Island Mode Operation of 3 phase BESS with Single Phase Loads

[kingpb]

Simple setup for discussion would be a 480Y/277V solidly grounded utility service. The facility loads are predominantly 3 phase with not more than about 20% phase to neutral loads. The BESS system is rated 480V and is furnished with, what they call, neutral forming transformers (NFT) to serve the phase to neutral load. The NFT is essentially a Y-grd/delta (High / low) 1:1 transformer where the Y side is solidly grounded.

Doing this, creates a situation where the the service entrance neutral and ground are bonded together, and there now exists another location where the neutral and ground are bonded. Additionally, if the BESS is operating in island mode, the main service conductors breaker is open, but then any ground fault could still come back on either grounds to the neutral. So the question is how to maintain compliance with the NEC by only having one location where the ground and neutral are bonded together. The only thing I can think is you have to be able to switch the main service neutral (4 pole breaker) when in Island mode which would be no different than say a standby generator, and when in parallel operations you need to be able to open the connection between neutral and ground at the NFT.

 

[jaggedben]

I don't see why the neutral has to be grounded againat the NFT. I work with systems with NFTs all the time now, and the nuetral just remains grounded at the service. They are single-phase NFTs but that makes no difference from a code perspective. If it makes a difference from an engineering perspective that's above my head.

But anyway, I agree with your last point. As with any standby generator you either switch the neutral and ground the standby generator separately, or you don't switch the neutral and you don't ground it separately. One wrinkle here might be whether it is legit to have separate components doing the switching, if they are not part of a listed assembly.

 

[synchro]

This document shows the neutral bonded to ground only at the service as jaggedben has mentioned.
Relay controlled breakers are also shown that protect the grounded-wye/delta transformer that's in parallel with the wye secondary of the service transformer.

https://www.rockymountainpower.net/content/dam/pcorp/documents/en/pp-rmp/customer-generation/Effective_Grounding_for_Customer_Generation_Facilities.pdf

 

[kingpb]

https://www.rockymountainpower.net/content/dam/pcorp/documents/en/pp-rmp/customer-generation/Effective_Grounding_for_Customer_Generation_Facilities.pdf

Excellent paper, thanks

 

[pv_n00b]

A grounding transformer to provide effective grounding is very different from a grounding transformer to drive a neutral in an ungrounded system. The design specifications a very different and the interconnection is different. Jaggedben is correct that you don't put in a N-G bond at the grounding transformer, just let the N-G bond at the service entrance do that job. When we are doing these we switch the grounding transformer in and out depending on the mode of operation, when fed from the grid the grounding transformer is disconnected, when disconnected from the grid the grounding transformer is connected in.