Siemens RL CB with STATIC TRIP III

[EmergencyPowered]

During a maintenance event, we were running four Detroit diesel 480v 1750kva units in parallel with approximately a 2000kw resistive load. This has been done frequently and with absolutely no problems. This time one generator CB tripped, activated the bell alarm. The LCD display on the trip unit indicated the GROUND FAULT flag. The trips are RMS TLSI and do not have ground fault protection. This test was repeated three times with the exact same failure. We swapped the trip units between two generators and the problem did not occur. After putting the trips back into the original units, we could not recreate either. The MFG test kit showed the trip to function perfectly. Injecting max test current in GFI mode correctly did not trip CB. as no GFI feature. We replaced trip as a precaution. Any ideas...this is a highly valued site!
Thanx, Steve

 

[alehman]

Sounds like a software error in the trip unit. Have you consulted Siemens?

 

[rcw retired EE]

Are the generators groudned individually? There could be circulating currents between generators caused by voltage imbalances that are read as a ground fault by a zero sequence CT on the generator neutral. Maybe voltages were balanced better the other times?

Other problems we've seen are unequal saturation of CT's during high inrush currents when synchronzing or when supplying inrush current to a motor or transformer. The three phase currents were summed and fed to a ground fault relay. Normally all three pahse currents summed to zero. Any imbalance current was seen as a ground fault by the relay. But during high currents, unequal saturation of the CT's supplied unequal currents to the realy, causing a false trip.

To help you any further, we would need to know more about your system.

 

[EmergencyPowered]

The trip unit does not have ground fault protection. I am wondering if Siemens would know of and/or admit if they have a firmware problem with this generation of trip units. Should I come right out and ask when I call them or what would you suggest Alehman?

 

[alehman]

Most manufacturers issue service notification bulletins when problems are uncovered. Sometimes that doesn't happen until or unless a correction is available.

Hopefully you have a local rep whom you know and can help. I would call your rep and ask him determine the best method to report the problem. Sometimes the rep can directly contact whomever in the engineering group is responsible.

It would be advisable to follow up in writing just to make sure you are 'on record'.

 

[EmergencyPowered]

Thanks, I think I will do just that!