Restricted Earth Fault Protection 1

[fb599]

I have experienced two 33/11kV mal operations due to REF in the last two weeks due to external through faults. I'm checking the stability resistor settings and high impedance voltage settings. I saw a post for a spreadsheet that could make life easier, does anyone have a copy of this please ?

 

[RalphChristie]

First of all, can you confirm that your CTs are correct? In most of the cases where I was involved on nuisance REF-operations, it was due to incorrect CT behaviour. (especially the neutral CT)

Do a ratio test, a magnetizing test and a polarity test on each CT in the scheme and ensure they are correct. If the CTs are easily obtainable, it is actually very easy to test them.

I do have a home-made excel-sheet to do the calculations, it is not difficult to create it.


V max = I x (R ct + R wires)

where:
V max = max voltage
I = through fault current seen from secondary side of CT
R ct = internal resistance of CT
R wires = Largest value of lead resistance between relay and CT

If you have a stabilizing resistor

V setting = V max - (R resistor x i )

where
V setting = setting voltage
R resistor = Value of series resistor
i = operating current of relay





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