Ground fault on 3-wire ATS transfer? 1

[peebee]

We have experienced a strange problem at a new installation:

We have a 3-wire ATS fed from a 3-wire grounded 3-wire 480-volt service, as well as a 3-wire grounded emergency generator source. ATS is reported to have midpoint-off position, although delay setting is unknown (currently being verified). ATS loads are (4) 3-wire elevators.

As part of comissioning, the generator was manually started, and a manual transfer to emergency was initiated. The emergency breaker tripped on ground fault, the ATS automatically retransferred to utility, and utility then also tripped on ground fault.

The breakers were reset and power was restored with no other corrective action. Feeders insulation had previously tested OK.

We have not been able to recreate the problem since, transfers have been OK. However, a concern remains about future failures of the emergency power system.

Some possible explanations come to mind:
1) Intermittent motor fault
2) Intermittent feeder fault
3) Super-low setpoints on ground fault pickup (or sensor) -- we're currently checking this.
4) Bad filter or TVSS in elevator controller
These two would seem to be easy enough to test for -- any specific recommendations on specific tests?

Here's some other explanations, although they all seem less probable:
5) Harmonics -- but zero-sequence current harmonics are not possible with 3-wire system, are they?
6) Insufficient mid-point delay on ATS -- but that would only show up as an overcurrent, not as a ground fault, right?

Any thoughts on this? Any of the above that we can eliminate as impossible? Any other potential problems I might have overlooked?

 

[peebee]

Red star for rbulsara. Thanks!

 

[peebee]

You know, I was never quite sold on zone interlocking. It seemed like a good idea, but not quite worth the cost.

Now, though, I think I'm somewhat biased against it -- if it kills the time delay, it would seem to increase nuisance tripping frequency, and therefore be a BAD thing.

Any comments on that?

 

[steveaz]

Disabling the zone-interlock on the feeder breaker seems to solve the problem. We have the GF time delay on the feeder circuit breaker set at .2 seconds and the GF delay on the main set at .4 second now so I don't believe we will have aprblem with the main tripping.

I've attached an email we received from the manufacture.
"...even though we will disable the feeder breaker zone interlock, the feeder if it senses the gnd fault will still send the blocking signal to the main breaker. So in other words because of the coordination of the two breakers if the feeder breaker senses a gnd fault it will wait .2 sec to trip, if the feeder does not trip in that time the main breaker will trip in .4 seconds if the fault level exceeds 1200 amps which the main breaker is set at."

 

[rbulsara]

Peebee:

Thanks for the star...)

I beleive only thing zone interlocking acheives is that it will block the main until feeder trips. The modern static trip units have made the zone interlocking obsolete, imho.

I beleive with modern trip units with adjustale current and time delay you can easily coordinate breakers

ZI may have had its use with less smarter trip units or breakers with a separate GF relay and a shunt trip.

To me ZI is not worth it.

Side note: There are some differences between manufactures as to how easily a ZI can be defeated one installed.