Short Circuit Analysis of Closed Transition ATS 2

[PaultheBigFinn]

My company designed synchronizing switchgear to bring four 1 mW generators on line to a single bus to supply power to a large hospital's emergency and stand-by systems. We calculated the available fault current from the generators and specified the short circuit withstand rating well above this at 65 kAIC. We did not design the distribution switchgear which coincidentally was also rated 65 kAIC.

A third engineer was hired by the hospital to perform a short circuit analysis of the hospital’s large and fairly complicated power distribution system. They insist that because the ATSs are closed transition, the equipment must be rated for the utility's short circuit contribution as well as the generator's and as such the rating of the switchgear can not be less than 85 kAIC. This in spite of the fact that the time both systems are exposed to each other is less than 100ms for each transition.

Interestingly the SKM modeling software does not have separate model for closed transition ATS, only ATSs in general. The engineer who did the study constructed his own using two switches connected to a common bus.

I found no exception in the code for such a short time, but then there is no discussion of the impact of ATSs either. Is there a precedent for this?

BTW there are ~31 million seconds in a year and if the system was tested and or used once a week the entire exposure time would be about 10 seconds.

Any thoughts would be appreciated.

 

[jghrist]

I think there are three somewhat separate issues:

1. What will the AHJ allow? In the absense of a clear standard saying that you can apply equipment above its rating for a very short period of time, the AHJ will probably not allow it. If you can't convince the AHJ, then items 2 and 3 don't matter.

2. Can the system be operated safely? If the system is operated so that there is no one close to the equipment during a closed transition, then I'd say yes. It will probably be difficult to assure than there will never be anyone close to the equipment when the transition from generation to utility source occurs. If you can't, then probability doesn't come into play and you have to furnish adequately rated equipment.

3. What are the economic consequences? If there is a bolted fault close in on a feeder during the 100 ms closed transition, the breaker may fail to clear the fault and the switchgear may be destroyed. The probability may be low enough to justify saving money in the initial installation. Consider that nothing goes from a de-energized state to an energized state when the system is transferred from generation back to utility source; the switching function does not really increase the probability of a fault.

 

[burnt2x]

If using longer cables does not appeal to your designers, how about using Is limiters?

 

[xxjohnh]

Ïm medium voltage some engineers use current limiters made by ABB to be able to live with 1000 MVA switchgear at 15 kV levels, rather than go to 33Kv. Common FPSO application. More conservative designs use a reactor shunted by the current limiter to maintain the circuit if the limiter blows. This function is normally done on low voltage by current limiting fuses. if you are forced to design to the closed transition maximum, insert some current limiting fues.

 

[PaultheBigFinn]

Thanks for the suggestions. I have been looking at current limiting fuses. The problem I see is is their impact on coordination.