3-pole ATS and breaker trip during transfer

[HYDL]

We recently installed a new Asco closed transition ATS to feed a new data center at a hospital. To test the system, the electrician shut off the utility breaker feeding the transfer switch and the generator started up and the ATS transferred to emergency power. After the test was over, the utility breaker was turned back on and after a period of time the ATS transferred back to utility using it's closed transition feature. Upon transferring, the 800 amp breaker up stream of the 225 amp breaker serving this ATS on the normal side tripped. The 225 amp breaker does not have GFP, but the 800 amp does and is on the lowest setting.

The ATS is 3-pole. The ATS Feeders are 480 volt 3-phase, 3-wire.

I am looking for ideas as to why that breaker tripped during ATS transfer. GF on the lowest setting make me think that it probably did trip on GF.

 

[rbulsara]

Most likely out of sync closing or excessive circulating currents. Also why is a 225A breaker feeding an 800A ATS?
What was the load?

Involve someone more experienced engineer in the project.


Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Bad sync? Possibly.

Where is the generator neutral conductor connected to earth? A system with the neutrals connected to earth in more than one location could well trip the earth fault protection, especially if it is sensitive.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

First, is this a true closed transition, or one of the ultra fast open transition switches. The ultra fast open transition switches may generate transients that may fool a sensitive GFI trip. An automatic closed transition can also cause some funky transients.
The generator ground connections may also cause issues.
Not enough information to say for sure what the problem may be.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Re-reading the post, I now see that the 800A CB is upstream of 225A CB. This may very likely be related to too low GF setting, depending upon the GF current sensing and grounding scheme. There should be an indication on the 800A CB trip unit or GF relay to verify, whether it indeed tripped on GF.

It is possible that just adjusting the GF can resolve the issue, but can be more to the issue than what you told us. Testing of a closed transition ATS should be supervised by an experienced engineer or things can get really ugly.

Typically closed transition ATS are also required to be set up to open one if the source breakers, if the ATS cannot complete the transfer in less than 0.5 sec maximum. Usually they should do that in less than 0.1s. Make sure this is in place.

Rafiq Bulsara

http://www.srengineersct.com

 

[Ceast]

2 questions 1) did you check the gen freq before the transfer test? 2) does the 800A breaker have a electronic trip unit?

If the gen freq is permissive (not driven into sync) this is going to be a on going problem. I've seen an ASCO CTS ATS In Phase Monitor allow a close with a very fast slip freq. By the time the switch operated it was far enough out to cause an OC trip. You want to tweek the gen freq so it's slightly faster than the Normal source. Very slowly rolling through the window. Easily checked with a Fluke.

Do you really need the In Phase Monitoring?

 

[waross]

When you sync a generator to an energized bus, you want the incoming generator to pick up a little load so as not to go into motoring. You want the generator to be a little fast.
Here the generator has the load, the grid source is the new or incoming source. You may want the generator frequency to be just a little slow so that the grid picks up the load and the generator doesn't motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ceast]

One more point on the In Phase Monitor type switches. If the gen freq is closely matched to the Normal source it may not drift into the sync window for minutes. I've seen them sit there for 20 minutes. I've instructed customers to check their gen (depending on the governor type) matching speed weekly.

FWIW I've never been a big fan of the permissive speed matching transition. Some governors hold the freq setpoint for months, some don't. Modern digital controls are much better than the old analog, goes without saying I guess...