Transformer feeder protection

[skm]

For a transformer feeder, is earth fault protection required if differential protection has already been applied. If yes,any tips on how the relay cordiantion is done?

 

[manmadhan]

Many advantages are realised when a restricted earth fault relay ( a simple earth fault relay does not give a good protection for a star connected primary with impedance grounding) is used in conjunction with overall differential protection of transformers especially in resistance and solidly grounded systems.<br>
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As an example, consider a transformer with the neutral of its star connected winding earthed through a 1PU ( i.e ratings of the transformer and resistor are equal) resistsnce. The differential protection with an effective setting of 20% detects faults in only approx 42% of the winding from the line end. If the setting is say 25%, then the percentage of protected winding drops to say 36-38%.<br>
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The restricted earth faultsensitive protection is instantaneous in opeartion. so there is no coordination or grading considerations. The REF relay covers virtually all of the winding<br>
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[kelvin]

From a rural overhead distribution perspective it is normal to install 3 phase overcurrent and earth fault on each feeder. Normal setting for earth fault in this case is 10% of the overcurrent setting and deffinate time of .5 seconds. Of course with better earthing you may get away with greater than 10% thus reducing the occurence of feeder trippings from small customer lines with fuses above the earth fault settings.<br>
REF protection is indeed very desirable for protection of the transformer, cabling and switchgear within the sub zone and is very sensitive.

 

[SphincterBoy]

If you do employ the earth (ground) fault protection, which
is normal, and if the feeder provides back up to another
substation feeder, and if the phase/earth protective
relays are all-in-one, MAKE CERTAIN YOU PUT IN A SWITCH
TO SHUTOFF THE EARTH PROTECTION, otherwize you risk tripping
the earth relays (blackout) during backup switching.

For a brief period, both substation transformers will be
connected, and the 'potential' phase imbalance will cause
earth currents to flow, tripping earth relays, which originally
were intended to protect against line-ground faults, not phase
imbalances.

With this scheme, you can instruct your linemen to shutoff
the earth relay during switching, and turn it back on when
finished. That way the lights stay on.

Experience 101

 

[dpc]

Ground fault protection is still desirable and useful, even with differential protection. The transformer differential will (you hope) only respond to faults within the differential zone. The ground fault relay can provide backup protection for any ground faults on the entire system.

Typically, the ground fault protection is coordinated for selectivity with downstream protective devices just like phase relaying.

dpc