Relay response with CT Saturation
Relay response with CT Saturation
(OP)
I'm looking for information on how the instantaneous element on an GE IAC-53 electromechanical relay might be expected to respond to a highly saturated CT secondary current.
My previous understanding was that these induction cup instantaneous units would normally operate in the first 1/2 cycle or so before the CT saturated.
I understand the distorted and reduced secondary current that results from saturation but I'm trying to determine if the saturation could be so severe that the IT element would not operate at all.
If anyone has any practical experience or test data, I'd be interested - thanks.
My previous understanding was that these induction cup instantaneous units would normally operate in the first 1/2 cycle or so before the CT saturated.
I understand the distorted and reduced secondary current that results from saturation but I'm trying to determine if the saturation could be so severe that the IT element would not operate at all.
If anyone has any practical experience or test data, I'd be interested - thanks.





RE: Relay response with CT Saturation
RE: Relay response with CT Saturation
The breaker did not trip on a fault and there are conflicting reports on whether or not the relay target was dropped.
It would be nice simple answer to the problem, but I'm trying to make sure that it isn't too simple.
RE: Relay response with CT Saturation
RE: Relay response with CT Saturation
We know the magnitude of the fault current (6000 A)from the upstream digital relays that eventually cleared it (after 20 cycles). The fault was transient, so the location was never determined for sure.
I don't have any doubt that the CT saturated, since it was only a 150:5, and the time element was set on the 0.5 amp tap - this is a burden of over 4 ohms. But the expected response of the old IT unit is still a little unclear to me.
RE: Relay response with CT Saturation
RE: Relay response with CT Saturation
Another common problem in metal-clad is the use of multi-ratio CTs - like a 1200/5 set on the 150/5 tap.
RE: Relay response with CT Saturation
You can do a magnetizing/excitation curve test on the CTs to determine the knee point voltage. (secondary induced voltage against exciting current)
With Vs = IL x (Zct + Zburden + Zlead) you can determine the highest secondary voltage the CT can produce without saturation.
where:
Vs - secondary induced voltage
IL - Maximum secondary current
Zct - internal impedance of CT-winding
Zburden - Connected burden impedance
Zlead - Lead impedance
In your case you'll just have to measure the burden impedances of the instantaneous element, the secondary leads and the CT-winding.
Probably you have done this, but assure that all the other elements in the protective scheme are in a working order. From the most common causes I've found on trip failures (with instantaneous units) are:
Damaged relay contacts
Sticky shunt trip coils
No back-up power
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RE: Relay response with CT Saturation
The relays were bench-tested after this and worked fine. The breaker was replaced with a spare and it tested OK.
I wish I had more info on the CTs, but this gear is 40 years old and without pulling out the breaker, there is no way to know for sure what is in there. And they are not interested in any more outages at this point.
RE: Relay response with CT Saturation
You don't need to pull the CT to perform an excitation test. You just need to get to the secondary leads with the primary open. See IEEE C57.13.1.
RE: Relay response with CT Saturation
Agreed. I was thinking about pulling the breaker to physically look at the CTs and perhaps a nameplate. But the test might actually be safer.
This gear will be replaced in another year or two, so probably no one (except me) would be interested in investing in CT testing.
Thanks.
RE: Relay response with CT Saturation