTL has also been designated as the nodal agency for implementation of activities for energy conservation. Over the years DTL has evolved as a most dynamic performer, keeping pace with the many – fold challenges that confront the ever-increasing demand – supply situation and achieving functional superiority on all fronts. The transmission losses were brought down from 3.84% in 2002-03 to 0.72% in 2005-06.

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Delhi being capital and the hub of commercial activities in the northern region, the load requirement has been growing at a much faster pace. Added to which, 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 vision 2021, aiming to make Delhi a global metro politic and world-class city demands greater infrastructure to enrich many service sector. Understanding the fact that the power is the main component in infrastructure development, DTL has been responsibly playing its role in establishing, upgrading operating and maintaining the EHV (extra high voltage) network and arranging bulk power supply to distribution licensees. DTL has also been assigned the responsibility of running the SLDC (State load dispatch centre), which is an apex body to ensure integrated operation of power system in Delhi. DTL has been playing a vital role in encouraging energy conservation. Thus also ensuring saving of power and smooth loan management in this regard. The Energy Efficiency & Renewable Energy management has been established at DTL.

Existing Transmission Network

The existing network of DTL consists of a 400KV ring around the periphery of Delhi interlinked with the 220 KV network spread all over the city. The salient features of the network: -

Parameters 400Kv 220Kv

No. of Sub stations 2 22

Transformation Capacity

2205 6300

Transmission lines length

227 556

Power Purchase:

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DTL purchase power from various sources in order to meet the power requirement of licensees. The main sources of procurement are:

o IPGCL (Indraprastha Power Generation Corporation Limited) owns

1) Indraprastha Power Ltd.

2) Rajghat power house

3) Gas Turbine Power House

o Pragati Power Corp. Ltd

o BTPS of NTPC Ltd.

o Central Generating companies, NTPC Ltd.

o Through bilateral arrangements with generating stations.

Power Selling

DTL has five licensees for distribution of power in Delhi:

o BSES Rajdhani Power Ltd.(For south and west Delhi)

o BSES Yamuna Ltd.(For east and central Delhi)

o New Delhi Municipal Council (For New Delhi Area)

o MES (For Cantonment Board Area)

DTL Objectives

o Arrange sufficient power to meet the growing needs of the NCT of Delhi.

o Increase load reception capacity at 400 KV and 220 KV grid sub-stations.

o Establish new 400 KV and 220 KV and 220 KV grid sub-station. o Maintain and upgrade existing Sub- Station & Training lines.

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o Establish transparent and responsive interface with Licensees, Suppliers and Employees.

o Regularly trained employees for up gradation of their skills.o Quality assumes through in house Quality Assure System

New Projects

1. 400/220 KV Sub-Stations1) Maharani Bagh2) Mundka3) 4th 315 MVA Power Transformers at Bamnauli S/S.4) ETC of 220 KV S/S DSIDC Bawana5) ETC of 220 KV S/S MASJIDH MOTH6) ETC of 8 NO.S 220 KV GIS BAYS at Maharani Bagh S/S

SUB-STATION

A substation is precisely defined as an assembly of apparatus installed to control transmission and distribution of electric power.A substation is an intermediate link between the generating station and consumer. It may be defined as the assembly of apparatus, which transfers the characteristics of electrical energy from one, form to another for example one voltage to another. This sub-station receives power from incoming lines from generating plant. The electrical energy is generated at low voltage link 6.6 kV or 11 kV, through higher voltage to 33 kV are also possible due to economic consideration low voltage is converted to high voltage like 220 kV 400 kv for transmission purpose.This can be done with the help of transformer.The consumer apparatus are made up of low voltage, so this voltage is again to be stepped down to the required voltage at substation. There may be two or three voltage levels in sub-stations depending upon incoming and out going line voltage level.

The electrical substation design is influenced by following aspects:

1. Rated voltage of incoming and outgoing lines.

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2. Total MVA to be transferred

3. Geographical area available

4. Step up and step down

5. Switching substation

6. Receiving substation

7. Distribution substation

8. Industrial substation

Difference b/w GRID and GRID SUBSTATION

GRID is a technical word used for the interconnection of power received from more than one place. It is a network of main lines for distribution of electricity.

GRID SUBSTATION is a substation where power from more than one place is interconnected through equipments.

CLASSIFICATION OF SUBSTATION

The classification of sub-station is based on several aspects:-

1. CLASSIFICATION BASED ON VOLTAGE LEVEL :

A sub-station is named in accordance with its higher voltage level i.e. a 220 kV sub-station has higher voltage level of 220 kv standard rated voltages in power frequency phase to phase A.C. voltage. There is generally two or more voltage level in sub- station .the sub-station is designated after higher voltage level i.e. a 220kv buses besides say 66 kv, 33 kv , 11 kv , buses.

The bus bars are either in two or three horizontal planes so as to permit proper connection and clearances. Three level sub- stations are more compact and complex.

Conventional open terminals sub-station is very common at all voltages above 11 kV.

2. CLASSIFICATION BASED ON APPLICATION

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A substation can be classified on the basis of functional requirement related with applications. Some of types include:-

1) Substation in generating stations, receiving

Stations, Distribution system.

2) Factory substation.

3) A.C. / D.C. conversion substations.

4) Sub-Station for load centre.

3. CLASSIFICATION BASED ON PHYSICAL LOCATION

A substation can be classified on the basis of orientation of its equipments and physical location as under:-

A. Outdoor substation.B. Indoor substation.

A. OUTDOOR SUB-STATION: -

In this type of sub-station, the substation equipments are installed in open yard and hence the name outdoor substation.

a)TRANSMISSION SUB-STATION:-

Such substations are those which transmit electrical energy and above including 400KV. Transmission substations have a different design than distribution on pole mounted substations which supply electrical energy at relatively low voltage. The transmission substations are designed to receive and transmit large blocks of electrical energy amounting to several hundred thousand KV. Such substations are generally installed outside the city. The equipment

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required for such substations such as high KV transformers bus bars for supporting conductor for taking the connections, supporting structure for CTs and CVTs, circuit breakers, Isolators and isolator with switches. Usually outdoor type substations are used for primary and secondary transmission purpose. Those substations are cheaper than indoor type substations. The rating of transformer used in such substations varies from 10 MVA to 315 MVA. The cooling generally used is of natural cooling type up to 10MVA.

ADVANTAGES OF OUTDOOR SUBSTATION

1. The fault location is easier since all the equipment is with in view.

2. The extension of the installation is easier.3. The time required for the erection of substation is less.4. Less amount of building material is required.5. The construction work required is switchgear installed is low.

B. INDOOR SUB-STATION: -

In these substations the electrical equipment is installed with in the building of substation. Indoor substation are usually for a voltage up to 11KV but can be erected 66KV volts when the surrounding atmosphere is contaminated with impurities , such as metal corroding gases and funs , conductive dust etc. The major factor in the design of indoor substation is the minimization of the fire risk; section align by fire resisting wall is usually necessary and fire extinguishing apparatus must be installed carbon dioxide is a very effective fire extinguishing medium and causes no damage to sound equipment the gas is stored in cylinder and released automatically by fusible plugs. This type of substation is having two transformers say each of 500KV. The usual primary voltage is 11KV and the secondary is 400/220 volts. On primary side the switch gear, which will be installed, consist of oil current breakers only. The supply is given to the primary side of the transformer and the secondary is connected to low voltage bus bars. Several feeders connecting the large consumers of electricity on the distribution system of a particular area leave from the bus bars through the enrolling equipment. The panel for each feeder consists of a switch isolator and a circuit breaker.

ADVANTAGE OF INDOOR SUBSTATION

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1. When fault occurs on any section of the bus that section can be isolated without affecting the supply from other section.

2. Repair and maintenance of any section of bus bar can be carried out by de-energizing that section only the possibility of complete shut down.

FUNCTION OF A SUB-STATION: -

A substation may be required to perform one or more of the following function.

o To switch on or off power lines the operation is known as switching operation.

o To raise the lower voltage, the operation is known as voltage transformation operation.

o To convert A.C. in to D.C. or vice versa, the operation is known as power converting operation.

o To convert frequency from higher to lower or vice-versa, theoperation is known as frequency operating operation.

o To improve power factor by installing synchronous condensers at the end of the line. Operation is known as power factor correction operation.

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.

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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

Used for EHV lines to improve

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line.2. Provided with bypass C.B and protect spare gaps.

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 Associate with protection To control all

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PANEL relays locate in big hall. equipment of substations.

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SINGLE LINE DIAGRAM OF 220/66/11 KV NAJAFGARH

SUB-STATION NEW DELHI.

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220/66/11 KV NAJAFGARH S/S AT A GLANCE

Najafgarh S/S is set up according to load requirement of South Delhi. It covers almost 70 % of the load requirement of South Delhi. The whole system is based on Ring main distribution system of 400kv. 220 kV Najafgarh S/S is located at Najafgarh (Powerhouse), and is an ideal place for distribution of power to the local area.

The 220 KV substations not only serve the purpose of providing domestic supply but also feed (serves as input) various substations at various stages. Thereby it not only serves the purpose of providing supply to domestic (household) but also to various commercial hubs located in close vicinity viz., Dwarka.

There are 66 KV outdoor yards which serve purpose of stepping down supply from BTPS incomer feeder to 220 KV Najafgarh. They also serve purpose for distribution of power to various areas as depicted in Single Line Diagram.

This S/S is divided in the following main parts:

1. 220 kV outdoor yards.2. 66 kV outdoor yards.3. 11 kV indoor yards.4. Control Room.5. SCADA.6. Battery Room.7. Main Office.

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THE 220 KV YARD

At Najafgarh S/S two EHV lines each of 220 kV are coming from BTPS (Badarpur Thermal Power Station), and are incomer to 220 kV yard. These are named BTPS incomer no.1 and BTPS incomer no. 2 respectively.

o Two buses of 220 KV each run in parallel from this very incomer feeder of 220 KV.

o There are 9 ways out of these buses.

o The ways 1, 2, 3, 4 are used to make 66 KV yard.

o The ways 5, 6 are used for 220 KV incomer.

o The ways 7 is for bus - coupler, for load sharing and line protection.

o The ways 8, 9 for making 66 KV yard.

o The two buses of 220 KV are joined by bus selection for each out going way.

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o The two transformer of 50 MVA 220/66/11 are used to step down the voltage from 220 KV to 66 KV. These transformers are located on way 1 and way 2. These transformers are single-phase 50 MVA star connected and form 66 KV bus in secondary of transformer. This makes total number of single-phase 50 MVA transformers equal to 6.

o Similarly way 3 and 4 are given to 100 MVA transformer of rating 220/66/11 KV. These are 3 – phase power transformer that gives 66 from its secondary, thus making bus no. 2 of 66 KV yard.

o Transformer no. 3 is TELK, India made and transformer no. 4 is EMCO India made.

o A PT to each bus is connected, to measure voltage in the line.

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THE 66KV YARD

o There are two incomers of 66 KV coming from 220KV yard transformers on way 8 and 9.

o From both the transformers two bus of 66 KV is made to run in 66 KV yard and is named as bus 1, and bus 2.

o Both transformers of 100 MVA, 220/66/11 KV each is BHEL make.

o There are 5 outgoing feeders connected to 66 KV bus 1 and bus 2 through bus selection.

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

o One of the transformers is BHEL make and named transformer no. 1 and other is CROMPTON GREAVES MAKE and is named transformer no. 2.

o There are 3 pairs of Shunt capacitor of 20 MVAR each in parallel, and is connected through bus selection to improve the power factor. These are named no. 1, no. 2 and no. 3.

o Shunt capacitor no. 3 is fixed to bus 2 and shunt capacitor no. 1 & 2 are joined by bus selection.

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 CB (Circuit Breakers) is aligned in the circuit for tripping whenever any fault occurs in the circuit.

o There are 144 shunt capacitor and 6 reactors.

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11 KV INDOOR YARDS

o There are two incomers of 11 KV coming from 66 KV yard transformers of 20 MVA 66/11 KV

o From both the transformers two bus of 11 KV is made to run in 11 KV yard and is named as bus 1, and bus 2.

o There are 10 outgoing feeders connected to 11 KV bus 1 and bus 2 .The feeders in sequence as in the 11 KV yard are:

o On first half bus:o Kalkaji , A - 12 o Kalkaji , A – 10o Girinagar 1o DDA phase 2o Local Transformer

o On second half bus : o Alaknandao Girinagar 2o Kalkaji A – 8o Tara Apartmento Govindpuri

o There is a 5 MVAR Shunt capacitor connected to it to improve power factor.

o A bus coupler is there for load sharing and line protection.

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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).

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.

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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:

1. Single bus-bar arrangement.2. Single bus-bar system with sectionalisation.3. Double bus-bar arrangement.

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1. 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.

2. 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 shut down. This arrangement is used for voltage upto 33KV.

3. DUPLICATE 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.

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TRANSFORMERS

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.

POWER TRANSFORMERS

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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.

The chief elements of construction are:

(a) Magnetic Circuits: This comprises limbs, yokes and clamping structures.

(b) Electrical circuits: Which constitutes the primary, secondary and (if any) tertiary windings, formers, insulation and bracing devices.

INSULATIONS:

The insulation between the H.V. and L.V. windings, and between L.V. winding and core, compresses Bakelite paper cylinders or elephantine wrap.

The insulation of the conductors may be of paper, cotton or glass tape being used for air-insulated transformer.

LEADS AND MATERIALS:

The connections to the windings are copper rods or bars, insulated wholly or in part, and taken to the bus-bars directly in the case of oil cooled transformers. The shape and size of the conductors are of importance in very high voltage systems, not on account of the current carrying capacity, but because of dielectric stresses, corona, etc. at sharp bends corners with such voltages.

The power transformer consists of:

1. Transformer core2. Windings

3. Tank

4. Conservator

5. Bushing

6. Air Cell

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

8. Cooling Equipments

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1. 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.

2. 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 helical b. 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.

3. 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.

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4. 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.

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5. 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.

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6. 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.

7. 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 load When 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

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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. Sequence of Tap Changing

8. 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

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for air –cooled transformer and oil to water heat exchangers for water cooled transformer.

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

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PROTECTION OF TRANSFORMER

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:- Double element relays can be used in detecting miner fault in a Transformer The 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

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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.

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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 to 3. 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.

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Maintenance & 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.

THERMO SIPHON FILTER Thermo 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

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

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

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12. Temp. rise winding (degC)

55 55 55

HV 1050Kvp-460kv r.m.s

LV 325 Kvp-140 Kv r.m.s

TV 170 Kvp- 70 Kv r.m.s

HVN & LVN 95 Kvp – 38 Kv r.m.s

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INSTRUMENT TRANSFORMER

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 Power.

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1 . CURRENT 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.

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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

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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) Differential d) 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 ....................

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2. POTENTIAL 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.

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3. Capacitor Voltage Transformer:-

Capacitor P.T in which primary voltage is applied to a series capacitor group. The voltage across one of the capacitor is taken to aux. wdg. S4 of PT is taken for protection & measurement. Generally capacitor voltage transformer (CVT) are used at sub-stations because for line voltmeters, synchoronoscope & protective relays. In CVT the capacitor connected in series acts as potential divider provided that the current taken by the primary is negligible as compared to the current passing series capacitor. However burden current becomes larger a ratio error and also phase error is produced. “Tanning” carries out compensation. The reactor connected in series with the burden is adjusted to such a value that a supply frequency it resonates with the sum of two capacitors. H.V. capacitor is enclosed in porcelain housing.

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SPECIFICATIONS OF A CVT IN 220 KV YARD:

Mfg by: WS insulation of India limited.

X1 - X2 Y1 – Y2 Z1 - Z2

V (primary) 220000/root (3) 220000/root (3) 220000/root (3)

V (secondary) 100/root (3) 100/root (3) 110/ root (3)

VA 150 150 150

CL 5.0 0.5 5.0

Intermediate voltage 20/root (3)Total Output Simultaneous 450Output Max. 750VA at 50 deg AMB

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Operating Voltage 220/root (3) to 245/root (3)

Voltage factor 1.5 - 30 sec.Test voltage 1 min 460 kvImpulse Withstand voltage 1.2/ 50 mew sec.1050 kvpInsulation Class - AImpulse Withstand voltage 1.2/ 50 mew sec.1050 kvpHF capacitance 4400pf - +10% - -5%Primary Capacitance 4840pf - + 10% - - 5%Secondary capacitance 48400 pf - + 10 % - 5%Insulation Class - A

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

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

o Space Factor o Insulation Security o Standardization o Ease of Maintenance o 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.

CIRCUIT BREAKERS

Circuit breakers are switching and current interrupting devices. Basically a circuit breaker comprises set of fixed and movable contacts. The contacts can be separated by means of an operating mechanism. The separation of current carrying contacts produce an arc, the arc is extinguished by a suitable medium such as dielectric oil,

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vacuum, sf6 gas. A circuit breaker is am equipment, which can open or close circuit under all conditions.It can be defined as an electrical device, which protects the system from short circuits or overloads with the help of relays. In case, circuit breaker is not of adequate capacity, its failure may result in failure of power, shut down, injury and damage to property.The NAJAFGARH S/S DELHI is equipped with following type of circuit breaker:

(a) SF6 circuit breaker(b) Vacuum circuit breaker(c) Miniature Oil circuit breaker

(a)SF6 circuit breakers : operate to switch electric circuits and equipment in and out of the system. These circuit breakers are filled with compressed sulfur-hexafluoride gas which acts to open and close the switch contacts. The gas also interrupts the current flow when the contacts are open.

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( SF6 circuit breakers )

(b) Vacuum circuit breaker:In this breaker Vacuum is used as arc quenching. Vacuum circuit breaker score over the conventional oil circuit breaker as regard to no of maintenance free operation, fault handling capacity, freedom from the fire hazard and minimum inspection requirement.

(c)Miniature Oil Circuit Breaker : In such CB insulating oil is used as an arc quenching media. The contacts are opened under oil and then arc is struck between them. The heat of arc evaporates and surrounding oil and dissociates it in substantial oil of gases at high pressure.

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(Miniature Oil Circuit Breaker)

PROTECTION

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INTRODUCTION: -

The different schemes adopted for the protection of various equipment of power system against 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 grounds C. 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

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 wires

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2. Earth screens 3. Lightning arrestors of surge diverters

GROUND-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.

EARTHING 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.

LIGHTNING ARESSTOR OR SURGE DIVERTER: -

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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 arrestor 2. Horn gap arrestor3. multi gap arrestor4. Thyrite arrestor5. Electrolytic arrestor6. Oxide film arrestor7. Expulsion type arrestor8. Value type arrestor

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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.

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VARIOUS OTHER KINDS OF PROTECTION

1 .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.

2 .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) or Residual 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.

3. 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

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possible causes of the circuit breaker failure is the failure of the trip-battery due to inadequate maintenance.

RELAY

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 attraction o Electromagnetic induction

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

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 Relay 4. Buccholtz relay5. Gas operated relay

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RELAYS OF 20 MVA transformer in Najafgarh S/S:

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

FEEDER RELAYS of Najafgarh s/s:

o Out of step blocking relayo Directional current relayo Directional earth fault relayo Fuse failure relayo Auxiliary relay typeo Tripping relayo Instantaneous Earth Fault relay

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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.Fig.2 shows if there is some external fault F, than the current flowing on the two sides of relay are equal and hence the relay will not sense the fault. 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.

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.

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CONTROL PANEL

Control panel is a panel in which all the controlling alarms and announcations and metering are located the main function or characterize of control panel as under.The control room is one of the important components of a sub - station. Control room is used to control the working of battery in battery room needed for DC supply, and protecting devices present in the yard of sub-station.

There are mainly five main sets of panels in the control room they are:(a)220 KV panel set.(b)66 KV panel set.(c) 11 KV panel set.(d)Battery control panel set.

FEATURES OF THE CONTROL ROOMo Different panels of different Circuit Breaker, Transformer,

Lightning arrester, Isolator, Current Transformer, Potential Transformer, and Shunt-Capacitor are coming to control room.

o In each set of panels there is at least one over-current and one earth fault relay.

o From control panel we can know what is happening in the sub-station yard.

o We can control, start, regulate or switch off the main circuit from control panels.

o The relaying equipment is installed in the control room.o The diagram of main connection are given in the front face of

the panel their diagram indicate the position of the Circuit Breaker and isolators.

o From observing the control panel we get the idea which break is open or closed.

o The whole controlling system is dependent upon the DC supply system of battery in battery room.

All control wiring and protections, interlocking and metering are done through control panel.The purpose an electrical power system is to generate and supply electrical energy to the consumer safely and reliably. The purpose of a protective system is to isolate the fault section of the power system as quickly as possible from the healthy plant.

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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.

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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

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In this S/S There are 110 batteries of 2 volts each.

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.

CLEARANCES IN THE LINES

Clearance is the shortest distance between two conducting point in air measured by stretched thread.

Rated Voltage kv (r.m.s)

Min clearance phase to earth (mm)

Minimum clearance phase to phase(mm)

6.6 kv 140 17811 kv 178 22915 kv 216 26722 kv 279 33033 kv 381 43166 kv 685 786110 kv 1068 1219132 kv 1270 1473

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220 kv 2082 2368400 kv 3065 5750

CAPACITOR BANK

Capacitor bank

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 loss

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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.

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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.

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CONCLUSION

It has been really a knowledgeable experience

pursuing training at DTL, 220 KV Najafgarh 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.

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It was definitely a knowledgeable experience and

pride to be a part of 220 kv Najafgarh 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 Najafgarh, New Delhi was a learning

experience.

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