One Ner per Substation for two Transformers

[Indy]

Hello,
I have a question regarding the use of one Neutral Earthing Resistor per substation for two transformers. Can anyone point me in the direction of websites/ literature that would show the operational philosophy of this set up compared to one neutral earthing resistor per transformer.

Thank you.

 

[RRaghunath]

I have seen one NER used with 6nos Diesel generating paralleled on the same bus.
Each generator neutral has its own Disconnecting switch and then the neutrals are paralleled to be taken to the common NER.
The plant interlocking logic ensures that only one generator neutral is connected to NER at any given point of time.
I believe the same can apply to transformers as well.

 

[Parchie]

We have 8 diesel engine generators with each output transformers' neutral all connected to a common NER. It's done every time.

 

[Indy]

Thanks for this. Am I correct that only one transformer/generator can be connected to the NER at the one time and this is achieved through disconnect switches and logic.

Thanks

 

[Parchie]

No. All that means is that there will only be one NER connected to the ground no matter how many parallel generators are online at any moment. Your NER design should be based on the worst configuration your system will ever be (maximum number of generators connected to the bus). Design logic to effect that scenarios.

 

[RRaghunath]

Indy,
That's right. Only one generator is connected to NER/ground at any point of time. This is a necessity for Low/Medium resistance earthed generators in order to avoid circulating currents.
In case of transformers, there is no concern of circulating currents but you would not like to keep the neutral of the transformer that is under shutdown/maintenance tied to the other transformer that is live (from safety point of view).

 

[VTer]

@RRaghunath,

Can you please elaborate? If multiple generators are operated in parallel and only one generator is connected to the NER, how are the rest of the generator neutrals grounded?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[RRaghunath]

VTer,
"..how are the rest of the generator neutrals grounded?" - the rest of generators are not grounded.