Transformer feeder protection
Transformer feeder protection
(OP)
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?
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Transformer feeder protection
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RE: Transformer feeder protection
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%.
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
RE: Transformer feeder protection
REF protection is indeed very desirable for protection of the transformer, cabling and switchgear within the sub zone and is very sensitive.
RE: Transformer feeder protection
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
RE: Transformer feeder protection
Typically, the ground fault protection is coordinated for selectivity with downstream protective devices just like phase relaying.
dpc