INDUSTRIAL TRAINING

POWER GRID CORPORATION OF INDIA LIMITED

Patna 400/220 KV Sub station

Ramnik Ujjwal NIT DELHI

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PATNA 400/220 KV SUB

STATION

GAURICHAK

Project Report

Submitted by :

Ramnik Ujjwal

131300048

Nit Delhi

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ACKNOWLEDGEMENT

I wish to express my sincere thanks to Mr. K.M. Sharma (DGM Patna Substation)

for allowing me for this training programme and providing me with all necessary

facilities.

I wish to express my gratitude to Mr. Uday Shankar (Senior engineer Patna

substation), for taking out his precious time for providing enormous knowledge

on transmission lines and for giving us an insight of the substation and various

equipments.

I would like to thank Mr Mohsin Raza and Mr. Ghanshyam Gupta for his proper

Guidance, inspiriting words, discussions & support throughout my training

period.

I would like to thank my friends who helped me a lot in finalizing this project

within limited time frame.

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Table of Contents

1. Powergrid 09

1.1 Introduction 09

1.2 Features 09

1.3 History 10

1.4 Vision 10

1.5 Mission 10

1.6 Objectives 11

1.7 Quality 11

1.8 Patna Substation 12

2. Substation 13

2.1 Classification 13

2.1.1 According to Service Requirements 13

2.1.2 According to Constructional Features 13

2.1.3 According to Operational Voltage 14

2.2 Single Line Diagram 14

2.3 Patna Substation 14

3. Equipments in Power Systems 16

3.1 Power Transformer (Interconnected Transformer I.C.T) 16

3.2 Instrument Transformer 18

3.2.1 Current Transformer 18

3.2.2 Types of Current Transformer 19

3.2.3 Capacitive Voltage Transformer (C.V.T.) 20

3.3 Busbar 21

3.3.1 Typical Bus Configuration 21

3.3.1.1 Single Bus 21

3.3.1.2 Sectionalised Bus 22

3.3.1.3 Breaker and a Half 23

3.3.1.4 Double Breaker and Double Bus 24

3.4 Isolator 25

3.5 Relay 26

3.6 Circuit Breaker 26

3.7 SF6 Circuit Breaker 27

3.8 Reactor 28

3.9 Wave Trap 28

3.10 Lightning Arrester 28

4. Transformer 30

4.1 Components of Transformer 30

4.1.1 Core 31

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4.1.2 Windings 31

4.1.3 Transformer oil 32

4.1.4 Conservator 33

4.1.5 Breather 33

4.1.6 Tap changer 33

4.1.7 Cooling Tubes 34

4.1.8 Buchholz Relay 34

4.1.9 Explosion Vent 35

4.2 Working of Interconnecting Transformer 35

5. Subsystem Protection 36

5.1 System Grounding 36

5.1.1 Earth Resistance 36

5.2 Step Potential and Touch Potential 36

5.2.1 Step Potential 37

5.2.2 Touch Potential 37

5.3 Formation of Subsystem Earthing 38

5.4 Lightning Protection 38

5.4.1 Shield Wire 38

5.4.2 Earth Wire 39

5.5 Transformer and Reactor Protection 39

5.5.1 Buchholz Relay 39

5.5.2 PRV (Pressure Relief Valve) 40

5.5.3 Differential Protection 41

5.5.4 Backup O/C and E/F Protection 41

5.5.5 Restricted E/F Protection 41

5.5.6 Over fluxing Protection 41

5.5.7 Oil Temperature alarm/temp. 41

5.5.8 Winding temp alarm/temp. 42

5.6 Low oil Level alarm 42

5.6.1 Auto reclose Relay 42

5.6.2 Over Voltage Relay 43

6. Transmission Line Protection 44

6.1 Introduction 44

6.2 Distance Relay 44

6.3 Main-I Protection 44

6.4 Main-II Protection 45

6.5 Bus Bar Protection 45

7. Maintenance 46

7.1 Maintenance of Transformer 46

7.2 General Maintenance 47

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7.3 Filtration and Degassing of Transformer oil 48

8. Testing 51

8.1 Transformer Testing 51

8.1.1 Tests done at Factory 51

8.1.2 Tests done at Site 51

8.1.3 Type tests on Transformer 51

8.1.4 Routine Tests on Transformer 51

8.1.5 Special Tests on Transformer 52

8.2 Tests on Transformer 52

8.2.1 Transformer Winding Resistance Test 52

8.2.2 Transformer Ratio Test 52

8.2.3 Magnetic Balance Test 53

8.2.4 Magnetising Current Test of Transformer 53

8.2.5 Short Circuit Test 53

8.3 Testing of CT 53

8.4 Transformer Oil Testing 54

8.4.1 DGA (Dissolved Gas Analysis Test) 54

8.4.2 Acidity Test 54

8.4.3 Tan Delta Test 55

8.4.4 BDV (Breakdown Voltage Test) 55

8.4.5 Water Content Test 55

8.4.6 Furan Test 56

8.4.7 Interfacial Tension Test 56

8.4.8 Viscosity Test 56

8.4.9 Flash Point Test 57

9. NTAMC and RTAMC 58

Conclusion 59

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Table of Figures

Figure 1: Patna Substation at night 12

Figure 2: Single line diagram of Patna Substation 14

Figure 3: Power Transformer 16

Figure 4: Bushing 17

Figure 5: Radiator 17

Figure 6: Silica gel breather 18

Figure 7: Current Transformer 18

Figure 8: Dead Tank Current Transformer 19

Figure 9: Live Tank Current Transformer 20

Figure 10: Capacitive Voltage Transformer 21

Figure 11: Busbar 21

Figure 12: Single Bus 22

Figure 13: Sectionalised Bus 23

Figure 14: Breaker and a Half 24

Figure 15: Double breaker Double bus 24

Figure 16: Isolator 25

Figure 17: Circuit Breaker 27

Figure 18: SF6 Circuit Breaker 27

Figure 19: Wavetrap 28

Figure 20: Lightning Arrester 29

Figure 21: Autotransformer 30

Figure 22: Transformer 30

Figure 23: Core 31

Figure 24: Conservator 33

Figure 25: Tap Changer 34

Figure 26: Step potential and Touch Potential 37

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Figure 27: Shield wire 38

Figure 28: Buchholz Relay 40

Figure 29: Buchholz Relay 40

Figure 30: Pressure vs Oil Flow Rate 47

Figure 31: Oil Filtration Machine 49

Figure 32: Dissolved gas Testing Equipment 54

Figure 33: Acidity Testing Equipment 54

Figure 34: Tan Delta Testing Equipment 55

Figure 35: Breakdown voltage Equipment 55

Figure 36: Water Content Testing Equipment 55

Figure 37: Interfacial Tension Testing Equipment 56

Figure 38: Viscosity Testing Equipment 56

Figure 39: Flashpoint Testing Equipment 57

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Table of Tables

Table 1: Equipment Description: 400 KV 15

Table 2: Equipment Description: 220 KV 15

Table 3: Limits of Various Tests 57

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1. Power Grid 1.1 Introduction

Power Grid Corporation of India Limited (PGCIL), is an Indian state owned electric utilit ies

company headquartered in Gurgaon, India. It is one amongst the largest Power Transmiss ion

utilities in the world. Power Grid is playing a vital role in the growth of Indian power sector by

developing a robust, Integrated National Grid and associating in the flagship programme of the

government of India to provide Power for all. An innovation in Technical & Management has

resulted in coordinated development of power transmission network and effective operation

and management of regional and national grid.

POWERGRID, as the Central Transmission Utility of the country, is playing a major role in

Indian Power Sector and is also providing Open Access on its inter-state transmission system.

POWERGRID has planned to create a strong and vibrant national grid in the country in a

phased manner to ensure optimum utilisation of generating resources, conserve eco-sensitive

right of way and accommodate uncertainty of generation plants. Strengthening of National Grid

is planned in a phased manner through consolidation of inter-regional connection framework,

so to support the anticipated generation capacity programme of about 88,000 MW during the

XII plan.

Based on its performance POWERGRID was recognized as a Mini-Ratna category-I Public

Sector Undertaking in October 1998 and conferred the status of “Navaratna” by the

Government of India in May 2008.

1.2 Features

1.2.1 Transmission

The company owns and operates about 130.020 circuit kms of Transmission lines at 765 KV,

400 KV, 220 KV and 132 KV EHAC & 500 KV and 800 KV HVDC levels and 210 substations

with Transformation capacity of about 259,163 MVA as on 31st May,2016. This gigantic

transmission network spread over length and breadth of country, is consistently maintained at

an availability of over 99%.

1.2.2 Consultancy

It provides transmission related consultancy to more than 150 domestic clients, It has global

foot prints in 18 countries and catering more than 20 clients.

1.2.3 Telecom

POWERGRID has started telecom business as “POWERTEL”.

It owns and operates approx. 36,563 kms of telecom network. Points of Presence in approx.

352 location and has intra-city network in 105 cities across India.

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1.3 History

In 1980, the Raj Adhyaksha Committee on Power Sector Reforms submitted its report to the

Government of India suggesting that extensive reforms were needed in the Indian power sector. In 1981, the Government of India took a policy decision to form a National Power Grid, which

would pave the way for the integrated operation of the central and regional transmiss ion systems.

In October 23, 1989 under the Companies Act, 1956, the National Power Transmiss ion Corporation Limited was formed, and assigned the responsibility of planning, executing,

owning, operating and maintaining the high voltage transmission systems in the country. In October 1992, the National Power Transmission Corporations name was changed to Power Grid Corporation of India Limited, as we know of it today.

POWERGRID was incorporated in 1989 and based on its impeccable performance, Govt. Of

India categorised it as the Mini ratna Category–I PSU w.e.f. Oct’98. Further, recognizing the role of POWERGRID in the overall development of Indian power sector and its consistent performance as per benchmark parameters stipulated by Department of Public Enterprise

(DPE) “Navratna status” was conferred to POWERGRID w.e.f. 1st May, 2008.

POWERGRID was listed on Indian Bourses in Sept 2007 and the Company came with Follow

on Public offer in November 2010. Presently Govt. of India holding is 69.42% and the balance

30.58% is held by Institutional Investors and public.

1.4 Vision

World Class, Integrated, Global Transmission Company with Dominant Leadership in

Emerging Power Markets Ensuring Reliability, Safety and Economy.

1.5 Mission

It will become a Global Transmission Company with Dominant Leadership in Emerging Power Markets with World Class Capabilities by:

World Class: Setting superior standards in capital project management and operations for the industry and ourselves

Global: Leveraging capabilities to consistently generate maximum value for all

stakeholders in India and in emerging and growing economies.

Inspiring, nurturing and empowering the next generation of professionals.

Achieving continuous improvements through innovation and state of the art technology.

Committing to highest standards in health, safety, security and environment.

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1.6 Objectives

The Corporation has set following objectives in line with its mission and its status as Central

Transmission Utility to:

Undertake transmission of electric power through Inter-State Transmission System.

Discharge all functions of planning and coordination to Inter-State Transmiss ion System with-

State Transmission Utilities Central & State Government

Generating Companies Regional Power Committees Authorities & Licensees

Any other person notified by the Central Government in this behalf.

To ensure development of an efficient, coordinated and economical system of inter -

state transmission lines for smooth flow of electricity from generating stations to the load centres.

Efficient Operation and Maintenance of Transmission Systems.

Restoring power in quickest possible time in the event of any natural disasters like

super-cyclone, flood etc. through deployment of Emergency Restoration Systems.

Provide consultancy services at national and international levels in transmission sector

based on the in-house expertise developed by the organization.

Participate in long distance Trunk Telecommunication business ventures.

Ensure principles of Reliability, Security and Economy matched with the rising / desirable expectation of a cleaner, safer, healthier Environment of people, both affected

and benefited by its activities.

1.7 Quality

POWERGRID is playing a strategic role in Indian Power Sector development by establishing & maintaining the power transmission infrastructure which carries about

half of the power generated in the country. POWERGRID has been instrumental in providing and managing a smooth, efficient and reliable grid operation in the country. Since 2009, the Grid Management and operation has been entrusted to Power System

and Operation Corporation limited (POSOCO, a complete subsidiary of POWERGRID).

POWERGRID is committed to Environment preservation and sustainab le

development. Though transmission projects are non-polluting, the Company has

developed a detailed corporate strategy document - “Environmental and Social Policy

and Procedures (ESPP)” in 1998 and has updated it over time through wide

consultations with social bodies, local communities, Govt. agencies to keep pace with

the best International standards. The policy outlines the Company’s approach and lays

out management procedures and protocols to deal with environment and social issues,

relating to transmission projects, thus keeping its commitment to the environment. The

policy and the initiatives taken by the Company for sustainable development of

transmission system have been applauded by the multilateral funding agencies like The

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World Bank & ADB like all its other projects across the country. The Company has

also been pro-active in bringing out the first ‘Sustainability Report’ in power sector, for

measuring, disclosing and being accountable to internal and external stakeholders. This

has paved way for promoting excellence in organizational performance and towards

achieving sustainable development.

1.8 Patna Substation

The Patna sub-station is located on Patna-Gaya road in Gaurichak village, in sampatchak block

of Patna district (803206). The station is located at a distance of 10 km from the state capital

Patna city and 25 km from ER-1, Head Quarter.

The 400/200 KV POWERGRID, Patna Sub-Station has been constructed by POWERGRID as

a part of transmission system associated with Kahalgaon stage-II phase_1 (2X500 MW) STPP.

The Patna substation has a two transformer each of capacity 315MVA and caters the power

supply to the state capital Patna through the BSEB system. Further it interconnects with

northern region through four 400 KV Patna-Balia (quad) transmission lines for supply of ISGs

power to the new grid. Subsequently, the station is also connected with Barh Thermal power

project, NTPC.

Figure 1:Patna Substation at Night

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2. Substation 2.1 Classification

2.1.1 According to service requirements

Transformer sub-station: Those sub-station which change the voltage level

of electrical supply is called Transformer sub-station.

Switching sub-station: This sub-station simply performs the switching

operation of power line.

Power factor correction S/S: This sub-station which improves the p.f. of the

system are called p.f. correction s/s. these are generally located at receiving

end s/s.

Frequency changer S/S: Those sub-stations, which change the supply

frequency, are known as frequency changer s/s. Such s/s may be required for industrial utilization

Converting sub-station: That sub-station which change A.C power into D.C.

power are called converting s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding etc.

Industrial sub-station: Those sub-stations, which supply power to individua l

industrial concerns, are known as industrial sub-station.

2.1.2 According to constructional features

2.1.2.1 Outdoor Sub-Station: For voltage beyond 66KV,

equipment is invariably installed outdoor. It is because for such Voltage the clearances between conductor and the space required for switches, C.B. and other equipment becomes so great that it is not economical to install the equipment

indoor.

2.1.2.2 Indoor Sub-station: For voltage up to 11KV, the equipment

of the s/s is installed indoor because of economic consideration. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for

voltage up to 66KV.

2.1.2.3 Underground sub-station: In thickly populated areas, the

space available for equipment and building is limited and the cost of the land is

high. Under such situations, the sub-station is created underground. The design of underground s/s requires more careful consideration.

The size of the s/s should be as small as possible.

There should be reasonable access for both equipment & personal. There should be provision for emergency lighting and protection against

fire. There should be good ventilation.

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2.1.3 According to Operating Voltage

High Voltage Substations (HV Substations) - involving voltages between 11 KV and 66 KV.

Extra high voltage substations (EHV Substations) - involving voltages

between 132 KV and 400 KV and

Ultra-high voltage substations (UHV Substations) - operating on voltage above 400 KV.

2.2 Single Line Diagram Power Systems are extremely complicated electrical networks that are geographically spread over very large areas, In fact, the power systems are so complex that a complete conventiona l

diagram showing all the connections is impractical. A one-line diagram or single line diagram (SLD) is a simplified notation for representing a three phase power system.

The Single Line Diagram for Patna substation is shown below:

Figure 2: Single Line Diagram of Patna Substaton

2.3 Patna substation

2.3.1 Salient Features Transformer Capacity = 2*315 MVA, 400/220/33KV Reactor Capacity – 1 nos. 80 MVAR

-2 nos. 50 MVAR Line Reactor

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-2 nos. 63 MVAR Line Reactor -2 nos. 125 MVAR Bus Reactor

Land Area = 43.70 Acres

2.3.2 Equipment Description: 400 KV

Table 1: Equipment Description: 400 KV

S.No. Equipment Details Quantity

1. 315 MVA ICT 2

2. 50 MVAR Line Reactor 2

3. 63 MVAR Line Reactor 2

4. 80 MVAR Line Reactor 1

5. Circuit Breaker (3 Phase) 23

6. 125 MVAR Bus Reactor 2

7. Isolator 66

8. Current Transformer 81

9. CVT (1 Phase) 36

10. Lightning Arrester (1 Phase) 45

11. Wave Trap 10

12. Earth Switch 67

2.3.3 Equipment Description: 220 KV Table 2: Equipment Description: 220 KV

S.No. Equipment Details Quantity

1. Circuit Breaker (3 Phase) 8

2. Isolator 29

3. Current Transformer (1 Phase) 24

4. CVT (1 Phase) 6

5. Lightning Arrester (1 Phase) 12

6. Wave Trap 4

7. Earth Switch 25

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3. Equipment in Subsystem A substation is an assembly of various electrical equipment connected to step down electric power at higher voltages and to clear faults in the system. The various electrical equipment

used in the substation are as follows:-

3.1 Power Transformer (Inter connected

Transformer I.C.T) This is the costliest equipment of substation. ICT is used to step down the EHV transmiss ion Voltage (400KV) to HV transmission voltage (220KV). Normally 315 MVA auto transformers

are being used. The secondary winding provides 220 KV voltages and other 33 KV voltage (tertiary winding). Usually tertiary winding is connected in closed delta formation and can be

used for auxiliary station supply purpose. In practice, it is preferred to installed three phase ICT as far as possible however in case of hilly terrain, where due to transportation limitations, three single phase units are installed.

A transformer is a device that transfers electrical energy from one circuit to another through

inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying Magnetic field through the secondary winding.

With transformers, however, the high cost of repair or replacement, and the possibility of a

violent failure or fire involving adjacent equipment, may make limiting the damage a major objective. The protection aspects of relays should be considered carefully when protecting transformers. Faults internal to the transformer quite often involve a few turns. While the

currents in the shorted turns are large in magnitude, the changes of the currents at the termina ls of the transformer are low compared to the rating of the transformer.

Figure 3.1: Power Transformer

There are different parts of a transformer given below:

Bushing: This maintains the incoming and outgoing connection of a

transformer.

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Figure 4: Bushing

Radiator: This is used to radiate the heat of a transformer when transformer is

heated up at a certain level.

Figure 5:Radiator (source: Internet)

Oil temperature meter: This meter indicates the temperature of transformer

oil. If temperature crosses a certain level, then it makes an alarm.

Temperature meter: This meter indicates the temperature of transformer

windings. If temperature crosses a certain level, then it starts the winding fans.

Oil level meter: This meter indicates the oil level of transformer. If oil is low than a certain amount it makes an alarm that means that transformer have to

feed oil.

Silica gel: It works like breathing. There has a little amount oil under the silica gel which suck the moisture of air and further sends this air to silica gel which further sucks the rest of the moisture of the air.

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Figure 6:Silica Gel Breather (source: Internet)

Exchanger: Regulate voltage through winding selection between primary &

secondary side.

PRD (Pressure relief device): release the oil pressure by releasing oil when oil pressure is high.

3.2 Instrument Transformer They are devices used to transform voltage and current in the primary system to values suitable for measuring instruments, meters, protective relays etc. They are basically the current

transformers and voltage transformers.

3.2.1 Current transformers: Current transformer is similar in construction to single phase power transformer and obeys the same fundamentals laws but primary current of CT is not controlled by the connected load in

secondary circuit. In facts it is governed by the current in the main circuit viz. line/transformer to which is connected. A typical 400/220 KV CT has five cores which is used for following functions: -

Figure 7:Current Transformer (source: Internet)

Core 1: Busbar I protection

Core 2: Busbar II protection

Core 3: Metering

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Core 4: Main I Protection

Core 5: Main II Protection The metering core of CT is of accuracy class of 0.5 whereas the other cores having accuracy

of PS class which is a special protection class for which Knee point Voltage and max. exciting current is specified.

3.2.2 Types of Current Transformer

There are basically two kinds of CT’s: -

Dead Tank Type: - Primary conductor (insulated) extended to tank at bottom.

Figure 8:Dead Tank Current Transformer (source: Internet)

Live Tank Type: - Primary conductor and Secondary windings in top tank.

Secondary cables brought down to Terminal Box at bottom.

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Figure 9:Live Tank Type Current Transformer (source: Internet)

Main Functions of Current Transformer:

Electrically isolates the instruments and relays from High Voltage side.

Measures and monitors current.

Used in Measuring Power flow.

Senses abnormalities in current for system protection.

3.2.3 Capacitive Voltage Transformer (CVT’s): It is used for providing small representative voltage of primary system for metering and

protection applications. CVT consists of coupling capacitors, intermediate voltage transformer, High frequency coupling terminal. The H.F terminal is used for PLCC purpose. The CVT has three cores which are utilized as follows.

Core 1: Main I protection

Core 2: Main II protection

Core 3: Metering.

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Figure 10: Capacitive Voltage Transformer (Source: Internet)

The accuracy class of protection core is 3P and metering core is 0.5.

3.3 Busbar In electrical power distribution, a bus bar is a thick strip of copper or aluminum that conducts

electricity within a switchboard, distribution board, substation or other electrical apparatus. Bus bars are used to carry very large currents, or to distribute current to multiple devices within switchgear or equipment. Bus bars are typically either flat strips or hollow tubes as these shapes

allow heat to dissipate more efficiently due to their high surface area to cross sectional area ratio. The size of the bus bar is important in determining the maximum amount of current that

can be safely carried.

Figure 11: Busbar

Bus bar may either be supported on insulators, or else insulation may completely surround it. Bus bars are protected from accidental contact either by a metal enclosure or by elevation out

of normal reach. Bus bars may be connected to each other and to electrical apparatus by bolted or clamp connections.

3.3.1 Typical Bus Configuration

3.3.1.1 Single Bus

Figure shows the one-line diagram of a single bus substation configuration. This is the simplest of the configurations, but is also the least reliable. It can be constructed in either of low profile or high-profile arrangement depending on the amount of space available. In the arrangement

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shown, the circuit must be de-energized to perform breaker maintenance, which can be overcome by the addition of breaker bypass switches, but this may then disable protection

systems.

Single Bus Advantages:

Lowest cost

Small land area

Easily expandable

Simple in concept and operation

Relatively simple for the application of protective relaying

Figure 12: Single Bus (source: Internet)

Single Bus Disadvantages:

Single bus arrangement has the lowest reliability

Failure of a circuit breaker or a bus fault causes loss of entire substation

Maintenance switching can complicate and disable some of the protection schemes and

overall relay coordination.

3.3.1.2 Sectionalised Bus

Figure shows the layout of a sectionalized bus, which is merely an extension of the single bus layout. The single bus arrangements are now connected together with a center circuit breaker that may be normally open or closed. Now, in the event of a breaker failure or bus bar fault,

the entire station is not shut down. Breaker bypass operation can also be included in the sectionalized bus configuration.

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Figure 13: Sectionalised Bus (source: Internet)

Sectionalized Bus Advantages:

Flexible operation

Isolation of bus sections for maintenance

Loss of only part of the substation for a breaker failure or bus fault

Sectionalized Bus Disadvantages:

Additional circuit breakers needed for sectionalizing, thus higher cost Sectionalizing

may cause interruption of non-faulted circuits.

3.3.1.3 Breaker and a Half A breaker-and-a-half configuration has two buses but unlike the main and transfer scheme,

both busses are energized during normal operation. This configuration is shown in Figure. For every 2 circuits there are 3 circuit breakers with each circuit sharing a common center breaker.

Any breaker can be removed for maintenance without affecting the service on the corresponding exiting feeder, and a fault on either bus can be isolated without interrup ting service to the outgoing lines. If a center breaker should fail, this will cause the loss of 2 circuits,

while the loss of an outside breaker would disrupt only one. The breaker-and-a-half scheme is a popular choice when upgrading a ring bus to provide more terminals.

Breaker-and-a-Half Advantages:

Flexible operation and high reliability

Isolation of either bus without service disruption.

Isolation of any breaker for maintenance without service disruption.

Double feed to each circuit.

Bus fault does not interrupt service to any circuits.

All switching is done with circuit breakers.

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Figure 14: Breaker and a Half (source: Internet)

Breaker-and-a-Half Disadvantages:

One-and-a-half breakers needed for each circuit.

More complicated relaying as the center breaker has to act on faults for either of the 2

circuits it is associated with.

Each circuit should have its own potential source for relaying Substation Configura t ion

Reliability.

3.3.1.4 Double Breaker Double Bus The final configuration shown is the double breaker – double bus scheme in figure. Like the

breaker-and-a-half, the double breaker-double bus configuration has two main buses that are both normally energized. Here though, each circuit requires two breakers, not one-and-a-half. With the addition of the extra breaker per circuit, any of the breakers can fail and only affect

one circuit. This added reliability comes at the cost of additiona l breakers, and thus is usually only used at large generating stations.

Figure 15:Double Breaker Double Bus (source: internet)

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Double Breaker-Double Bus Advantages:

Flexible operation and very high reliability

Isolation of either bus, or any breaker without disrupting service

Double feed to each circuit

No interruption of service to any circuit from a bus fault

Loss of one circuit per breaker failure

All switching with circuit breakers

Double Breaker-Double Bus Disadvantages:

Very high cost – 2 breakers per circuit

3.4 Isolator

In electrical systems, an isolator switch is used to make sure that an electrical circuit is completely de-energized for service or maintenance. Such switches are often found in electrica l distribution and industrial applications where machinery must have its source of driving power

removed for adjustment or repair. High-voltage isolation switches are used in electrica l substations to allow isolation of apparatus such as circuit breakers and transformers, and

transmission lines, for maintenance. An isolator can open or close the circuit when either a negligible current has to be broken or made or when no significant voltage change across the terminals of each pole of isolator occurs.

It can carry current under normal conditions and can carry short circuit current for a specified time. They can transfer load from one bus to another and also isolate equipments for

maintenance. Isolators guarantee safety for the people working on the high voltage network, providing visible and reliable air gap isolation of line sections and equipment. They are basically motorized i.e. motor does the closing and opening of the isolator. Isolators are

distinguished as “off load” and “on load” isolator.

Figure 16:Isolator

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3.5 Relay A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrica l

isolation between control and controlled circuits), or where several circuits must be controlled by one signal. Relays with calibrated operating characteristics and sometimes multip le

operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".

Types of relays:

Electromagnetic attraction relay

Electromagnetic induction relay

Thermal relay

Buchholz relay

Numerical relay

Over current relay

3.6 Circuit Breaker A circuit breaker is an automatically operated electrical switch designed to protect an electrica l

circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either

manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear

designed to protect high voltage circuits feeding an entire city. The type of the Circuit Breaker is usually identified according to the medium of arc extinct ion. The classification of the Circuit Breakers based on the medium of arc extinction is as follows:

Air break Circuit Breaker.

Oil Circuit Breaker (tank type of bulk oil)

Minimum oil Circuit Breaker.

Air blast Circuit Breaker.

Vacuum Circuit Breaker.

Sulphur hexafluoride Circuit Breaker. (Single pressure or Double Pressure).

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Figure 17: Circuit Breaker

3.7 SF6 circuit breaker SF6 is inert gas the property of this gas the higher pressure and temperature its dielectr ic strength will be SF6has two gas chamber when contract is close the pressure is two chamber have the same pressure but when the contract is open then one of the chamber get totally close

and other remain open ,there is a narrow channel between two chamber and when contract open the SF6 flow a plane of high pressure region to the low pressure region there will be turbulence

of SF6.At zero current the turbulence of SF6 absorb all the ions and since it is flowing from a narrow region hence it provide high dielectric strength but there is problem that the pressure of SF6 is not always remain fixed due to leakage in the cylinder of SF6 so there is pressure gauge

as well as alarm attached with it. Whenever pressure decreases the alarm ringing and the gas is refilled to increase pressure.

Figure 18: SF6 Circuit Breaker (Source: Internet)

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3.8 Reactor It is similar in appearance and used for absorbing the reactive power from the system. When the system voltage is high. It has air core, oil filled ONAN type. Generally 50 and 63 MVAR shunt reactor are used with both the LINE/BUS both non-switchable/ switchable type shunt

reactors are in use.

3.9 Wavetrap It is an inductor having tuned LC circuit, which is mainly used for PLCC purpose. It offers very high impedance to high frequency PLCC signals does not allow them to enter in S/Y and offers very low impedance for frequency currents.

Figure 19: Wavetrap

3.10 Lightening arrestors A lightning arrester is a device used on electrical power systems and telecommunicat ions systems to protect the insulation and conductors of the system from the damaging effects of

lightning. The typical lightning arrester has a high-voltage terminal and a ground termina l. When a lightning surge (or switching surge, which is very similar) travels along the power line

to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth. In telegraphy and telephony, a lightning arrestor is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near

them. Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the

Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by

lightning or is near to a lightning strike.

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Figure 20:Lightning Arrester

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

of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage

spikes in incoming power lines can damage electrical home appliances.

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Figure 22:Transformer (source: Internet)

4. Transformers The Transformer being used in the Patna substation is an Auto transformer, which is also

named as Interconnecting Transformer of the substation (ICT).

An autotransformer (sometimes called auto

stepdown transformer) is an electrica l

transformer with only one winding. The

“auto” prefix refers to the single coil acting

on itself and not to any kind of automatic

mechanism. In an autotransformer, portions

of the same winding act as both the primary

and secondary sides of the transformer. In

contrast, an ordinary transformer has separate

primary and secondary windings which are

not connected.

4.1 Components of a Transformer

The following are basic components of a Transformer: -

Core

Windings

Transformer oil

Tap changer

Conservator

Breather

Cooling Tubes

Buchholz Relay

Explosion vent

Figure 21:Auto Transformer (source: Internet)

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4.1.1 Core

Core is used to support the windings in the transformer. It also provi des a low reluctance path to the

flow of magnetic flux. It is made up of laminated soft iron core in order to reduce eddy current loss

and hysteresis loss. The composition of transformer core depends on such factors as voltage, current,

and frequency. Diameter of the transformer core is directly proportional to copper loss and is inversely

proportional to iron loss. If diameter of the core is decreased, the weight of the steel core is reduced

which leads to less core loss of transformer and copper loss increase. The vice versa happen when the diameter is increased.

Figure 23:Core (source: Internet)

The construction of a transformer is dependent upon how the primary and secondary windings are

wound around the central laminated steel core. The two most common and basic designs of

transformer construction are the Closed-core Transformer and the Shell-core Transformer.

In the “closed-core” type (core form) transformer, the primary and secondary windings are wound

outside and surround the core ring. In the “shell type” (shell form) transformer, the primary and

secondary windings pass inside the steel magnetic circuit (core) which forms a shell around the

windings.

4.1.2 Windings

Transformer windings form another important part of a transformer construction, because they

are the main current-carrying conductors wound around the laminated sections of the core. In

a single-phase two winding transformer, two windings would be present as shown. The one

which is connected to the voltage source and creates the magnetic flux called the primary

winding, and the second winding called the secondary in which a voltage is induced as a result

of mutual induction.

If the secondary output voltage is less than that of the primary input voltage the transformer is

known as a “Step-down Transformer”. If the secondary output voltage is greater then the

primary input voltage it is called a “Step-up Transformer”.

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The type of wire used as the main current carrying conductor in a transformer winding is either

copper or aluminium. While aluminium wire is lighter and generally less expensive than copper

wire, a larger cross sectional area of conductor must be used to carry the same amount of current

as with copper so it is used mainly in larger power transformer applicatio ns.

Small kVA power and voltage transformers used in low voltage electrical and electronic

circuits tend to use copper conductors as these have a higher mechanical strength and smaller

conductor size than equivalent aluminium types. The downside is that when complete with

their core, these transformers are much heavier.

Transformer windings and coils can be broadly classified in to concentric coils and sandwiched

coils. In core-type transformer construction, the windings are usually arranged concentrica l ly

around the core limb as shown above with the higher voltage primary winding being wound

over the lower voltage secondary winding.

Sandwiched or “pancake” coils consist of flat conductors wound in a spiral form and are so

named due to the arrangement of conductors into discs. Alternate discs are made to spiral from

outside towards the centre in an interleaved arrangement with individual coils being stacked

together and separated by insulating materials such as paper of plastic sheet. Sandwich coils

and windings are more common with shell type core construction.

Helical Windings also known as screw windings are another very common cylindrical coil

arrangement used in low voltage high current transformer applications. The windings are made

up of large cross sectional rectangular conductors wound on its side with the insulated strands

wound in parallel continuously along the length of the cylinder, with suitable spacers inserted

between adjacent turns or discs to minimize circulating currents between the parallel strands.

The coil progresses outwards as a helix resembling that of a corkscrew.

4.1.3 Transformer oil

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent

electrical insulating properties. It is used in oil-filled transformers, some types of high-

voltage capacitors, fluorescent lamp ballasts, and some types of high-

voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing,

and to serve as a coolant.

The oil helps cool the transformer. Because it also provides part of the electrica l

insulation between internal live parts, transformer oil must remain stable at high temperatures

for an extended period. To improve cooling of large power transformers, the oil-filled tank may

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have external radiators through which the oil circulates by natural convection. Very large or

high-power transformers (with capacities of thousands of kVA) may also have cooling fans,

oil pumps, and even oil-to-water heat exchangers.[1]

Large, high voltage transformers undergo prolonged drying processes, using electrical self-

heating, the application of a vacuum, or both to ensure that the transformer is completely free

of water vapour before the cooling oil is introduced. This helps prevent corona formation and

subsequent electrical breakdown under load.

4.1.4 Conservator

Conservator conserves the transformer oil. It is

airtight metallic cylindrical drum which is fitted

above the transformer. The conservator tank is vented

to the atmosphere at the top and the normal oil level

is approximately in the middle of the conservator to

allow expansion and contraction of oil during the

temperature variations. It is connected to the main

tank inside the transformer which is completely filled

with transformer oil through a pipeline.

4.1.5 Breather

The natural air always consists of more or less moisture in it and this moisture can be mixed

up with oil if it is allowed to enter into the transformer. The air moisture should be resisted

during entering of the air into the transformer, because moisture is very harmful for transformer

insulation. A silica gel breather is the most commonly used way of filtering air from moisture.

Silica gel breather for transformer is connected with conservator tank by means of breathing

pipe. The silica gel breather of transformer is very simple in the aspect of design. It is nothing

but a pot of silica gel through which, air passes during breathing of transformer. The silica gel

is a very good absorber of moisture. Freshly regenerated gel is very efficient, it may dry down

air to a dew point of below − 40°C. A well maintained silica gel breather will generally operate

with a dew point of − 35°C as long as a large enough quantity of gel has been used. The picture

shows a silica gel breather of transformer.

4.1.6 Tap Changer

A tap changer is a connection point selection mechanism along a power transformer winding

that allows a variable number of turns to be selected in discrete steps. A transformer with a

variable turns ratio is produced, enabling stepped voltage regulation of the output. The tap

selection may be made via an automatic or manual tap changer mechanism. The output voltage

may vary according to the input voltage and the load. During loaded conditions the voltage on

Figure 24:conservator (source: Internet)

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the output terminal fall and during off load conditions the output

voltage increases. In order to balance the voltage variations, tap

changers are used. Tap changers can be either on load tap

changer and off load tap changer. In on load Tap changers the

tapping can be changed without isolating the transformer from

the supply and in off load tap changers it is done after

disconnecting the transformer. Automatic tap changers are also

available.

4.1.7 Cooling Tube

Cooling tubes are used to cool the transformer oil. The Transformer oil is circulated through

the cooling tubes. The circulation of the oil may either be natural or forced circulation. In

natural circulation, when the temperature of the oil raises the hot oil naturally moves to top and

cold oil moves downwards. Thus the oil keeps on circulating through the tubes. In forced

circulation, an external pump is used for circulating oil.

4.1.8 Buchholz Relay

Buchholz relay in transformer is an oil container housed the connecting pipe from main tank

to conservator tank. It has mainly two elements. The upper element consists of a float. The

float is attached to a hinge in such a way that it can move up and down depending upon the oil

level in the Buchholz relay Container. One mercury switch is fixed on the float. The alignment

of mercury switch hence depends upon the position of the float.

The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge

just in front of the inlet (main tank side) of Buchholz relay in transformer in such a way that

when oil enters in the relay from that inlet in high pressure the alignment of the baffle plat e

along with the mercury switch attached to it, will change.

In addition to these main elements a Buchholz relay has gas release pockets on top. The

electrical leads from both mercury switches are taken out through a moulded terminal block.

Figure 25:Tap Changer (source:

Internet)

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4.1.9 Explosion Vent

Explosion vent is used to expel the boiling oil in the transformer during heavy internal faults

in order to avoid the explosion of the transformer. During heavy faults the oil rushes out of the

vent. The level of the explosion vent is normally maintained above the level of the conservatory

tank.

4.2 Working of Interconnecting Transformer

Within each transformer case are three extra voltage transformers, one for each

phase. Each has its own conductor in and conductor out.

A transformer is a laminated steel core surrounded by two different layers of

wire windings. One set of windings has fewer loops of wire than the other. One

with fewer loops is low voltage set of winding. The LV winding sit between

core and HV winding.

The current comes into transformer through a conductor that joins on the HV

windings, this induces a magnetic flux in the iron core. This magnetic flux

induces a current in the lower voltage windings.

Because there are fewer complete loops of wire in the second set of windings,

the current sent out from them has a lower voltage from the one that came in.

To adjust the final voltage further we can tap changers. These reduce the number

of winding on the LV side further by changing the location of the conductor that

leads out of the transformer.

The sinusoidal current wave reduces as the tap changer connects to fewer

windings reducing the voltage.

Thus electricity comes in at one voltage and goes out at the other at a different voltage.

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5. Subsystem Protection

5.1 Substation Grounding The sole purpose of substation grounding is to protect the equipment from surges and lightning

strikes and to protect the operating persons in the substation. Hence intentional grounding system is created by laying grounding rod of mild steel in the soil of substation area. All

equipments/structures which are not meant to carry the currents for normal operating system are connected with main earth mat .The substation grounding system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the

non-current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Grounding of surge arresters is through the grounding system. The function of

substation grounding system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and low earth resistance.

The earthing system in a substation:

Protects the life and property from over-voltage.

To limit step & touch potential to the working staff in substation. Provides low impedance path to fault currents to ensure prompt and consistent operation of protective device.

Stabilizes the circuit potentials with respect to ground and limit the overall potential rise.

Keeps the maximum voltage gradients within safe limit during ground fault condition inside and around substation.

5.1.1 Earth Resistance:

Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth

potential with respect to a remote earth fault. Persons touching any of the non-current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each

structure, transformer tank, body of equipment, etc, should be connected to grounding mat by their own earth connection. Generally lower earth resistance is preferable but for certain applications following earth resistance are satisfactory

Large Power Stations – 0.5 Ohm Major Power Stations - 1.0 Ohm

Small Substation – 2.0 Ohm In all Other Cases – 8.0 Ohm.

5.2 Step Potential and Touch Potential Grounding system in an electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’.

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5.2.1 Step Potential: Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault

current through the grounding system.

5.2.2 Touch Potential:

Touch potential is a potential difference between the fingers of raised hand touching the

faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e. The Touch

Potential should be very small.

Figure 26:Step Potential and Touch Potential (source: Internet)

Usually, In a substation a surface layer of 150 mm of rock (Gravel) of 15 mm to 20 mm size shall be used for the following reasons:

To provide high resistivity for working personnel.

To minimize hazards from reptiles.

To discourage growth of weed.

To maintain the resistivity of soil at lower value by retaining moisture in the under laying soil.

To prevent substation surface muddy and water logged.

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5.3 FORMATION OF SUBSTATION EARTHING:

The main earth mat shall be laid horizontally at a regular spacing in both X & Y

direction(9m) based upon soil resistivity value and substation layout arrangement. The main earth mat shall be laid at a depth of 600 mm from ground. The earth mat shall be connected to

the following in substation. Lightning down conductor, peak of lightning mast

Earth point of S A, CVT

Neutral point of power Transformer and Reactor

Equipment framework and other non-current carrying parts.

Metallic frames not associated with equipment

Cable racks, cable trays and cable armour.

5.4 LIGHTNING PROTECTION

The protection from the lightning is done with the help of shield wire and lightning mast (high lattice structure with a spike on top).

5.4.1 Shield wire Shield wire lightning protection system will be generally used in smaller sub stations of: Lower voltage class, where number of bays is less, area of the substation is small, & height of

the main structures is of normal height. The major disadvantage of shield wire type lightning protection is, that it causes short circuit in the substation or may even damage the costly equipment’s in case of its failure (snapping).

Figure 27: Shield Wire (source: Internet)

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5.4.2 Earth wire Overhead power lines are often equipped with a ground conductor (shield wire or overhead

earth wire). A ground conductor is a conductor that is usually grounded (earthed) at the top of the supporting structure to minimize the likelihood of direct lightning strikes to the phase

conductors. The ground wire is also a parallel path with the earth for fault currents in earthed neutral circuits. Very high-voltage transmission lines may have two ground conductors. These are either at the outermost ends of the highest cross beam, at two V-shaped mast

points, or at a separate cross arm. By protecting the line from lightning, the design of apparatus in substations is simplified due to lower stress on insulation. Shield wires on

transmission lines may include optical fibres (OPGW), used for communication and control of the power system. 7/3.66 mm wire is used for providing earthing in lightning mast and towers. The main function of Earth wire/Ground wire is to provide protection against direct

lightening strokes to the line conductors or towers.

In addition, ground wire reduces the induced voltage on parallel telecom lines under fault condition Ground wire must meet the following requirements:

It must be able to carry the maximum lightening current without undue overheating.

It must be strong mechanically.

It must be high enough to afford protection to all the line conductors. This function is

called shielding.

It must have enough clearance above the line conductors at mid-span to prevent a side

flashover to a line conductor.

Tower footing resistance should be low.

5.5 TRANSFORMER AND REACTOR

PROTECTIONS

5.5.1 Buchholz Relay:

This relay is located in pipe between Transformer. tank and conservator and protects/warns for incipient faults internal to the Transformer. Main faults in this group

are core insulation failure, loss of oil and winding. Turn to turn short.

The relay has two elements; upper one is a float with a mercury switch. lower elements consist of baffle plate and a mercury switch. Due to incipient minor faults gas is

produced which in turn reduces oil level in relay and upper float sinks causing its mercury switch to close and alarm is initiated.

In case of severe fault likes phase to earth or ph˗ph short circuit or faults in OLTC a surge of oil results and it strikes a baffle plate which causes second mercury switch to

close and trip command to Transformer. CB is initiated.

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Figure 28: Buchholz Relay (source: Internet)

Figure 29: Buchholz Relay (source: Internet)

5.5.2 PRV (Pressure Relief Valve) protection:

On transformer tank two pressure relieving valves are provided which opens

whenever the pressure inside the Transformer. Tank increases beyond designed value.

The operation of PRV involves loss of substantial Transformer. oil and this protection

operates whenever heavy gas is generated inside the transformer tank because of severe insulation failure in core or transformer windings.

A Micro switch closes on PRV operation which energizes an aux. relay whose N/O

contact closes and trips the Transformer. HV & LV CBs.

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Inter –tripping of HV & LV CB is incorporated.

5.5.3 Differential protection:

Low impedance type static percentage biased restraint differential relay MBCH13 is provided for detecting Phase to earth fault and phase to phase fault internal to

Transformer. and terminal faults.

The relay has an operating time of 10-25 sec. and provides high stability against heavy

thorough faults, magnetizing inrush current and over fluxing conditions.

The setting range of differential current is 10-50% of in .generally relay diff. current is

set at 20% in. The relay also has a high -set ranging feature and varies from 4 in at normal load condition to 8 In at heavy thorough faults .

Inter -tripping of HV & LV CB is incorporated.

5.5.4 Back up O/C and E/F protection:

Directional O/C and E/F backup protection against external/internal short circuits and

excessive O/L is provided by CDD31 IDMT relay.

5.5.5 Restricted E/F protection:

CAG14 high impedance relay is employed to provide restricted to E/F protection for

transformer windings.

This is also a differential protection where 3 line CT output are paralleled and balanced

against neutral CT output. The differential current setting is kept at minimum i.e.10% In .

Inter -tripping of HV & LV CB is incorporated.

5.5.6 Over fluxing protection:

GTTM relay operating of V/F ration is used to protect Transformer against over fluxing

condition. this occurs when large quantum of Transformer. Load is disconnected which results in the rise of transformer Primary voltage and its exciting current.

5.5.7 Oil temperature alarm/trip:

Oil temperature is measured and indicated by oil temperature indicator When

temperature reaches 85degree C alarm is initiated. If left unattended and reaches 95-degree C trip command to HV & LV CBs is given.

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5.5.8 Winding temperature alarm/trip:

Winding temp. is indirectly measured by adding equivalent temperature generated by flow of load current in transformer.

For this a turret CT is used to supply load current to a thermister whose resistance changes according to temperature rise due to current flow .

Generally an aux. relay contacts are used for initiating winding temp. alarm /trip .

When temperature reaches 90degree C first aux. relay operates and its N/O contact

closes the supply ckt . of cooler fans start them.

When temperature reaches 95 degree C second aux. relay operates and its N/O contact

close the supply ckt .of cooler oil pump and pump starts.

When temperature reaches 100 degree C third aux. relay operates and its N/O contact

closes the alarm ckt.

When temperature reaches 110 degree C fourth aux. relay operates and its N/O

contact closes the trip ckt to the Transformer. HV & LV CBs.

5.6 Low oil level alarm:

Whenever oil level in the transformer drops below designed /specified safe operation level an alarm is initiated through a micro switch connected with MOG installed at

transformer main tank which operates an aux. relay to initiate alarm in the control room.

The relay has stepped time distance characteristics for three independent measuring

zones, having quadrilateral shaped mho characteristics.

The max operating time of the relay for zone -1 fault is 40 ms for all types of 30-75 degrees.

The relay schemes include timers for zone -2&3 are having continuously variable setting of 0-3&0-5 seconds respectively.

It is suitable for carrier aided tripping.

It is having power swing blocking protection for blocking the tripping in zone 2&3 but

tripping can be permitted in zone -1, if desired.

The relay has memory circuit to ensure correct operation during close 3-phase fault or

switch on to fault feature (SOFT).

The relay also provides Distance to fault measurement.

5.6.1 Auto –reclose Relay:

VARM relay is provided which is suitable for 1/3 phase auto -reclosing.

The relay has continuously variable Dead time setting range of 0.1-2 sec.

The relay scheme has reclaim timer setting of 25secs.

The relay scheme is of single shot type.

The scheme has provision for assigning priority in both CBs in case of 11/2 breaker

scheme to allow closing of main CB first.

The relay scheme has facility for selecting check synchronizing or dead line charging

feature for which SKD and VAG type relays provided respectively.

SKD relay have response time of 200 ms and a continuously variable timer with range

0.5-5 seconds. The max. phase angle setting is 35 degrees and max. voltage difference setting is 10%.

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VAG type relay have two sets of relay and each set is able to monitor the 3-phase

voltage where one set is connected to line CVTs with fixed setting of 20% of rated voltage and other set is connected to bus CVTs with a fixed setting of 80% of rated voltage.

5.6.2 Over Voltage Relay:

VTU type relays provided with two stages.

1st stage has IDMT characteristics and adjustable setting of 100-170% of rated voltage

with a timer of 1-60 seconds.

2nd stage has instantaneous characteristics and adjustable setting of 100-170% of rated

voltage with a timer of 0-200ms. Voltage setting of 110% &150% and Timer setting of 4 seconds & 30 ms are kept for

1st & 2nd stage respectively.

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6. TRANSMISSION LINE PROTECTION

6.1 Introduction: All relay are electrically isolated from main power system and receive measuring quantities

from CT, CVT etc. The following basic properties are desired of any relay/protection scheme: - Selectivity - also known as discrimination means relay should chose correctly which portion

to isolate and trip nearest CB only. - Stability- means relay should remain unaffected by load conditions and external faults.

- Speed- means relay should operate quickly for minimizing the damage due to fault. - Sensitivity- relay should have minimum operating current/voltage to operate.

6.2 Distance Relay:

Normally this relay scheme is employed for line protection. Since impedance of line is

proportional to its length, therefore relay measures impedance and based on this decides whether faults lies within its zone or not. Accordingly relay either operates or restrain.

To ensure correct co-ordination between distance relay in a system it is customary to

choose a relay setting of 80% of line impedance for Zone-1 setting. In Zone-2 setting reach of relay is extended to 1st line +50% of next shortest line.

Zone-3 setting is kept as 1st line +2nd longest 25% of third shortest line.

6.3 Main-I Protection:

In 400KV line this protection is provided by a numerical distance scheme such as EPAC3000.

This relay is self-monitoring type, non-switched scheme with separate measurements for all ph-ph and ph-earth faults.

The relay has self-diagnostic features.

The relay is having stepped time distance characteristics for three independent

measuring zones, having quadrilateral shaped Mho characteristics. The max.

Operating time of the relay for zone-1 fault is 40 ms for all types of faults. The relay

has independent R/X settings and adjustable characteristics angle of 30-75 degrees.

The relay scheme has timers for Zone-2&3 are having continuously variable setting of

0-3 & 0-5 seconds respectively.

It also has an off-set feature with 10-20% of Zone-3 to cover backward zone faults

between relay and busbar.

The relay has memory circuits to ensure correct operation during close 3-phase fault or

switch on to fault feature (SOTF).

It is suitable for carrier aided tripping.

It is having power swing blocking protection for blocking the tripping in zone 2&3 but

tripping can be permitted in zone-1, if desired.

The relay also provides distance to fault measurement.

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6.4 Main-II protection:

In 400KV line this protection is provided by static modular type distance scheme such

as OPTIMHO.

This relay is non-switched scheme with separate measurements for all ph-ph and ph-

earth faults.

6.5 BUS BAR PROTECTION

The bus bar faults generally involve one phase and earth and large no. of bus bar faults

result from human error when safety earthing done for maintenance work is not removed before charging the bus.

The requirement of bus bar protection is high speed of operation (of the order of one cycle) to limit the consequential damage and maintain system stability.

The other important requirement of bus bar protection is that it should be completely stable.

To achieve this 2 independent measurement are taken by two differential relay systems being energized from separate cores of CTs. One relay is applied to each busbar/bus

section and second relay is applied to both buses/sections and called check system.

The tripping of busbar/section is only initiated if both i.e. its busbar/section relay has

operated and check busbar relay has also operated.

The busbar protection is based on circulating differential current measurement

principle. The CT output of all the zone/section feeders/ckts are connected in parallel and relay measuring element is connected between phase and neutral.

The busbar protection covers phase as well as earth faults in that zone and setting for

operation is kept same for both.

The selective tripping of bus zone/bus section involved with fault is achieved through

the position of aux. contact of bus isolators of faulted feeder.

For 400KV busbar two identical low impedance biased differential protection scheme

MBCZ are employed.

It is a modular solid state relay having very fast operating time of less than 20ms.

Separate modules for feeders, tie, Bus coupler, zone measuring and bus selection isolators are assembled to make the scheme functional.

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7. Maintenance 7.1 Maintenance of Transformer

Generally EHV class power Transformers and shunt Reactors are dispatched

from manufacture’s works filled with dry/nitrogen and fitted with Impact

Recorder. During transportation by rail/road, positive pressure of N2/dry air

needs to be maintained to minimise the ingress of atmospheric moisture.

After Receipt at Site, N2 pressure and Dew points should be checked after

receipt of transformer at site. It should be within permissible band.

Oil samples shall be taken from oil drums/tankers received at site and sent to

our Lab for oil parameter testing.

If transformer is to be stored up to three months after arrival at site, it can be

stored with N2 filled condition. N2 pressure and dew point to be monitored on

daily basis so that chances of exposure of active part to atmosphere are avoided.

In case of storage of transformer in oil-filled condition, the oil filled in the unit

should be tested for BDV and moisture contents once in every three months.

During erection, the active part of transformer should be exposed to atmosphere

for minimum time and either dry air generator should be running all the time or

dry air cylinders may be used to minimize ingress of moisture. After erection

activity, the transformer should be filled with dry nitrogen during night.

The pre-commissioning tests after erection shall be carried out in accordance

with latest technical specifications issued by CC/Engg., Pre-Commissioning

procedures/formats issued by CC/OS and Guidelines/ instructions from the

equipment manufacturer.

The test will be carried out only after obtaining approval of head of O&M of

the region based on a proposal of execution site in consultation with Regiona l

OS.

Figure 30: Pressure vs Oil Flow Rate (source: Internet)

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7.2 General Maintainence

Dirt and Dust: - The external transformer surfaces shall be inspected regular ly;

and when required cleaned from dust, insects, leaves, and other airborne dirt.

Rust damages, touch up paintings: - A regular inspection of the external

surface treatment (i.e. paint) of the transformer should be carried out. Possible

rust damages are removed and the surface treatment restored to original state

by means of the primer and finish-paints of the transformer to minimise the risk

of corrosion and its subsequent spreading. These checks also include looking

for signs of oil leaks on gasket areas and weldered areas containing oil. The

touch-up paint as and when required as per site condition and re-painting is

advised once in five years or as per the manufacture’s recommendation.

Silica Gel Breather Check: - In order to prevent severe deterioration of the

silica gel charge, it is recommended that it is replaced when half to two thirds

of the gel has become saturated and turned pink color. Failure to do so will

severely retard the drying efficiency of the breather. The silica gel can be

reactivated whilst in its charge container or it can be emptied into a shallow

tray. It should then be heated in a well ventilated oven at a temperature of 130°C

to 138°C until the entire mass has achieved the bright blue color. Alternative ly,

the silica gel can be dried in hot sun for a period of 8 hours or more to regenerate

it.

Thermo-Vision Scanning of Transformer: - A Thermo-Vision camera

determines the temperature distribution on the surface of the tank as well as in

the vicinity of the jumper connection to the bushing.

Checks on Gas Pressure Relay: - There are two types of sudden pressure

relays. The most common type is mounted under the oil and a similar type that

is mounted under the oil and a similar type that is mounted in the gas space.

Internal arcing in liquid filled electrical power equipment generates excessive

gas pressures that can severely damage transformer and present extreme

hazards to personnel. The sudden pressure relay is intended to minimise the

extent of damage by quickly activating protection system.

Checks on Bushings: - After one month of service and on yearly basis, bushing

porcelains shall be cleaned from dust and dirt regularly. If the bushing is

damaged or heavily polluted / contaminated, leakage current becomes

excessive, and visible evidence may appear as carbon tracing (treeing) on the

bushing surface (may require binoculars for viewing). Flashovers may occur if

bushings are not cleaned periodically. Check the bushing oil level by viewing

the oil-sight class or oil level gauge. When the bushing has a gauge with a

pointer, look carefully, because the oil level should vary a little with

temperature changes. If oil level is low and there is no visible external leak,

there may be an internal leak around the lower seal in to the transformer tank.

Page | 48

If possible, re-fill the bushing with same oil in consultation with manufacturer

and carefully monitor the level and the volume it takes to fill the bushing to

proper level.

7.3 Filtration and Degassing of Transformer oil

The plant is suitable for treating transformer oil by first heating it and then passing it through

specially designed filter and then subjecting it to vacuum treatment which dehydrates and

degasifies the oil to achieve following improvements after filtration as per IS 335-1993 for

new transformer oil purification and IS 1866 for used transformer oil filtering.

Break Down Voltage

Moisture Content

GAS Content

Suspended Particles

Acidity

Tan Delta

The plant shall generally confirm to is-6034-1989 and its latest revision.

The transformer oil filtration plant shall be designed for high vacuum and low temperature of

oil for achieving required results.

The transformer oil filtration machine shall be mobile type mounted on four pneumatic wheels

with towing and lifting arrangement, Roller Mounted or Skid Mounted as per requirement. The

plant shall be weather proofed and shall be suitable for outdoor use. The casing are provided

with doors of CRCA sheets, hinged on fabricated frame work, angles and channels to have

access to operational controls and inspection windows etc. The equipment shall be enclosed

and protected against climatic conditions.

All components shall have adequate strength and rigidity to withstand normal conditions of

handling transport and usage and shall be free from edges or corners to avoid injury to

operating personnel in normal conditions of use. The design of the oil purifier plant shall be

Page | 49

such that if required the part/s can easily be replaced. Proper guarding arrangement shall be

provided on all such parts which due to their position and nature of operation are liable to cause

accidents.

Figure 31: Oil Filtration Machine

For filtration plant are generally high vacuum type consisting of high vacuum creating system

comprising of high vacuum capacity Rotary oil seal type pumps and Mechanical booster pump,

which create high vacuum for degasifying dissolved gases and moisture contains form the

transformer oil.

Before degasifying the oil is heated upto the desired temperature and then it is filtered through

specially designed filters like magnetic strainer, press filter, bag filter, EDGE Filter, mic ro

filter, Ironic Reaction Column, etc as per requirement for degasification process. Oil is sprayed

out for degassing where gases and moisture being abstracted through high vacuum system.

After degasifying the process oil is being taken out by discharge pump from the degassing

chamber under vacuum.

Page | 50

The filtration plants are used for online as well as off-line transformer oil filtration of various

capacity transformers.

These processes of dehumidification of transformer oil and removal of gasses is executed in

the degassing chamber. The dissolved water oil separation or dissolved gas oil separation is

possible at reduced pressure, i.e. vacuum, due to difference of boiling point of water, gas and

transformer oil. In the process of separation of gases from the oil it becomes important to retain

the aromatic hydrocarbons so that the original properties of the oil are retained. When the water

level in the oil is above saturation level of the transformer oil, oil is observed in free water.

Removal of free water can be done by power driven centrifuge or by coalescing princip le,

where the latter is more effective and economical in practice.

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8. Testing 8.1 Transformer Testing

For confirming the specifications and performances of an electrical power transformer it has to

go through numbers of testing procedures. Some tests are done at manufacturer premises before

delivering the transformer. Mainly two types of transformer testing are done at manufacturer

premises- type test of transformer and routine test of transformer. In addition to that some

transformer tests are also carried out at the consumer site before commissioning and also

periodically in regular & emergency basis throughout its service life.

8.1.1 Tests done at Factory Type tests - Type test of transformer confirms main and basic design

criteria of a production lot.

Routine Tests - Routine tests of transformer is mainly for confirming

operational performance of individual unit in a production lot. Routine

tests are carried out on every unit manufactured.

Special Tests - Special tests of transformer is done as per customer

requirement to obtain information useful to the user during operation or

maintenance of the transformer.

8.1.2 Tests done at site

Pre-commissioning test - The transformer testing performed before

commissioning the transformer at site is called pre-commissioning test

of transformer. These tests are done to assess the condition of

transformer after installation and compare the test results of all the low

voltage tests with the factory test reports.

Periodic / Condition monitoring Test

Emergency Test

8.1.3 Type Tests on Transformer

Transformer winding resistance measurement

Transformer ratio test.

Transformer vector group test.

Measurement of impedance voltage/short circuit impedance (principa l

tap) and load loss (Short circuit test).

Measurement of no load loss and current (Open circuit test).

Measurement of insulation resistance.

Dielectric tests of transformer.

Temperature rise test of transformer.

Tests on on-load tap-changer.

Vacuum tests on tank and radiators.

8.1.4 Routine Tests on Transformer

Transformer winding resistance measurement.

Transformer ratio test.

Page | 52

Transformer vector group test.

Measurement of impedance voltage/short circuit impedance (principa l

tap) and load loss (Short circuit test).

Measurement of no load loss and current (Open circuit test)

Measurement of insulation resistance.

Dielectric tests of transformer.

Tests on on-load tap-changer.

Oil pressure test on transformer to check against leakages past joints and

gaskets.

8.1.5 Special Tests on Transformer

Dielectric tests.

Measurement of zero-sequence impedance of three-phase transformers

Short-circuit test.

Measurement of acoustic noise level.

Measurement of the harmonics of the no-load current.

Measurement of the power taken by the fans and oil pumps.

Tests on bought out components / accessories such as buchhloz relay,

temperature indicators, pressure relief devices, oil preservation system

etc.

8.2 Tests on Transformer

8.2.1 Transformer Winding Resistance Test

Transformer winding resistance measurement is carried out to calculate the I2R losses and to calculate winding temperature at the end of a temperature rise test. It is carried out as a type

test as well as routine test. It is also done at site to ensure healthiness of a transformer that is to check loose connections, broken strands of conductor, high contact resistance in tap changers,

high voltage leads and bushings etc.

There are different methods for measuring of transformer winding, likewise

Current voltage method of measurement of winding resistance.

Bridge method of measurement of winding resistance. Kelvin bridge method of

Measuring Winding Resistance.

8.2.2 Transformer Ratio Test

The performance of a transformer largely depends upon perfection of specific turns or voltage

ratio of transformer. So transformer ratio test is an essential type test of transformer. This test

also performed as routine test of transformer. So for ensuring proper performance of

electrical power transformer, voltage and turn ratio test of transformer one of the vital tests.

Page | 53

8.2.3 Magnetic Balance Test

Magnetic balance test of transformer is conducted only on three phase transformers to check

the imbalance in the magnetic circuit.

Procedure of Magnetic Balance Test of Transformer

First keep the tap changer of transformer in normal position.

Now disconnect the transformer neutral from ground.

Then apply single phase 230 V AC supply across one of the HV winding

terminals and neutral terminal.

Measure the voltage in two other HV terminals in respect of neutral terminal.

Repeat the test for each of the three phases.

In case of auto transformer, magnetic balance test of transformer should be repeated for LV

winding also.

There are three limbs side by side in a core of transformer. One phase winding is wound in

one limb. The voltage induced in different phases depends upon the respective position of the

limb in the core. The voltage induced in different phases of transformer in respect to neutral

terminals given

8.2.4 Magnetising Current Test of transformer

Magnetizing current test of transformer is performed to locate defects in the magnetic core

structure, shifting of windings, failure in turn to turn insulation or problem in tap changers.

These conditions change the effective reluctance of the magnetic circuit, thus affecting the

current required to establish flux in the core.

8.2.5 Short Circuit Test

It determines the series branch parameters of the equivalent circuit of Real Transformer. It is

conducted on HV side where the LV side is short circuited. The supply voltage should circulate

rated current through the transformer and voltage is a small percentage of the nominal voltage,

wattmeter readings indicate the total copper loss of transformer at full load.

8.3 Testing of CT

Ratio test

Secondary core winding resistance measurement test

Insulation Resistance measurement Test

Capacitance measurement

Magnetising Current Test

Polarity test

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8.4 Transformer Oil Testing

The insulation oil of various types of transformers fulfils the purpose of insulating as well as

cooling.

Generally, highly refined mineral oil is used as transformer oil which is of two types:

Naphthenic oil

Paraffinic Oil

Transformer oil deteriorates through Aging, Moisture or any types of faults so it should be

tested periodically. The Transformer oil tests are: -

8.4.1 DGA (Dissolved Gas Analysis Test)

It is the study of dissolved transformer oil insulating materials in

transformer break down to liberate gases within the unit. The

distribution of these gases can be related to the type of electrica l

fault and rate of gas generation indicate severity of fault.

8.4.2 Acidity Test

The acidity test is used to estimate the total acid value of the

transformer insulating liquid. As acid value increases the value

of insulating quality of oil decreases.

Figure 32: Dissolved gas testing

equipment (source: Internet)

Figure 33: Acidity Testing Equipment (source: Internet)

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8.4.3 Tan Delta Test

It is used to measure the deterioration of

cables and the aging process and helps us to

estimate the remaining life. It is also called

loss angle test.

8.4.4 BDV (Breakdown Voltage Test)

The BDV test is a physical test that

measures the break down voltage of insulat ion

liquid. It serves as an indicator to the presence of

contaminating agents like water, dirt etc.

8.4.5 Water Content Test

Figure 36: Water Content testing Equipment (source: Internet)

Figure 34: Tan Delta Testing Equipment

Figure 35: Breakdown voltage Equipment (source: Internet)

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It is used to check the water content in transformer oil.

8.4.6 Furan Test

By measuring the quantity of furans present in transformer oil quality of paper insulation can

be inferred with high degrees of precision.

8.4.7 Interfacial Tension Test

This test indicates the presence of polar compounds. These compounds

are considered by some to be an indicator of oxidation.

8.4.8 Viscosity Test

The flow characteristics of oil depend upon the viscosity.

And cooling depends on Fast heat exchange and thus fast

flow of oil.

Figure 37: Interfacial

Tension Testing Equipment (source: Internet)

Figure 38: Viscosity Testing Equipment

Page | 57

8.4.9 Flash Point Test

The maximum temperature after which the coil

catches fire is called flash point, so this test is done

to avoid burning of oil.

Limits of Various Tests

Table 3: Limits of Various Tests

Std name BDV Water

Content

Resistiv

ity

Tan

Delta

IFT Acidity Flash

Point

Viscos

ity

DGA

IS 1866 in service (power

transformer 400KV)

50 20 0.1 0.2 15 0.3 125

IS 1866

Before Charging

(power transformer 400KV)

60 10 6 0.01 35 0.03 140 27 0.89

IS 335

(Fresh Oil)

30 50 35 0.002 40 0.03 140 27 0.89

Figure 39: Flash Point Testing Equipment (source: Internet)

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9. NTAMC & RTAMC

The emphasis on the power sector to ensure the growth in GDP has brought in many

changes in the business environment of Power Sector. The transmission sector being the integral part of, is also facing multiple challenges like competitive bidding for

transmission project, lack of experienced manpower, stringent demands by the regulator etc.

The technological development couple with falling prices of communication system and information technology provides us the opportunity for virtual manning of

Substation thereby optimizing the requirement of skilled manpower and managing the asset with the available skilled workforce.

Thus, state of the art computerized control centers NTAMC & RTAMC with associated telecommunication system and adapted substation for enabling remote centralized

operation, monitoring and control of POWERGRID Transmission system has been proposed.

The aim is to have completely unmanned substation except security personnel. The

operations of the substations will be done from a remote centralized location i.e. NTAMC. The RTAMC will co-ordinate the maintenance aspect of the substation from a centralized location and will act as a backup to the NTACM for operation. The

maintenance activities would be carried out by maintenance service hub (MSH). One MSH will cater to the requirements of 3-4 substations in its vicinity in coordination

with the respective RTAMCs.

The substations and various control centers will be connected by redundant broadband

communication network through POWERGRID (Telecom) communication links.

Telecom Department to provide high speed communication links between NTAMC, RTAMCs and Sub-stations.

The Connectivity Status has been finalized in association with LD&C department and

NTAMC group. More links have to be planned by LD&C for total protection. Bandwidth requirement and Connectivity Scheme finalized. At stations where this connectivity is not possible, leased lines will be hired from other telecom operators up

to the nearest connection point.

Total 192 Substation connectivity will be planned in 2 phases. Phase-I 120 Sub Stations

Phase-II 72 Sub Stations

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Conclusion

By Doing Summer Training in Patna Substation, I came to know about How the

transmission and distribution of Power lines are carried out in our country. I got to know about the construction, maintenance and different protections in a line

and substation. I was exposed to different types of equipment like isolators, circuit breakers, relays, etc used in substation and I also came to know how the

signals from the equipment kept in switchyard are brought in the control room for monitoring as well as controlling it from control room. I was also taught

about how the equipment were controlled if any fault occurs in them.