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Mainly a delta/delta power transformers are used i think it has no effect on relays...........
1) Delta/ Star Trafo:-
A) As we know Delta side provides Zero sequence Isolation Between Star Winding & Source, so residually Connected E/F Relay ( REF )in Delta side will not operate on star side Earth-fault. So it’s operation remain restricted in Earth fault in Delta side.
Remedial Action :-
Sensitive- Instantaneous Earth-Fault Relay in High-impedence mode without requiring any Time CO-ordination with Star side backup protection.
B) In Delta/star transformer Phase-Phase fault in Star side produce2:1:1 current distribution on the Delta side, so it is essential to provide3O/C Element on the delta side ( as against2 Element) for faster clearing through the phase side sensing highest current in Back-up mode.
C) At the same time there must be differential protection above5 MVA rating, along with winding ,oil ,Prv trip.
D) Delta/ Star transformer are most commonly used transformer.
E) Also Delta winding provides30 Electrical Phase shift, it is advantageous, because starside voltage got increased, due to30 degree electrical shift.
2) Delta /Delta Trafo :-
A)There is no shift between primary & secondary because of connection, so we gate better sinusoidal wave on the secondary side. VLine = V phase. Better used in long EHV transmission.
B) In Delta-Delta Transformer beause of absence of Neutal ,more insulation is required & voltage reached between winding & core will be equal to full load line voltage in case of earhtfault in any phase . So residual over voltage & Sensitive instantenious Earth fault relay should be the back-up protection.
C) At the same time there must be differential protection above5 MVA rating, along with winding, oil, Prv trip etc.
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