INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

CT polarity check for transformer ct

CT polarity check for transformer ct

(OP)
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




RE: CT polarity check for transformer ct

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

RE: CT polarity check for transformer ct

(OP)
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.

RE: CT polarity check for transformer ct

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.

RE: CT polarity check for transformer ct

The transformer core behaved like an iron-cored inductor. Good ol' Ldi/dt rears its head.

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

RE: CT polarity check for transformer ct

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.

RE: CT polarity check for transformer ct

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.

RE: CT polarity check for transformer ct

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.

RE: CT polarity check for transformer ct

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close