CT Ratio Test

[KenAlmon]

I have just worked a job site where the requirement was to ratio test the CT's at a number of test currents (400:5 A CT, tested at 100, 200, 300, and 400 A primary current).

I am familiar with testing CT's for ratio for different taps, but only 1 point per tap.

Does anyone know of this practice and what the value is of testing at numerous currents?

Thanks

Ken

 

[scottf]

The only value for testing at these different currents would be for an accuracy test, which I doubt you were set-up to do in the field. And even for an accuracy test, the normal points would be at 10%, rated, and rating factor, at both minimum and maximum burden rating.

Are these CTs being used for protection or metering?

 

[KenAlmon]

For protection scottf, and so we are not that interested in accuracy other than not saturating to excess on fault currents. The main thing would be just to confirm the ratio. So, as you say, probably 1 current would prove that... We also do mag curves (applied voltage at different values vs current into the CT secondary winding) to prove "C" protection accuracy class.

Thanks for your response.

Ken

 

[busbar]

Would 100, 200, 300, and 400 Ampere-Turns meet the spec?

No offense meant to anyone, but if no particular secondary burden is called out, it sounds like a waste of time.

 

[xabproject]

Dear Ken,

Most of the O/E think that by making the contractor mobilize a BIG injection kit the contractor's money is correctly spent! I have done from 100A injection for 3000/1A CT (just one point, with acceptance criteria being the secondary current nearly meeting the specified ratio) to 10%, 50% and 100%or1000A whichever smaller to a mixed setup with 600-400/1A and 3000-2500/1A CTs!

Technically speaking, the Magnetization current test and the Rct test complementing the ratio CHECK test at 10%(and not accuracy test) certify the correctness of the CT delivered to site. Burden measurement / loop resistance measurement during the CT circuit check test and ofcourse the polarity tests complete all that is required to certify Current Circuit correctness. But then who has time to convince the stubborn reps of reputed O/Es?!

I have also done magnetization current test on METERING cores to prove that the cores are indeed metering cores!

Best regards