MV/LV Transformer Protection

[AVR_1.0pu]

Is it standard practice to place inter-trips on the MV and LV breakers when grading across say 1 2MVA tx feeder? - Assuming a single transformer single bus arrangement (no bus couples or parallel tx feeds).

In such a case, my understanding is that the MV breaker should grade with the largest LV outgoing feeder and does not have to grade with the LV incoming feeder (as it doesn't matter which trips first)?

Looking to understand thoughts?

AVR

 

[RRaghunath]

Inter trip of LV breaker from HV is standard with all transformer installations when the trip happens to be due to fault in the transformer (that operates transformer protection installed on primary side).
Grading of HV protection with that of LV incomer is not necessary done as it isolates one and the same source.
I know some clients who don't provide any protection in the LV incomer except stand by earth fault protection that operates using LV neutral CT input.

 

[AVR_1.0pu]

Thanks RRaghunath,

This is what my understanding was too; there is no benefit from this coordination as I can see?.

1) Under this circumstance am I correct in saying that the HV breaker must also protect for a 3 phase short circuit at the LV transformer terminals before the LV incommer (limited by the tx only impedance)?

2) Could Earth fault protection be either provided by
a) the HV Breaker phase protection
b) SEF /REF on the neutral point?

 

[RRaghunath]

That's right AVR.
There is bound to be an IDMT Overcurrent protection on transformer primary. That will be coordinated with the outgoing feeders from LLV switchboard and hence may not be fast acting for faults in case of transformer secondary faults, including those in the cables between transformer and the switchgear.
Anyway, transformer is designed to withstand short circuit on its secondary terminals for 2s (IEC 60076).
For earthfaults, neutral CT based earth fault protection works. Of course, this is also coordinated with the LV switchboard outgoing feeders and hence cannot be fast acting. HV phase overcurrent protection will also operate provided the fault yields sufficient current.
If you have REF protection on LV side, that is helpful. But, it may not be standard for LV transformers of this size.

 

[AVR_1.0pu]

Much appreciated, thanks for your help.