connections_of_overcurrent_relay_part_2

7/29/2019 Connections_Of_Overcurrent_Relay_part_2

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Connections Of Overcurrent Relay (part 2)

http://electrical-engineering-portal.com/connections-of-overcurrent- relay-part-2 Februray 3, 2013

Connections Of Overcurrent Relay (part 2)

Continued from first part:Types and Applications Of Overcurrent Relay (part 1)

Connections of Overcurrent and Earth Fault Relays

1. 3 Nos O/C relay for overcurrent and earth fault protection

Its used for:

3-phase faults the overcurrent relays in all the 3-phases act .

Phase to phase faults the relays in only the af fected phases operate.

Single line to ground faults only the relay in the faulty phase get s the fault current andoperates.

Even then with 3 overcurrent relays, the sensitivitydesired and obtainable with earth leakageovercurrent relays cannot be obtained in as much as the high current settingwill have t o benecessarily adopt ed f or the overcurrent relay t o avoid operation under maximum load condition.
http://electrical-engineering-portal.com/connections-of-overcurrent-relay-part-2http://electrical-engineering-portal.com/working-principle-of-earth-leakage-circuit-breaker-elcb-and-residual-current-device-rcdhttp://electrical-engineering-portal.com/protection-and-relays-in-main-circuit-boardhttp://electrical-engineering-portal.com/types-and-applications-of-overcurrent-relay-1http://electrical-engineering-portal.com/connections-of-overcurrent-relay-part-2


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3 Nos O/C Relay for Over Currentand Earth Fault Protection

3 No O/C Relay+ 1 No E/F Relay forOvercurrent and Earth Fault Protection

Over current relays generally have 50% to 200% current settingwhile earth leakages over current relays have either10% to 40%or 20% to 80% current settings.

One important thing to be notedhere is that the connection ofthe star points of bot h the C.T. secondarys and relay windingsby a neutral conductor should be made.

A scheme without the neutral conductorwill be unable t o ensurereliable relay operation in the event of single phase to earthfaults because the secondary current in this case (without star-point interconnect ion) completes its circuit t hrough relay andC.T. windings which present large impedance.

This may lead to failure of protection and sharp decrease inreduct ion of secondary currents by CTs.

It is not suf f icient if the neutral of the CTs and neutral of the relays are separately earthed. Aconductor should be run as stated earlier.

2. 3 No O/C Relay+ 1 No E/F Relay for Overcurrent and Earth Fault Protection

The scheme of connection for 3 Nos Over current Relay 1 No Earth Fault Relay is shown in figurebelow.

Under normal operat ing condit ions the three phase faultcondit ions and current in the 3-phase are equal andsymmet rically displaced by 12 Deg. Hence the sum of thesethree currents is zero. No current flow through the earth faultrelay.

In case of phase to phase faults (say a short between R and Yphases) the current f lows f rom R-phase up to t he point offault and return back through Y phase. Thus only O/L relays inR and Y phases get the f ault and operate.

Only earth faults cause currents to flow through E/L relay. Anote of caution is necessary here. Only either C.T secondarystar point of relay winding st ar point should be earthed.

Earthing of both will short circuit the E/L relay and make itinoperative for faults.

3. 2 No O/C Relay + 1 No E/F Relay for Over Current and Earth Fault Protection

The two over current relays in Rand B phases will respond to phase f aults. At least one relay willoperate f or fault involving t wo phase.
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2 No O/C Relay + 1 No E/F Relay forOver Current and Earth Fault

Protection

Transmiss ion Line , Bus Bar & Transformer scheme

For fault involving ground reliance is placed on earth faultrelay.

This is an economical version of3-O/L and 1-E/L type ofprotection as one overcurrent relay is saved. With theprotection scheme as shown in Figure complete protectionagainst phase and ground f ault is aff orded.

Current TransformerSecondary Connections

For protect ion of various equipment ofExtra High Voltageclass, the star point on secondarys of CTshould be made asfollows for ensuring correct directional sensitivity of theprotect ion scheme.

Transmission Line , Bus Bar and Transformer:

For Transmission Lines Line side

For Transformers T ransformer side

For Bus bar Bus side

Generator Protection:

Generator Protection Generator Side

The above method has to be followedirrespect ive of polarit y of CTs on primary side.

For example, in line prot ect ion, if P1 is towardsbus then S2s are to be shorted and if P2 istowards bus t hen S1s are to be shorted.

Standard Overcurrent and Earth Fault

Protection

No Name of theEquipment

Protection
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Generator protection scheme

1 11 KV Feeders A) 2 No Over Currentand one no EarthFault IDMT relays

B) 2 No Inst antaneousOvercurrent (highest )and one noInstantaneous Earthfault relay

2 8 MVACapacity ORTwoTransformer ina Substation(Irrespect ive ofcapacity)

HV side: 33 KVBreaker (Individual orGroup Control with 3Over Current and OneEarth Fault IDMTrelaysLV Side: Individual 11KV Breakers with 3Over Current and OneEarth Fault IDMT

relays

3 8 MVA PowerTransformer

Differential relays ORREF relays on LV side

4 Only one PTRin a SubStation (Lessthan 8 MVA)

HV Side: HG fuseLV Side: 11 KVBreaker with 3 OverCurrent and one E/FIDMT relay

connections_of_overcurrent_relay_part_2

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