BARC

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A REPORT ON
STUDY OF ELECTRICAL POWER SYSTEM OF TAPS-3&4
AND IT’S PROTECTION
AT
TARAPUR ATOMIC POWER STATION- 3&4(NPCIL), MAHARASHTRA
Done under the guidance of
Shri A.N. THAKUR
SME (E)
TAPP- 3&4
BY: -
SIDDHARTH JAIN
(0801EE101060)
Shri G. S. Institute of Tech. & Science,
Indore (M.P.)
JUNE, 2013

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ACKNOWLEDGEMENT
A comprehensive report always requires the goodwill, encouragement and support of many
people. The all-round aspect thinking that an engineer has to have can hardly be gained through
books and classes. The exposure to industries, learning and fulfilling their requirements make me
feel more confident about my knowledge, and such learning process is very motivating make me
feel more confident about my knowledge, and such learning process is very motivating to keep
learning more. Right from the design aspects, protection schemes, calibration and maintenance to
troubleshooting are part of our knowledge.
All this was possible with great support and guidance of employees at TAPS- 3&4, the love and
tenderness that these people have shared with me is invaluable. I am wholeheartedly indebted to
them. I dedicate this project to the employees and friends at electrical maintenance unit (EMU),
TAPS-3&4.
I would like to extend my gratitude to
Shri R.K. Gargye, SD TMS
Shri R.P. Tomar, SD TAPS-3&4
Shri A.N. Thakur, SME (E)
ShriAshwin Kumar Yadav, SO/E
Shri Tapas Kumar Dey, SO/E
ShriNishantDhimole, SO/E
ShriSanjay Panday, SO/D
ShriNayan Shah, SO/D
I would also like to extend my special gratitude to Shri H.T. Gayiker,Shri Rahul Sapkale SA/D
and Shri M.M. Raut TM/D for their support and cooperation.

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TABLE OF CONTENTS
1) Introduction to TAPS-3&4 and India’s nuclear programme
1.1) How a Nuclear Reactor Works
1.2) Importance of the Project
1.3) About the Project
1.3.1) Plant Layout
1.3.2) Unique Features of This 540 MWe Plant
1.4) India’s Nuclear Programme
1.4.1) Pressurized Heavy Water Reactor (PHWR)
1.4.2) Nuclear Fuel Cycle
2) Electrical system
2.1) General Description of Electrical System:
2.2) Objectives of Electrical System
2.3) Classification of Electrical System
2.4) Description of Station Auxiliary Power Supply System
2.4.1) Class IV Power Supply System
2.4.1.1) 6.6 KV System
2.4.1.2) 415V System
2.4.2) Class III Power Supply System
2.4.3) Class II Power Supply System
2.4.4) Class I Power Supply System
2.5) Unit Auxiliary System Voltage Levels
2.6) Nomenclature Adopted For Taps-3&4 Electrical Systems
2.7) Redundancy
2.8) System operation
2.8.1) Normal operation
2.8.2) Shut down condition
2.8.2.1) Unit shut down condition

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2.8.3) Operation Under Off Normal Condition
2.8.3.1) Shut down of one UT
2.8.3.2) Shut down of two UTs
2.8.3.3) Shut down of SUT
2.8.3.4) Fault on any of the 6.6kV buses C41 to C44
2.8.3.5) Fault on any of the 415V buses D41 to D46
2.8.3.6) Tripping of any one of the auxiliary transformers
2.8.4) Operational transients
2.8.4.1) 6.6kV auto/ manual transfer operations
2.8.4.2) Manual transfer
2.8.5) Auto-Transfer scheme
2.8.5.1) Automatic fast transfer (SUT to UTs)
2.8.5.2) Automatic fast transfer (UT to SUT)
2.8.5.3) Automatic slow transfer
2.8.5.4) Emergency transfer scheme
2.8.5.4. A) 6.6 kV class-III, 415v class-III bus supply and
feeder restoration
2.8.5.4. B) EMTR initiation for 415 V class-III buses
2.8.5.4. C) EMTR initiation for 415 v class-II buses
2.8.5.4. D) Emergency transfer panel
3) Power ups system
3.1) General description
3.2) Components of system
3.3) Operations
3.4) Design basis
3.5) Technical particulars of power ups
4) Gas insulated switchyard
4.1) TAPP-3&4 Grid
4.1.1) 400 KV Switchyard

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4.1.2) 220 KV Switchyard
4.2) Details of electrical equipment
4.2.1) Capacitive voltage transformer
4.2.2) SF6 gas circuit breaker
4.2.3) Lighting arrestor
4.2.4) Current transformer
4.2.5) Electromagnetic potential transformer
4.2.6) Disconnecting switches/Earthing switches
4.3) Switching Scheme Adopted
4.4) Operation Principle
4.5) SF6 Monitoring
4.6) Ratings OF 220 KV & 400 KV GIS
4.7) Hot line washing system for insulators
4.8) Why GIS housed indoor switchyard is selected for tapp-3 & tapp-4?
5) Electrical protection
5.1) Purpose of Electrical Protection
5.2) Essential Qualities of Electrical Protections
5.3) Types of Protective Relays
5.3.1) Electromagnetic relays
5.3.2) Static relays
5.3.3) Digital relays
5.3.4) Numerical relays
6) Transformers and its protection
6.1) Transformer Faults
6.2) Transformer Protection
6.2.1) Transformer Instantaneous Over-Current Protection
6.2.2) Transformer Differential Protection
6.2.2.1) Principle of Transformer Differential Protection.
6.2.2.2) Basic Considerations for Transformer Differential protection

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relay settings.
6.2.3) Transformer gas (Buchholz) relay
6.2.3.1) Generation of Gas Due to Faults
6.2.3.2) Operation of a Transformer Gas Relay
6.2.4) WTI and OTI Protection
6.2.4.1) OTI and WTI at TAPS
6.2.5) Overfluxing Protection:
6.2.6) Transformer Ground Fault Protection
6.2.6.1) Residually Connected Earthfault Protection
6.2.6.2) Neutral Connected Earthfault Protection
6.2.6.3) Restricted Earthfault Protection
6.3) Characteristic Transformer Faults
6.4) Condition monitoring of transformers
6.5) Choice of Transformers at TAPS-3&4
6.5.1) Why such GT is chosen?
6.5.2) Why such Unit-transformer is chosen?
6.5.3) Why such type of SUT is chosen?
6.6) Protection of transformer at TAPS-3&4
6.6.1) Protection of GT (Generator Transformer)
6.6.2) Protection for UT
6.6.3) Protection for SUT
7) Generator and generator protection
7.1) Requirement and Functions
7.2) Description
7.2.1) Overall System Layout
7.2.2) Layout of Individual System
7.3) Design Features
7.4) Protections of Generator
7.4.1) Nature of faults in generators and their protection
7.4.1.1) Stator winding faults & protection

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7.4.1.2) Overcurrent protection
7.4.1.3) Overvoltage protection
7.4.1.4) Undervoltage protection
7.4.1.5) Rotor Earth Fault protection:
7.4.1.6) Loss of Excitation (Field Failure):
7.4.1.7) U/O Frequency:
7.4.1.8) Unbalance Loading:
7.4.1.9) Prime mover failure - Reverse power protection:
7.4.1.10) Overspeed Protection:
7.4.1.11) Protection against inadvertent energisation:-
7.4.1.12) Overfluxing
7.5) Generator Circuit Breaker
7.5.1) General Construction Features:
7.5.2) Capabilities of the GCB
7.5.3) Contacts
7.5.4) GCB Specifications:
7.5.5) Air blast type Generator Circuit Breakers
7.5.5.1) Operating Mechanism
7.5.6) Importance and Advantages of using GCB in TAPS-3&4 systems
7.6) Generator protection scheme at TAPS-3&4
7.7) Replacement of REF protection scheme at TAPS-3&4
7.7.1) New relay scheme
7.7.1.1) Installation
7.7.1.2) Testing for REF
8) Motor protection
8.2) Motor Faults and settings
8.2.1) Thermal Overload Protection (49)
8.2.2) Short circuit Protection (50/51)
8.2.3) Start Protection

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8.2.3.1) Excess long Start Protection (48)
8.2.4) Stalling Protection
8.2.5) Negative Phase sequence Protection
8.2.6) Earth fault Protection
9) Vacuum circuit breaker
9.1) Dielectric strength of Vacuum
9.2) Electric arcing in Vacuum
9.3) Phenomena associated with breaking at current zero
9.4) Practical design of vacuum interrupters
9.5) Suitability of Vacuum CB for MV
9.6) Vacuum CB at TAPS- 3&4
9.6.1) CB control
9.6.2) Circuit Breaker Positions
9.6.3) Operating Mechanism
9.6.3.1) Anti-pumping:
9.6.3.2) Trip-free
9.6.3.3) Spring charging & breaker closing mechanism
9.6.4) VCB Particulars
10) Distance protection
10.2) Factors influencing distance protection
10.3) Principles of distance relays
10.4) Relay performance
11) Conclusions
12) Appendix
13) Bibliography

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1) INTRODUCTION TO TAPS-3&4 AND INDIA’S NUCLEAR
PROGRAMME
Tarapur Atomic Power Project, Unit-3&4 is the first twin unit module of 540 MWe capacity
each in India. These units are from pressurized Heavy Water Reactor family. These reactors
are indigenously developed and designed. The twin unit module design has been chosen based
upon the overall techno-economic considerations. Its operation will be supported by supply of
heavy water from the heavy water plants and fuel from the Nuclear Fuel Complex, both under
the Department of Atomic Energy.
Tarapur Atomic Power Project-3&4 is located on the West Coast of the Arabian Sea. The site is
located near the existing Tarapur Atomic Power Station-1&2. The nearest railway station is
Boisar at a distance of 12 Km from the site, which is on the main trunk railway line from
Mumbai-Delhi. The site is well connected by road and is around 30 Km away from the Bombay-
Ahmedabad National Highway.
1.1) HOW A NUCLEAR REACTOR WORKS:
A Nuclear Reactor is a source of heat, which is produced by self-sustained and controlled chain
reaction within the reactor core. The geometrical boundaries within which the nuclear fuel,
moderator, coolant and control rods are arranged to facilitate production and control of the
nuclear reaction to provide heat energy at desired rate is called the reactor core.
The natural uranium is used as a fuel in our Pressured Heavy Water Reactors. Uranium has a
natural property to emanate radio-active particles. This element has 3 isotopes i.e. U-238, U-235
and U-234. Only the isotope U-235 which is around 0.7% in the natural uranium is important for
energy production. When thermal neutron strikes the atom of U-235, fission of U-235 atom

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takes place breaking it up into two or more fragments. During this process enormous heat
energy is generated along with production of two to three fast moving neutrons.
These fast moving neutrons are slowed down in the presence of moderator (heavy water) and its
probability to cause further fission with uranium atom increases. This process continues and
self-sustained chain reaction is maintained. This provides the constant heat energy source. The
energy produced in this process is proportional to the neutron density in the reactor core. Thus
the reactor power is regulated by controlling the absorption of the excess neutrons in the core.
The heat produced in the reactor is used to generate light water steam at high pressure, which
drives the turbo-generator to produce electrical energy.
1.2) IMPORTANCE OF THE ATOMIC PROJECT:
The states in the western zone are located at considerable distances from the coal fields and coal
linkages and transport bottlenecks are becoming difficult day by day. Potential for the
development of hydro-electric power is also limited and is further vitiated by ecological
problems in setting up of the hydro-electric projects. It is in the light of these circumstances that
the addition of 1080 MWe to the western grid from nuclear power is to be viewed. It has been
visualized that there is going to be a magnificent industrial growth in western zone and addition
of 1080 MWe electric power generated by TAPP-3&4 will meet demand as a base load station in
western zone of India.
1.3) ABOUT THE PROJECT:
1.3.1) Plant layout:
a) The Principle features of the layout are:
b) The layout is based on the concept of independent operation of each unit with some of the
common facilities for the reason of economy.
c) All safety related systems and components are placed in separate buildings/structures of
appropriate design including seismic considerations.

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d) All safety related systems and components are protected from low trajectory missiles
emanating from turbine. This has resulted in locating the turbine building at an angle
with reference to reactor building axis.
e) A common fuelling machine head calibration and maintenance facility between the two
reactor units is provided, communicating via fuelling machine air lock and passage
leading to each RB. This facility has been located in service building, which is common
for both units. This facility would reduce the down time of the plant.
f) Reactor Auxiliary Building is located very near to Reactor Building to avoid long piping
lengths.
g) A separate Control Building has been provided as a common facility.
h) A separate emergency control room has been provided for each unit in service buildings
as a back up.
i) Emergency power systems have been provided in two station auxiliary buildings for each
unit for higher reliability.
j) Space around the reactor buildings has also been considered for erection facility for
heavy equipments.
1.3.2) Unique features of this 540 MWE plant:
a) 220 KV & 400 KV gas insulated indoor switchyards (GIS) 400 KV used for power
evacuation system.
b) Introduction of generator CB between generator and GT System divided in two
independent divisions one fed by UT and other fed by SUT
c) Totally independent EMTR for both divisions.
d) 4 DG sets/unit
e) Safety related systems of each division housed in separate buildings qualified for SS
1.4) INDIA’S NUCLEAR PROGRAMME:
To utilize large uranium and thorium reserves in the country for electricity generation, India has
been following a three-stage nuclear power programme, which aims at the development of

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a) Pressurized Heavy Water Reactors, (PHWR) based on natural uranium.
b) Fast breeder reactors utilizing plutonium-uranium fuel cycle, and
c) Breeder Reactors for utilization of thorium.
1.4.1) Pressurized heavy water reactor (PHWR)
In the PHWR based nuclear power programme, India has attained commercial maturity. The
design of 220 MWe PHWR has been standardized and scaled up to 540 MWe. This has been
further scaled up to 700 MWe. Self-reliance has been achieved in the whole ambit of PHWR
technology, and associated fuel cycle, starting from mining and ore processing, fuel fabrication,
fuel reprocessing and waste management, including heavy water production.
Table-1: Nuclear Power Stations in Operation
Unit – Location Reactor Type Capacity (MWe)
Tarapur Atomic Power Station-1&2 BWR 2×160
Tarapur Atomic Power Station-3&4 PHWR 2×540
Rajasthan Atomic Power Station-1&2 PHWR 100, 200
Rajasthan Atomic Power Station-3&4 PHWR 2×220
Madras Atomic Power Station-1&2 PHWR 2×220
Narora Atomic Power Station-1&2 PHWR 2×220
Kakrapar Atomic Power Station-1&2 PHWR 2×220
Kaiga Generating Station-1&2 PHWR 2×220
Total 3900
With a total capacity of 3900 MWe, 16 atomic power reactors (table-1) are in operation in the
country. Currently, 7 nuclear power reactors with a total capacity of 3380 MWe are under
construction. These include, two 220 MWe PHWRs each at Rawatbhatta and Kaiga, two 100
MWe pressurized water reactors at Kundakulam (Tamil Nadu), and one 50 MWe fast breeder
reactor at Kalpakam and two newest 700MWe reactors at Kakrapar, Gujarat. The current share
of nuclear power generation as a percentage of total electricity generation in the country is 3 %.

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1.4.2) Nuclear fuel cycle
India has acquired comprehensive capability in the PHWR design, construction and operation of
associated plants/facilities covering the entire nuclear fuel cycle of the nuclear power programme
based on PHWRs.
Nuclear fuel fabrication for power and research is done at Nuclear Fuel Complex, Hyderabad
and BARC respectively.
There are 7 heavy water plants in the country that are based on ammonia-hydrogen exchange and
hydrogen sulphide-water exchange technologies. The latter has been developed indigenously.
Through continuing research, BARC has developed heavy water upgrading technology on
commercial scale. Based on this technology, at present 23 upgrading/final enrichment towers are
in operation at various sites.
The Indian nuclear power generation programme is based on closed cycle approach that involves
reprocessing of spent fuel and recycle of plutonium and uranium-233 for power generation. The
development of fuel reprocessing technology had commenced from inception of DAE’s nuclear
power programme. DAE has a pilot plant for fuel reprocessing at Trombay and industrial scale
plants at Tarapur and Kalpakam. BARC has successfully developed technology for vitrification
for radioactive waste.

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2) ELECTRICAL SYSTEM
2.1) GENERAL DESCRIPTION OF ELECTRICAL SYSTEM:
The electrical system of this project is divided mainly into two sub systems. Main power output
system and station auxiliary power supply system. Main power output system transmits the
power generated by 500 MWe generators at 21 KV from generator terminals to the switchyard
through generator transformer, which steps up voltage upto 400 KV before evacuation to grid.
Station auxiliary power supply system provides power supply for various station loads required
for start up operation, safe shutdown and maintaining the unit. The main sources of power supply
are from startup transformer interconnected with 220 KV grid and unit transformers (2 No.)
connected with output terminals of main generator. Station auxiliary power supply is also
divided into various classes namely CL-IV, CL-III, CL-II, and CL-I, depending upon
requirement of availability / reliability of power supply for various loads. Diesel generators and
battery banks are provided as backup power supplies.
2.2) OBJECTIVES OF ELECTRICAL SYSTEM
The electrical power system for TAPP-3&4 is designed to provide for
The following objectives:
a) To evacuate the power generated from the turbo generators to the off site grid connected
to the station at 400 KV switchyard.
b) To provide required quality of power to the station axillaries through start-up transformer
(SUT) and/ or GT/UT combination and in case of emergency on site diesel generator sets
and uninterruptible power supply systems.

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c) To provide emergency electric power supply to safety system of the station during
simultaneous occurrence of postulated initiating events and single failure of any
active/passive electric component/system.
d) To provide station emergency electric power system with reliable off site power from at
least two transmission lines preferably connected to two generating stating stations.
e) To provide fast transfer systems, emergency transfer systems and load shedding schemes
so that electrical power supply is restored within the interruption time permitted by the
connected loads.
f) To provide operational flexibility.
g) To provide necessary isolations, alarms and indications for safe operation maintenance of
electrical equipment.
h) To provide fire protection and safety.
i) To provide earthing of electrical system and equipment for personnel and system safety
and isolation of defective system.
j) To provide surge suppression, lighting protection.
k) To provide adequate lighting during plant operation and during emergency.
2.3) CLASSIFICATION OF ELECTRICAL SYSTEM
Power supply system for Nuclear power station is classified into four categories depending
upon the requirement of continuity of power supply to the loads.
a) Class-IV Power Supply:
Alternating current power supply to auxiliaries, which can tolerate prolonged interrupt
without affecting safety of reactor, is classified as class-IV. This supply is the normal
power supply drawn from switchyard through SUT and or GT/UT combination.
b) Class-III Power Supply:
Alternating current power supply to auxiliaries, which can tolerate short interruptions (up
to one minute), is classified as class-III power supply. Under normal conditions this

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power supply is derived from class-IV and on loss of class-IV power supply, on-site
standby diesel generators provide the back up.
c) Class-II Power Supply:
Alternating current power supply to auxiliaries, which require un-interrupted power
supply, is called class-II power supply. Under normal conditions electrical power is
derived from class-III buses through AC/DC rectifier together with DC to AC inverter. A
battery bank provides direct back up power so that class-II power is available even when
supply to class-III or rectifier is not available.
d) Class-I Power Supply (Dc Supply):
Dc power supply to loads which require uninterruptible direct current power supply.
Normally direct current power is derived through a AC to DC rectifier connected to class-
III. Battery backup is provided so that direct current power supply continues to be
available even when class-III or rectifier fails.
2.4) DESCRIPTION OF STATION AUXILIARY POWER SUPPLY
SYSTEM:
Station auxiliary power supply system (SAPSS) Provides power supply to various station
auxiliary loads required for start-up, shut down and running operations of the unit. The class IV
SAPSS has been divided into two divisions, one division (Division I) supplied from unit
transformers (UTs) and the other division (Division II) supplied from start up transformer (SUT).
Interconnections are provided between Division-I and Division II at all voltage levels except
415V CL.III and CI.IV to feed the loads belonging to the other division in case of total or partial
loss of power to that division. The buses, transformers and MCCs in Division-I are given odd
numbers and Division-II even numbers. Supply sources in each division can independently meet
the entire station demand under normal and abnormal conditions of one unit operation.

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2.4.1) Class IV Power Supply System
Under normal condition of operation, the power to all the auxiliary loads is fed from class IV
power supply system. Class IV Power supply system for each unit derives its power from UTs
(two Nos.) and SUT (one No.). UTs are two winding transformers of 21 KV/6.9 KV, 35 MVA
rating each, connected to Generator terminals through a Generator Circuit Breaker (GCB). SUT
is a three winding transformer (70/35/35 MVA, 220/6.9/6.9 KV and unloaded delta for
suppression of harmonic currents), which is connected to 220 KV grid and supplies power to
class IV system at 6.6KV.
The class IV power supply has two levels of voltages supplying power at:
a) 6.6 KV, 3-phase for motors of rating 200KWand above.
b) 415 V, 3-phase for motors 200 KW rating.
2.4.1.1) 6.6 KV System
This system consists of four numbers of buses with each switchgear bus fed from UTs or SUT
directly. The startup/auxiliary power of the unit will be derived through GT/UT and/or SUT.
Major loads connected on this system are Primary Coolant pumps; Boiler feed pumps,
Condensate pumps, CW pumps, Chillers etc. One 6.6 KV feeder will be provided for supplying
loads in waste management plant and D20 & Upgrading plant from unit-4.
2.4.1.2) 415 V System
This system consists of six numbers (6Nos.) of buses supplied through six 6.6 KV/433V, 2 MVA
transformers for feeding power to auxiliary loads. 415V loads in service building, CW pump
house and DM plant will be supplied from 415V; Class IV local MCCs. MCCs located in DM
plant will be supplied from Unit-4. To maintain the continuity of the supply with minimum time
of interruption when any one of the six transformers fails, a hot standby transformer is provided
to supply the load of the affected bus, which will be switched in manually.

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2.4.2) Class III Power Supply System
This system derives its power from class IV, 6.6 KV system under normal condition of
operation. This system consists of four numbers (4 Nos.) of 6.6kV buses, each backed up by a
DG set, and four numbers (4 Nos.) of 415 V buses.
2.4.3) Class II Power Supply System:
Class II, 415V, emergency power supply system provides uninterrupted A.C. power to the loads
connected to this system. This system consists of 2 Nos. of 415V buses which derive power from
power UPS.
2.4.4) Class I Power Supply System:
This system provides 220V DC uninterrupted power to the DC loads. This is further divided into:
a) 220V DC power supply
b) 220V DC control supply
2.5) Unit Auxiliary System Voltage Levels:
Following voltage levels have been adopted for TAPP-3&4 Electrical systems.
21KV (AC) : Input to unit Auxiliary transformer/ Unit
generation voltage.
6.6 KV (AC) : Unit main power buses, DG sets,
motors above 200KW rating &
auxiliary transformers
415V (AC) : Distribution buses, motors below 200
KW rating
240V (AC) : Single-phase loads like, control power
supplies, recorders, lighting, space
heaters, receptacles.
220V (DC) : Control power to circuit breaker, DG
controls, emergency lighting etc.
24V (DC) : Controls, annunciations, indications
Involving main control room control
logic, remote operation etc.

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Figure1-class III power supply system

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2.6) NOMENCLATURE ADOPTED FOR TAPP-3&4 ELECTRICAL
SYSTEM
A) Capital Alphabets have been used to designate various voltage levels. Following is the list of
voltage designation.
Voltage level Alphabet
i. 400KV (AC) A
ii. 220KV (AC) B
iii. 6.6KV (AC) C
iv. 415KV (AC) D
v. 220V (DC) E
vi. 240V (AC) F
vii. 24V (DC) G
B) Numerical have been used to designate different classes
Class Numerical
IV 4
III 3
II 2
I 1
C) Grouping:
For designating equipments in div-1, odd numbers are used & for Div-2, even numbers are used.
Example: 52410- BU- C 4 3
3 – Bus number in division-1
4 – Class –IV
C – 6.6 kV
B – Bus
52410 – System USI

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2.7) REDUNDANCY:
Each division will be functioning independently irrespective of availability of other Division.
This arrangement is provided to ensure that failure of one of the equipments, does not affect the
operation of the other equipments in general, and operation of safety related equipment in
particular.
In each division there are safety related equipments and non-safety related equipments. Each
safety related equipment power supply system is provided with alternate source of feed so that
the equipment comes back into operation within the specified limit of time.
Equipments are duplicated and supplied from Div-1&2 so that when one equipment is out of
service, the stand by will be operational to ensure the system operation.
2.8) SYSTEM OPERATION
2.8.1) Normal operation
Under normal operating condition the Main generator is synchronized with the grid and the
station supply is available through UTs and SUT. Under this condition, total auxiliary load is
shared by UTs & SUT equally. Buses C41 and C43 will be fed from UT1 &UT2 respectively
and buses C42and C44 will be fed from SUT and hence the breaker between C41 & LV1 of
SUT, C43 & LV2 of SUT, C42 & UT1 and C44 & UT2 will remain open CL .IV-CL. III ties
between buses C41 & C31, C43 &C32, C42 & C32 and C44 & C34 will remain closed. 415V
buses will be energized through respective auxiliary transformers. Reserve transformer will be
on hot stand by and ties between reserve transformer and 415V CI. IV buses (D41 to D46) will
remain open.

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2.8.2) Shutdown condition
2.8.2.1) Unit shut down condition
Under this condition power is derived from 400kV grid through GT-UT combination and / or
from 220 KV grid through SUT.
2.8.3) Operation Under Off Normal Condition
2.8.3.1) Shutdown of one UT
Under this condition, the load taken by the running UT will not change. The load catered to by
the shut down UT will be transferred to corresponding SUT winding with the respective 6.6kV
breaker from SUT closed.
2.8.3.2) Shutdown of two UTs:
It is not envisaged that both the UTs will be on forced outage simultaneously during the unit
operation. However under this condition all the station load is supplied from SUT.
2.8.3.3) Shutdown of SUT:
Under this condition all the loads of a unit are supplied from the UTs.
2.8.3.4) Fault on any of the 6.6kV buses C41 to C44:
Consider fault on bus C41. Bus will be isolated by protection. Fast transfer will be blocked by
operation of corresponding incomer breaker lockout relay. All the motor loads on this bus will
trip on under voltage. If standby loads corresponding to running loads on this bus are available
on the other buses they will start automatically. All the feeders except feeders to MCCs, lighting
load centers and other load centers on corresponding 415V switchgears will trip on under voltage
and if standby loads corresponding to running loads are available on other buses they will be
started automatically.

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Loads on corresponding MCCs will also trip and interposing relays (3C) will be deenergized.
Loads will be restored manually after restoration of power supply to the bus.
2.8.3.5) Fault on any of the 415V buses D41 to D46:
Consider fault on bus D41. This bus will be isolated by protection. All the feeders except
feeders to MCCs, lighting load center and other load centers on this bus will trip on under
voltage. Loads on corresponding MCCs will trip and interposing relays (3C) will be
deenergized. Loads will be restored manually after restoration of power supply to the bus.
2.8.3.6) Tripping of any one of the auxiliary transformers:
In the event of tripping of any one of the auxiliary transformers all the feeders except feeders to
MCCs, lighting load center and other load centers on the corresponding bus will trip on under
voltage. If standby loads are available on the other buses they will start automatically. Loads on
corresponding MCC’s will trip and interposing relays (3C) will be deenergized. Hot standby
transformer will be switched in manually. Loads will have to be started manually after
restoration of supply.
2.8.4) Operational transients
2.8.4.1) 6.6kV auto/ manual transfer operations
For starting of the unit the power to the buses C41 and C43 is drawn through GT/ UTs and for
the buses C42 and C44 is drawn through SUT and the unit is synchronized with the grid by
closing GCB. For loss of supply to Class IV 6.6kV system from either SUT or UTs provision
has been made for high speed transfer of loads from one source to another (UTs to SUT or SUT
to UTs).

26
2.8.4.2) Manual transfer:
For planned transfer of loads from one source to other (SUT to UTs and UTs to SUT), the two
sources will be momentarily paralleled by closing the incoming sources breaker and the outgoing
source breaker will be tripped automatically. If the outgoing source breaker fails to trip the
incoming source breaker will trip automatically after a preset time delay. The time for which the
buses are paralleled is very small and hence during this time the possibility of a fault is remote.
2.8.5) Auto-Transfer scheme:
The purpose of auto transfer is to achieve automatic transfer of 6.6KV, Class-IV auxiliary power
supply, in the event of failure of either of the two feeds (from unit transformer or start-up
transformer) due to faults in feeder. The auto transfer scheme shall consist of:
a) Fast transfer scheme and
b) Slow transfer scheme.
2.8.5.1) Automatic fast transfer (SUT to UTs):
Immediately after the tripping of SUT by protections, the loads fed from SUT will be transferred
to UTs by automatic fast transfer under the following conditions:
a) No fault on bus undergoing transfer i.e. Bus incomer breaker lockout relay in reset
condition.
b) Residual bus voltage is above preset value.
c) Angular difference between residual bus voltage and incoming voltage is less than preset
value. (It may be noted that the angular separation between the voltages of the buses
under normal conditions before transfer will depend on the proximity of interconnection
between the 400 kv and 220 kv systems). The above transfer is a high-speed dead
transfer with a dead to time of about 2 cycles (40 milliseconds).

27
2.8.5.2) Automatic fast transfer (UT to SUT):
After the tripping of UTs by protection, the loads fed from UTs will be transferred to SUT. Fast
transfer is initiated only in case of faults in UT, GT and interconnecting bus duct up to GCB. In
case of unit trip GCB will be opened and the power to UT buses is maintained through GT/UTs.
Advantages of fast transfer
The fast transfer schemes proposed in above will have the following advantages:
a) All motors will reaccelerate quickly and consequently all essential services will be
available immediately.
b) The voltage dips during change over will be of momentary nature only.
c) High inrush currents in individual motors as well as in auxiliary system are reduced.
d) Since the residual bus voltage does not go down perceptibly, the 415 V motor contactors
do not drop off during the change over sequence. Thus restarting of 415V motors after
the change over can be avoided.
e) There will be no perceptible flicker in the lighting systems, and hence fluorescent and
HPMV lamps, which are susceptible to voltage dips, will remain unaffected.
2.8.5.3 ) Automatic slow transfer:
In the event of failure of fast transfer the slow transfer can be carried out after the voltage of the
bus undergoing transfer goes below 20%.In case other system conditions demand the
disconnecting of loads during Slow transfer, the to the affected bus will be restored
automatically after disconnecting the loads. Restoration of the loads will be carried out
manually thereafter
In the event of electrical faults in start-up transformer or in the zone covering generator
transformer and unit transformers, buses fed by one of these sources are automatically
transferred to healthy source by simultaneous tripping of faulty source and closing of incomer
breaker from healthy source. The fast transfer scheme receives it initiating signals from the
lockout relays of GT, UT, SUT, 400 KV bus to which GT is connected, 220KV bus to which
SUT is connected etc. Auto transfer initiation from UT to SUT takes place on energisation of

28
relay UTX by 86A2-I, 86A2-II and 400KV BUS diff. Relay 4 along with GCB 52 X contact.
Auto-transfer initiation from SUT to UT takes place on energisation of relay SUTX by SUT
protection main and backup lockout relays, 220 KV Bus diff. Protection and LINE-1, LINE-2
protection Lock Out relays as shown in the scheme.
The scheme proposed for fast transfer shall be high-speed dead transfer with a dead time of about
2 cycles (40 milli seconds) after considering the difference between closing time and opening
time of breakers. If the above minimum bus dead time of 2 cycles is not achieved with the
available close and trip time of 6.6 KV breakers, closing of the healthy side breaker shall be
delayed accordingly. This is achieved by Timers UT-T1 and SUT-T1.
Synchronism between the faulty supply and incoming supply (represented by the 6.6 KV bus
voltages) is checked by synchronism check relay (25) and if it is permissive, the fast transfer
shall take place. The fast transfer shall be completed within a set time; otherwise it will be
blocked. This is achieved through timer UT-T2 and SUT-T2.
In case fast bus transfer fails, the change over shall be achieved by slow bus transfer scheme
provided the voltage of affected bus has fallen below 20% of rated voltage This is achieved by
energizing relays C41-27-UTX, C43-27-UTX, C42-27-UTX, C44-27-UTX (UT to SUT transfer
initiation) and C41-27-SUTX, C43-27 SYTX, C42-27-SUTX, C44-27-SUTX (SUT to UT
initiation) through 2/27-3 of respective 6.6 KV bus and 52X & 86 contacts of supply breaker.
Subsequent to fast transfer, if both healthy and faulty source breakers remain closed
simultaneously, both breakers will be tripped instantaneously. This is achieved through relays
UT-C41X, UT-C43X, UT-C42X, UT-C44C (UT to SUT transfer) and SUT-C41X, SU-C43X,
SUT-C42X and SUT-C44X (SUT to UT transfer).

29
2.8.5.4) Emergency transfer scheme
EMTR scheme is initiated for any of the following conditions:-
a) Loss of normal class-IV supplies to any one or more number of 6.6 KV class-III buses.
b) Loss of supply to 415 class-III buses due to 6.6 KV/433 V auxiliary transformer failures.
c) Failure of ups in class-II system/ups static bypass beyond preset duration.
d) Loss of class-II supply.
Sensing of the above conditions is done either by detecting under-voltage on the bus when there
is no bus fault or directly by checking equipment failure at EMTR logic diagrams.
2.8.5.4. A) 6.6 kV class-III, 415 class-III bus supply and feeder restoration
On initiation of EMTR, all motor feeder breaker and other predefined loads are tripped and
reclosing is blocked by under-voltage lockout relay. The closing of each feeder is blocked until
the blocking feature is reset by the breaker hand switch manually or by restoration of sequence
contact of emergency transfer panel. This sequence is required after permanent supply is
available to class-III buses.
The sequence is initiated by EMTR logic. This energises number of software timers, each of
which is set at a time step of 4 seconds. This contact of each timer will give permission to close
the corresponding class-iii feeder. After the last restoration of loads is done by timer it
automatically reset EMTR scheme.
2.8.5.4. B) EMTR initiation for 415 V class-III buses
Loss of voltage on any of the 415 V class-III buses initiates emtr. EMTR is also initiated on
tripping of any one of the 6.6 KV/433 V auxiliary transformers normally supplying to a 415 V
class-III bus. EMTR restores power supply to the affected 415 class-III bus by closing the
standby transformer secondary side circuit breaker after checking for conditions such as
healthiness of the bus, availability of breaker etc. After power supply is restored to the affected
bus, EMTR restores the loads in a predetermined sequence.

30
2.8.5.4. C) EMTR initiation for 415 v class-II buses
Loss of voltage on class-II bus initiates EMTR. EMTR closes the class-III-class-II tie breaker
and restores supply to the affected class-ii bus. Prior to closing the class-III-class-II tie breaker,
emtr will check for healthiness of the bus, availability of breaker etc. EMTR will also start DGs
of that division but the dg will be manually connected to the relevant class-III bus by operator
action.
2.8.5.4. D) Emergency transfer panel
Two emergency transfer panels are provided for each unit of TAPP-3 and TAPP-4. One
emergency transfer panel is dedicated for each division. Emergency transfer scheme for one
division is completely independent from emergency transfer scheme for the other division.
Functions of any division emergency transfer scheme are independent of other division scheme
with no communication between them. Each EMTR has two redundant PLCs running in parallel
all the time.
The output issued to the field is generated by combining the output of both the PLCs in such a
manner that n case of failure of any one plc, required function is met by the other healthy plc.
For this purpose, normally open output contacts of the two PLCs are connected in parallel and
normally closed contacts are connected in series.

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3) POWER UPS SYSTEM
3.1) GENERAL DESCRIPTION
The ups system provides uninterrupted ac power to the loads connected to class-II buses. Class-II
system is divided into 2 divisions (i.e. bus D21 & bus D22) which are located in SABs.
Each division in CL-II power supply system consists of one power ups set and its associated
switchgear.
Each division derives its power from the following sources:
1. From 415 V class –III power supply through ups set under normal condition.
2. Through battery & ups inverter under class-iii power failure condition or under ups
rectifier failure condition.
3. Directly from 415 V class –III power supply system when power from ups is not directly
available.
Tie-breakers, one at each end, are provided between buses D21 &D22. Similarly tie-breakers,
one at each end are provided between buses D33 and D21 and also between D32 and D22.
For each power ups system, one 400 KV DC switchgear is provided in between power ups and
its associated power battery bank. The connection between power ups and 400v dc switchgear
through dc bus duct. Similarly, connection between 400v dc switchgear and power battery are
through dc bus duct. One 500 v dc boost charger for each division is provided for off-line boost
charging of power batteries. The boost charger is connected to the battery bank through the 400v
dc switchgear.
3.2) COMPONENTS OF SYSTEM:-
The static ups system consists of 4 systems:
1. Rectifier assembly
2. Inverter assembly
3. Static-by pass switch
4. Controller for the above 3 assembly

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Rectifier assembly:
It is used to charge battery and provide input supply to inverter. Two six pulse fully controlled
SCR bridges are employed to provide regulated dc supply.
Inverter assembly:
Converts dc output of rectifier into ac voltage with the help of IGBT based microprocessor
controlled PWM inverters.
Static- by pass switch:
Two back-to back thyristors for each phase are provided as static by-pass switch. All six
thyristors are fired simultaneously during emergency requirement. These switches enable to and
fro transfer of load from the inverter output to bypass supply without any interruptions of supply
to load.
Power battery:
Each power battery set is rated to supply entire class-ii power supply system loads of both the
divisions for a period of 30 minutes.
3.3) OPERATIONS:
Class –II power supply system derives the power from class-III power supply through ups set.
Both the tie breaker between buses D21 and D22 are kept open. In case of tie breaker between
class-III and class-II buses of a division, the breaker at class-III end is normally closed and
breaker at class-II end is open. Each ups and switchgear is designed to meet the entire class-II
system load requirement during normal and emergency condition.

33
3.4) DESIGN BASIS:
Power ups systems are designed considering the following criteria:
1. The ups panel can operate satisfactorily for input voltage of 415v ± 10% and frequency
of 50 Hz ±5%. Output voltage and frequency of the ups will follow the input voltage and
frequency within a specified band and will be in synchronism with the input to facilitate
the changeover to static bypass and vice-versa.
2. The transient output voltage fluctuations are within ± 5%, steady state voltage is ± 1% of
set value and steady state output frequency variation is ± 0.5 % of set value.
3. Neutral of output transformer are solidly earthed.
4. Static by-pass facility is provided.
5. For calculation of ups rating, a design margin of around 25% is considered on the
maximum base load on ups.
6. For the purpose of sizing of ups, continuous operation of steady load and starting of
biggest motor (on the class-ii 415 v) with soft starter is considered. Simultaneous starting
of 4 ECCS valve motors on the remaining loads connected to the buses are considered.
7. While feeding 100% linear load, the output filter shall limit harmonic distortion
generated by ups within 4% and any single harmonic within 3%.
8. Under normal condition ups, will feed normal load and float charge its battery. Ups set
will also be designed such that it will be able to feed normal load while equalizing
charging its battery.
3.5) TECHNICAL PARTICULARS OF POWER UPS:
Normal ratings
Capacity 650 KVA
Voltage 415 V
Current 3*904
Maximum and minimum power factor for operation 0.18 lag to .85 lag at 650 Kva

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4) GAS INSULATED SWITCHYARD
The main output system consists of a 400 KV switchyard for evacuation of 1080MWe power
generated from the two units. The 400 KV switchyard will serve as an additional start-up source,
as Generator Circuit Breaker is provided between Generator and Generator Transformer. The
220 KV switchyard is provided to obtain start-up power for the station Auxiliary loads through
the start-up Transformer.
The purpose of the above separate switchyards at different voltage levels is mentioned below:
400 KV Switchyard:-
a) For evacuation of generated power
b) For obtaining start-up power supply via GT-UT Transformer.
220 KV Switchyard:-
a) For obtaining power supply via start-up Transformer / Start-up.
Gas insulated switchyard is provided for both the switchyards. In this type of switchyard, all the
switchgear components (like CTs, PTs, CBs etc.) and all live connections are enclosed in
metallic enclosures filled with pressurized SF6 gas. The complete switchgear will be housed in a
building with adequate ventilation and space for maintenance. The attached single line
diagrams– 1&2 show the layout of 400 KV & 220 KV switchyards.
4.1) TAPP-3&4 Grid
Both the 400 KV & 220 KV switchyards, the transmission lines & the transformer feeders
constitute the TAPP-3&4 grid.
The transmission lines from the switchyards are connected to the grids of Maharashtra State
Electricity Board, which in turn is further connected to the Electrical Power Networks of Gujarat,
Chhattisgarh Madhya Pradesh, Goa & Union territories and forms the Western regional grid.
Four Nos. of 400 KV lines & 2 Nos. of 220 KV lines are envisaged for TAPP-3&4 grid.

35
Two Nos. of 400 KV lines will be terminated at 400 KV PADGHE--- substation at a distance of
108 KM and another two Nos. will be terminated at 400 KV Boisar sub-station situated about 10
KM from our site.
Regarding 220 KV lines, one number will be terminated at Boisar sub-station and another as tie
line to TAPS-1&2.
Fig: 4.1 Power Evacuation (400 kV) & 220 kV System

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4.1.1) 400 KV Switchyard:
The configuration of 400 KV switchyard is as follows
a) No of Feeder bays 4
b) No of GT bays 2
c) Bus coupler bay 1
d) Bus PT bay 1
e) Space for future bays 2(one for ICT and one for feeder)
Electrical equipments present are:
a) Part of main bus.
b) Bay bus
c) Current transformer
d) Voltage transformer
e) Isolators
f) Lightning arrestor
g) Earth switches
h) Wave traps
i) SF6/Air
j) Bushing.
4.1.2) 220 kV Switchyard:
The configuration of 220 kV Switchyard is as follows:-
a) No of feeder Bays = 2
b) No of Transformer Bays = 2
c) Bus – coupler bay = 1 Bus PT bay = 1
d) Space for future bays = 2 (One for 1 CT & one for line feeder).
Electrical equipments are similar to those mentioned for 400 KV in section 4.1.1

37
Fig 4.2 – 400KV SWITCHYARD

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Fig: 4.4 Respective positions of the four types of switchgear in terms of the current to be
broken and of the number of operations to be conducted.
4.2) DETAILS OF ELECTRICAL EQUIPMENT:
4.2.1) Capacitive Voltage Transformer:
The capacitive voltage transformer consists of a consists of a capacitive potential divider and an
inductive medium voltage circuit. The inductive part is immersed in mineral oil and hermitically
sealed with an nitrogen cushion inside a steel tank. One, two or three capacitor units are mounted
on the steel tank and are used as capacitor potential divider. They consist of condenser stacks
with paper foils as dielectric under mineral oil with a nitrogen gas cushion and are hermitically
sealed.
The CVT are provided on all the three phases. Each outgoing line from the switchyard has its
own potential transformer. Line potential transformers are of capacitor type. Capacitor potential
transformers are used with power line carrier communication system (PLCC) and are suitably for
a PLCC system frequency range of 40 KHz to 500 KHz.

40
Type of CVTs:-
This line CVTs are of single phase type and each unit is connected between respective phase and
earth. Each CVT has 3 secondary winding and winding connection will be of phase to ground.
Function:
Each CVT has 3 secondary winding and used as:
1. Core-1: metering and synchronizing.
2. Core-2: back, over current protection and CVT fail protection.
3. Core-3: earth fault directional core protection.
4. CVT is used with power line carrier communication (PLCC).
5. CVT is used for carrier inter tripping with PLCC.
Table 4.1-Technical details of CVT used in 200 kv and 400 kv switchyard
200 kv 400 kv
Make BHEL, Bhopal BHEL, Bhopal
Installation Outdoor outdoor
Frequency 50 Hz 50 Hz
No. of secondary voltage 3 3
Rated phase voltage 200/√3 v 400/√3 v
Highest system voltage 245/√3 v 420/√3 v
Rated secondary winding and the method
of connection
First winding 110/√3v (star grounded) 110/√3 v(star grounded)
Second winding 110/√3v (star grounded) 110/√3 v(star grounded)
Third winding 110/√3v (star grounded) 110/√3v (star grounded)
Highest voltage capacitor 4840 pf 4656 pf
Rated voltage factor continuous 1.2 1.2
Rated voltage factor short time 1.5 1.5
Rated voltage factor time duration 30 sec 30 sec
Intermediate voltage capacitor 48400 pf 80000 pf

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4.2.2) SF6 gas circuit breaker:-
200 kV GIS circuit breaker (HB9 type) and 400 kV GIS circuit breaker (HB10 type)
Each CB comprises:
1. 3 metal clad breaker poles, each pole being actuated by its operating mechanism, one
supporting frame for the three poles.
2. Each pole is provided with one single break interrupt of the single pressure puffer type
with separate contact system for carrying continuous current and for arching whereby
control erosion is reduced to a negligible level ensuring long life.
3. Simplicity of interrupt operation: the moving contact with a compression cylinder, which,
during tripping operation generates the pressurized SF6 gas, required for arc quenching
4. Only minor over voltage of switching of small inductive currents, owing to optimized
interruption process which prevents current chopping.
Properties of SF6:
1. In pure form it is inert, exhibits exceptional thermal stability and has excellent arc
quenching properties as well as exceptional high insulating properties, one of the
most stable component, non-flammable, non-toxic and odorless.
2. Its density s more than that of air and heat dissipation in it is also much more than that
in air. At the atmospheric pressure the dielectric strength is about 2.4 times that of air
at about 3 kg/cm2
it is same as that of oil.
3. There is some decomposition of gas after the long periods of arcing. However such
decomposition is very little and has no effect upon dielectric strength and interrupting
capability. The solid arc product formed by arcing is metallic fluoride which appears
in the form of fine gray powder. This powder has high dielectric strength under dry
condition as existing in the breaker. A good quality absorbent is used in the apparatus
to remove most of the gaseous decomposed by-products so the level of this gaseous
by-product is kept very low.

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4.2.3) Lighting arrestor
Provided in 220 kV and 400 kV transmission line and power transformer lines in switchyard and
transformer yard for suppression strokes in transmission lines. A surge monitor counter is located
on the lighting arrestors on which 3 color band are there. Each of these bands has its own
significance. The criteria for different bands are as follows:
 Green- healthy
 Yellow-precaution
 Red- first check counter, if counter is healthy, then clean insulator and if still red then
remove arrestor.
Surge monitor is provided on all the three phases and s located on the mounting structures of
lighting arrestors.
4.2.4) Current transformer
Constructional details
 The CT is of the ring type. The straight conductors passing through the cores act as a
single turn primary winding.
 The secondary winding on the cores are braced inside a retaining frame and are so
inserted with the screening cylinder in the cast aluminium enclosure.
 The number of cores to be accommodated in an enclosure depends upon the primary
current, the accuracy class and the required specification.
 The ends of secondary winding are brought into the terminal box through a gas tight
bushing plate.
 CT consists of one or more magnetic cores on which the secondary turns are wound.
 The inter layer insulation is made up of synthetic film.
 The phase conductors of GIS form the primary winding.
 The magnetic cores are mounted on a sheath like metallic armature, which ensures a good
distribution of the electric field.
 The phase bar passes through this armature.

43
 The cores are carefully sealed in order to withstand the mechanical vibrations occurring
during the transmission and when in use in GIS.
 Each winding may offer several ct ratio.
 The CT terminal block consists of a barrier insulator with several outlets.
4.2.5) Electromagnetic potential transformer
They are used to transform high tension line voltage to low voltage in order to supply appropriate
voltage to measuring to instruments, meters, relays and other similar apparatus. They can be used
with the voltmeters for voltage measurement or they can be used in combination with current
transformer for wattmeter or watt-hour meter measurements. They are also used to operate
protective relays and similar devices.
Other features:
1. There are one set of VT for each 220 kV bus bars. Each set consists of three single phase
VTs. Each 220 kV VTs is of electromagnetic type having 400 VA. These have the ratios
of (220 kv/√3)/110/√3, 110/√3/110√3 volts and 0.2, 3p, 3p class respectively. These VTs
are star/ star connected to ground.
2. The 400 kv EMPT are also electromagnetic type with 100 va rating 0.2 and 3p and
400kv/√3:110/√3:110/√3:110/√3.
3. The active part of the VT is formed by a rectangular core consisting of one or more
magnetic steel sheet on which the secondary turns and high voltage windings are wound.
4. Pressurized sf6 gas insulates the high voltage from the conducting parts.
5. The inter layer insulation of the primary winding is made of a synthetic film. Selected
from its dielectric properties, its thermal stability and its low moisture absorbency.
6. The windings are manufactured and the active parts assembled in an air conditioned
workshops. Considerable care is taken to avoid pollution by dust particles. In order to
eliminate moisture a vacuum is created inside the enclosure and then it is dried before
filling the gas.

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Application:
EMPT are connected to potential coils of directional relay associated with bus differential
protection, synchronizing check relays and other metering and recording instruments.
Secondary core allocation:
The secondary terminals are brought and terminated in a terminal box fitted to the EMPT bottom
220 KV GIS EMPT. 3 cores are provided for each EMPT in 200 kV GIS.
220 KV GIS EMPT:
1. Core1: used for synchronization, bus voltage and frequency measurement, biling and
metering.
2. Core2: bus u/v protection, fuse failure protection, o/fd fluxing protection and o/c
protection.
3. Core3: directional grounding over current protection.
400 KV GIS EMPT:
2 cores are provided for each EMPT in 400 kV GIS
1. Core1: used for synchronization, bus voltage and frequency measurement, billing and
metering.
2. Core2: fuse failure, DR, under voltage protection
4.2.6) Disconnecting switches/Earthing switches
The DS and ES are motor operated and are capable of remote operation from control room as
well as local control panel. es are provided on the line, transformer and bus, EMPT isolators.
These are electrically interlocked with the man isolators so that they are not closed to the earth
when the system is charged. These may only be operated after de-energisation and in the absence
of current.
The DS are capable of making and breaking
 Magnetizing current of the EMPT
 Capacitive current of the bus and short connections.

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General features of the disconnectors (220 KV GIS)
Depending upon the geometrical course of the current path, 3 different types are used:
In-line disconnectors
L-shaped disconnectors
T-shaped disconnectors
General features of ES (220 KV GIS)
Two types of ES are available:
1. Maintenance type (slow operating): for earthing isolated sections of switch gear for
protection of personnel during maintenance and overhauls or erection.
2. High speed type (fast closing, slow opening):
 For earthing high capacities cables, overhead lines.
 For interrupting capacitive and inductive currents from parallel overhead lines.
 For safely earthing even line equipment, if operated inadvertently.
4.3) SWITCHING SCHEME ADOPTED
The switching scheme adopted for both the 400 KV & 220 KV switchyards is Double Bus
scheme with bus-coupler and bypass isolators for transformer feeders only.
The general features associated with this scheme are as follows:-
a) Very good operational flexibility.
b) Total shutdown of switchyard due to bus-faults is ruled out as the same are of totally
enclosed type.
c) Transformer feeder breaker can be taken out for maintenance without affecting the
circuit.
d) Any main bus can be taken out for maintenance without affecting the associated circuits.
e) Future expansions are possible without prolonged shutdowns.
f) Reliability is more as most of the switchgear components are of totally enclosed type.

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4.4) OPERATION PRINCIPLE:
General organization of the GIS:
The GIS consists of the electro technical power equipment, the local control and the monitoring
equipment.
The GIS equipment is made up of bays.
Each bay contains:
1. all the devices attached to the wiring diagram busbar components, circuit breaker
2. and the various disconnectors,
3. The total control monitoring cubicle for the bay devices.
4. Local bay control panel
Control cubicle:
The local bay control panel contains;
1. the single phase mimic diagram of the bay,
2. the control switches of the switching devices,
3. the position indicating lamps of the switching devices,
4. the local remote mode selector switch,
5. the alarm panel board with signaling lamps.
The alarm panel signals any disturbance on:
1. SF6 gas monitoring
2. Circuit breaker monitoring,
3. Disconnectors monitoring,
4. Auxiliary supplies.
Bay control mode:
The control mode determines the control possibilities of the devices in each bay (disconnector,
earthing switch and circuit breaker).

47
Priority:
Priority is normally given to continuity of supply. Operation is thus designed to respect this
priority. For example if a monitoring function reveals a failure, an alarm is triggered, but bay
operation is not interrupted.
Dependability:
Dependability is ensured by the interlocking functions between bay device operations. The
interlocking functions only allow operations without risk for personnel and equipment.
4.5) SF6 MONITORING:
As gas density is not an easily measurable physical quantity, the specific mass of gas in the
compartment is replaced by pressure brought to a reference temperature of 200 C expressed in
relative value and brought to an atmospheric pressure of 0.1013 MPa. The term “pressure”
means “corrected pressure brought to a temperature of 200 C and to an atmospheric pressure of
0.1013 MPa and characterizes the specific mass of gas in the specific conditions of use. Gas
pressure in the devices and GIS substation compartments determines breaking and insulation
withstand.
Compartment pressure is normally monitored at two levels:-
1. The first level indicates an acceptable pressure drop. It is placed slightly above minimum
operating pressure, the equipment retains its properties and normal operating conditions
remain unchanged. At this stage the operator must check and top up the compartment.
2. The second level indicates the minimum operating pressure. Under this pressure the
devices dot not retain their insulation properties and the circuit breaker does not retain the
breaking properties. Appearance of the second level includes automatic change in
operating conditions, and device locking or circuit breaker tripping. Operation conditions
are then determined by the chosen priorities. At this stage operating personnel must check
the monitoring circuits and if necessary place part of the substation out of operation and
top up the compartment.

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Table 4.2 -Ratings OF 220 KV & 400 KV GIS:
S. No Description Values for 220 KV Values for 400 KV
1 Rated voltage 220 KV 400KV
2 Rated frequency 50Hz 50Hz
3 Rated current (In) Feeder &
busbars
1250 A 162000
4 Rated breaking capacity
current (Icc)
40KA – 3s 162000
5 Rated short time withstand
current (Ith)
40KA 40KA
6 Rated operating sequence O-0.3s-CO-3min-CO O-0.3s-CO-3min-CO
7 Rated insulation level
8 Lightning impulse (UW) 1050KV 1425
9 Power frequency (Us) 460 KV – 1min 520
11 Filling Pressure (Pn) 0.70 0.43
12 First stage alarm pressure 0.62 0.37
13 Second stage alarm pressure 0.60 0.36
14 Auxiliary voltage services
rated values.
15 Control devices 110V DC
16 Motor 110 V DC
17 Heating and Lighting 220 V (ac) - 50 HZ.

49
4.6) HOT LINE WASHING SYSTEM FOR INSULATORS
Many electric networks are located near sources of extreme pollution. Contaminants like salt,
dust and sand collect on insulators. The contaminants when mixed with moisture in the air can
significantly reduce the effectiveness of the power line insulators. If the pollutants are not
cleaned off of the insulators, they will form conductive bands or dry bands. These conductive
bands enable leakage (also known as creep age) current to jump across the insulator dry band to
dry band. If too much time lapses without washing, there will ultimately be a short circuit to
ground called a flashover.
Flashovers are bad. Flashovers can destroy the power company’s apparatus and will produce a
momentary electric blackout which means the customer is out of electricity until the snag can be
corrected. These faults are usually brief in nature and will vary irregularly with no noticeable
pattern. This can be extremely frustrating to everyone involved in trying to resolve the crisis.
In TAPS 3&4 hot line washing is specifically designed to meet the particular requirements for
the 400/220 KV switchyards. Each switchyard has its own special features which includes a
variety of insulators performing different tasks. These have varying shapes, sizes and power
ratings and therefore demand differing design of insulator washing. Spray-rings can take form of
circular, square or rectangular rings having a number of specially designed nozzles mounted
them on them. The numbers of nozzles depend on the size and the rating of the insulators.
Each nozzle is accurately designed to achieve the required spray pattern and the direction onto
the insulator. Each spray-ring with its associated nozzles and supporting clamps is tailor made to
each type of insulator.
The spray-rings are arranged in washing zones to enable economic pump set and pipe work
sizing. It also provides identification of particular area in which insulators are washed together.
Zoning obviously takes into account various electrical consideration and is arrange to minimize
the possibility of flashover due to over-spraying.

50
It is important to have fast actuating valves so that nozzles form the correct pattern instantly. The
use of low conductivity water is imperative in hot line insulator washing system. Our design
includes a monitoring system, which constantly measures the conductivity of the dm water
between the storage tank and the wash pumps. The maximum conductivity permissible is pre-set
and if exceeded stops the washing sequences instantly.
Operation of HLW is carried out manually from wash control panel. The frequency of wash is
determined by the rate of pollutant build up, which in turn depend upon the location of site and
time of years.
Wash zone allocation:-
It is not practical to wash all the insulators in a substation together, as this would require a large
pumping and pipe work system. For practical purposes insulators are washed n group or zones.
The zones may be a full bay of equipment that s a feeder bay or similar equipment within the
bay, or equipment which is very close together where it is impossible to wash one item of
equipment without over spraying onto the adjacent equipment.
Washing of overhead equipment should be carried out separately from washing ground mounted
equipment to prevent water flowing back from high –level equipment and out through the
nozzles for the ground equipment at the end of washing cycle.
The allocation of wash zone is therefore dependent on various criteria such as water
requirements, relative position of equipment and relative level of equipments. For tarapur,
wormald fire system recommend a total of 46 wash zones of the 400 KV equipment and 20 wash
zones for the 220 KV equipment.

51
4.7) WHY GIS HOUSED INDOOR SWITCHYARD IS SELECTED FOR
TAPP-3 AND TAPP-4?
Reliability:-
In case of GIS equipment failure rate is negligible; faults due to external influence and climatic
pollution are nearly zero except at the termination.
Installation:-
For GIS simple as factory assembled equipment are delivered, whereas for conventional outdoor
it is time consuming and in case of indoor it is not only time consuming but also require very
large size enclosure which are difficult to construct.
Maintenance requirement:
GIS requires negligible and normally limited to operating mechanism maintenance whereas
conventional outdoor and indoor switch yard require continuous maintenance.
Hotline washing:
For GIS it is limited to outdoor line take-off bushings and CVTs. No separate DM water plant
required. In case of conventional outdoor regular hot line washing is required throughout the
year and separate dm water plant. In case of indoor switchyard some quantity of hotline washing
is required for outdoor insulated and wall bushings.
Safety:-
GIS is almost completely safe since all parts except termination are enclosed. In case of outdoor
and indoor switchyard all part exposed, personnel safety needs continuous attention.
GIS housed indoor switchyard was finally selected, GIS because of many advantages stated
above and indoor because of plant’s close proximity to Arabian sea which can leads to heavy
salt deposition on insulator and heavy rains during the monsoon.

53
5) ELECTRICAL PROTECTION
In generating stations, all electrical circuits and machines are subject to faults. A fault is
generally caused by the breakdown of insulation between a conductor and ground or between
conductors due to a variety of reasons. The result is a flow of excess current through a relatively
low resistance resulting in severe damage unless cleared quickly. The majority of systems and
devices in our stations are three phase which can experience faults of categories:
• Phase to ground
• Phase to phase
• Three phase, with or without ground.
In this module, we will discuss the purpose and essential qualities of electrical protection
schemes, the types of faults that can be expected and various means of protecting equipment
against them.
5.1) PURPOSE OF ELECTRICAL PROTECTION:
The function of protective relaying is to ensure the prompt removal from service of a faulty
electrical system component, thereby protecting that part and the remainder of the electrical
system from damage and electrical instability.
Every item of electrical equipment must have some form of electrical protection, which will
remove electrical power from the equipment in the event of it becoming faulty or overloaded.
This is necessary to ensure that:
a) Damage is minimized on the faulty equipment and any damage is not allowed to spread
to other equipment. For example, if a fault occurs in a motor, we want to isolate the
motor before damage occurs to the bus supplying the motor.
b) Unaffected equipment remains in service. Continuing on the previous example when a
fault occurs in a motor, we only want the motor to trip (not the entire bus), while still
providing power to the unaffected equipment on that same bus.

54
c) Equipment operating limits are maintained. Again using the motor as an example, most
motors are designed to run in an overload condition for at least a short duration without
experiencing damage. However, we must remove the electrical power when the overload
gets too great, preventing damage to the equipment.
d) Electrical system stability is maintained. As discussed in the previous module on
generators, an un-cleared or slow-clearing fault will make the electrical system unstable.
Instability will cause the break-up of the electrical system until stability is obtained.
Inevitably there is loss of generation capability and disruption to large amounts of
electrical equipment.
5.2) ESSENTIAL QUALITIES OF ELECTRICAL PROTECTIONS:
Having looked at the fundamental purpose of electrical protection, we should cover the four
main building blocks that are used to meet these requirements:
Speed:
When electrical faults or short circuits occur, the damage produced is largely dependent upon the
time the fault persists. Therefore, it is desirable that electrical faults be interrupted as quickly as
possible. High-speed fault detecting relays can now operate in as little time as 10 milliseconds
and output relaying in 2 milliseconds. The use of protection zones minimized the requirement for
time-delayed relaying.
Reliability:
The protective system must function whenever it is called upon to operate, since the
consequences of non-operation can be very severe. This is accomplished by duplicate A and B
protections and duplicate power supplies.
Security:
Protections must isolate only the faulted equipment, with no over-tripping of unaffected
equipment. This is accomplished by the use of over-lapping protection zones.

55
Stability:
It is defined as the quality of the protection system by the virtue of which the protective system
remains inoperative & stable under certain specified condition such as system disturbance,
through fault, transients etc. Design aspects like biased differential scheme & harmonic restraint
relay add to stability of the transformer protection system.
Selectivity:
The protective relaying should select the faulty part of the system & should isolate as far as
possible only faulty part from the remaining healthy system.
Discriminating quality of protective system enables it to distinguish between normal & abnormal
condition, and abnormal condition within protective zone & abnormal condition elsewhere.
Sensitivity:
The protection must be able to distinguish between healthy and fault conditions, i.e., to detect,
operate and initiate tripping before a fault reaches a dangerous condition. On the other hand, the
protection must not be too sensitive and operate unnecessarily. Some loads take large inrush
starting currents, which must be accommodated to prevent unnecessary tripping while still
tripping for fault conditions. The ability of relaying to fulfill the sensitivity requirement is
improved by the use of protection zones.
Redundancy:
To ensure operation of the protection under fault conditions redundancy can be obtained with the
following alternative scheme listed in the order of merit for availability and to ensure tripping.
1) One out of two : Highest
2) Two out of three : 2nd Highest
3) Single relay scheme: 3rd Highest
4) Two out of two : Lowest
To achieve complete redundancy, it may be necessary to provide, for each of the redundant
scheme, separate cores of instrument transformers, DC power source and communication

56
channel and trip relays. Further, trip contact of lockout relays and trip relays. Further, trip contact
of lockout relays and trip coils of circuit breakers may have to be duplicated for each scheme to
the extent possible.
However, considering the cost involved in providing complete redundancy the following levels
of redundancy is adopted for the different protective scheme.
400KV Line:
Two main protections operating on different principles are provided. Each scheme has dedicated
core of current transformers and dedicated core of voltage transformer. DC supply for main 1&2
will be from 2 different DC sources, Circuit breakers will have two trip coils one for each main
protection will have a separate carrier channel. This will amount to one out of two schemes.
220 KV Line:
One main protection and one back up protection with dedicated core of current transformers, and
dedicated core of voltage transformer are provided. DC supply is from two different Dc sources.
Circuit breakers have two trip coils and separate trip relays for each scheme. The main protection
will have a separate carrier channel. This approximates to one out of two schemes. However, the
speed of clearing of back up protection will be slower than the main protection.
Transformer Protection:
One main protection and one back up protection with dedicated core of current Transformer are
provided. DC supply will be separate for main and back up protection.
Generator Protection:
The protection relays of generator are divided in to two groups Groub-1 and Group-2. Each
group is provided with separate DC supply and trip relays. These trip relays will operate on
separate trip coils of 400KV and generator breakers, and common trip coils of unit transformer
and Exciter field circuit breakers.

57
The classifications of the various protections in two groups are done in such a way to ensure that
the availability of any group would be adequate to ensure tripping of generator under any fault
condition. Thus the arrangement will approximate to one out of two schemes. Further separate
control batteries and DC will feed each group supply boards. These batteries will have inter-
connection facility to avail supply to the DC board from other battery, when the associated
battery is not available.
Bus-Bar Protection:
Bus-bar protection is with main and check zone feature with dedicated core of current
transformers. Each zone will have one main protection. Check zone protection will be common
to all the zones. Each will have DC supply from different sources.
5.3) Types of Protective Relays:
Fig. 5.1 Trip Circuit
5.3.1) Electromagnetic relays:
These relays are either of attracted armature or induction cup or induction disc versions. They
possess mechanical inertia and therefore take longer time to operate as compared to static relays.
Besides, the burden imposed by these relays on the CT and VT are substantial. These relays are
provided as back-up relays for station electrical Auxiliary Systems.

58
5.3.2) Static relays:
These relays use solid state devices to process the input signals in analog form. The burdens
imposed on CTs by these relays are very low compare to Electromagnetic relays. It is possible to
obtain higher speed of operation with static relays. Static relays are already in use in various
power stations of the country and are gradually replacing Electro-magnetic version, which are
being taken out of manufacturing range by most of the manufacturers.
5.3.3) Digital relay:-
Digital protection relays introduced a step change in technology. Microprocessors and
microcontrollers replaced analogue circuits used in static relays to implement relay functions.
Compared to static relays, digital relays introduce A/D conversion of all measured analogue
quantities and use a microprocessor to implement the protection algorithm. The microprocessor
may use some kind of counting technique, or use the Discrete Fourier Transform (DFT) to
implement the algorithm. However, the typical
Microprocessors used have limited processing capacity and memory compared to that provided
in numerical relays. The functionality tends therefore to be limited and restricted largely to the
protection function itself.
5.3.4) Numerical relays:
These are programmable version of solid state relays based on digital signal processing by
microprocessors. The main advantage is in its modular architecture allowing the same unit to be
programmed in to different types of relays. Processing is carried out using a
Specialised microprocessor that is optimised for signal processing applications, known as a
digital signal processor or DSP for short. Digital processing of signals in real time requires a
very high power microprocessor. In addition, the continuing reduction in the cost of
microprocessors and related digital devices (memory, I/O, etc.) naturally leads to an approach
where a single item of hardware is used to provide a range of functions (‘one-box solution’
approach). By using multiple microprocessors to provide the necessary computational

59
performance, a large number of functions previously implemented in separate items of hardware
can now be included within a single item.
Figure 5.2 typical numerical relay

60
6) TRANSFORMERS AND ITS PROTECTION
Transformers:
For each 500 MWe unit, protections for the following transformers are considered.
A) Generator transformer (GT)
B) Unit transformer (UT)
C) Start-up transformer (SUT)
6.1) TRANSFORMER FAULTS:
Transformer faults are generally classified into following categories:
a. Winding and terminal faults
b. Core faults
c. Tank and transformer accessory faults
d. On–load tap changer faults
e. Abnormal operating conditions
f. Sustained or unclear external faults
Winding faults
A fault on a transformer winding is controlled in magnitude by the following factors:
a) source impedance
b) neutral earthing impedance
c) transformer leakage reactance
d) fault voltage
e) winding connection
Core Faults
A conducting bridge across the laminated structures of the core can permit sufficient eddy-
current to flow to cause serious overheating. The bolts that clamp the core together are always
insulated to avoid this trouble. If any portion of the core insulation becomes defective, the
resultant heating may reach a magnitude sufficient to damage the winding.

61
Tank Faults
Loss of oil through tank leaks will ultimately produce a dangerous condition, either because of a
reduction in winding insulation or because of overheating on load due to the loss of cooling.
Overheating may also occur due to prolonged overloading, blocked cooling ducts due to oil
sludging or failure of the forced cooling system, if fitted.
6.2) TRANSFORMER PROTECTION:
Transformers, of course, are somewhat more difficult to provide electrical protection than a
section of solid electrical bus. This is such because:-
a) Transformers have high magnetizing inrush currents when energized.
b) Transformers can vary the ratio of input to output current via off-load and under-load tap-
changers.
c) The input and output current is often not the same phase relationship (sometimes has Y.∆
transformation)
d) Transformers will be affected by over-fluxing (high volts/hertz).
e) They will be affected by over-temperature.
To examine transformer protections, we will build on the similarity to bus protections just
discussed. Transformers utilize duplicate protections and the protection zone (similar to buses)
can been seen in the figure below
Figure 6.1 – Transformer protection zone

62
6.2.1) Transformer Instantaneous Over-Current Protection:
Fuses commonly protect small distribution transformers typically up to ratings of 1MVA at
distribution voltages. In many cases no circuit breaker is provided, making fuse protection the
only available means of automatic isolation. The fuse must have a rating well above the
maximum transformer load current in order to withstand the short duration overloads that may
occur. Also, the fuses must withstand the magnetizing inrush currents drawn when power
transformers are energized. High Rupturing Capacity (HRC) fuses, although very fast in
operation with large fault currents, are extremely slow with currents of less than three times their
rated value.
Transformer ratings Fuse
KVA Full load current(A) Rated current(A) Operating time at
3*ratings (s)
100 5.25 16 3
200 10.5 25 3
315 15.8 36 10
500 26.2 50 20
1000 52.5 90 30
Table 6.1- Typical Fuse Ratings
Plain over current and earth fault protection utilizing IDMT relays are used primarily to protect
the transformer against the effects of external short circuits and excess overloads. The current
settings of the protection must be above the permitted sustained over load allowance and below
the minimum short circuit current. The ideal characteristic is the extremely inverse (CDG14) as
it is closely approximates to the thermal curve of the transformer.
The protection is located on the supply side of the transformer and is arranged to trip both the
HV and LV circuit breakers. In many cases the requirements for protecting the transformer and
maintaining discrimination with similar relays in the remainder of the power system are not

63
compatible. In these circumstances, negative sequence filter protection or under voltage blocking
may be used to obtain the desired sensitivity.
High set overcurrent cut-Off:
On small transformers where the main protection is provided with overcurrent devices and where
the transformer is fed from one end only, a high set instantaneous relay is utilized to provide
protection against terminal and internal winding faults.
The relay is set to be above the short circuit level on the secondary (load ) side of the transformer
and below that for a terminal fault on the primary (supply)side of the transformer.
On choosing the type and setting of the high set relay, it is important to consider the magnetizing
inrush currents under normal switching, offset fault currents and starting currents of motors. The
first two problems can be overcome by using a relay sensitive only to fundamental frequency
currents, while the third is overcome by setting the relay above the maximum starting current
level.
6.2.2) TRANSFORMER DIFFERENTIAL PROTECTION:
6.2.2.1) Principle of Transformer Differential Protection:
Similar to bus protections, transformers are protected by differential relays. Inter-winding faults
(short circuits) and ground faults within power transformers can be detected by this protection
scheme. Failure to detect these faults and quickly isolate the transformer may cause serious
damage to the device.
A differential relay is basically an instantaneous over current relay that operates on the difference
of current flowing into and out of the protected zone. For transformers the differential protection
(Figure 6.2) is basically the same as that for a bus but there are certain differences that we will
look more closely at. These differences are a direct result of three characteristics or a
transformer.

64
a) A transformer has a turns ratio so the current in is not really equal to the current out. The
current transformers are not likely exactly matched to the transformer turns ratio so there
will always be an unbalance current in the operating coil of a transformer differential
relay.
b) Transformers require magnetizing current. There will be a small current flow in the
transformer primary even if the secondary is open circuited.
c) A transformer has an inrush current. There is a time period after a transformer is
energized until the magnetic field in the core in alternating symmetrically. The size and
the length of this inrush depend on the residual field in the core and the point in the ac
cycle the transformer is re-energized. In large transformers in might be ten or twenty
times the full-load current initially and it might take several minutes to reduce to
negligible values.
Transformer differential relays have restraint coils as indicated in Figure below. The value of the
operate current has to be a certain set percentage higher than the current flowing in the restraint
coils.
Figure 6.2
The current is very high. The restraint coils also prevent relay operation due to tap changes,
where the ratio of transformer input to output current can continuously vary. One other item
included in transformer differential relays but not shown in the diagram is second harmonic
restraint.

65
When transformers are first energized there is over-fluxing (saturation) of the core and the large
inrush energizing current has a distorted waveform. This waveform is described as having high
second harmonic content. The transformer differential relays make use of this known fact and
add in extra restraint when it detects this second harmonic. This extra feature prevents the
transformer from tripping due to magnetizing current when being energized, but does not add
any time delay.
Because the differential relay will not operate with load current or faults outside the protected
zones (through faults), it can be set to operate at a low value of current thereby giving rapid
operation when a fault occurs. There is no need to time delay the operation of the relay and
therefore a fast acting type of relay can be used.
6.2.2.2) Basic Considerations for Transformer Differential protection relay settings
a) Line current transformer primary ratings:
The rated currents of the primary and the secondary sides of a two winding transformer
will depend on the MVA rating of the transformer and will be in inverse ratio to the
corresponding voltages. For three winding transformers the rated current will depend on
the MVA rating of the relevant winding. Line current transformers should therefore have
primary ratings equal to or above the rated currents of the transformer windings to which
they are applied.
b) Current transformer connections:
The CT connections should be arranged, where necessary to compensate for phase
difference between line currents on each side of the power transformer. If the transformer
is connected in delta/star as shown in figure, balanced three phases through current
suffers a phase angle of 30 degree which must be corrected in the CT secondary leads by
appropriate connection of the CT secondary windings.
When CTs are connected in delta, their secondary ratings must be reduced to 1/ √3
times the secondary rating of star connected CTs, in order that the currents outside the
delta may balance with the secondary currents of the star connected CTs.

66
c) Inter posing CTs (ICT’s) to compensate for mismatch of Line CTs:
Besides their use for phase compensation, interposing CTs may be used to match up
currents supplied to the differential protection from the line CTs for each winding. The
amount of CT mismatch which a relay can tolerate with out mal-operation under through
fault conditions will depend on its bias characteristic and the range over which the tap
changer can operate. If the combined mismatch due to CTs and tap changer is above the
accepted level, then interposing CTs may be used to achieve current matching at the mid
point of the tap changer range.
For the protection of two winding transformers interposing CTs should ideally match the
relay currents under through load conditions corresponding to the maximum MVA rating
of the transformer.
6.2.3) Transformer Gas (BuchHolz) Relay:
The transformer gas relay is a protective device installed on the top of oil-filled transformers. It
performs two functions. It detects the slow accumulation of gases, providing an alarm after a
given amount of gas has been collected. Also, it responds to a sudden pressure change that
accompanies a high rate of gas production (from a major internal fault), promptly initiating
disconnection of the transformer. An incipient fault or developing fault usually causes slow
formation of gas (the process of gas formation is discussed later in this section).
Examples of incipient faults are:
• Current flow through defective supporting and insulating structures;
• Defective joints at winding terminals causing heating;
• Minor tap changer troubles; and
• Core faults.
A major fault is one that results in a fast formation of a large volume of gases. Examples of such
faults are:
• Shorts between turns and windings; and
• Open circuits, which result in severe arcing.

67
Failure to disconnect the transformer under fault conditions can result in severe equipment
damage from high gas and oil pressures and the effect of the electrical fault.
Figure-6.3 Buchholz relay mounting arrangement
6.2.3.1) Generation of Gas Due to Faults:
Internal transformer electrical faults result in the production of ionized gases. A significant
volume of gas is frequently generated in the early stages of a fault by rapid oil breakdown. The
generated gases rise through the oil to the top of the equipment and collect in the gas relay.
Once a sufficient volume of gas has accumulated, an alarm is generated by contacts within the
gas relay. In the event of a gas alarm, it is necessary to sample and analyze the gas being
generated. This analysis, together with knowledge of the rate at which gas is accumulating, will
determine the proper course of action. If a fault is thought to be developing, the device must be
removed from service. Ignoring this early warning sign can lead to severe equipment damage as
the fault progresses.

68
6.2.3.2) Operation of a Transformer Gas Relay:
A typical transformer gas relay consists of two chambers, each performing a distinctive function.
The relay assembly consists of a gas accumulation chamber mounted directly over a pressure
chamber. The accumulation chamber collects slowly produced gases. A float located in this
partially oil-filled chamber moves as the gas volume increases. It operates an alarm switch when
the amount of gas collected reaches a specified level. An indicator coupled to the float also
provides a means to monitor the rate at which gas is being generated.
Sudden pressures, such as oil circulating pump surges, are normal operating events and the relay
must be set to ride through them. In practice, it is necessary to make sure the relay is set to
operate at about 7 KPa (1 psi) above the maximum oil circulating pump surge pressure.
Dangerously high pressure increases from major faults are relieved by an explosion vent on the
top of the transformer tank. This is basically a diaphragm sealed pipe with its open end directed
away from the transformer. A significant increase in pressure bursts the diaphragm and
discharges gases and hot oil with a possibility of resulting fire.
6.2.4) WTI and OTI Protection:
Heat is generated in a power transformer by current flow in the primary and the secondary
windings as well as internal connections due to I2R losses. At low loads, the quantity of heat
produced will be small. But, as the load increases, the amount of heat becomes significant. At
full load, the windings will be operating at or near their design temperature. The nameplate on a
transformer will provide information on the maximum allowable in-service temperature rise for
its windings and connections and will indicate what method of cooling is employed to remove
the heat generated under load. A temperature of about 105°C is considered to be the normal
maximum working value for large power transformers, based on an assumed maximum ambient
temperature of 40°C.

69
The winding temperature is sensed and indicated by a winding temperature gauge/alarm
assembly.
6.2.4.1) OTI and WTI at TAPS:
Similar Indicators are used for oil temperature indication and winding temperature indication.
The instrument operates as OTI when its sensing bulb is mounted in an oil filled pocket located
in the hottest oil of an oil immersed transformer.
The instrument operates as WTI, when a proportionate load current of the transformer is passed
through the thermal image device; the instrument integrates the simulated temperature rise of the
thermal image device and the top oil temperature measured by the sensing bulb.
6.2.5) Overfluxing Protection:
Increase in power frequency voltage causes increase in working magnetic flux, thereby increases
the iron loss and magnetizing current. The core and core bolt get heated and the lamination
insulation is affected. Overfluxing protection is provided for generator transformer and feeder
transformer where a possibility of over fluxing due to sustained over-voltages exists. The
reduction in frequency also increases the flux density and consequently, it has the similar effects
as those due to over-voltage.
The expression for flux in a transformer is given as:
Φ α V / f
Where, Φ = flux, V = applied voltage, f = frequency
and all are p.u. values. When V/f exceeds unity, it has to be detected. Usually 10% of
Overfluxing can be allowed without damage. If V/f exceeds 1.1, overfluxing protection operates.
Overfluxing does not require high speed tripping and hence instantaneous tripping is undesirable
when momentary disturbances occur. But the transformer should be isolated in 1-2 min if over
fluxing persists. The Overfluxing setting is IDMT in general cases.

70
6.2.6) Transformer Ground Fault Protection:
Earthfault protection of transformer can be in one or more types such as:
a) Residually connected earth fault protection.
b) Neutrally connected earth fault protection.
c) Restricted earth fault protection.
6.2.6.1) Residually Connected Earthfault Protection:
Delta windings and ungrounded star windings are best protected by zero-sequence overcurrent
relays (Earth fault relays) supplied by CTs situated at the terminals of power transformer.
Such relay can only operate for a ground fault in the transformer winding since it does not have
an earth connection through which it can supply an external fault.
The relay is usually instantaneous but must be of high impedance type if supplied with residually
connected CTs in the three phases. The high impedance relay is required to prevent wrong
operation of the relay on false residual currents during heavy external fault between phases due
to transient differences in the CT outputs. An ordinary overcurrent relay is acceptable if it is
supplied by a core-balance type CT because in this case, the magnetic conditions of the CTs are
the same for all the three phases.
6.2.6.2) Neutral Connected Earthfault Protection:
The relay is connected across the secondary of a CT whose primary is connected in the neutral to
earth connection of a star connected transformer. The fault current finds a path through the earth
and earth to neutral connection of the transformer.
The magnitude of the earth fault current is dependent on the type of earthing and the location of
the fault.
In both the above types of protection the zone of protection can not be accurately defined. The
protected area is not restricted to the transformer winding alone. The relay may sense an
earthfault beyond the transformer winding depending upon position of the source.

71
Hence, such protection is called unrestricted earthfault protection. In residually connected
earthfault relays where the zone of protection is not restricted to transformer winding only and in
neutral connected earthfault relays, IDMTL earthfault protection co-ordinated with down stream
is to be provided.
6.2.6.3) Restricted Earthfault Protection:
When the primary winding is delta connected or star connected without neutral earthing,
earthfaults on secondary side are not reflected on the primary side as the zero sequence
impedance between the primary and secondary is infinite (i.e., open). In such cases an earthfault
relay connected in the residual circuit of 3 CTs on primary side will operate on internal faults in
primary winding only. During external ground fault the sensitivity of low impedance relay is
limited by the fact that the magnetizing current of the neutral CT is three times that of each of the
If this resistance balance does not exist, it can theoretically be remedied by adding resistance on
the neutral CT side, but this is not the practice because the balance would not hold during
transient conditions or if the neutral CT saturates. The proper solution is to use a stabilizing
resistance in series with low impedance relay or to use high impedance relay.
This protection is based on high impedance differential principle, offering stability for any type
of fault occurring outside the protected zone and satisfactory operation for faults within the zone.

72
6.3) CHARACTERISTIC TRANSFORMER FAULTS:-
Table – 6.3 transformer faults
Type of faults Causes Effects Actuation of
Buchholz relay
Major components
of gases evolved
High energy
discharges
(arcing)
Short circuits in
the windings.
External short
circuit from parts
at potential to
earth.
Breakdown
between the
windings.
Breakdown
through the oil
between the bare
conductors
Pyrolytic
decomposition
of insulating
oil.
Formation of
oil carbon.
Decreases in
the flash point
of oil.
Suddenly
actuations
Methane, Hydrogen
& Acetylene
(Carbon monoxide if
solid insulation is
also involved.)
Continuous
sparking at
breaks
Bad contact
action of
connections to
metallic parts
with floating
potential.
Slight
decomposition
of insulating oil
Actuation after a
long period
Hydrogen,
Acetylene, Methane
is less than in (a)
Discharges
between selector
contacts
Due to surfaces
layer of foreign
materials
High energy
partial
discharges with
tracking
Poor
impregnation.
Presence of
cavities in the
insulation.
Electrical
overstressing of
the insulation.
Ionization
processes.
(Excitation and
dissociation of
hydrocarbon
molecules by
collision with
high-energy
electrons, ions,
atomic
hydrogen etc.)
Low energy
partial
Poor
impregnation or Ionization Methane, Hydrogen.

73
discharges
without tracking
cavities in the
insulation
processes
Thermal faults:
Overhauling to
temperature
between 150°C
& 300°C (Hot
spot).
Excessive
magnetic
losses.
Insufficient
cooling.
Slight
decomposition
of oil.
After a long
period
Hydrogen,
Methane,
Ethylene,
(Ethylene
predominates),
No Acetylene.
Local
overheating
(300°C to
1000°C)
High circulating
currents in the
core
Decomposition
of oil with
formation of oil
carbon
After some time
Hydrogen,
Methane,
Ethylene,
(Ethylene
predominates),
No Acetylene.
Local
overheating
beyond 1000°C
Shorting links
between core
laminates
Decomposition
of oil with
formation of oil
carbon.
Destruction of
organic
insulation.
Melting spots,
Core burn,
melted
conductors and
scorching
points.
Actuation of
relay after
accumulation of
little quantities
of gases.
Hydrogen,
Methane,
Ethylene,
(Ethylene
predominates),
Considerable
amount of
Acetylene.
6.4) CONDITION MONITORING OF TRANSFORMERS
It is possible to provide transformers with measuring devices to detect early signs of degradation
in various operator can make a better judgement as to the frequency of maintenance, and detect

74
early signs of deterioration that, if ignored, would lead to an internal fault occurring. Such
techniques are an enhancement to, but are not a replacement for, the protection applied to a
transformer. A typical condition monitoring system for an oil immersed transformer is capable of
monitoring the condition of various transformer components as shown in Table 5.3
Table 6.4- condition monitoring of transformer

75
6.5) CHOICE OF TRANSFORMERS AT TAPS-3&4
6.5.1) why such GT is chosen?
Type: 3 Nos. of single-phase transformers
OFAF, OFAN (60%)
YNd 11,
210 MVA (1Phase rating)
Reasons for adopting three single phase units:
a) No weight and transport height constraints:
A single unit of 3 phases, 630 MVA capacity would be immense in size and weight and,
if any attempt is made to decrease the size, height has to be increased for the same flux
density of core. Further the cooling arrangement will have to be very elaborate for entire
coverage
b) Easy repair /replacement of single unit;
As amount of oil in one unit is less compared to single unit, replacement / draining of oil
will take less time and thereby reduce the total downtime. Further, with a spare single
unit available, the downtime is reduced, in case of shutdown.
c) No common failure probability:
No question of transformer phase to phase faults, as the units are physically separate.
Each unit has its own cooling arrangement. Therefore, cooling unit failure of one unit
will not lead to total outage; the total units can remain in service at reduced power.
(OFAN-80%, ONAN = 60%). Easy identification of phase to earth, inter-turn faults as
each unit has its own buchholz relay.

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