CT application for Generator Differential Protection

[sykimk]

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.

 

[scottf]

sykimk-

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.

 

[redtrumpet]

Consider the probably high X/R ratio of the generator and the large fault current that may be available. It may not be possible (or practical) to size the cts large enough to avoid saturation when accounting for remanent flux and dc offset. Then the unequal performance of the mismatched cts will be a factor. If the ct characteristics are not matched, then you will have to be that much more diligent in setting the slope to account for the mismatch.

 

[ypo]

You should consider first the Differential protection relay manufacturer recommendations and requirements for CT's characteristics.
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.

 

[ypo]

In addition: calculated Vk should be the point before saturation of yor CT's excitation curve!

 

[dpc]

If you have C400 and C800 CTs, I don't think you'll have much problem with an electronic differential relay. These are both capable CTs. Problems sometimes occur with C50 (or worse) CTs. But as Scottf said, the burdens on modern relays is so low, that even this is generally not a problem.

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.

 

[alehman]

Problems can arise due to unequal saturation characteristics when high current levels are present (e.g. load inrush). I have experienced this problem with electronic relays. It is important to match CT's as closely as possible. They should be from the same manufacturer and product line if possible.

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=31&catid=168&id=489&typeId=9

 

[scottf]

alehman-

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.

 

[alehman]

My statement was just a caution based on recent experience. My problems pertained to C200 and C400 CT's however. We recently spent considerable time troubleshooting false trips with GE's engineers and eventually replaced the CT's which corrected the problem.

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.