Hi David,

Thanks for taking the time to think about htis problem. Yes, the system is very much like the one you have sketched. We have 110/35/10kV YN/yn0/d11 transformers and other substations with 35/10kV or 35/6kV YN/d11 transformers (why they don't just make it simple and use D/yn11 I have not been able to find out). The YN/yn0 "earthnig transformers also provide local substation supply and are earthed on bot sides, the MV side being earthed through a 70 ohm resistance. (I don't really think tht htis is really necessary and solid earth would do). I had not thought about the tank providing the balancing "delta". Referring to your sketch. The earth fault current will divide into three, in phase, currents each equal to the zero sequence current. thus three in-phase fluxes will be developed in the core limbs and flux balance will not be achieved, which is why I came ot the conclusion that this would provide a high resistance path for earht faults and possibly earth faults wouold not be cleared. This would also cause an MV neutral shift at the substation and may overstress the MV windings of the "earthing" transformer and LV equipment in the substation.

The balancing effect of the steel tank is a good idea. I have not heard anything and neither have I had any liuck consulting "google" to find examples of such a connection. As such I am still very much tempted to recommend to the client that they follow standard practice and use interconeected star for the earthing transformers. 

If you hear of anything else on this subject I would be pleased to hear.

regards,

Geoff

Hi David,

Thanks for taking the time to think about htis problem. Yes, the system is very much like the one you have sketched. We have 110/35/10kV YN/yn0/d11 transformers and other substations with 35/10kV or 35/6kV YN/d11 transformers (why they don't just make it simple and use D/yn11 I have not been able to find out). The YN/yn0 "earthnig transformers also provide local substation supply and are earthed on bot sides, the MV side being earthed through a 70 ohm resistance. (I don't really think tht htis is really necessary and solid earth would do). I had not thought about the tank providing the balancing "delta". Referring to your sketch. The earth fault current will divide into three, in phase, currents each equal to the zero sequence current. thus three in-phase fluxes will be developed in the core limbs and flux balance will not be achieved, which is why I came ot the conclusion that this would provide a high resistance path for earht faults and possibly earth faults wouold not be cleared. This would also cause an MV neutral shift at the substation and may overstress the MV windings of the "earthing" transformer and LV equipment in the substation.

The balancing effect of the steel tank is a good idea. I have not heard anything and neither have I had any liuck consulting "google" to find examples of such a connection. As such I am still very much tempted to recommend to the client that they follow standard practice and use interconeected star for the earthing transformers. 

If you hear of anything else on this subject I would be pleased to hear.

regards,

Geoff