33kV/11kV delta delta transformer

If it's delta delta, how do you provide an earth reference to the secondary side?

   - Andy.

My thinking is that the incoming cable screens are connected to the room earthing so as to provide earth fault path to upstream MSS transformer but admittedly I am not sure on validity of this approach. 

I may well be picturing your setup wrongly, but in the case of an earth fault, I'm not seeing that your earth fault loop would actually be a loop (circuit) - so no fault current and so no disconnection. A picture might help...

   - Andy.

I assume this is UK practice, and the 33kV and 11kV side will be distributed 3 wires, no neutral and have earth fault detection and protection of some kind ?  At the EHT side you sometimes find autotransformer but I assume this is double wound ?
you will need some sort of earthing transformer on the secondary side  Delta star is the UK usual, with either solid earth or L or R...

from page 25 of https://www.nationalgrid.co.uk/downloads-view-reciteme/361558

Mike

I think you have indeed drawn the problem.

Mike.

Mike/Andy

1. Thanks for the advice and references that is understood. I can now see need for creation of own earth using star transformation on secondary probably with neutral earthing resistor.