Single-Ratio vs Multi-Ratio CT's 1

[CuriousElectron]

Hi All,
For differential protection, are there any concerns with using one set of CT's having a single ratio and the other set of CT's multi-ratio?
I don't believe that should be a problem, as long as the same ratios are wired up to the relay.
Multi-ratio gives us more flexibility, but from protection stand-point, is single-ratio preferred over a multi-ratio units?

Thanks,
EE

 

[davidbeach]

It depends. For high impedance differential you want the CTs matched as closely as possible. For low impedance differential the relay can be told the CTR of each CT.

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

 

[cranky108]

Usually it is nice to set the CT as high as possible, but it is possible to be limited by the taps available (transformer differential example) so the multi ratio is nice that you can have the flex-ability if you find yourself in that case.

Example a 115 to 12.47 transformer differential. Low side with a CTR at 2000/5. Because of the tap limits on the relay the high side should be set at 1600/5, and not the 2000/5 the CT may allow.

 

[dpc]

Accuracy class differences could be a concern. At least in the ANSI world, the relaying accuracy rating of a MR CT is based on the full winding. So a C200 2000/5 tapped at 1000/5 is closer to a C100 accuracy. Some consulting firms specify MV switchgear with 2000/5 MR CTs on every breaker, then connect to the 200/5 tap for all the feeders. Not really good practice, IMO. In most cases they get away with it, but in dfferential applications the accuracy class of the CTs can matter.

 

[cranky108]

Yes consultants do that, but maybe not that bad.
I take them to task with a calculation of the effective C rating, then the burden, and knee point.
And ask are you sure this will work? If so you put your PE stamp on it for our insurance company.
Most don't want to certify something that does not work.

 

[CuriousElectron]

Thanks for all the replies!
In my opinion, selecting the CT's manufactured by the same vendor for the differential scheme should also be strongly considered. Based on the C57.13 standard, the CT's exhibit a maximum of 3% ratio error if loaded under their maximum rated secondary burden. If there is a mismatch in the ratio error of both CT's(based on the differences of material properties&manufacturing), I wonder how that would affect the scheme; the digital relay may already be programmed to account for this.
Thanks,
EE

 

[dpc]

What is the relaying accuracy class of the actual CTs? For relaying CTs, the issue is the performance of the CTs under short circuit conditions, when the fault current may be many multiples of the nominal current rating of the CT. If one CT saturates, it can cause the differential CT to misoperate.