What possible root cause Phase C current spiked two time of Phase A & B current?

[TONY2023]

Hi All,

I would like to get your input on this topic. We had two gensets were running in paralleling to provide about 4MW, 13.8KV to 5000hp motor and pump. While the system was running, the switchgear breaker got tripped for fault current on Phase C when I reviewed the Event file from genset protection SEL relay. My question is how can I find out the root cause of high current fault on Phase C about two times higher than the current of Phase A & B? Based on your experience, what really caused the Phase C's current two times higher than the current of Phase A & B? Thank you,

 

[electricpete]

Questions:
[ol 1]
[li]I assume the trip was phase time overcurrent?[/li]
[li]Do you know how long did the overcurrent last?[/li]
[li]Were any other flags received? If so which ones[/li]
[li]What other protections are provided (the ones that didn't trip)?: Differential? Ground fault? (and if so how is it sensed?) Unbalance/neg sequence?[/li]
[li]Can you tell if the current from the three phases sum to 0? (either on an instantaneous basis at a given time or a vector basis). I don't recall exactly what an SEL gives you but maybe a 3-phase SEL relay computes the sum of the three phase currents.[/li]
[li]That reminds me, what type of SEL relay?[/li]
[li]Is your current value corroborated by any other indications? (voltage decrease on a bus, independent indication from a different ct, etc).[/li]
[li]Do you expect any indications if voltage is severely unbalanced?[/li]
[li]have you done any electrical testing to investigate the event (insulation resistance and resistive balance from breaker low side)? [/li]
[/ol]

 

[davidbeach]

Post the event reports.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[electricpete]

> We had two gensets were running in paralleling to provide

There was no transition of lineup (paralleling) in process when this occurred, right? If so I think you mean running in parallel (I'm not criticizing, just making sure we understand)

> the switchgear breaker got tripped for fault current on Phase C

I assume the switchgear breaker you're referring to is the motor breaker just upstream of the motor but downstream of the bus where the generators are paralleled, correct?

 

[TONY2023]

Sorry, I didn't do a good job on stating the overall system. Let me summarize it one more time. There are two gensets tie to common bus of switchgear breaker and the other side of breaker stab connects to medium voltage VFD to control the 5000hp motor and pump. In normal operation, we start up two gensets to run paralleling operation by closing its genset breaker to common bus of switchgear when both gensets are in synch. We know the two gensets are running in paralleling operation without any issue. Then, the MVVFD sends a signal to close the switchgear breaker to run the motor to turn the pump.
I know we had a few trips on phase overcurrent (50) from the SEL751A on switchgear as we load the motor when closing the breaker on switchgear. I know there was inrush current from the MVVFD' transformer which caused the trip. So, I changed or increased the time delay from .2 sec to 1 sec on SEL751A. We don't have this issue now. When the gensets were running in paralleling to provide power to motor/pump to operate at 8 bpm. There was no issue. As we increase to pump 10 bpm, the switchgear breaker tripped. I reviewed the SEL751A event report, and I found that Phase C current was two time higher than Phase A & C current, and SEL tripped on phase current (50P1P).

 

[electricpete]

ok, so the generators were running in steady state in parallel and the breaker that tripped was downstream of the common bus.

And we have new information - there is a vfd in the picture.

50P1P sounds like some kind of instantaneous. Labeled Arc-Flash Phase Overcurrent here

https://selinc.com/api/download/2893/

With my limited knowledge of VFD's and SEL's, I should step aside and let others answer. David asked for event reports so you should think about posting those. It would also obviously help to know if you have done any testing (or successfully run the motor afterwards for that matter) and what other trip features on that SEL are enabled that didn't activate.

 

[waross]

A point for investigation.

I know there was inrush current from the MVVFD' transformer which caused the trip

Residual magnetism may cause current transients on one phase of a transformer.
The residual depends on the point on wave that the current is applied and occurs on random energizations.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

Yeah, event reports. And now not only the filtered events, but also the raw events. One glance at the raws and fault vs. inrush is exceedingly obvious.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Marke]

Part of the bus charging mechanism of the VFD??

Rotate the phases around and see if the trip moves to a different phase.

Mark Empson
Advanced Motor Control Ltd

 

[Ethnan]

I would have suggested you check your insulation resistance of Phase C on motor side and also on generator side. Compare your result to that of Phase A and B. Comparatively low insulation resistance will gradually show its effect as you increase load. But your phase over current trip on the breaker suggests otherwise, that this is not insulation-resistance related. You may need to check your VFD. That's my strongest suspicion. In reporting to a superior however, he will expect you do all of these tests-IR test and also check the CTs. CTs hardly go bad though.

 

[waross]

Did this happen once or does it happen frequently?
Does this happen on turn on or when the motor is running normally?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!