basic power system protection

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POWER SYSTEM POWER SYSTEM PROTECTIONPROTECTION

Fundamentals of Fundamentals of Protection PracticeProtection Practice

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Why power system need the Why power system need the protection system?protection system?

Severe disruption to normal routine of modern Severe disruption to normal routine of modern society is likely if power outages are frequent or society is likely if power outages are frequent or prolonged.prolonged.

Many items of equipment are very expensive, Many items of equipment are very expensive, and the complete power system represent a very and the complete power system represent a very large capital investment.large capital investment.

Fault may represent a risk to life and/or property.Fault may represent a risk to life and/or property.


5Other

3Control equipment

10Instrument transformer

12Transformer

15Busbar

10Underground Cable

45Overhead line

% of faults% of faultsType of equipmentType of equipment

3


2Other

5Series Fault

10Three Phase Short Circuit

5Two Phase Short Circuit

18Two Phase to Earth

60One Phase to Earth

% of fault% of faultType of faultsType of faults

What can the protection system do?What can the protection system do?

Increase emphasis on reliability and Increase emphasis on reliability and security of supply.security of supply.

Prevent the diverse items of equipment Prevent the diverse items of equipment from the severe damage by detect and from the severe damage by detect and disconnect elements of the power system.disconnect elements of the power system.

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More fundamentalMore fundamentalHowever, is the power system should operate However, is the power system should operate

in a safe manner at all times. No matter how well in a safe manner at all times. No matter how well designed, faults will always occur on a power designed, faults will always occur on a power system.system.

The provision of adequate protection is The provision of adequate protection is therefore on integral part of power system therefore on integral part of power system design. design.

As requirements of reliability and economic are As requirements of reliability and economic are largely opposed, power system design is largely opposed, power system design is inevitable a compromiseinevitable a compromise

Protection SystemProtection SystemProtection system is a complete arrangement of Protection system is a complete arrangement of protection equipment and other devices required to protection equipment and other devices required to achieve a specified function based on a protection achieve a specified function based on a protection principal.principal.

Protection equipment is a collection of protection device Protection equipment is a collection of protection device ( relay, fuse, etc.) excluded are device such as CT( relay, fuse, etc.) excluded are device such as CT’’s, s, CBCB’’s, contactor, etc.s, contactor, etc.

Protection scheme is a collection of protection Protection scheme is a collection of protection equipment providing a defined function and including all equipment providing a defined function and including all equipment required to make the scheme work (i.e. equipment required to make the scheme work (i.e. relays, CTrelays, CT’’s, CBs, CB’’s Batteries, etc.)s Batteries, etc.)

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Protective RelayProtective RelayRelay maybe classified according to the Relay maybe classified according to the technology used.technology used.

electromechanicalelectromechanicalstaticstaticdigitaldigitalnumericalnumerical

The different type have somewhat different The different type have somewhat different capabilities due to limitations of technology capabilities due to limitations of technology used. used.

Type of protective relayType of protective relaya relay that responds to single quantitya relay that responds to single quantitya relay that responds to several quantitiesa relay that responds to several quantitiesa single relay containing several elements, each responding a single relay containing several elements, each responding independently to a different quantityindependently to a different quantity

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ANSI / IEC Relay SymbolsANSI / IEC Relay Symbols

59N59NNeutral point Neutral point displacement relaydisplacement relay3232Directional Directional

overpower relayoverpower relay

5959OvervoltageOvervoltage relayrelay2727UndervoltageUndervoltagerelayrelay

5555Power factor relayPower factor relay2626OvertemperatureOvertemperaturerelayrelay

51V51VVoltage Voltage

restrained/controlled restrained/controlled overcurrentovercurrent relayrelay

2121Distance relayDistance relay

51N51NDefinite time earth Definite time earth fault fault overcurrentovercurrent

relayrelay1414UnderspeedUnderspeed relayrelay

51G51GInverse time earth Inverse time earth fault fault overcurrentovercurrent

relayrelay1212OverspeedOverspeed relayrelay

IEC 60617IEC 60617ANSIANSIDescriptionDescriptionIEC60617IEC60617ANSIANSIDescriptionDescription

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Z <

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cos ϕ >

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O I

ANSI / IEC Relay SymbolsANSI / IEC Relay Symbols

8787Differential relayDifferential relay

81O81OOverfrequencyOverfrequency relayrelay5151Inverse time Inverse time overcurrentovercurrent relayrelay

81U81UUnderfrequencyUnderfrequencyrelayrelay5050Instantaneous Instantaneous

overcurrentovercurrent relay relay

7979AutorecloseAutoreclose relayrelay4949Thermal relayThermal relay

7878Phase angle relayPhase angle relay4747Negative sequence Negative sequence voltage relayvoltage relay

67N67NDirectional earth Directional earth fault relayfault relay4646Negative sequence Negative sequence

relayrelay

6767Directional Directional overcurrentovercurrent relayrelay3737Undercurrent relayUndercurrent relay

6464EarthEarth--fault relayfault relay3737UnderpowerUnderpower relayrelay

IEC 60617IEC 60617ANSIANSIDescriptionDescriptionIEC 60617IEC 60617ANSIANSIDescriptionDescription

P <

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Zones of ProtectionZones of Protection

To limit the extent of power system that is To limit the extent of power system that is disconnected when a fault occur.disconnected when a fault occur.

G

Zone 1 Zone 2 Zone 3 Zone 4

Zone 4

Zone 6

Zone 5

7

G

G


Ideally the zones of protection should overlap. So that no part Ideally the zones of protection should overlap. So that no part of the power system is left un protected.of the power system is left un protected.

The point of connection of the protection with the power The point of connection of the protection with the power system usually defines the zone and corresponds to the system usually defines the zone and corresponds to the location of CTlocation of CT’’s.s.


Unit protection will result in the boundary being a clearly Unit protection will result in the boundary being a clearly define close loop.define close loop.

Zone maybe un restricted, the extent or reach will depend on Zone maybe un restricted, the extent or reach will depend on measurement of the system quantities.measurement of the system quantities.

G

G

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Principal of ProtectionPrincipal of Protection

ReliabilityReliabilityOperate under all required condition, and refrain from Operate under all required condition, and refrain from operating when so required.operating when so required.

Incorrect operation can be attributed to one ofIncorrect operation can be attributed to one ofIncorrect design / settingIncorrect design / setting

Design: Due consideration must be given to the nature, Design: Due consideration must be given to the nature, frequency and duration of fault, all relevant parameters of frequency and duration of fault, all relevant parameters of the power system and type of protection equipment used.the power system and type of protection equipment used.Setting: The setting are chosen for protection relays and Setting: The setting are chosen for protection relays and system which take in to account the primary system, fault, system which take in to account the primary system, fault, load levels etc. The characteristic of power system load levels etc. The characteristic of power system changes with time change in load etc. Therefore, setting changes with time change in load etc. Therefore, setting value of relay may need to be checked at suitable intervals value of relay may need to be checked at suitable intervals to ensure that are still appropriate. to ensure that are still appropriate.


Incorrect installation / testingIncorrect installation / testingInstallation: The complexity of interconnections of Installation: The complexity of interconnections of many systems and their relationship to the remainder many systems and their relationship to the remainder of the installation may make checking difficult.of the installation may make checking difficult.

Deterioration in serviceDeterioration in serviceThe time between operations of protection relays The time between operations of protection relays maybe years rather than days. During this period maybe years rather than days. During this period defects may have developed unnoticed until revealed defects may have developed unnoticed until revealed by the failure of the protection to respond to a power by the failure of the protection to respond to a power system fault. For this reason, relays should be system fault. For this reason, relays should be regularly tested in order to check for correct regularly tested in order to check for correct functioning. functioning.

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SelectivitySelectivityTo trip only those circuit breakers whose operation is To trip only those circuit breakers whose operation is required to isolate the fault. The property of selectivity required to isolate the fault. The property of selectivity tripping is also called tripping is also called ‘‘discriminationdiscrimination’’ and is achieved and is achieved by two general methods.by two general methods.

Time GradingTime GradingProtection systems in successive zones are arranged to Protection systems in successive zones are arranged to operate in times that are graded through the sequence of operate in times that are graded through the sequence of equipments so that upon the occurrence of a fault although a equipments so that upon the occurrence of a fault although a number of protection equipments respond, only those relevant number of protection equipments respond, only those relevant to the faulty zone complete the tripping function. The others to the faulty zone complete the tripping function. The others make incomplete operations and then reset.make incomplete operations and then reset.


Unit SystemsUnit SystemsThe protection systems that respond only fault conditions The protection systems that respond only fault conditions occurring with in a clearly defined zone, it does not occurring with in a clearly defined zone, it does not involve time grading, is relatively fast in operation. The involve time grading, is relatively fast in operation. The speed of response is substantially independent of fault speed of response is substantially independent of fault severity.severity.

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StabilityStabilityThe ability of protection system to remain The ability of protection system to remain unaffected by conditions external to the unaffected by conditions external to the protected zone, for example through load protected zone, for example through load current and external fault conditions.current and external fault conditions.


SpeedSpeedThe function of protection systems is to isolate faults The function of protection systems is to isolate faults on the power system as rapidly as possible.on the power system as rapidly as possible.

The main objective is to safeguard continuity of supply by The main objective is to safeguard continuity of supply by removing each disturbance before it lead to widespread loss removing each disturbance before it lead to widespread loss of synchronism and consequent collapse of power system. of synchronism and consequent collapse of power system. As the loading on a power system increase the phase shift As the loading on a power system increase the phase shift between voltages and different between voltages and different busbarsbusbars on the system also on the system also increases, and therefore so does the probability that increases, and therefore so does the probability that synchronism will be lost when system is disturbed by a fault, synchronism will be lost when system is disturbed by a fault, protection must thus operate as quickly as possible.protection must thus operate as quickly as possible.

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However speed of operation must be However speed of operation must be weighed against economy. weighed against economy.

Distribution circuits which do not Distribution circuits which do not normally require a fast fault clearance, are normally require a fast fault clearance, are usually protected by timeusually protected by time--graded systems. graded systems.

Generating plant and EHV systems Generating plant and EHV systems require protection gear of highest attainable require protection gear of highest attainable speed.speed.


SensitivitySensitivityThis is a term frequently used when referring This is a term frequently used when referring to the minimum operating level ( current, to the minimum operating level ( current, voltage, power etc.) of relays or complete voltage, power etc.) of relays or complete protection scheme. protection scheme.

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Primary and BackPrimary and Back--Up ProtectionUp Protection

The reliability of a power system has been The reliability of a power system has been discussed earlier, including the use of more than discussed earlier, including the use of more than primary ( or main ) protection system operating primary ( or main ) protection system operating in parallel. in parallel.

In the event of failure or nonIn the event of failure or non--availability of availability of the primary protection some other means of the primary protection some other means of ensuring that the fault is isolated must be ensuring that the fault is isolated must be provided. These secondary systems are referred provided. These secondary systems are referred to as to as ‘‘backback--up protectionup protection’’..

BackBack--Up ProtectionUp Protection

Local backLocal back--up protectionup protectionThis is achieved by protection which detect an This is achieved by protection which detect an unun--cleared primary system fault at its own cleared primary system fault at its own location and which then trip its own circuit location and which then trip its own circuit breakers, e.g. timebreakers, e.g. time--graded over current relay.graded over current relay.

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R1 R2

A

BD

E

C

++

Local backLocal back--up protectionup protection


R1 R2

A

B

C

D

E

++-

Local backLocal back--up protection ( Breaker failure relay )up protection ( Breaker failure relay )

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Remote backRemote back--up protectionup protectionThis is provided by protection that detects an This is provided by protection that detects an unun--cleared primary system fault at a remote cleared primary system fault at a remote location and then issue a local trip command location and then issue a local trip command e.g. the second or third zones of distance e.g. the second or third zones of distance relay.relay.


FR1R2R3123

Current

Time

R1

R2

R3

fault

T1T2T3

Remote backRemote back--up protectionup protection

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The extent and type of backThe extent and type of back--up protection up protection applied will naturally be related to the failure applied will naturally be related to the failure risks and relative economic importance of the risks and relative economic importance of the system.system.

For distribution systems where fault clearance times For distribution systems where fault clearance times are not critical, time delayed remote backare not critical, time delayed remote back--up up protection maybe adequate.protection maybe adequate.For EHV systems, where system stability is at risk For EHV systems, where system stability is at risk unless a fault is cleared quickly, multiple primary unless a fault is cleared quickly, multiple primary protection systems, operating in parallel and possibly protection systems, operating in parallel and possibly of different type ( e.g. distance and unit protection ) of different type ( e.g. distance and unit protection ) will be used to ensure fast and reliable tripping.will be used to ensure fast and reliable tripping.

basic power system protection

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