Transformer differential protection - Need your wisdom
Transformer differential protection - Need your wisdom
(OP)
Transformer: 230kV(Yg)-115kV(Yg)-34.5kV (Delta with a zig-zag grounding transformer).
At 34.5kV side, there are two wind power generation feeders and a capacitor bank feeder. CT is 1200:5 C400.
Available 3 phase fault is about 8kA and 1LG fault is about 5kA.
Relay: SEL-387-5
Question: Is there any porblem to combine CTs of the two wind power generation feeders to one CT inputs of SEL-387-5, i.e., one restrain element? The CT burden is low and no satuaration is expected. Combining source feeders seems no problem.
Thank you for your response.
Regards,
Bing
At 34.5kV side, there are two wind power generation feeders and a capacitor bank feeder. CT is 1200:5 C400.
Available 3 phase fault is about 8kA and 1LG fault is about 5kA.
Relay: SEL-387-5
Question: Is there any porblem to combine CTs of the two wind power generation feeders to one CT inputs of SEL-387-5, i.e., one restrain element? The CT burden is low and no satuaration is expected. Combining source feeders seems no problem.
Thank you for your response.
Regards,
Bing






RE: Transformer differential protection - Need your wisdom
RE: Transformer differential protection - Need your wisdom
RE: Transformer differential protection - Need your wisdom
Nice point, thank you.
In our case, the wind power contribution is about 300 Amp per feeder for a three phase fault. By comparing to 8kA total 3 phase fault level, can the removal of the small portion of restraint currents be allowed and still have correct selectivity for an out-of-zone feeder fault?
Thanks again.
Regards,
Bing
RE: Transformer differential protection - Need your wisdom
One thing needs to clarify is
SEL-387 only has four sets CT inputs.
The above case has five sets CT required for 230kV, 115kV, 34.5kV feeder 1, 34.5kV feeder 2, and 34.5kV capacitor bank.
So, that's why a paralleling CTs is considered.
Thank you,
Regards,
Bing
RE: Transformer differential protection - Need your wisdom
RE: Transformer differential protection - Need your wisdom
RE: Transformer differential protection - Need your wisdom
I agree that you should not parallel CTs on sources in general, but when a source is 5% of the total fault current, as in your case, I think you can easily ignore the common recommendation and go ahead and parallel the CTs.