4160 Motor Protection
4160 Motor Protection
(OP)
I have a fused motor starter for a 4kV motor protected by a GE Multillin 369. The 4.16kV system is High Resistance Grounded. The other motors in the same 4.16kV MCC are having motor phase differential (87M). The MPR is having a trip relay to activate the 86M and further de-energize the contactor coil and opens the vacuum contactor. My comments:
1. Can we disable both 50 and 87 functions since setting
them would make the contactor interrupt the phase fault
current (where the contactor is not rated to interrupt)
and furthermore, that's the power fuses' job right?
2. The MPR is having a 50G element connected to a core
balanced CT. It seems not needed (50G) to me since the
grounding system is HRG; however, it seems the client
insists on it...can anyone tell me as why maybe?
3. If indeed a ground protection trip is required on this
motor supplied from a system with HRG, what type of
ground fault protection scheme is required?
Self-balancing Differential? or a 50G connected to a
core-balanced CT is more than adequate?
Thanks for any comments and suggestions
1. Can we disable both 50 and 87 functions since setting
them would make the contactor interrupt the phase fault
current (where the contactor is not rated to interrupt)
and furthermore, that's the power fuses' job right?
2. The MPR is having a 50G element connected to a core
balanced CT. It seems not needed (50G) to me since the
grounding system is HRG; however, it seems the client
insists on it...can anyone tell me as why maybe?
3. If indeed a ground protection trip is required on this
motor supplied from a system with HRG, what type of
ground fault protection scheme is required?
Self-balancing Differential? or a 50G connected to a
core-balanced CT is more than adequate?
Thanks for any comments and suggestions





RE: 4160 Motor Protection
2. They want it because they don't know any better. For a true HRG system with ground fault current limited to maybe 2 or 3 amps, it is doubtful the the 50G via flux summation CT will ever operate. It won't hurt anything to have it in service.
3. For a HRG system, if there is a ground fault, there will be voltage relay at the grounding resistor that will trip or alarm, however the system is designed.
RE: 4160 Motor Protection
If you rely on the protection at the main secondary breaker for the transformer (building incoming breaker) then you will shut the plant down when the motor develops a ground fault.
RE: 4160 Motor Protection
LionelHutz, I will verify this with Process and Operations if they prefer to shutoff the pump in the event of a ground fault on the pump motor or continue service and send people to trace the fault.
The motor is a 3000HP, 4kV Induction Motor
By the way, we also observed from the single line diagram that both motor Metering (IQDP-4130) and MPR (GE SR369) have common phase CTs (3 x 600/5A), The Meter took its CT input from these 600/5A CTs and interposing CTs (unknown ratio but I think might be either 5/5A or 5/1A) is connected along the main CT circuit and feeds the MPR CT input.
Is it possible that the 600/5A Phase CTs are metering class or protection class? Is this set-up not going to create errors on either devices? I was thinking of installing a separate CT for the other device but we are still considering cost and constructability.
Thanks
RE: 4160 Motor Protection
RE: 4160 Motor Protection
By the way, jst for information, this motor feeder circuit was used to feed a VFD controlled 1000HP Motor. It was decided to replace the motor with a 3000HP and we are removing the VFD+isolation transformer and re-use and re-wire this feeder to be direct on-line.
RE: 4160 Motor Protection
RE: 4160 Motor Protection
RE: 4160 Motor Protection
If the client will insist using for alarm, then I would assign one relay output for it on the MPR and probably propose to be wired on an amber lamp local to the MCC with labelled as "Ground Fault".
The existing HRG is already having a common alarm to the annunciator and DCS.