Standardisation - Connection of Windfarm 38kV Transformers

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Hi all
An issue has arisen in relation to the Electrical Design that we use in the WIndfarms for 38kV connections. It is in relation to the connections of the Grid connected transformer and whether it should be star(38kV)-star (20kV) or star(38kV)-delta (20kV). Both configurations have been used by contractors on Windfarms to date. The generator Transformers are connected in Delta (20kV) - Star (690V).

I have discussed with engineers and the following The 3 configurations possible are as follows:

1.) Grid Transformer Yynd0 and Turbine Transformer Dyn11, in this situation because there is a tertiary winding on the grid transformer an Earth Fault protection relay can be used to detect a fault on the 20kV system. This is standard for ESB Networks using 38/20kV transformers.

2.) Grid Transformer Yyn0 and Turbine Transformer Dyn11, in this situation because there is no delta tertiary winding on the grid transformer an earth fault on the 20kV system can only be sensed using an Neutral Voltage Displacement Relay (NVD), an earth fault relay would not pick up due to a low zero phase sequence impedance current.

3.) Grid Transformer Yd11 and Turbine Transformer Dyn11, in this case the 20kV neutral is floating as it is not solidly grounded, however again an NVD can be used to detect an earth fault on the 20kV system.

I think that 1.) is the most safe system because the Earth Fault protection will operate quicker than the NVD, and we have a duty as designers to design the safest possible system therefore this is the one we should use. But if you take into account the fact that the windfarm is all underground cable and it is private property is the system of using the NVD equally as safe? And for standardisation purposes which one should we adopt? Can anybody help me out with this? Also just to point out the Distribution Code does not specify what we should use.

many thanks for any comments

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The size/complexity of the 20kV system is not given, so I’m not sure if these comments are completely appropriative. The second and third methods seem to use “59G” type relaying, and a caution is that the relay will operate for a fault anywhere in the 20kV system.

In ANSI regions, application of such high-resistance grounding is not too common above 5kV-class. Typically, ground-fault currents are <10 amperes, so zero-sequence overcurrent relaying likely applicable to case 1 won’t be operable in cases 2 and 3.