CT Rating vs overcurrent

To all,

At the moment we are using a 5P20 / 500 / 1 A current transformer. Now we are calculating a primary current of 527.1A. The question is: Can we still use the 5P20 / 500 / 1 A CT or should we go for the 5P20 /

600 / 1 A CT because of continuous overcurrent.

When 5P20 / 500 / 1 A can still be used but not continuously what would be the maximum alowable operation time for this CT.

Data: voltage level =34.5kV I= 527.1A continuous. frequency = 60Hz

Does someone know were we can also find the solution according IEC, because in IEC 60044-1 it is not clear stated how to handle continuous primary overcurrents vs CT rated current.

Any advise is welcome.

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Not familiar with that particular CT, but shouldn't the manufacturers data answer this? In the US, CTs have nameplate ratings, such as 500:5 but also an overload rating. The utility where I worked purchased metering CTs with an overload factor of 3x, meaning that the CT could carry, and still be accurate for, a current 3 times the nameplate current.

Charles Perry P.E.

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Charles Perry

Charles Perry a écrit :

yes but in this case it is a continuous overload that is asked, this must be avoided 'cause the transformer will be continuously saturated what make the output signal no more linear. regards

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I agree, because of continuous overload, the transformer is going to saturate, causing nonlinearities in measurement. I would opt for a higher rated CT, but, as previous posters mentioned, see if you can contact the manufacturer or get a data sheet. It may be that the saturation current is higher than 500A, but you'll also have to consider that this high continuous current will heat up the CT, causing further nonlinearities. It'll heat up quite a bit, so you may really need a higher rated/bigger CT. Some things to consider.

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Carlton Stedman

Depends on the CT. The CTs I refer to were meant to run a continuous overload of 3 times rated current.

The overload rating indicated the range over which the CT would still meet meter accuracy requirements, so no appreciable distortion. The CTs with such a rating are much more expensive, but much cheaper than sending a meter tech to change them out every time a customer's load increases.

May IEC spec CTs are made to much lower standards.

Charles Perry P.E.

Reply to
Charles Perry

Continuous current of 1.05 times the rated current is ok for a 5P20 CT with less than rated secondary burden. For a short time this CT can handle 20 times the rated current , which is 10 kA, with an amplitude error of 5 % with the rated burden.

More than rated overload will heat up the CT. No saturation. Short circuit fault currents about an above 20 times the rated current with rated burden will cause saturation.

If the burden is less than rated, this CT can handle event higher currents than 10 kA.

(Remove the two Roentgen-ray characters to reply.)

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