RESTRICTED EARTH FAULT. 3

[a430]

Restricted Earth Fault.I have A90MVA 330/132KV Transformer with act ratio of 300/1A on the primary and the bushing,ie the star point, and a ct ratio of 400/1A on the secondary and tne secondary star point,the transformer trips on line faults,on the primary but very stable on the secondary.All efforts to find the cause is proving abortive.The transformer is without differential protection, because it was wired many years ago.

 

[RalphChristie]

Does it trip on primary line faults?
I assume your primary star point is solidly earthed?

If your CT-polarity is correct (and especially the polarity of the CT in the star point) then it seems as if your current divides during primary line-earth faults. You have multiple earthing points (at the source and at the 90MVA trsf) and during an earth fault a portion of the fault current flows up in your 90MVA transformer's star point.

In my opinion you can either disconnect the star point (if the neutral is not degraded), disconnect the primary REF or set it to a less sensitive value.

 

[a430]

The transformer trips on primary line faults, and is solidly earthed.

 

[a430]

Thanks Ralphchristie,please more imformation on nuetral degraing.

 

[a430]

Thanks Ralphchristie,please more imformation on nuetral degrading.

 

[RalphChristie]

a430

Due to transformer costs, transformer manufacturers tend to degrade the insulation towards the neutral point. This is especially true on HV/EHV-transformers. The higher the voltage, the more insulation you have to use, and by degrading it you can save on costs. (More insulation = more money) I can be wrong with the number, but I think above 66kV almost every transformer's insulation towards neutral is degraded.

If the neutral point is indeed degraded, you have to earth it. When not earthed, the voltage oscillations at the neutral end tend to be much more severe than when the neutral is earthed.

Regards
Ralph

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

 

[ScottyUK]

Hi a430,

I have been on a couple of wild goose chases with very similar scenarios. I think Ralph has also been chasing wild geese from his comment about neutral CTs! I've posted about our problems elsewhere in Eng-Tips, but in summary:

The first problem was traced to a reversed neutral CT. Normally the neutral CT does not carry any appreciable current and the relay scheme is stable, but when a ground fault occurs the NCT contrbution opposes the faulted phase current and causes relay operation. It is an absolute pig of a problem to identify - we resorted to a primary injection test with sensitive instrumentation in the CT secondaries and in the relay coil connection.

We had a second slightly different problem on a different transformer where two NCTs had been swapped: one CT was a Class X type designed for a high impedance REF relay, and the other was intended for the standby earth fault relay and had a much lower kneepoint voltage. The scheme became unstable during heavy through faults when the local grid was hit by lightning. We believe the SBEF CT was saturating causing the REF scheme to become unstable. Connecting the correct CT in the scheme fixed the problem.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!