CTs in isophase bus ducts 2

[alpha75]

how are the CTs mounted installed in the isophase bus ducts? my thought is to provide a transition compartment where the CTs are mounted thru the bus.

i am not in practice with MV power generation so I cannot visualize this situation.

thanks for your comments.

 

[jbartos]

Suggestion: Follow the CT manufacturer installation instruction or application notes. There are various CTs and various CT installations possible or be more specific.

 

[busbar]

This may not directly address your question, but alternatives are described in IEEE Std C37.102-1995 …AC Generator Protection, it seems that use of generator- and GSU-transfomer-mounted CTs may be suitable for bus protection, and possibly more cost-effective compared to isophase-bus mounting. Two schemes are described.

87U Differential relay for overall unit and transformer.

87T Differential relay. Primary protection for the GSU or UAT transformer. May be used to provide phase fault backup for the generator in some station arrangements. The zone may be extended to cover the generator bus using cts from the generator and Unit Auxiliary Transformer when lowside cts are not available.

 

[ScottyUK]

Normal practice for isolated phase bus ducts is to minimise the number of discontinuities in the structure, which maximises the screening effect of the enclosure and thus minimises the external magnetic field. CTs are sometimes fitted in IPB's, but special care is required to provide massive bonding - typically welded - across the outer enclosure from one side of the CT to the other, but without passing through the CT. Only the busbar itself must pass through the CT, or image current cancellation will occur and your CT will measure the differential between the bar current and image current, which might be interesting to see how good your bus duct is but will be of little use for measuring bar current!

Busbar's comments represent typical practice on IPBs associated with large generator conenctions to their associated step-up grid transformers. The is always a discontinuity at the end of an IPB, so introducing a CT there does not further degrade the performance. You also have more chance of getting access to a CT mounted in a bushing cubicle than one installed mid-way along an IPB.




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[alpha75]

thanks again to all the postings! gives me valuable info.