Parallellin CT's
Parallellin CT's
(OP)
I have an application where I need to use a breaker that has four sets of 300/5 CT's in its bushings. I need a much lower CT ratio (around 75/5 would be ok). Is there any reason why we cannot connect the CT's in parallel to acheive an effective ratio of 75/5? Is this something that anyone has done before, or seen done?






RE: Parallellin CT's
RE: Parallellin CT's
Is this for metering?
RE: Parallellin CT's
What do you mean by "is the secondary current under a fault going to be a problem"?.
Thanks.
RE: Parallellin CT's
Paralleling CTs is sometimes done for metering, but metering CTs saturate at much lower currents than protection CTs.
Not recommended. Change the CTs or fit interposing CTs.
RE: Parallellin CT's
Or if you consiter the cost of the transformer to be small, and the risk low, you can just not try so hard to protect it.
RE: Parallellin CT's
Continuous - 15A
1 Second Thermal - 500A for 1 second, 1250A for one cycle. It would take a fault of extremely long duration for 200A to be of any concern.
I'd far rather see the CTs paralleled if they can't be replaced than to see interposing CTs. I find it hard to believe that there is only one relay that needs to be connected to the CTs of that breaker though. One of the CTs just needs to be replaced with one of a better ratio.
RE: Parallellin CT's
RE: Parallellin CT's
This is to prevent relay non-operation on CT saturation.
RE: Parallellin CT's
RE: Parallellin CT's
I still say don't do it if the fault level is significant.
RE: Parallellin CT's
Also, when using bushing CTs, getting ratios this low is tough to do while maintaining some suitable protection accuracy rating.
It's just not possible to provide a 75:5A CT on a bushing, since that would only be 15 secondary turns.
I'm a CT guy and not really a relay engineer, but from my perspective, the correct way to protect this installation would be to use free-standing CTs, where a lower ratio is possible...assuming you can't use the 300:5A CTs without paralleling them or have ruled out using an aux CT (which I'm not sure exactly why you have).
RE: Parallellin CT's
RE: Parallellin CT's
RE: Parallellin CT's
David. Im fully agree with you. It's standard solution in this case. STAR.
Best Regards.
Slava
RE: Parallellin CT's
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If we learn from our mistakes I'm getting a great education!
RE: Parallellin CT's
These relays are continually failing (open circuit) - this relay is normally very reliable - and only fails when used in this manner.
We think that being an electromechanical relay with a large burden at the low setting, the voltage from the CT is damaging the relays (any other theories would be appreciated !). We are looking to replace the relays in this area.
I guess with a modern relay, the buden presented to the CT is low, so it wouldn't be a problem - I just thought I would mention it.
RE: Parallellin CT's
What is the protection rating, ratio, nominal primary current of the CT and input voltage rating of the relay?
Also, failing open circuit is generally an indication of over-current and not over-voltage.