138 Bus Diff - Different CT accuracy class 1

[Sn00ze]

When using differential bus protection, one of the TX's CT accuracy calss is different than the others (0.3B1.8 @800:5)

How does this affect protection? During fault conditions it will saturate at different times? the other CTs are

http://files.engineering.com/getfil...a252-4b13-b25e-2355edad39d7&file=IMG_6704.JPG

 

[mgtrp]

0.3B1.8 accuracy class is a metering CT, unless it has a dual-rating as a protection CT as well then it is not suitable for use with bus differential at all!

 

[davidbeach]

It matters considerably whether or not you're doing low-impedance or high-impedance bus differential.

 

[Sn00ze]

Understood, it is too bad because that is the only spare CT for it. haha

My other question is. Has anyone ever seen that "XXXX" rating next to it? im not sure what that means?

 

[bacon4life]

Also kind of weird that CT A doesn't have any sort of rating listed.

David- Can you please explain a bit more? I hadn't realized a metering rated CT could be used for any kind of differential.

 

[Sn00ze]

@Bacon4life: yes it is weird, the digital copy (Autocad drawings) does show the 2.5L400 @ 1200:5 rating on the empty space next to it. so maybe it is a printing error?

 

[davidbeach]

I wasn't particularly focusing on the metering CT bit, maybe the point is moot.

More generally if you can avoid saturation on a low-impedance scheme unequal, even radically unequal, saturation characteristics don't matter. On a high-impedance scheme they absolutely matter. There's also the case where it only saturates for in-zone faults (mostly transformer diff) that the saturation becomes less of an issue.

 

[bacon4life]

Having two sets of metering CTs and no relaying CTs on the high side would be strange. Mixing different relay class CTs for low impedance schemes can be ok as David mentioned, but metering class CTS is not advisable.

As part of commissioning the transformer, there may have been CT ratio/saturation tests conducted. If not, the tests are pretty easy to run next time the transformer is out for maintenance.

 

[scottf]

I believe the "XXXXX" means that since the 0.3B1.8 rating is given at the lower tap, it is exceeded at all higher ratios. However, I'm not sure why they wouldn't just write 0.3B1.8 again.

 

[Sn00ze]

Thanks for the input guys. much appreciated. I ended up moving the TX prot to the circuit breaker that is on the bus side and the bus protection on the top TX CT. This has been quite insightful for me. Thanks for the explanation David, I always enjoy learning something from your comments.

 

[stevenal]

As modified, do you now have CTs of similar ratios and accuracy? And is the relay impedance high or low?

 

[Sn00ze]

Yes, the CT are protection class and the ratios are satisfactory. Relay is low-impedance