Mal Operation of Earth Fault Protection in Generators

[thureinaung27]

In last week, one of my friends asked me about his problem that occurred in his work as fellow. Actually, it was beyond my knowledge and I am much appreciate if anyone can solve his problem.

" During commissioning of the standby generator power system, a problem was identified with mal-operation of earth fault protection on each of the generators. There appeared to be no earth fault mal-operation problems when the system was operating from the mains utility power supply. The standby power system comprised three diesel generators operating in parallel supplying the 11kV site distribution network. When the generators were operating the system, neutral was earthed via a resistor bank. It was noticed that during the operation of the plant, prior to the earth fault protection operating, the resistors were getting very hot - exceeding their thermal rating - even when the generators were not fully loaded ".

Thanks with regards.
thureinaung27

 

[itsmoked]

A whole lot of informational holes in that description big enough to drive a truck thru!

Did the problem only occur once all three were running or was there no problem unless all three were running?

Transformers between the generators and the 11kV?

What's the method of switching from Standby to Utility?

Certainly sounds like a solid earth fault somewhere, like one phase grounded.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[slavag]

Hi.
In additional to Keith Q:
1. What is a type of EF protection?
2. What is current and time setting of 50,51N?
3. Are you connected all DG to same bus or each one have
separted GSU xfr?
4. If you connect all DG on same bus: Are you earted via
HGR all DG or you provide only one earth point?
Regards.
Slava

 

[slavag]

Next Q.
If your standby system operated as island: What is a type of neutral earthed (grounding)of generators?

 

[jghrist]

What kind of transformer connection is there between the system and the utility? Possibly the generators were providing unbalanced load to the utility system. Is the system neutral grounded at more than one point when operating in parallel with the utility?

 

[RRaghunath]

thureinaung27,
It appears to be a matter of circulating currents due to multiple earthing of generator neutrals.
Could it be that only one generator neutral is to be earthed (through resistor) during standby operation but all the generator neutrals are earthed (through the resistors ofcourse) at the time when maloperation occurred.