Will DC current through a window/donut type CT damage the CT in anyway? 11

[edison123]

If DC current flows through a window/ring/donut type current transformer shown below, will the CT get damaged?

Will its magnetic properties get affected?

Can it be used again for AC current?

Thanks for inputs.

Muthu

http://www.edison.co.in

 

[scottf]

PRC...I would strongly disagree with this statement:

"2) A similar myth is, oil filled EHV CT may explode in case secondary circuit is broke open inside the CT. There may be arcing due to secondary opencircuit, but the arc energy is so low to create any CT explosion."

 

[prc]

Scottf, why do you are disagreeing? I have great respect for your views on Instrument Transformers and am ready to change my views if you give reasons.
I have mentioned my logic; In the field, I have seen reports of CTs with open secondary for months without CT exploding. Raghunath also reported a case, The arcing must have been going on for some time, but CT did not fail violently.

 

[oldfieldguy]

Older CT's used to be susceptible to magnetization due to the DC offset of starting inrush. Older electromechanical relays managed to work through the few cycles of distorted waveform before things evened back out.

The CT's I experienced with this problem were part of a motor differential scheme and were installed in the 1940's. We switched from vintage electromechanical protection to microprocessor-based protection, saw issues where the input from the old CT's did not match the input from a more modern set. The relay took issue with that.

old field guy

 

[scottf]

prc-

Sorry, I was pressed for time yesterday.

If an open circuit on the CT's secondary occurs at the "right spot" it can damage the insulation in certain designs. This can vary substantially with the rating of the CT's core. In general, the higher the ratio and the higher the protection rating, the higher the open-circuit voltage. The higher the open-circuit voltage, the more chance there is to arc to different points.

Over the long-term, the continuous arcing can lead-to gas build-up either leading to an over-pressure situation or deteriorating the oil.

Also, an open-circuited CT causes the core to go into deep saturation. That saturated core can generate a lot of heat...again that varies with the design of the core. That excess heat generated from inside the core housing (on a head-type CT) can lead to primary insulation failure and/or an over-pressure situation.

 

[edison123]

Thanks, prc. The OEM did not find any difference between pre and post demagnetization. May be because the DC current was only 30% of rated AC current?

Muthu

http://www.edison.co.in

 

[prc]

Scottf, I agree with the points mentioned by you. But in real life, I have never seen a violent CT failure due to open circuiting of CT secondary. The arc energy from a CT secondary open circuit will be too low to generate substantial gas. The arc energy has to come from primary through core and capacitance. Probably in exceptional cases as you mentioned, it may happen. I had seen cases where CT secondary went open-circuited for years!

 

[scottf]

prc-

As I said, it all depends on where the open-circuit point is.

As for the arc energy, I'm not so sure about that. When a large protection CT core/coil is open-circuited, the peak open-circuit voltage can be quite high (40kVp and higher) and the arcing can do substantial damage to secondary terminal blocks. I've seen Marthon-type terminal blocks completely melted due to an open secondary terminal set.

I've also seen large oil-filled CTs fail due to an internal open circuit that damaged insulation and generated enough gas to cause the pressure relief plate to open on the top of the CT.

 

[prc]

edison123, Thank you for that information. Residual magnetism in the core will get dissipated within weeks or months.

scottf, Don't think, I am argumentative. I feel the chances of a failure sequence as described by you are remote. Let us say the secondary lead ( maybe 2.5 mm2) breaks inside and an arc occurs across it. Even a gap of a few millimeters across the gap will be able to stand the open-circuit voltage developed across the gap. Copper will melt and such a gap will be developed within a short period.

 

[edison123]

Thanks, prc.

Muthu

http://www.edison.co.in

 

[edison123]

Update: The CT's have been commissioned on load and are working fine. Thank you all again for your tips.

Muthu

http://www.edison.co.in