REF protection for Transformer LV windings

[RRaghunath]

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!

 

[davidbeach]

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

 

[PUNGLU]

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

 

[ScottyUK]

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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If we learn from our mistakes I'm getting a great education!

 

[jefferslot]

Thought you might find the link useful if you are involved with REF systems