Tranaformer Differential 1

[ukzn]

i was just wondering if anyone can help me, by explaining the old Transf. Diff pricipal, using inter-posing CT's. i have not worked on such a system until now. i understand the protection with the new electronic relays but not when you introduce electro-mechanical relays, with iner-posing CT's.

 

[RalphChristie]

Hi Neelan

The main function of an interposing CT is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main CTs.

Interposing CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where this is necessary.

Regards
Ralph Christie





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[tinfoil]

Also, if you have a wye-delta, or a delta-wye transformer, you may need interposing or auxilliary (the term varies) CT'
s to cancel back out the 30° phase shift from prim to sec.

You can connect the 'delta-side' CTs as a'star', and the 'star-side' CTs as a 'delta' to match the phase angles for the electromechanical relay to compare.

As already noted, you can correct for high side / low side turns-ratio current magnitudes with the same CTs.

 

[jghrist]

Section 16 if the Areva Protective Relaying Application Guide (PRAG) at

http://www.areva-td.com/servlet/Con...9&tab=Contacts and Downloads&id=1056536208254

explains differential protection including the use of interposing CTs.

Also see GE's Art & Science of Protective Relaying, Chapter 11 at

http://www.geindustrial.com/multilin/notes/artsci/index.htm