Current in relay earth fault element 2

[paulcolesec]

We have a 10MVA 33000/11000 volt transformer connected to a 27-panel 11000 volt board. The transformer is fitted with various types of relays including a GEC K-Series 3-phase and earth fault relay.

Recently, it has been noticed that a current reading of 4 amperes can be seen on the 'Neutral' (Io). The currents in the phase elements are approximately 200A.

Relays on other transformers and feeders connected to the same 27-panel board all have a zero reading of current in the 'Neutral'.

Is this indictative of a pending earth fault?

Many thanks in anticipation,

Paul Cole

 

[DanDel]

How are you measuring the neutral current? Is the switchboard 3-phase 4-wire(does it actually have a neutral bus)?

 

[paulcolesec]

DanDel,

Thanks for the reply.

The system is 3-phase 3-wire. 'Neutral' is the terminology used by GEC Alsthom to describe the current in the earth fault element, it has the symbol Io.

Paul

 

[DanDel]

How is the earth-fault element connected to the system PTs? And what are the pickup and delay settings?

 

[busbar]

Offhand, it seems like 4 amperes CT residual primary current is of no significant concern compared to 200 amps per phase in a transformer winding of ~525 amperes.

Have harmonic contents been measured? The 4 amperes is probably just "garbage," and normally shouldn't have any undesirable effects on the system.

 

[dpc]

I tend to agree with busbar. This appears to be a residual value of CT current and there is often some slight imbalance in the CT secondary currents, even for a balanced system. In addition, the relay may be registering triplen harmonic currents. I'm not sure if this particular relay provides filtering of harmonic currents or not.

You may want to check the pickup setting for this element to make sure it isn't in danger of operating.

 

[jbartos]

Suggestion: In high resistance grounding system grounding, an Io increase of 4 amps could be a premonition of some deterioration in insulation or an increase in harmonic content.