Hello , Can I use CT accuracy clas 4

[Ihsan Abbas Oudah]

Hello ,
Can I use CT accuracy class C800 for differential protection? Transformer data 11.5/6.6 kV ,25 MVA .

 

[RRaghunath]

C800 CT has a knee point voltage of 800V which is pretty good. Also, verify CTs at the other side of the transformer.
What relay are you using for transformer differential protection? The relay manual comes with a formula for CT sizing. Check both side CTs for adequacy with the CT sizing formula in the manual and decide.

R Raghunath

 

[wcaseyharman]

Normally I would use the CT ratio, available fault and the CT burden to calculate the adequacy of CTs. Based on what I’ve seen installed C800 should be more than adequate for your application, but without calculating I wouldn’t say for sure.

 

[davidbeach]

Probably a bit of overkill unless you're trying to run extremely long secondaries into a fleet of electromechanical relays. C800 is about as good as CTs get, although I have heard tell about C1200 CTs on some 500kV breakers.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[che12345]

Ihsan Abbas Oudah (Electrical)(OP)28 Oct 23 18:31
"...can I use CT accuracy class C800 for differential protection? Transformer data 11.5/6.6 kV ,25 MVA"
You had already received numerous learned advice. I wish to point out the CT requirements for Transformer differential protection; for your consideration.
1. CT basic requirements:
a)secondary I1 = I2 approx. Phase angle 1 = phase angle 2. Note: modern microprocessor relay can have these differences modified/matched by programming.
b) for through-fault stability, knee-point voltage Vk1 = Vk2 under system short-circuit kA level. Note: independent of the exact magnitude of the voltage.
c) the CT1 internal resistance + external resistance = CT2 internal resistance + external resistance approx. Note: modern microprocessor relay can have these differences modified/matched by programming.
d) refer the relay OEM CT selection recommendation.
2. Additional information: There are two sets of CTs and these are usually located some distance apart. Do NOT earth them locally. Wire the insulated earthing wires to a common point isolated from earth. Connect this ONE point to the Earth.
3. Opinion: as whether suitability of a C800 is dependent on fulfillment of above mention conditions.
3.1 Most recent microprocessor relays are designed to accept CT IEC 5P20 with programming to match the differences.
Che Kuan Yau (Singapore)

 

[b_gibb]

I've been down the rabbit hole on this sizing. It was good for my understanding but I'd recommend not doing that. Just stick to the recommendations of the relay manual. It's not so much saturation that is the concern but rather mismatched CTs (saturating at different times, etc.).

ben
bengibb.ca
powerdesignerpro.com