Effect of installing surge caps on motor ground protection parameters?
Effect of installing surge caps on motor ground protection parameters?
(OP)
Does the addition of surge caps on motor terminals demand the need to adjust ground fault (trip) settings in motor protective relay?





RE: Effect of installing surge caps on motor ground protection parameters?
Yes, a very short time delay will likely be required. I have used 0.1s before, and this seems to work just fine. BTW that was a 16,000hp 13.8kV synch motor supplied from a system that has a 50A NGR.
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Effect of installing surge caps on motor ground protection parameters?
But what is the reason for this delay?
RE: Effect of installing surge caps on motor ground protection parameters?
If I recall correctly, that w/o the TD the 50G element (on the MultiLin MPR) would pick up on instantaneous GF on every attempt to energize the motor. When the TD was introduced, the tripping stopped. It was obviously the initial charging of the surge-caps that was the issue.
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Effect of installing surge caps on motor ground protection parameters?
RE: Effect of installing surge caps on motor ground protection parameters?
I hadn't stopped to think about this aspect before (at least not recently enough to recall).
During across the line start, how to analyse capacitor currents. As a first simple model, I'd be inclined to say that all three phase to ground voltages ramp up to their prescribed value in the same delta-T. Assuming the three phase-to-ground voltages sum to zero, the computed dv/dt's under above assumption would also sum to zero. But didn't consider things like LC ringing (rather than smooth ramp), CT saturation, non-simultaneous closing, etc which could throw a wrench in the works.
Any thoughts to further describe this? Does anyone have links to literature describing adjustment of instantaneous ground setting for capacitor "inrush" currents.
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(2B)+(2B)' ?
RE: Effect of installing surge caps on motor ground protection parameters?
If it works, don't fix it.
I suspect that the CT arrangement may make the difference.
One CT will probably not have saturation issues.
Three individual CTs may have saturation issues and require a delay.
Comments?
Bill
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"Why not the best?"
Jimmy Carter
RE: Effect of installing surge caps on motor ground protection parameters?
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(2B)+(2B)' ?
RE: Effect of installing surge caps on motor ground protection parameters?
I would think that a GF relay would have a tough time to pick up any surge-cap energization issues with residually connected CTs. I had assumed that he had a zero-seq CT in this situation.
Perhaps the OP could provide some additional information?
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)