Sizing Current Transformers
Sizing Current Transformers
(OP)
I am in the process of sizing current transformers for a 20MVA 69kv-4.16kv power transformer. I am calculating the 69kV full load current to be 167amps and the 4.16kV full load current to be 2776amps. The low side current transformer will be used for differential protection. Can anyone recommend how I would size the current transformers based on the above information?






RE: Sizing Current Transformers
Voltage Rating: 69kV Current: 200 A
VR: 5kV Current: 3000A
I'm not sure what the standard voltage rating is that encompasses 69kV, but 4160 would be 5kV. 200A and 3000A are standard current ratings.
RE: Sizing Current Transformers
For example, for metering on the 69 kV circuit, a 100:5A with an RF of 2.0 (or better yet, a 75:5A with an RF of 3.0) would be better than a 200:5A ratio.
RE: Sizing Current Transformers
RE: Sizing Current Transformers
Ratio also depends on the type of differential relay. An older electro-mechanical type requires a closer matching of the ratios, than modern digital relays.
Check the relay instruction book for suggested ratios.
RE: Sizing Current Transformers
Also you have to check the CT knee voltage requirement for the used relay. Both CTs must has the same
RE: Sizing Current Transformers
RE: Sizing Current Transformers
http://www.selinc.com/techpprs/6027.pdf
http://www.selinc.com/techpprs/6107.pdf
http://www
RE: Sizing Current Transformers
You should check that neither CT will saturate for the maximum through fault.