Parallel operation of Utility to generator via LV distribution

[alex444]

Hi all,

I would like to get some feedback on paralling substations.

We have the following substation operation and configuration:
- 13.8 kV Generator (12MW) capable of tying to Utility grid at the 13.8 kV level
- Misc double ended substations (13.8kV - 480Y) with sync relays.

We had the following scenario: The Generator-Utility Tie breaker was not able to close due to control wiring problems.

Question: Could I have transferred the loads of the double-ended substations (the ones that have sync relays) by paralling the two sources (Gen/Utility) without any impact to the electrical system?

Non-issues: double-ended substations are matched and substation gear is properly rated.

Issues/Concerns: Can I damage/impact the distribution system by tying the Gen-Utility via the 480V system? Will the Gen stay sync'd to the Utility or will there be current swings in the 480V system due to the generator trying to pick up load?

Personally I think evertything should be alright .... but you never know ....

Thanks for any input/suggestions y'all can provide.

 

[jghrist]

If I understand this correctly, you would normally parallel the generator with the utility directly at 13.8 kV, but now you would be paralleling through a 13.8 - 0.48 kV transformer and backfeeding through another 13.8 - 0.48 kV transformer. The problem I see is that you probably don't have the proper interface protection for serving through what is now a 0.48 - 13.8 kV step-up transformer. I assume that the transformer is delta on the 13.8 kV side and wye on the 480 volt side. If the utility line were to have a single phase fault and trip the source substation breaker, you might not detect the fault and disconnect from the utility.

Also, if you are exporting power to the utility, it is now going through two transformers that may not be rated for the load.

 

[alehman]

Are you talking about a brief parallel period or sustained operation? It may be possible, but I don't think this would be a good arrangement for sustained operation unless you can assure all the required protective relaying is in place and you have a means to control power flow through the transformers.

If you only want to parallel briefly for the purposes of a non-interrupted transfer to generator, this should be possible. You will need a good quality and properly configured sync check relay and a means to match voltage. Also, you should have reverse power relays.

Your utility company no doubt has protective relaying requirements for this type of operation that would need to be met.

 

[jbartos]

Suggestion: Some power distributions are designed for bi-directional power flows; however, proper protection schemes and system grounding schemes have to be in place.