11kv Neutral-Earthing

[genhead]

We have a small (6mw) powerhouse, generating at 415v and stepping up to 11kv through several transformers, some of which have star-connected hv, and others delta. To simplify the system, I want to have only one neutral-earthing resistor connected via an earthing transformer at the 11kv switchboard (the common point where all incoming 11kv connects). This will provide a common reference-point, and an earth-fault current return path for all transformers on the system (star and delta).
My main concern is what to do with the neutral connection on the star-wound transformers. Can I leave them disconnected ('floating').

 

[230842]

My feeling is that it's higly recommendable to connect all 415 V star centers to one common neutral line, even in case that secondary, (or terciary) winding is delta connected. The reason is to provide a circulation path for the third (and multiples of three) harmonics of current.
Concerning 11 kV sides, if interconnected, I think all star centers must be interconnected too to one neutral line and this latter connected to ground for ground fault detection purposes.
In any case, no floating star centers on the 11 kV side are advisable because unbalanced loads may displace neutral voltage from ground producing significant overvoltages on one or two of the three lines.
I apologize for my bad english butI hope this information is usefull to you. Julian

 

[peterb]

Assuming that the 11 kV system is a 3-wire system (no neutral and no phase-neutral loads), there is no reason to interconnect all of the neutrals.
Check that the neutral bushings are rated for full line voltage to earth, as they may be subject to this value during earth faults. In some cases, I have seen it recommended that a surge arrestor be connected to unconnected neutrals.
You didn't mention what earthing is presently in place on the 11 kV system - is it presently resistance earthed? You will need to do a study to confirm operation and coordination of the 11 kV earth fault protection schemes.