CT polarity check for transformer ct 1

[tem1234]

Hi,

While testing polarity check on transformer ct and the battery applied to the primary, a dangerous voltage and arcing may be generated while disconnecting the battery from the transformer winding. This happened to a very experienced technician last week.

I just want to be sure that i understand why this happen. Is it that the reactor of the transformer is opposed to current variation.

This isn't happening habitually, but this time a 36 V instead of a lower voltage battery was used because of some reading problem by the ampmeter connected at the secondary.

How do you check polarity safely?

Thanks

 

[ScottyUK]

Was this a wound primary CT? Or did you drive DC current through the transformer windings to pass it through a conventional CT?

 

[tem1234]

It is a conventional bushing CT. A battery was connected momentarily between H1 and H0. A miliammeter was connected to the secondary to read the deflection.

 

[marks1080]

When putting a voltage on the primary winding you should always make sure when you are removing the voltage you gradually dial it back to zero. Don't apply a voltage and then just remove it, you will leave a residual charge on the core. This, aside from causing your working to draw an arc while removing test leads, can also cause a DC shift which can screw with other testing you will want to do.

If you think this might be the cause of your issue, just reapply the DC voltage and gradually (slowly) dial it back to zero. Remove your test leads at this point. This should be standard practice whenever doing this work.

 

[ScottyUK]

The transformer core behaved like an iron-cored inductor. Good ol' L^di/_dt rears its head.

Be careful, with a bigger core that could have turned out nasty for the commissioning tech involved.

 

[stevenal]

Link

Perhaps the low impedance ammeter is the problem. Usually this check is done with an analog voltmeter. Residual magnetization shouldn't be an issue, since the switch isn't closed long enough for current to rise.

 

[DTR2011]

There are a number of commercial CT test sets that test polarity with AC voltage. An AC voltage is applied to the secondary of the CT and the polarity (and ratio) is measured from the primary. One could accomplish this with a variac and an oscilloscope or phase angle meter as well. The magnitude of the voltage applied to the secondary of the CT depends on the CT ratio and the sensitivity of the measuring equipment, but it can generally be accomplished with approximately 100 VAC or less.

 

[bacon4life]

Finding an analog meter these days is pretty challenging, so we switched to exclusively using the CT test sets DTR mentioned several years ago. A 36V battery seems pretty big.

In the past when used to use a battery, the contact was a just a momentary tap of the battery lead to the bushing using an insulated hot stick. Hopefully you do not connect, measure and then disconnect the battery. If you have magnetized the CT, you will need apply a voltages of alternate polarity and decreasing magnitude to demagnetize it.

 

[vijaypandiyan]

you can use a 9V battery and a galvanometer connected in secondary side. with one pulse from 9V battery will show +ve deflection in galvanometer. If polarity is reversed, it will show -ve deflection.

Thanks,
Vijay