4160 Motor Protection 2

[nightfox1925]

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

 

[dpc]

1. In general, the contactor should be prevented from attempting to interrupt short circuits beyond its limited rating. Some MV starters provide a built-in time delay such that the contactor will remain closed for a time period after the control circuit stops the motor to allow the fuse to clear. In this case, the 87 function can be implemented in the 369. Or you can program a time delay into the differential function, or you can block the differential trip if the current gets too high. Many internal motor fault could trip the differential at levels of current that the contactor can handle. I'm not sure if the 369 can be set to block the diff function for high current faults. 87M protection is generally only used on very large motors.

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.

 

[LionelHutz]

The 50G is there to selectively trip the motor on a ground fault. The 369 can use a 50:0.025 CT and this CT on the 369 can detect a few amps of ground fault current.

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.

 

[nightfox1925]

Thanks dpc and LionelHutz for your early responses.

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

 

[dpc]

The meter and relay have very low burdens, so having them both on the CTs shouldn't be an issue. However, you really need relaying CTs for this application. Having the meter on the relaying CTs is not a problem since this is not a revenue meter.

 

[nightfox1925]

Hi dpc, do we still need that interposing CT connected to the MPR which derived from the 600/5A main CT? or we may wire them (MPR, Meter) in series on the 600/5A main CTs.

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.

 

[davidbeach]

Protection must be on protection CTs, non-revenue metering does just fine on protection CTs. Aux or interposing CTs are a relic of the electromechanical era and generally don't provide any benefit on numeric/microprocessor relays or meters. Provide test switches on the CT circuit around each device and hook them all up in series.

 

[dpc]

As David Beach says, the aux CTs may have been required to match CT ratios for the differential relaying if this had been implemented with an electromechanical relay. If you don't see a need for them, you can probably get rid of them, which would be a good thing.

 

[nightfox1925]

Thanks to all your recommendations. I am actually pursuing to have the 50G and 87 disabled.

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.