Power System Protection - CDEEP Engineering... · Relays Setting and Coordination Summary Power System Protection S.A.Soman Department of Electrical Engineering IIT Bombay Fundamentals

Power SystemProtection

S.A.Soman

Principles ofOvercurrentProtectionLimitations of a Fuse

Algorithm forOvercurrent Relay

Time MultiplierSetting

Classificationof OvercurrentRelayLack of CoordinationProblem

IEC StandardEquations

IEEE StandardEquations

Pick upCurrentSettingGuidelines

Primary and Back upRelays

Setting andCoordination

Summary

Power System Protection

S.A.Soman

Department of Electrical EngineeringIIT Bombay

Fundamentals of Overcurrent Protection


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1 Principles of Overcurrent ProtectionLimitations of a FuseAlgorithm for Overcurrent RelayTime Multiplier Setting

2 Classification of Overcurrent RelayLack of Coordination ProblemIEC Standard EquationsIEEE Standard Equations

3 Pick up Current SettingGuidelinesPrimary and Back up RelaysSetting and Coordination

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Limitations of a Fuse

It is difficult to control the time to trip.

This makes primary backup relay coordination difficult.

Once fuse melts, it has to be replaced forreenergization of the equipment.

Thus it is difficult to have remote operation.


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Algorithm for Overcurrent Relay

Set reference or threshold for discriminatingovercurrent Iref .

Measure device current I.

Compute the ratio abs( IIref

) which is known as the PlugSetting Multiplier.

Trip the device, if PSM is above the threshold. Thethreshold is always greater than 1.


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Time Multiplier Setting

Overcurrent relays normally play the dual roles ofprimary and backup protection.

Backup action requires a wait state.

By providing the additional setting feature of TMS, thiscan be achieved.

TMS is defined as the ratio TTm

, where T is the desiredrelay operating time for a given PSM and Tm is thecorresponding operating time at TMS=1.

Relay operating time can be varied by varying the TMSsetting.


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Classification of Overcurrent Relay

Instantaneous relayIts action is very fast.It has only pick up setting.It does not have any TMS setting.This relay is not suitable for backup protection.

Time delayed definite time relay

It can be adjusted to issue trip decision after a specifieddelay.This delay is independant of PSM value.It has both time setting and pick up setting.It is used for short length feeders.


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Classification of Overcurrent Relay

Instantaneous relayIts action is very fast.It has only pick up setting.It does not have any TMS setting.This relay is not suitable for backup protection.

Time delayed definite time relayIt can be adjusted to issue trip decision after a specifieddelay.This delay is independant of PSM value.It has both time setting and pick up setting.It is used for short length feeders.


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Overcurrent Protection Using Definite Time Relay

R1 trips without any intentional time delay.R2 has to coordinate with R1 and its operation isdelayed by CTI.R3 has to back up R2 and it is again time delayed.


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Lack of Coordination Between Fuse and DT relay

DT relay R1 has to backup fuse.

After point X relay actsfaster than fuse.

This lack of coordinationis due to differentcharacteristics of fuseand relay.

This lead to thedevelopment of inversecharacteristics forovercurrent relay.


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Classification of Overcurrent RelayInverse definite minimum time relay

It is inverse in the initial part and tends to approach adefinite minimum operating time characteristics as thecurrent becomes high.This is the most widely used characteristics.


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IEC Standard Equations for Inverse Characteristics

Standard Inverse Characteristics

t = TMS × 0.14(I/Is)0.02 − 1

Very Inverse Characteristics

t = TMS × 13.5(I/Is)− 1

Extremely Inverse Characteristics

t = TMS × 80(I/Is)2 − 1


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IEEE Stanard Equations for Inverse Characteristics

Moderately inverse: t = TD7

{(0.0515

(I/Is)0.02−1

)+ 0.114

}Very inverse: t = TD

7

{(19.61

(I/Is)2−1

)+ 0.491

}Extremely Inverse: t = TD

7

{(28.2

(I/Is)2−1

)+ 0.1217

}US CO8 Inverse: t = TD

7

{(5.95

(I/Is)2−1

)+ 0.18

}US CO2 Inverse: t = TD

7

{(0.02394

(I/Is)0.02−1

)+ 0.01694

}


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IEEE Standard Curves


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Faults are classified into two:

Phase fault which do not involve ground.Earth fault which involve ground.


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Guidelines for Setting Pickup Current for Phase FaultRelays

Pickup current should be above the maximum loadcurrent.

Pickup current should be below minimum fault current.

Pickup current should also be below the minimum faultcurrent of the feeder that it has to back up.

The back up relay also should provide time for primaryrelay to operate. This time interval is known ascoordination time interval.


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Algorithm for Identifying Primary and Backup Relays

Identify leaf nodes androot nodes.

Find the parents of theactive child nodes.

The correspondingrelays should back upthe leaf node relays.

Update active childnodes to correspondingparent node.


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Algorithm for Setting and Coordination of OvercurrentRelays in a Radial System

Initialize the coordination tree.

Are there any leaf nodes except the root? If yes, go tostep 3, else go to step 7.

Identify the leaf node in coordination tree.

If the PSM of these relays are not set so far, set theirPSM.

Identify the parents of leaf nodes in step 3. Computetheir PSM and TMS for backup protection andcoordination.

Delete the leaf nodes. Update the coordination tree andgo back to step 2.

Coordination activity is complete.


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Summary

Limitations of fuse protection.

Fundamental principle of overcurrent relay.

Plug setting multiplier and time multiplier setting.

Classification of overcurrent relay.

Primary relay and back up relay.

Setting and coordination of overcurrent relays.

Documents

Power System Protection - CDEEP Engineering... · Relays Setting and Coordination Summary Power System Protection S.A.Soman Department of Electrical Engineering IIT Bombay Fundamentals