KVuorio
Electrical
- Apr 19, 2007
- 8
We have a case where is some nuisance tripping of protective devices during the start of big motors (several 3,45kV 800kW induction motors). When starting big 3,45kV motors the feeder circuit breakers in 690V MCC are tripping occasionally? We have been investigating the problem and have been able to eliminate some possible causes. The voltage quality is good enough (THD approx 2…3%), voltage dip in starting conditions is approx 5…6%, this is also good enough. Also the protective devices are not tripping due to wrong short circuit and overload settings.
We have come to a conclusion that the nuisance tripping may have something to do with ground currents. There are several transformers in the system, 13,2/3,45kV transformers are low resistance grounded through 400 Amps NRG, 13,2/0,69kV transformers are high resistance grounded through 2 Amps NRG. There is also one 690/400V transformer which is solidly grounded (is there any restrictions combining high resistance grounding and solidly grounded systems?).
In the 690V switchgear there is IPC DSP MKII ground fault protection system. System led’s are indicating normal state (NORMAL lamp lit, PHASE ALARM lamps ON steady, All other indicators OFF). Now when I push the FEEDER GROUND CURRENT pushbutton I have a reading of 100% in two feeders? What does this situation tell me, do I have a solid one phase ground fault? And do I have a solid one phase ground fault in another feeders same phase too? But why DSP is not showing me any alarm, instead it is showing normal condition?
We have come to a conclusion that the nuisance tripping may have something to do with ground currents. There are several transformers in the system, 13,2/3,45kV transformers are low resistance grounded through 400 Amps NRG, 13,2/0,69kV transformers are high resistance grounded through 2 Amps NRG. There is also one 690/400V transformer which is solidly grounded (is there any restrictions combining high resistance grounding and solidly grounded systems?).
In the 690V switchgear there is IPC DSP MKII ground fault protection system. System led’s are indicating normal state (NORMAL lamp lit, PHASE ALARM lamps ON steady, All other indicators OFF). Now when I push the FEEDER GROUND CURRENT pushbutton I have a reading of 100% in two feeders? What does this situation tell me, do I have a solid one phase ground fault? And do I have a solid one phase ground fault in another feeders same phase too? But why DSP is not showing me any alarm, instead it is showing normal condition?