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

CERTIFICATEThis 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. Debasish Mishra.

Assistant Professor

Department of Electrical Engineering

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DECLARATIONI, 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 .

Th is report is submitted in the part ia l fu lfi lment 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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INDEXABSTRACT

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 DevicesAbstractAs 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

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Components of Protection SystemAn automatic protection system has mainly six components:

1. Instrumentation Transformer Current Transformer(CT) Potential Transformer(PT)

2. Protective Relay3. 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

UsageCurrent 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

Relays by functions

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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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Sensing Element

Comparison

Element

Control Element

Relay

To trip or signal circuit(CB)

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 Common trip breakers

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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

SF 6 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

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