Generator for Backfeeding Industrial Plant

[Laplacian]

Background:
We are purchasing a 200KW genset for standby service duing a natural disaster at our plant. The genset is purely for logistic functions for making facility repairs before the utility can get back online. The bulk of the required loads are on one 480V substation, but there are required loads in two other 480V substations. These 480V substations are fed from our 13.8KV distribution system by 13.8KV-480V, delta-wye transformers that are solidly grounded at the X0 busing on the 480V side. The plant takes power from the utility at 13.8KV from the utility's 69KV delta-wye transformers that are low resistance grounded at the neutral.

Question:
Assuming the utility is isolated by opening the incoming circuit breakers, can the genset backfeed the 13.8KV plant distribution system? If so, what about grounding the 13.8KV side? Is a zig-zag transformer required? The NEC applies in this situation.

 

[davidbeach]

The utility would have a great interest in what you are doing to ensure that you can never energize their 69kV system. Delta on the 69kV, low resistance grounded wye on the 13.8kV side, is that correct? The open in coming breaker on the 13.8kV or 69kV? If on the 69kV, the transformer would continue to provide grounding for the 13.8kV system.

Much bigger problem, assuming you have some decent sized transformers, you may have a problem getting past the transformer inrush with such a small generator. You might well be better off with three generators, particularly when you are talking about such small units.

 

[Laplacian]

The utility won't be a problem, we have regular communication. Yes, delta 69kV, wye 13.8kV. Open 13.8kV breaker.

I did wonder if I could utilize the utility's ground by leaving our 13.8kV incomming breakers closed and have them open their 69kV breakers. The transformers are only 15MVA each with a closed tie breaker. They share the same NGR. One 15MVA transformer would work for this purpose. The larger utility connection we have would remain open.

 

[rbulsara]

Take davidbeache's advice...

 

[BJC]

You will also need some kind of a load shedding scheme. 200 KVA is not going to power up everything connected to your system. A good percentage of your generator is going to supply lossed to your transformer.

 

[waross]

In support of davidbeach and rbulsara;
I suggest a dedicated standby panel with an automatic transfer switch and an auto-start generator.
Do the same at each location.
A 200 Kw generator will be 250 KVA.
250 KVA / 15 MVA = 1.67% It is doubtfull if the generator would be able to energise this transformer or supply the losses. In addition I assume that there will be at least 3, 480/13.8k transformers of various sizes in your local distribution system.

I don't believe that the NEC will require a 13.8k delta system to be grounded, but your system design may require the low resistance ground.

You may easily burn up the price of a small diesel generator for one of the remote locations with engineering studies and cost estimates to prove that the most economical solution is three seperate sets.
respectfully