Parallel CTs 1

[mikeengurs]

I have a situation where an other feed needs to be added to a 28 MVA delta-to-Y transformer. The Transformer currently has 2 feeds, with 2 CTs paralleled for differential protection through HU differential relay. each CT is 4000:5A and connected in delta. I need to add a third feed. Do I have to add a third CT in delta configuration connected parallel to the other 2 CTs in the HU relay? What CT type and ratio should I use?

 

[DTR2011]

I need to add a third feed - Yes
Do I have to add a third CT in delta configuration connected parallel to the other 2 CTs in the HU relay? - Yes
What CT type and ratio should I use? - It would be recommended that you add exactly what you have for your existing ct's.

You don't have much flexibility with an existing HU scheme.

Food for thought... Adding a modern digital relay should be considered. There are tremendous benefits (fault records, communication, monitoring, lower maintenance, etc). If you plan on taking the transformer out of service for the upgrade you can plan for a retrofit of a digital relay package at the same time.