GE Multilin 750 - "Ground Time Overcurrent"
GE Multilin 750 - "Ground Time Overcurrent"
(OP)
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!
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!





RE: GE Multilin 750 - "Ground Time Overcurrent"
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.
RE: GE Multilin 750 - "Ground Time Overcurrent"
RE: GE Multilin 750 - "Ground Time Overcurrent"
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
www.cvoes.com
RE: GE Multilin 750 - "Ground Time Overcurrent"
RE: GE Multilin 750 - "Ground Time Overcurrent"
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
www.cvoes.com
RE: GE Multilin 750 - "Ground Time Overcurrent"
Is the Multilin 750 located on a feeder breaker to one of the generators?
RE: GE Multilin 750 - "Ground Time Overcurrent"
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
RE: GE Multilin 750 - "Ground Time Overcurrent"
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