CT test set plans

[tcoupekyle]

Anybody ever build they’re own CT test set? Got any plans on it?

[Kalbi_Rob]

Anybody ever build they’re own CT test set? Got any plans on it?

Yes, and yes. Easiest way to understand how to use a variac and 2 voltmeters is read the IEEE for testing CTs. That includes performing Ratio, Excitation, and Polarity.

[anonymous]

One important detail is discussed in Annex B of C57.13.1-2017: for excitation tests you want to use an averaging voltmeter, NOT a true RMS voltmeter. The waveform for CT excitation is not sinusoidal, it has a non-linear impedance which causes the voltage waveform to be distorted into a kind of sawtooth pattern. But the voltage in a CT secondary is proportional to the average of the magnetizing flux. Factory tests are made with averaging voltmeters. Using a true RMS voltmeter will give more weight to the harmonic distortion in the waveform compared to an averaging voltmeter, so you end up with higher voltage readings in the saturation region compared to factory tests. If you use a true RMS voltmeter, your knee will be in the right place, but the part of the curve to the right of the knee will have a steeper slope compared to the factory tests.

[tcoupekyle]

Yes, and yes. Easiest way to understand how to use a variac and 2 voltmeters is read the IEEE for testing CTs. That includes performing Ratio, Excitation, and Polarity.

Great info guys thank you. Any drawings?

[anonymous]

Great info guys thank you. Any drawings?

I think we're not allowed to copy out of the IEEE book, but if you can get a copy of C57.13.1 there are drawings for each test setup.

For ratio tests, the setup is a variac, an inline ammeter, and two voltmeters. The ammeter and one voltmeter are hooked up to the variac output, which you use to inject voltage into the CT secondary. The other voltmeter goes across the primary side. Jump the X2 terminal with the non-polarity (H2) side of the primary. Inject voltage (less than the knee point voltage), record readings on primary and secondary voltmeters. The voltage ratio is the turns ratio.

For excitation tests, test at multiple voltages below and above the knee point and record the current. Plot the results and it should match the factory curve.

Polarity test is a 9V battery and an analog meter. Meter goes on the primary. With the positive side of the battery on X1 of the secondary and the positive side of the meter on the polarity (H1) side of the primary, the needle will deflect upscale when the battery is momentarily connected to the secondary if polarity marks are correct. It will deflect downscale if the polarity marks are incorrect.

To demagnetize between tests, inject AC voltage higher than the knee point voltage into the CT secondary and slowly reduce it to zero.