We have a 3.5MVA 3.3kV generator powering a 3.3kV ring. The ring also has delta/star 11/3.3kV transformer connecting to our 11kV network. We are proposing to connect a 2.5MVA load to the 11KV network via a 11/0.415kV transformer. The supply authority also supplies power at 11kV. If the supply authority power fails we propose to back-feed power from the generator to the 11KV network and hence on to the 11/0.415 transformer. This will mean the two transformer's 11KV windings will be connected in delta/delta formation - is this a problem (isolated supply) and what is necessary for protection as there is no earth connection? Update: We have opted for a neutral earthing transformer that is switched in (via contactors) when the supply authority is disconnecred.

Edited: 22 December 2010 at 03:48 AM by swanless

Are you sure you want to connect 2.5 mVA to a single 11 kV/415 TX?

In any case, as far as I know you cannot backfeed onto the HV from the 6.6 kV as you have no earth on the HV side.

This used to be done in the past by earthing one of the conductors, but is now not allowed.

Hello Swanless.

With respect to the Delta earthing, you could introduce an earthing transformer (sometimes known as a zig zag because of the winding arrangement).