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Faulted 3ph delta connected motor tripping a 3ph wye connected motor

jsamuels (Electrical) (OP)
9 Jan 04 16:00
I work at a power plant and had two separate occasions where a 3 phase 480VAC 150HP delta connected motor tripped on ground fault (valid). A 3 phase 480VAC 150HP wye connected motor also nuisanced tripped (not valid). Both motors are connected to a ITE/ABB Kline loadcenter with a 13.8KV/480VAC delta/wye neutral grounded 1000KVA transformer. Ground fault protection for both provided by a ITE GR-5 ground fault relay and type GS ground sensor (core balance or "window" type CT). Relay pickup is 50A and time delay is .033 seconds. Would like some suggestions on what is causing the wye connected motor to also trip on ground fault when the delta connected motor has a valid ground fault.   
busbar (Electrical)
9 Jan 04 16:31

Is it possible that the wye-neutral in one motor is {inappropriately} grounded?  A fault elsewhere in the system could conceivably split between the grounded wye-connected transformer and motor stator.
jbartos (Electrical)
9 Jan 04 21:25
Suggestion: The delta connected motor experiencing the ground fault may unbalance the power supply. The unbalanced power supply may cause the other motor, wye connected, to be tripped by the ITE GR-5 ground fault relay.
jghrist (Electrical)
10 Jan 04 11:33
Even though the unfaulted wye-connected motor may feed the fault, the core balance window-type CT will not operate the ground fault relay unless the neutral is inappropriately grounded as noted by busbar.
electricpete (Electrical)
10 Jan 04 21:41
I have 4 thoughts.  Most likely to least likely.

1 - The ground suggested by busbar may be intermittent, arising from voltage transient associated with the 1st fault at the delta motor.

2 - Caps connected to ground at the wye motor.  Unbalanced transient voltages create unbalanced currents to ground through the caps.

3 - Cable capacitance to ground same effect as #2 - No, on 2nd thought how can we get 50 amps.

4 - Due to placement of cables within the core window it responds slightly differently to each of the 3 currents.  This offset current due to inaccurate sensing increases as current increases. High current from wye motor feeding delta motor fault pushes it over the edge.
jbartos (Electrical)
10 Jan 04 21:57
Suggestions to jghrist (Electrical) Jan 10, 2004 marked ///\\\
Even though the unfaulted wye-connected motor may feed the fault, the core balance window-type CT will not operate the ground fault relay unless the neutral is inappropriately grounded as noted by busbar.
///Not quite. See also the previous posting by "electricpete".
A fault to the ground may reduce voltage on one phase. A lower  voltage will unbalance leakages to the ground at the  "other motor" and some unbalanced ground fault current will be present that may cause the relay at the other motor to trip. Also, the motor may trip on the single phasing if the power supply to the other motor has high currents in the motor feeder two remaining phase conductors.\\\

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