Parallell CT for 7UT86 1

[Elektromange]

Hi,

I have an existing 7UT86 protection that measueres on one CT on the primary side of a three-winding 130/6.6 kV Transformer and includes the cable all the way up to the feeding switchgear (roughly 1 km) for differential protection. The feeding switchgear was before an ABC-switchgear but will be changed to a double-breaker scheme. This means I need two CTs on the primary side in order to cover the same zone as before. Could I put the two CTs on the double breaker switchgear in parallel so that I don´t need to change so much and get roughly the same protection zone as before? The reason is that I preferably don´t want to perform work at the site where the 7UT86 is placed.

Regards
M

 

[System Protection]

Yes, its possible.

I have no experience with the 7UT86.

In general, with parallel CTs on differentials you need to be careful as the restraint function may not work as well compared to individual CTs.

If you had say 10,000A of fault current flowing into one breaker and out the other breaker; (ie not flowing into the XFMR) with paralleled CTs the diff relay would see no restraint current as the current would sum to zero. With individual CTs the diff relay will be able to see that current flow and incorporate it into the restraint logic.

 

[slavag]

yes, it is standard solution for years