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Erroneous SE-330 Resistor Faults

Erroneous SE-330 Resistor Faults

Erroneous SE-330 Resistor Faults

(OP)
Has anyone out there had erroneous resistor faults from their Startco (Littelfuse) SE-330 NGR monitors?

I just had an outage where two SE-330's tripped on resistor fault (based on measuring a change of resistance greater than 2500 ohms). These units were properly commissioned and calibrated and were working fine.

Here are the facts of our case:

-Two parallel 138-13.8kV transformers with 25A NGRs in the secondary (delta-wye standard industrial arrangement).
-Both transformers were energized (and thus so were their secondaries) but one of the transformer's secondary breaker was open so the unit was offline
-A fault (that ultimately ended up 3LG) occured on the 13.8kV system, a substantial cable distance from the transformers.
-Both transformers tripped offline (primary breakers opening) indicating resistor faults on the SE-330, apparently/anecdotally, at the same time as this remote fault on the 13.8kV system (this is the only event that we consider, if not a probable cause, then an initiating factor
-The SE-330 resistor faults were immediately resettable
-Next, the faulted area was isolated from the 13.8kV system
-One of the two transformers was reenergized and again tripped on resistor fault (so while the remote fault on the 13.8kV system might have been an initiating condition, it no longer is the cause)
-The resistor fault was immediately resettable.
-This happened 5 times on one transformer and 2 times on the second transformer until finally, without any intentional change in the power system, the second transformer did not trip on resistor fault and stayed online. The second transformer was paralleled in and there has been no more issues since.
-For all these resistor faults, the ground current and resistor voltage (as captured in the SE-330 event record) were all equal to zero.


This thread is to examine erroneous resistor faults with the SE-330 and not how the troubleshooting of this problem went down. The electricians on call at the time had no idea what a resistor fault meant and thought they were dealing with a faulted feeder that they just needed to isolate. They did not allow adequate time (about 12 seconds) between transformer energization and closing in the seondary breaker to determine if the problem seemed to be associated with having JUST the transformer energized or if it was the transformer PLUS the 13.8kV distribution bus that triggered the problem...or was it adding load or....A lot of information was lost in their panic to get the power back on.

The salient issue though, as far as I can see, is that these Resistor Faults from the SE-330 ONLY occurred when the transformer was energized. The SE-330 monitors the resistor whether the transformer is energized or not. This fact that it only tripped when energized implies to me that there is some causal element associated with the secondary system being energized which as I understand it, should not be the case. I feel that this is reaffirmed by the fact that the resistor faults were also immediately resettable (since the fault caused the transformer to be deenergized).

Startco says that their relay is immune to secondary power system conditions and has only been known to erroneously trip in the presence of stray DC voltages.

Thoughts?
Similar experiences?

Thanks

RE: Erroneous SE-330 Resistor Faults

What is the ground fault trip and neutral voltage trip set for? I've seen it in the past with he se325's where they will indicate a resistor fault during a ground fault conditions based on the neutral voltage. This would explain why you do not get any resistor faults after the circuit has been de-energized as well.

I imagine you've probably solved this by now, but thought I'd share.

RE: Erroneous SE-330 Resistor Faults

(OP)
Nope, that wasn't it. Not only were the voltage settings way higher than the ground (set so high they never would have picked up), the event record downloaded from the SE330 shows Ground Current = 0 and NGR Voltage = 0 for all the resistor faults.

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