GE Multilin 750 - "Ground Time Overcurrent"

[SparkOmatix]

I have a switchgear (15kV, 95kV BIL, 1200A) that feeds a 5000kVA, 15kV to 4160V transformer that in turn powers a 5kV switchgear (1200A, 60kA RMS symmetrical). The 5kV switchgear feeds to 1750HP motors. When we attempt to start one while the other is running...it runs for about 5 minutes then the GE Multilin 750 (located in the 15kV gear) trips on "Ground Time OverCurrent".

The GE 750 is set as follows:

Phase:
Phase OC PU 1.4 (420A)
Ext Inverse 6; 1 (S;M)
Phase Inst OC PU 10 (3000A)

Ground:
Ground OC PU 2 (100A)
Ext Inverse 10; 1 (S;M)


Does anyone have any thoughts? Could this be a potentially serious issue or is it more likely an unbalance and/or setting issue?

Thanks in advance!

 

[JG2828]

It sounds like an unbalance.
Is this a new installation? If not, has anything changed recently?
What were the currents and angles at the time of the trip? You should be able to retrieve these from the relay front panel.

 

[SparkOmatix]

This is not a new installation, no. I am not aware of any recent changes. These two are not typically run simultaneously but have been successfully run simultaneously before. I'm looking into what changes have been made since such occurred. I will also look to find the current/angles at time of trip. Thanks for taking the time to share your thoughts and ask relevant questions.

 

[dpc]

Is the system solidly grounded or resistance-grounded? Is the ground overcurrent using a separate CT on the neutral to ground connection?

The 100 A pickup setting for the ground is fairly high. In the absence of any other data, I would tend to trust my relays and assume there may be a ground fault somewhere.



David Castor

http://www.cvoes.com

 

[SparkOmatix]

The 5000kVA transformer (located between the 15kV gear and the 5kV gear)is High-Resistance Grounded. The Ground Overcurrent is indeed using a seperate CT (50/5).

 

[dpc]

High-resistance grounded? It appears to be a low-resistance ground ed system. In a high-resistance system, the ground current is limited to probably less than 5 A. Something with 100 A of ground current is probably low resistance grounded.

Any generators connected directly to this 4160 V bus?

You might also take a look at the grounding resistor.

But I would be concerned about a ground fault somewhere.

David Castor

http://www.cvoes.com

 

[vandal06]

What value does the resistor limit ground current to?

Is the Multilin 750 located on a feeder breaker to one of the generators?

 

[rbulsara]

If 100A is within the rating of the NGR, I would suspect an actual fault. If not check the resistor and/or CT and relay wiring.

What is the NGR rating, resistance value? If it is good, even if there is a GF, NGR will not allow more than its rated current, unless Ohm's law fails.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Also you said:

When we attempt to start one while the other is running...it runs for about 5 minutes

Is this true regardless of which motor is running first? Does each of the motors works OK when running alone?

Rafiq Bulsara

http://www.srengineersct.com