Use of Star/Delta & Delta/Delta Power Transformer in Power System, their respective protective relays on both sides ?

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.