Ground Fault tripping - Parallel Motors

[khamoshie]

Dear Members,

I have recently come across a nuisance tripping issue that has me baffled. We have a 34.5kV switchgear feeding two Motors (call them A and D). Both motors are protected by GE 469 MMR and are exactly identical. Recently both motors tripped on Ground fault at the same time. I was called to the plant during the middle of the night and by the time I reached, the operators had already attempted to restart the motors since they are process critical. the sequence of events is as follows:

1:30 am both A and D tripped
1:50 am I get called
2:00 am motor D is started by operations crew and the pump is loaded up with no issues
2:15 am motor A is started and immediately both motors trip on GF. MMR of Motor A also had I> pickup alarm.
2:20 am, I reach the site and check the Insulation resistance of both circuits.

It was found that the cable connected to motor A had failed as the IR value to ground on Phase "C" cable was a few Kilo ohms, both motors and the cable connected to motor D was found OK.

3:15 am Motor D is started again without problems and has been running smoothly so far (more than 4 days in operation).

Later it was found that the cable failed due to problems with a joint approximately half way through the cable run. My question is How can a ground fault on one outgoing circuit affect a parallel feeder. I was initially speculating that since this is a single line to ground fault, Motor D's contribution to the short circuit current is seen by its protection relay and since the protection is non directional and probably too sensitive , it initiated a false trip. However looking at the protection settings this is unlikely.

The GF is implemented through CTs with CTR 50:5 (the usual) and the relay is set for 0.37xCT and 100ms delay. Motor contribution to the fault should be zero or near zero beyond 3-5 cycles (83ms or less for a 60Hz system) under this condition I do not think that the motor contribution caused the trip since at 100ms it should have definitely gone to zero.

Has anyone faced similar problem in the past? Also what could be the possible cause of the trip?

 

[Jraef]

Im thinking that if you had them set up for Residual GF protection, not Zero Sequence, the severe current imbalance and resulting voltage imbalance on the first motor may have triggered the GF trip in the other relay. That would typically be faster than any imbalance trip you had set up.

 

[khamoshie]

Im thinking that if you had them set up for Residual GF protection, not Zero Sequence, the severe current imbalance and resulting voltage imbalance on the first motor may have triggered the GF trip in the other relay. That would typically be faster than any imbalance trip you had set up.

Thankyou for the reply. The GFP is set up with residual connections, and I did have the same idea before, however my understanding (which may be wrong) is that if a voltage imbalance occurs on the bus due to one phase being resistance grounded the result will be a current unbalance on Motor D, and during an imbalance current flow in a three phase,four wire circuit, there should not be anoperation of any ground fault protective relay since the vector summation of the currents should still equal zero right?

 

[ATSman]

I am more familiar with the GMR 489 relay (generators) but if you will look at page 5-63 of the MMR 469 manual they talk about GF currents during startup and nuisance tripping and how the overreach filter feature is there to prevent it. It may be that the asymmetrial component of motor A somehow affected the phase currents of motor D causing a net current and GF trip from motor D relay. Although these times are very fast (1 cycle) and seem to be well under your 100ms setting.
I would talk to GE-Multilin and the motor manufacturer for recommendations and if tweeking of the relay settings may be the answer to your problem while still providing proper motor protection which I'm sure are not cheap!
See attached of the page mentioned above: