This document provides an overview of the Patna 400/220 KV substation owned by Power Grid Corporation of India Limited (PGCIL). It begins with background on PGCIL, describing its role in developing India's power transmission infrastructure. It then discusses classifications of substations and provides a single line diagram and description of the Patna substation. The document outlines various equipment used at substations like transformers, circuit breakers, buses, and protection systems. It also covers maintenance, testing procedures, and concludes with thanking those involved in the author's training at the Patna substation.

1. 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 TouchPotential 36
5.2.1 Step Potential 37
5.2.2 TouchPotential 37
5.3 Formation of SubsystemEarthing 38
5.4 Lightning Protection 38
5.4.1 Shield Wire 38
5.4.2 Earth Wire 39
5.5 Transformer and ReactorProtection 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 utilities
company headquartered in Gurgaon, India. It is one amongst the largest Power Transmission
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 transmission
systems.
In October 23, 1989 under the Companies Act, 1956, the National Power Transmission
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 Transmission
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 sustainable
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 individual
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 conventional
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 transmission
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 terminals
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 SectionalisedBus
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 Breakerand 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 interrupting
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 Configuration
Reliability.
3.3.1.4 Double BreakerDouble 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 additional 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 electrical
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 electrical
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

27. Page | 26
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 electrical
isolation between control and controlled circuits), or where several circuits must be controlled
by one signal. Relays with calibrated operating characteristics and sometimes multiple
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 electrical
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 extinction.
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).

28. Page | 27
Figure 17: Circuit Breaker
3.7 SF6 circuit breaker
SF6 is inert gas the property of this gas the higher pressure and temperature its dielectric
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)

29. Page | 28
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 telecommunications
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 terminal.
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.

30. Page | 29
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.

31. Page | 30
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 electrical
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 usedto supportthe windingsinthe transformer.Italsoprovidesalow reluctance pathto the
flow of magnetic flux. It is made up of laminated softiron core in order to reduce eddy current loss
and hysteresisloss.The compositionof transformercore dependsonsuchfactorsas voltage,current,
andfrequency.Diameterof the transformercore isdirectlyproportional tocopperlossandisinversely
proportional toironloss.If diameterof the core isdecreased,the weightof the steel core isreduced
whichleadstolesscore lossof transformerandcopperlossincrease.The vice versahappenwhenthe
diameterisincreased.
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
transformerconstructionare the Closed-core Transformer andthe Shell-coreTransformer.
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”.

33. Page | 32
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 applications.
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 concentrically
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 electrical
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

34. Page | 33
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)

35. Page | 34
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 plate
along with the mercury switch attached to it, will change.
In addition to these main elements a Buchholz relay has gas release pockets on top. The
electrical leads from both mercury switches are taken out through a moulded terminal block.
Figure 25:Tap Changer (source:
Internet)

36. Page | 35
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.

37. Page | 36
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’.

38. Page | 37
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.

39. Page | 38
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)

40. Page | 39
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.

41. Page | 40
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.

42. Page | 41
 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.

43. Page | 42
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%.

44. Page | 43
 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.

45. Page | 44
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.

46. Page | 45
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.

47. Page | 46
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 Regional
OS.
Figure 30: Pressure vs Oil Flow Rate (source: Internet)

48. Page | 47
7.2 General Maintainence
 Dirt and Dust: - The external transformer surfaces shall be inspected regularly;
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. Alternatively,
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.

49. 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

50. 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, micro
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.

51. 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 principle,
where the latter is more effective and economical in practice.

52. Page | 51
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 (principal
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.

53. Page | 52
 Transformer vector group test.
 Measurement of impedance voltage/short circuit impedance (principal
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 testof 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.

54. 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

55. Page | 54
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 electrical
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)

56. Page | 55
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 insulation
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)

57. Page | 56
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: ViscosityTesting Equipment

58. 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)

59. Page | 58
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

60. Page | 59
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.