Security factor metering current transformer.

[antxon]

I recently had a problem where a 51N protection saw a sawtooth oscilography (see attached file) and the main distance protecctión 21 didnt measure well beeing in a diferent winding.
In the subestation there where three windings for each phase two for protection and one for meassurement. Each winding made a paralel from two torodial current tranformers since there where two cables entering the GIS. Therefor 6 toroidals for each phase 4 for protecction and two for meassurement or metering. There where two types of toroidals 5P30 and 0,5 SF5.

On a visual visit we found the following,

One of toroidals por the 51n winding was a 0,5 SF5. and was in paralel with the one from the other cable 5P30. Besides there could be some lack of isolation in the windings terminals.

What is the error in a metering winding above 120% of the nominal current? Could you use a metering tranformer for protection if the SF is above the Icc.

Is there any problem using metering transformer and protection transformer in paralel to get the addition of the currents if it does not reach the Security factor?

Any clues why you can see a sawtooth oscilography (document attached)?

Thanks in advance Antxon

 

[waross]

It's been a few years since I was responsible for metering accuracy but I seem to remember that revenue class CTs maintain acceptable accuracy up to 200% primary current.
Protection CTs may trade off low current accuracy for the ability to meter quite high current levels before saturating.

Bill
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"Why not the best?"
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