All in one

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ACKNOWLEDGEMENT
The satisfaction that accompanies the successful completion of any task
would be incomplete without acknowledging the people whose constant
guidance and encouragement has crowned all the efforts with success.
I express my deep gratitude to Prof.Dr. N.R. Samal, Head of the
Department of Electrical Engineering for his support and inspiration
and to my guide Er. Debasish Mishra (Asst. Professor), Department of
Electrical Engineering for permitting me to present the seminar on
“SUBSTATION PROTECTION DEVICES”.
I would also like to extend my deep regards to the lecturers of
Electrical Department for their guidance and help. I am also very
grateful to my friends without whose support and help this would not
have been a success.
RAHUL KUMAR
REG.NO: 1201211280
ROLL.NO: 125069
DEPARTMENT OF ELECTRICAL ENGINEERING

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ORISSA ENGINEERING COLLEGEBHUBANESWAR
CERTIFICATE
This is to certify that Mr. Rahul Kumar, bearing Roll No: 125069
and University Registration No-1201211280, a student of 7th
semester has submitted the seminar report entitled “substation
protection devices” as per the requirement of the Biju Patnaik
University Of Technology in partial fulfilment of Degree of
Bachelor of technology (Electrical Engineering Department) for
the academic year 2012-2016.
Guided By:-Dr. N.R. Samal
Professor& HOD
Dean Academics
Department of Electrical Engineering
Er. DebasishMishra.
Assistant Professor
Department of Electrical Engineering

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DECLARATION
I, RAHUL KUMAR, hereby declare that the seminar
report Entitled “SUBSTATION PROTECTION DEVICES”
submitted to the OEC Bhubaneswar is a record of an original
work done by me under the guidance of Er. Debasish Mishra,
Assistant Professor in Electrical Engineering, O r i s s a
E n g i n e e r i n g C o l l e g e , B H U B A N E S W A R .
This report is submitted in the
partial fulfilment of the requirements for the award of
the degree of Bachelor of Technology in Electrical Engineering.
The results embodied in this thesis have not been submitted to
any other University or Institute for the award of any degree
or diploma.

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INDEX
ABSTRACT
INTRODUCTION
THREE PHASE DIAGRAM OF PROTECTION
COMPONENTS OF PROTECTION SYSTEM
CURRENT TRANSFORMER (CT)
POTENTIAL TRANSFORMER (PT)
PROTECTION RELAY
CIRCUIT BREAKER (CB)
LIGHTINING ARRESTER (LA)
ISOLATOR
CONCLUSION
REFERENCES

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Substation Protection Devices
Abstract
As we know the safety is very first requirement for any system. So
here I am discussing about some protection devices use in the
Electrical Power Substation.
Power-system protection is a branch of electrical power engineering
that deals with the protection of electrical power systems from faults
through the isolation of faulted parts from the rest of the electrical
network.
The objective of a protection scheme is to keep the power system
stable by isolating only the components that are under fault, whilst
leaving as much of the network as possible still in operation.
In Electrical Power Substation(switch yard) various type of protection
devices are used to protect from different fault.
Some of them are:
 Current Transformer(CT)
 Potential Transformer(PT)
 Protection Relay
 Circuit Breaker(CB)
 Lightning Arrester(LA)
 Fuse
When any type of fault occurs in the system, it is detected by Current
Transformer or Potential Transformer or Lightning Arrester. These devices give
the controlling signal to Relays biased with them. Relay decides whether fault
occurs in the particular part of system or not. When the faults occurs in any
section of the system, Protection Relay of that section comes in operation and
closes the trip of the Circuit Breaker, which disconnects the faulty section. The
healthy section continue to supply loads as usual and thus there is no damage to
the equipment and no complete interruption of supply.

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Introduction
Power-system protection is a branch of electrical power engineering
that deals with the protection of electrical power systems from faults
through the isolation of faulted parts from the rest of the electrical
network. The objective of a protection scheme is to keep the power
system stable by isolating only the components that are under fault,
whilst leaving as much of the network as possible still in operation.
Thus, protection schemes must apply a very pragmatic and
pessimistic approach to clearing system faults.
A substation is a part of an electrical generation, transmission, and
distribution system. Substations transform voltage from high to low,
or the reverse, or perform any of several other important functions.
Between the generating station and consumer, electric power may
flow through several substations at different voltage levels.
The apparatus or equipments and their associated auxiliaries
employed for controlling, regulating and switching on or off the
electrical circuits in the electrical power system is known as
“Protection devices” or “switchgear”.
An automatic Protection consists of the Relays, Circuit Breakers(CB),
Lightning Arrester(LA) and Fuse. When the faults occurs in any
section of the system, Protection Relay of that section comes in
operation and closes the trip of the Circuit Breaker, which disconnects
the faulty section. The healthy section continue to supply loads as
usual and thus there is no damage to the equipment and no complete
interruption of supply.
In this lesion we shall discuss about the various Protection Devices
used in Substation Protection System.

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Three-Phase Diagram of the Protection
CTs
VTs
Relay
CB
Control
Protected
Equipment

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Components of Protection System
An automatic protection system has mainly six components:
1. Instrumentation Transformer
 Current Transformer(CT)
 Potential Transformer(PT)
2. Protective Relay
3. Circuit Breaker(CB)
4. Lightning Arrester(LA)(Surge Diverter)
5. Isolators(Disconnecting switch)
6. Fuse
A typical Substation

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Current Transformer(CT)
A current transformer (CT) is used for measurement of
alternating electric current. Current transformers, together with
voltage (or potential) transformers (VT or PT), are known as
instrument transformers. When current in a circuit is too high to
apply directly to measuring instruments, a current transformer
produces a reduced current accurately proportional to the current in
the circuit, which can be conveniently connected to measuring and
recording instruments. Current transformers are commonly used in
metering and protective relays in the electrical power industry.
Design
Like any other transformer a current transformer has a primary
winding, a magnetic core and a secondary winding. The alternating
current in the primary produces an alternating magnetic field in the
core, which then induces an alternating current in the secondary
winding circuit. An essential objective of current transformer design
is to ensure the primary and secondary circuits are efficiently coupled,
so the secondary current is linearly proportional to the primary
current.
Basic operation of current transformer

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SF6 110 kV current transformer
Usage
Current transformers are used extensively for measuring current and
monitoring the operation of the power grid. Along with voltage leads,
revenue-grade CTs drive the electrical utility's watt-hour meter on
virtually every building with three-phase service and single-phase
services greater than 200 amperes.
CT used in metering equipment for three-phase 400-ampere electricity supply
Current Transformers are also used as the protection device including
with Relay and Circuit Breaker.

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Potential Transformer(PT)
Potential transformers (PT) (also called voltage transformers (VT))
are a parallel connected type of instrument transformer. They are
designed to present negligible load to the supply being measured
and have an accurate voltage ratio and phase relationship to enable
accurate secondary connected metering.
It gives the reference voltage to the Relay for Over-voltage or Under-
voltage Protection.

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Protective Relay
A Relay is a logical elements which process the inputs(mostly voltage
and currents) from the system and issue a trip decision if a fault
within its jurisdiction is detected.
Inputs to the Relays are
• Current from current transformer(CT)
• Voltage from voltage transformer(VT)
Principle of operation
Electromechanical protective relays operate by either magnetic
attraction, or magnetic induction. Unlike switching type
electromechanical relays with fixed and usually ill-defined operating
voltage thresholds and operating times, protective relays have well-
established, selectable and adjustable time/current (or other operating
parameter) operating characteristics. Protection relays may use arrays
of induction disks, shaded-pole magnets, operating and restraint coils,
solenoid-type operators, telephone-relay contacts, and phase-shifting
networks.
Types of Relay
According to construction
 Electromechanical
 Induction disc over-current relay
 Static relay
 Digital relay
 Numerical relay

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Relays by functions
 Over current relay
 Distance relay
 Current differential protection
 Directional relay
 Synchronism check
Basic elements of Relay
 Sensing Element
 Comparison Element
 Control Element

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A typical Protective Relay
Relay panel

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Circuit breaker(CB)
A circuit breaker is an automatically operated electrical switch
designed to protect an electrical circuit from damage caused by
overload or short circuit. Its basic function is to detect a fault
condition and interrupt current flow. Unlike a fuse, which operates
once and then must be replaced, a circuit breaker can be reset (either
manually or automatically) to resume normal operation. Circuit
breakers are made in varying sizes, from small devices that protect an
individual household appliance up to large switchgear designed to
protect high voltage circuits feeding an entire city.
Operation
The circuit breaker must detect a fault condition; in low voltage
circuit breakers this is usually done within the breaker enclosure.
Circuit breakers for large currents or high voltages are usually
arranged with protective rela pilot devices to sense a fault condition
and to operate the trip opening mechanism. The trip solenoid that
releases the latch is usually energized by a separate battery, although
some high-voltage circuit breakers are self-contained with current
transformers, protective relays and an internal control power source.
Once a fault is detected, within the circuit breaker must open to
interrupt the circuit; some mechanically-stored energy (using
something such as springs or compressed air) contained within the
breaker is used to separate the contacts, although some of the energy
required may be obtained from the fault current itself. Small circuit
breakers may be manually operated, larger units have solenoids to trip
the mechanism, and electric motors to restore energy to the springs.
Types of circuit breakers
 Low-voltage circuit breakers
 Magnetic circuit breakers
 Thermal magnetic circuit breakers

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 Common trip breakers
 Medium-voltage circuit breakers
 High-voltage circuit breakers
 Sulphur hexafluoride (SF6) high-voltage circuit breakers
 Disconnecting circuit breaker (DCB)
 Carbon dioxide (CO2) high-voltage circuit breakers
Oil circuit breaker

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Air circuit breaker
SF6 Circuit breaker

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Lightning arrester(LA)
A lightning arrester is a device used on electrical power systems and
telecommunications systems to protect the insulation and conductors
of the system from the damaging effects of lightning. The typical
lightning arrester has a high-voltage terminal and a ground terminal.
When a lightning surge (or switching surge, which is very similar)
travels along the power line to the arrester, the current from the surge
is diverted through the arrestor, in most cases to earth.

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Isolators
Isolators are used to isolate the high voltage from flow through line
into the bus. It allows only needed voltage and rest is earth by itself if
required.

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Conclusion
The objective of the protection devices(system) is to keep the power
system stable by isolating only the components that are under fault,
while leaving as much of the network as possible.
• There is no ‘fault free’ system.
• It is neither practical nor economical to build a ‘fault free’
system.
• Electrical system shall tolerate certain degree of faults.
Usually faults are caused by breakdown of insulation due to various
reasons: Short Circuit, High Voltage, system aging, lighting, etc.

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References
1.Power System Protection and Switchgear by Badri Ram
Tata McGraw-Hill Education, 01-Nov-2011
2. Fundamentals of Power System Protection By Yeshwant
G. Paithankar, S. R. Bhide
3. Electrical Power System Protection By C.
Christopoulos, A. Wright
4. Wikipedia