I was reading about how a tie breakers prevents paralleling of busing from two transformers to supply two loads. So normally these each feed their own load, but if one transformer fails, you can feed both loads from one transformer.

But what would be the result of paralleling bussing, like combining A phases of two different souces? I dont see any direct short happening. Any ideas on what would happen?

As long as the cycles are in sync you would have twice the current.

This happens when you parallel gensets.


EDIT: changed "sink" to "sync".

[This message has been edited by iwire (edited 02-03-2004).]

There is another situation that could occur also. If the transformers primaries are different sources, and one of these sources were to be turned off for servicing, you would have a reverse angle and some one working on the source of the supposedly dead side would be subject to full primary voltage.

I know this would be unlikely, just wanted to throw it out on the table.

Roger

OH,that's a good point, I was looking for the safety issue at hand.

Hopefully, the installation was engineered and commissioned to accommodate phasing and synchronization of the two sources, whatever they are. There should be construction drawings and acceptance test reports that bear this out. If the tie is at low voltage, a phasing check can be conducted with a power-rated voltmeter. Post again if you need more information.

Another aspect that should have been considered, if all breakers are simultaneously closed—a “closed transition”—the available fault current on the buses roughly doubles, and the gear should be so intended.

Electric utility substations quite often have such a scheme. We normally have low side bank breakers at the output of the two transformers and a bus tie breaker between the buses. If either transformer fails its low side breaker is opened and the bus tie breaker is closed. There are interlocks to make sure that a failed transformer is not energized from the low side.

If you lose one source and everything is paralleled, you find yourself in the position of trying to backfeed the down source by passing current through your equipment. Not at all healthy for your stuff.