Utility Back-feed 1

[Power2all]

We have 3X 4000hp MV motors. The idea was to have 2 VFDs so that the first two motors can be started by the 2 VFDs and if the 3rd is required, one of the already started motors can be placed cross the line and we can use the VFD to run the 3rd.
Problem is that utility company is concerned of back-feed from the motor placed across the line in the event of a fault and have basically said that a 3rd VFD is required to ensure no back-feed.
Any ideas on possible solutions to prevent back-feed?

 

[dpc]

Any motor running across the line is going to source fault current - for a few cycles. Are these induction motors or synchronous. Synchronous motors will contribute for a longer period of time.

I've never had a utility express this concern. It's possible that the fault current contributed by these large motors could be putting them over the rating of some of their equipment.

Unless this is a unique situation, I think the utility is being fairly unreasonable. It might be less costly to buy them a new recloser instead of another 4000 HP MV VFD.

This also means you are not allowed to ever run these motors on bypass in the event of a VFD failure.

 

[LionelHutz]

They're concerned that the motor will backfeed or that the VFD will backfeed when synchronously transferring the motor to/from utility? As for the first, every motor does that. As for the second, with a high load on the motor and other utility loads being powered the VFD won't be able to power itself for very long, likely no more than 5 or 10 cycles before tripping. Besides, the VFD won't be capable of sourcing current much higher than 2X to 3X it's rated output so it might just trip due to high load the moment the utility is lost.