Masterpact NW Breaker Trip Unit Reading Current High

[catserveng]

This one I've never seen before and hoping someone may have some insight.

I have a 1200 amp Masterpact NW breaker with a Micrologic 6.0A trip unit (CAT # S144B). The breaker is used as a generator breaker for a small engine based cogen unit. For over a year since recommissioning the plant everything has appeared to work fine.

Last week I recieved a call from the end user, saying unit would not make full load, and breaker kept tripping. There is also a Basler GPS100 installed and the settings done by the protection engineer were supposed to use the breaker protections as a backup to the GPS100, so if we did get a trip we could get info from the relay.

I went to site today, brought unit on line. After the unit starts and parallels, the control system takes it to 200 ekW to allow the engine and cooling system to warm up. The generator metering and the metering data from the GPS100 both indicate about 250 amps per phase (480 VAC and pf controller set to .97 lagging), the trip unit on the breaker initially showed about 60 amps higher per phase about 310 to 320 amps, then after a couple of minutes, ramped up to nearly 500 amps per phase. The gen metering and the GPS100 still showed about 250 amps per phase. The metering and the utility meter, plus the engine parameters all seem to agree, so I have a high confidence the unit is actually producing 200 ekW, but I have never seen a trip unit metering act that way before.

After unit warmed, brought load up to about 500 ekW, phase current indicated by trip unit nearly double what GPS100 and gen metering read, and just below trip level.

End user reports this appears to have started about two weeks ago, but hasn't been able to provide much bettter info.

I've dealt with a fairly large number of these breakers over the years but never ran into this issue. Pulled the breaker this afternoon and took it to a local NETA shop where they tested the breaker with a breaker test set, all readings and trip functions worked normally.

Have it in the back of my truck now, will reinstall and try again tomorrow, but would like to hear if anyone has had a similar experience with these breakers. I'm leaning towards a trip unit problem, but according to the local SquareD house it is two weeks away.

Any thoughts would be appreciated.

Thanks, Mike L.

 

[DTR2011]

You may want to check the current sensors. If there is some kind of shorted turns, this may explain the problem. If you have some kind of current source, try injecting and wiggling the sensor wires.

 

[rbulsara]

Was the breaker tested with primary injection method? Only that would test both the sensors and the trip unit. All three phase sensors going bad at the same time is unlikely.

If that tests OK, I have a feeling something is missing from the story.

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

Some mocrologic trip units have had issues with transients caising nuisance trips, check your date codes. See attachment.

 

[catserveng]

Yes, the breaker was tested by primary current injection. First a pretty standard test carried out by most NETA shops, then after discussiong what I was seeing in the field they hooked up so we had current (about 300 amps) on all three phases for about 5 minutes, trip unit was indicating proper readings.

The shop did not have a working full function test kit (bad cable)so they couldn't carry out some of the SquareD recommended secondary tests, but by using a breaker test set for primary injection it appeared ok.

Of course this is after taking it out of the cell, opening up and inspecting, checking internal wiring and connections, then putting in back of truck and driving a couple hours over some pretty rough roads to get it to the shop.

I'll put it back in the cell this morning and try again.

Zog, thanks for the product bulletins, I'll check the date code.

Mike L.

 

[HighVoltageZap]

Something simple. Is the relay rated the same as the breaker?

There was also a recall as someone else mentioned. I think that was a problem with the AP auto protect coming on and locking out the breaker.

 

[catserveng]

Called SquareD this morning and discussed at length, they had me make several checks, all of which were good. Reinstalled breaker and put into service, it read normally. I hate looking a gift horse in the mouth, but having a protective device I'm not 100% sure of bothers me.

Thanks to all who replied, we'll see how it does.

Mike L.

 

[dpc]

I know what you mean. Problems that go away on their own tend to come back on their own.



David Castor

http://www.cvoes.com

 

[GenSwgrEng]

I'm inclined to believe you had a bad connection on the trip unit's sensor plug. It may be that the connection was "fixed" in the process of removing and re-installing the breaker. With the breaker out of service, I would remove, inspect/clean and replace the sensor plug and re-test.

By the way, the current sensors on the NW breaker are Rogowski coils, not current transformers, and shorted turns are unlikely. Beside that, shorted turns would lead to a low current reading, rather than a high reading.



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