GF Trip Issue with Service Breaker

[powerwagon75]

Greetings all experts,

We have a customer who has an issue with one of the back up power systems at their site. This is a 480V 2MW genset and closed/soft transition transfer switch. Switch has the utility-side service breaker integral to the transfer switch cabinet. (MasterPact with a STR-38 trip unit)

Problem is when attempting to crank the genset, the utility service breaker trips at moment of starter disengagement. But, doesn't do it every time. All protective functions associated with the parallel transfer controls were isolated, leaving only the STR unit. Turning the GF settings to their highest points, helped, by allowing about 15-16 crank attempts, as opposed to the 2-3 it took to trip it at the normal settings.

Customer had the STR unit go bad about two years ago, and had it replaced.

Background: Issue arose when the customer thought the starters (dual starters) on the genset went bad because they weren't engaging, but just spinning. When our tech arrived, he found the customer had removed one starter. It checked out, and was reinstalled. Wiring stake-on connectors were flaky, and were replaced. Wiring verified against sister-units. It appears that perhaps the customer "zapped" the B+ pretty hard to ground at some point, maybe not realizing they needed to disconnect more than one ground lead with series-parallel batteries.

Genset uses two "safety relays", mounted on each starter main solenoid, that are operated through the "safety control." ( a box that senses the output terminal of each starter, and disengages both and re-trys, if both don't fully engage simultaneously.)

Does anyone see where an obvious problem would be? System has been in use since 2000. Unfortunately, every generator system on the site is different, so this is the only transfer/breaker set up like this.

Could a DC short have caused the GF sensing to go bad? I don't really see it--fault should have been local to the genset, at B+ to point of fault via a tool.

Would applying flyback diodes across the solenoids, and perhaps even the starter + terminal be overkill/ necessary? Customer doesn't want to believe the STR unit is bad again--and due to production concerns wouldn't let a breaker tech test it until their next shutdown.

Any thoughts are greatly appreciated


eric

 

[itsmoked]

Let me get this straight.. Looking at my outstanding quality sketch you are saying that the CB from the utility trips when you start cranking the generator?!?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[powerwagon75]

itsmoked,

Thanks for responding. No, it seems to trip, randomly, at the moment the starters are released, either by turning the manual start switch back to off, or letting it fire up and the speed switch disconnects them. Doesn't matter, how it starts, its that release that causes it. We can let it crank for just two seconds and stop, and when the starters drop out, you hear the breaker trip.

This is a three pole, momentary closed transition, soft parallel set up.

We elimated all sources of protective devices, save for the STR unit integral to the breaker. And it would trip randomly, usually after 2-3 start attempts. Setting GF settings to max allowed up to 16 attempts before tripping. Almost seems like a cumulative effect, by have no reasoning why that would be so.

eric

 

[alehman]

We've established that the breaker trips when the starter is disengaged. So back to Keith's question - is it the utility source breaker as shown in his sketch?

Is it always the same breaker? Do any other breakers trip or do any other breakers operate at the same time? By "STR" do you mean shun trip coil or trip unit? Does this breaker have an undervoltage release function? Any other voltage-based trip functions integral to the trip unit?

Where does the control power for the breakers come from? It is not unusual to use the engine starting battery as a backup control power source. A bad diode there could cause problems.

If there is no other communication to the breaker (com wiring to the trip unit, zone interlock, etc), I would first check for wiring errors for everything related to the trip unit (neutral sensors, aux contacts, etc). Check carefully as it is easy to overlook errors. If none are found, and control power problems are ruled out, I would have the breaker tested with primary injection.

 

[powerwagon75]

alehman

Yes, always the same breaker (the Utility Breaker, as previously stated). Apologies for not making that clearer.

Square D, Master Pact series breaker, STR-38 is the protective Trip Unit model installed in the breaker--the basic one: long/short time current functions, and GF residual protection.

Protective functions from switch gear all feed common to shunt trip coil (where the connection was temporarily eliminated during testing).

Customer had requested the orignal transfer switch manufacturer have a tech on site too, and they verified wiring connections. There is no one on site who looks at these systems at all, mis-wiring doubtful since it worked for 8 years.

This is a simple closed transition transfer switch with the service entrance breaker inside--it not part of the active operating system, and the only wiring to it is one set of A/B contacts for the display screen, the B+/B- supply, and shunt trip command wires.

I agree with your B+ control power logic. There is a DC convertor inside one of the door modules--that is where the breaker is supposed to pick up its power. Between genset B+ and control supply B+, all seemed were where they were supposed to be. Will try to verify breaker supply again when I get back on site. Customer is not willing to let folks work/look at it, unless they are in quarterly shut down mode. Agree with the breaker testing, too, if they can be convinced.

BTW, I just found some info a little while ago, that this STR-38 Trip Unit comes standard with thermal memory for the current and ground protection. That seems to coincide with the higher number of repetitive start attempts before it actually trips the breaker at the higher GF setting.

Thanks for the response,

eric

 

[Zogzog]

Thats why many specs require switchable thermal memory features, can cause problems like this.

 

[alehman]

Sounds to me like you should concentrate on the power supply. Can you put a scope on it/them?

 

[powerwagon75]

alehman,

Yes, we can, when allowed to go back out and work on it.

Very peculiar operation. I will revive/update the thread once we've been able to get hands-on again.

Thanks to all again for responding,

eric

 

[ANGIBEW25]

Hi Powerwagon75:
I have been considering this problem and I agree their may be a power suply issue, but I would like to mention some other considerations:
1. You mention that this installation is wired for Residual Ground Fault sensing.
a) Has the Neutral CT wiring connections been checked?
b) Have new Neutral connections been made on the Neutral Bus ahead of the Neutral CT?
I would visually inspect the Nuetral Bus, Neutral CT and Ground Bus bond points. Make sure the Neutral CT is connected to sense all of the Neutral Current in the system.
2. You mention that the trip unit was repaired in the past.
a) Is the repaired trip unit under warranty?
b) Some trip units will default to a much lower setting (20% of lowest available rating plug) if the rating plug is not properly installed.
I would suggest the purchase of a new rating plug for the next scheduled outage since this will be the most economical part to replace.
I have attached a file which describes the correct Residual Ground Sensing connections.

Good Luck
ANG