Generator Transformer Vector and earthing options 2

[Rodmcm]

Hi We have an application for two 11kV generators to supply a local load and export to a 33kV system. The specification for supply says that the generators must operate in island mode so in this mode the 33kV will have to be earthed. We expect the 11kV should also be earthed for the local supply . The generators will have neutral resistors. So options
a) Star Star with 33kV star switched to earth in island mode, 11kV star earthed
b) Delta Star, with a switched zig zag on 33kV for island mode, 11kV star earthed
c) Star Delta with 33kV star switched to earth and zig zag on 11kV
d) Star Delta with 33kV star switched to earth and 11kV floating

I would be interested in this groups comments

 

[Kiribanda]

Rodman,
Kiribanda:Your solution is correct if you can run the 11kV un-earthed. But what if you have to also Effectively Earth the 11kV when the generators are not running?
The 11kV local network should NOT necessarily to be EFF EARTHED unless it serves another set of Utility consumers.
Is your 11kV local distribution
1) confined to an industrial setup?
2) or also serving another set of Utility consumers?
Could you please post your SLD?

 

[ScottyUK]

Separate the 11kV load bus from the 11kV generator buses. Each generator operates on its own delta winding with a high impedance earth limiting fault current to about 5A - 10A. Use a standard earthing resistor for the 11kV system to limit the fault to whatever value you you are happy with, say 400A or so. Apply whatever earthing practice you require on the 33kV transformers.

Crude example SLD attached. Next time I'll use a ruler to draw it. ;-)

 

[odlanor]

davidbeach,
ungrounded system is a type of grounding recommended by IEEE Stds. but You hate it.
It's like you said: But that's just you

 

[Rodmcm]

Gentlemen
Thankyou for your comments. Its good to see that there are as many thoughts as engineers
Enclosed is an article that may be of interest

The methodology proposed in their hybrid system suits in terms of satisfying an effectively earthed system, discriminating with external rural faults,local generator faults, high resistance earthing of generators.

This methodolody could be applied to Y or D on the generator connection winding.
Which leaves the 33kV winding as a Y for effective earth of 33kV in island mode

http://ecmweb.com/bonding-amp-grounding/addressing-ground-faults-mv-generators