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Transformer sequence or differential current...what does this refer to

Transformer sequence or differential current...what does this refer to

Transformer sequence or differential current...what does this refer to

(OP)
I notice on a listing of the alarm points on our main transformer that "sequence or differential current" will trip the main. In a nutshell can you explain what this refers to? How do you measure it? And do they usually go together like this?  

RE: Transformer sequence or differential current...what does this refer to

Differential current measures the difference between incoming power against outgoing power.  In the event of an internal fault, the incoming power will be significantly higher than the outgoing power.  Since the voltage is known, all that is measured is current.  That said, internal faults are not small.  You have a lot of margin for error in your differential settings.

------------------------------------------------------------------------
If it is broken, fix it.  If it isn't broken, I'll soon fix that.

RE: Transformer sequence or differential current...what does this refer to

(OP)
If it internally faults why would the voltage stay the same?

RE: Transformer sequence or differential current...what does this refer to

Loss of voltage or over voltage does not necessarily mean that there is an internal winding fault, the very purpose of the differential protection.

Voltage may get low on an internally faulted phase, but UV protection, if provided, may have longer time delay to avoid nuisance tripping, the differential is generally set sensitive but only when output "current" is not the same as the input, which is much more reliable and faster.

(Btw, differential protection uses CTs only, hence only monitors current, not power.)

I am guessing, but sequence may be referring to overcurrents, e.g. negative sequence for detecting imbalance current , zero sequence for ground fault currents, etc.

Voltage also may n

Rafiq Bulsara
http://www.srengineersct.com

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