CT application for Generator Differential Protection
CT application for Generator Differential Protection
(OP)
Is it acceptable to use the different CT ratio for Genenerator differential protection?
That is, C400 is used at line side of generator and C800 is used at generator side, and those CTs are used for 87G.
As far as I know, the designation of "C" in ANSI means that it should guarantee 10% of error when 20 times rated primary currents are applied. It is similar to 10P20 (IEC rating).
Therefore, I think it is possible to apply this configuration for generator differential protection when I consider the actual characteristic of CT saturation and short circuts faults outside protective zone during setting of 87G. Furthermore, I think it is not necesary to worry about CT saturation because modern protective relays (digital type) usually require quite small burden.
Could you please advise whether my understanding is correct or not? If I missed anything else, please advise also.
Thanks in advance for your advices.
That is, C400 is used at line side of generator and C800 is used at generator side, and those CTs are used for 87G.
As far as I know, the designation of "C" in ANSI means that it should guarantee 10% of error when 20 times rated primary currents are applied. It is similar to 10P20 (IEC rating).
Therefore, I think it is possible to apply this configuration for generator differential protection when I consider the actual characteristic of CT saturation and short circuts faults outside protective zone during setting of 87G. Furthermore, I think it is not necesary to worry about CT saturation because modern protective relays (digital type) usually require quite small burden.
Could you please advise whether my understanding is correct or not? If I missed anything else, please advise also.
Thanks in advance for your advices.






RE: CT application for Generator Differential Protection
I think you mean to say CTs with differect accuracy ratings, not different ratios?
Most guidelines recommend using similarly rated CTs for differential protection, but in reality (especially with electronic relays), it typically doesn't matter, as long as the saturation characteristics of each CT is adequate for the fault current.
RE: CT application for Generator Differential Protection
RE: CT application for Generator Differential Protection
Your understanding about the accuracy class for relaying is correct.
Your use of CT,s sets C400 and C800 is strange. How do you explain it?
Common practice for CT's choice is: the maximum fault current times the total burden should give a voltage drop taking place on the proportional (line side) of the CT's excitation (saturation) curve.
You could use the following formula:
Vk = Ifault x (Rct + 2Rl+Rother)
Vk – kneepoint of CT's excitation (saturation) curve in Volts
Ifault – fault current: (24 or 50 times In) thus (24 or 50 or other is a multiplier coming from diff. protection relay manufacturer and your considerations of the gen. system fault current), (In is the nominal current of the CT's secondary, usually 5A or 1A)
Rct – resistance of CT's secondary in ohms
Rl – resistance of wires
Rother – resistance of relay and all other in the CT's secondary circuit.
RE: CT application for Generator Differential Protection
RE: CT application for Generator Differential Protection
But remember that if you are using multi-ratio CTs, the accuracy class applies only to the full winding. If you use a lower ratio, the accuracy class goes down proportionally.
RE: CT application for Generator Differential Protection
Also, generally multi-ratio CT's are not recommended for differential protection.
See product service bulletins at: http://www.geindustrial.com/cwc/products?pnlid=6&famid=...
RE: CT application for Generator Differential Protection
I don't know of many differetial protection applications (at least at HV levels) that don't use multi-ratio CTs. As is said below, you just have to be mindful that the accuracy performance is roughly linear to the number of secondary terms versus the full winding.
I agree that ideally, each CT's performance should be the same. However, for most differential schemes there should not be a problem with mixing C400 and C800 performance. It's hard to say for sure without knowing the exact details. You can look at the 2 excitation curves and see what the difference in excitation current is going to be at the fault level and see if that will cause a problem.
RE: CT application for Generator Differential Protection
I don't disagree multi-ratio CT's are in common use. I think my statement actually came from some place in Blackburn. My experience has been with manufacturers that like to use one model for everything. I don't think it's wise to use very low taps on high ratio CT's.