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Feeders parallel work

Feeders parallel work

Feeders parallel work

Is it permissible to work momentarily (time – 50 ÷100 ms) on parallel switchgear feeders (level voltage 10kV) when during this work the initial short circuit current is 35% higher then the initial short circuit current of switchgear. Time given above is necessary to change over between feeders circuit breaker and coupling circuit breaker.

RE: Feeders parallel work

Many plants have switchgear that is rated for the normal plant configuration, which is generally for non parallel operation of incoming power supplies.  There are strong economic arguments for doing this.

When you do a supply changeover as you propose, you parallel incomers for a very short time only and you are doing this at a time when there is no fault on your downstream system (i.e. downstream of any feeder circuit breakers on your switchboard).  For things to go bad, you would need a downstream fault to develop a few mS after you paralleled your incomers.  Whilst possible, I think that realistically it is not going to happen.  Also, especially with an automated transfer, one of your incomers is going to be in the process of opening anyway.

You could possibly have a look at introducing a small delay to all "instantaneous" trips on feeder circuit breakers.  This would ensure that the feeder would be tripping at about the same time as the incomer and would ensure that it would never be attempting to break the higher fault current available when the incomers were paralleled.

I expect there will be some differing views.

RE: Feeders parallel work

This is a common question and there are differing views. So far as I know, ANSI/IEEE doesn't address this situation. The conservative approach is to say cover the worst case because in general faults are more likely to happen when switching (because switching is commonly due to system faults, switching causes voltage transients which can initiate faults and circuit breakers fail most commonly when operating).

Another thought is to provide automatic controls on the coupling breaker(s) to prevent sustained parallel operation in the event of manual operation or primary control failure. You should look at all sources of fault current to evaluate each breaker and bus bracing. Consider significant motors and generators.

RE: Feeders parallel work

Suggestion: To make before break and to break before make are frequent application considerations of automatic transfer switches.
To apply the make before break requires that both supplies are synchronized.

RE: Feeders parallel work

If the higher short circuit current is still within the rating of the switchgear, then it's no problem.

If the higher short circuit current exceeds the switchgear rating, then this is an NEC violation, based on strict interpretation.  I don't think there is any exception for this.  If a fault occurred as you closed the tie breaker, it could cause a catastrophic failure.  

However, what you describe is commonly practiced, NEC violation or not.  

We have worked on projects where the owner negotiated a variance with the local inspector to allow this practice.  This generally requires automatic tripping after a fixed time delay to eliminate parallel condition and remote operation of the breakers, so that the operator is not required to be standing directly in front of the equipment.  

RE: Feeders parallel work

Suggestion: It is not advisable for the switchgear to be expected to clear higher than rated short circuit current or fault level. The switchgear is not tested for such application. It can disintegrate by short circuit caused electromechanical/electromagnetical forces. The switchgear manufacturers have a plenty of pictures documenting the underrated switchgear collapse due to higher than rated short circuit currents.

RE: Feeders parallel work

It all boils down to practical reality versus the ideal.  What do you do when you get to that incremental change in the transmission system that means the fault level in the distribution system is increased to the point where you can't parallel things for switching?  Spend millions fixing it, or live with the known risk?  Get in trouble (like a well known car manufacturer recently) for saying, "Well it'll cost x million to fix, and y million in compo if something goes wrong, Y< x, so we'll wait for the problem and pay up" then have a court penalise you for making such a cold-blooded analysis.  I wish I knew the answer (if there is one).  And who pays if your supplier does something to push your equipment over the edge?

Life is non-linear...

RE: Feeders parallel work

Suggestion: Fortunately, there may be a reasonably priced solution to limit the short circuit current levels. Utilities often use current limiting fuses which are not that expensive to apply.

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