CT's for Differential Relays of 230kV Delta Pri X'former 69kV Wye Sec 4

[leur2011]

Dear fellow Power folks,

For a power transformer that is protected in the primary with a 230kV breaker and a half CB's and 69kV CB at the secondary as shown on the attached SLD, the differential zone should include the high voltage and low voltage side circuit breakers, by connecting the transformer ANSI device 87T differential relays to current transformers on the LINE side of the 230kV breaker and a half CB’s and on the LOAD side of the GIS 69kV incomer CB. However, I was advised to install the CT's of 87T at the primary of the transformer instead at the line side of the 230kV CB. The feeder between the 230kV Sub and 230kV/69kV transformer is via underground cu/XLPE/AWA/PVC cables at approx. 1.1kM. They keep telling me that the 87L line differential relay at Sub 69kV will line transfer trip between 69kV substation to 230kV Substation via F/O cable to the upstream 87L relay in case there is an internal fault within the transformer.

In my understanding, the line diffential is totally different function with transformer differential and there must be a separate dedicated CT's of 87T at the line side of the 230kV breaker and a half CB. Could you please share your expertise?

 

[leur2011]

This means I need to install two indepandent CT of my 87L line differential at Sub 230kV one and a half CB (one CT each line side of 230kV CB). Maybe one CT for 87L at my Sub 69kV, the CT should be installed at the load side of the CB incomer of my GIS. Do I need two CT as well in my Sub 69kV?

 

[davidbeach]

One CT per breaker. So if your 69kV has only one breaker you only need one CT.

 

[leur2011]

With reference to my original post, our Sub 69kV is fed from two by 230kV pri transformer step down 69kV secondary. We have one 69kV GIS with two (2) incomer. The scheme uses two main buses (e.g. Bus 1 and Bus 2) with two disconnect switch of each feeder circuit breaker. Z bus configuration with normally closed bus tie circuit breaker.

Correct me if I am wrong, at Sub 69kV, one CT for the 87L per incomer is adequate.