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REF protection for Transformer LV windings

REF protection for Transformer LV windings

REF protection for Transformer LV windings

(OP)
High impedance REF protection for 415V windings of transformers can be two types - 5-CT scheme and 4-CT scheme.
I wonder why anye vendor should prefer 5-CT scheme to 4-CT scheme. User point of view, 5-CT scheme is more expensive and less sensitive.
Any thoughts!

RE: REF protection for Transformer LV windings

I can't even picture what a 5 CT scheme would be. But why high impedance when low impedance REF works so well?

RE: REF protection for Transformer LV windings

I wonder too how the 5 CT scheme is implemented. Do you mean to include an ICT?! There has been a shift towards low impedance schemes

RE: REF protection for Transformer LV windings

The fifth CT is used where it is not possible to place a CT on the common return to the transformer star point. One CT is on the earthing conductor, the other on the neutral. Together they equate (more-or-less) to the CT which would normally be on the neutral bushing of the transformer. I've only seen this on retro-fit applications where the transformer has both a neutral bushing and a separate earthing bushing. I can't think of any benefit other than avoiding the need to dismantle the transformer to fit a CT at the common return to the star point.

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