15a_setting of protection

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Setting of Protectiono/c & e/f relays

Protection Application Handbook

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Setting of Protection

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Setting of ProtectionPhase Overcurrent Protections Selectivity

Fault current

Time

t

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Setting of ProtectionPhase Overcurrent Protections Pick-up and reset

The relay does not reset

t

I

I stI reset

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Setting of ProtectionOvercurrent relays Current selectivity

Possible when impedance in between is substantial

Time selectivity Time starts adding and trip times will be very long

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Setting of ProtectionShort circuit protection

Current setting shall be Higher than maximum load current Shall be lower than minimum fault current Margin to load to be set considering reset ratio

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Setting of ProtectionInstantaneous stage for transformer

Setting shall be Higher than maximum inrush current

Should not reach > 80% of transformer

Complement to diff relay and buchholz

protection

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Setting of ProtectionDirectional short circuit protection

Directional current relay shall have 60 degrees

angle lagging

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Setting of ProtectionMargin to equipment capacity

Tripping should be before reaching thermal capacity of the object

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Setting of ProtectionEarth fault relays in solidly earthed systems

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Setting of ProtectionTime setting of inverse type relays

Margin 100-150 ms for short circuit protection 150-200 ms for earth fault relays

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Setting of ProtectionNetwork for selectivity plan

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Setting of ProtectionInformation required for selectivity plan

System diagram Maximum short circuit currents Minimum short circuit currents Maximum service currents Inrush currents for transformers, motors Relay characteristics Breaker characteristics

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Setting of ProtectionPhase and Ground over current protections The phase over current protection offers both circuit

local back-up protection and remote back-up protection for multi phase faults. They also offer protection for many faults involving ground if fault current is high enough to allow relay operation.

To cater to low current ground faults as a result of high ground fault resistance the ground over current protections are applied. They might also be applied, as main protection for high resistance faults where main protection does not offer required coverage.

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Setting of ProtectionPhase over current protections Settings

The protection must not operate for highest load current during both normal and abnormal power system operation.

The protection must operate for multiphase faults on the entire circuit under minimum plant conditions i.e. lowest fault current.

The protection should not operate before the main protection of the protected circuit

The protection must not operate before the circuit local back-up protection clears a fault in an adjacent circuit.

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Setting of ProtectionGround over current protections Settings

The ground over current threshold should be set to ensure detection of all ground faults, but above any continuous residual current under normal system operation. Continuous residual current may arise because of

Unbalanced series impedances of un transposed transmission circuits

Unbalanced shunt capacitance of transmission circuits Third harmonic current circulation.

15a_setting of protection

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