×
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!
  • Students Click Here

*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.

Students Click Here

Jobs

Transformer Questions (Demag, Tests, etc)
2

Transformer Questions (Demag, Tests, etc)

Transformer Questions (Demag, Tests, etc)

(OP)
I have a few questions regarding transformers:

1) When demagnetizing a three phase transformer core, is it necessary to do all three windings? Or, will the current through a single winding be sufficient to demagnetize the entire core?

2) How does the demagnetization process work? I know that using a car battery is one method. It is connected, disconnected, and then connected in reverse polarity for a few cycles. How does this demagnetize the core? I would think the core would remain magnetized with respect to the way it was last connected to the car battery. How do you get the random assortment of magnetic domains in the iron needed for demagnetization?

3) When a core is magnetized, I've heard that it will return to normal after being reconnected to the system frequency for a few minutes. This can be heard by listening to the decaying hum intensity after a transformer is energized. If a transformer is not connected to a load initially where saturation could cause issues, is the only concern with a magnetized core the potential inrush current?

4) I've heard of leakage reactance (short circuit impedance) tests being useful for monitoring winding shifts. If SFRA tests are performed, is the leakage reactance test repetitive and unnecessary?

5) Of the following internal tests, what would you rank as being the most important and indicative of transformer problems:

Dissolved gas analysis
Power factor of inter-winding resistance and windings to ground
Excitation current tests
Turns ratio
Winding resistance
Leakage reactance (short circuit impedance)
SFRA
Meg-ohm resistance tests & polarization index


6) CT test sets often calculate ratio by injecting current through the secondary side of the CT to prevent the need for a high current power supply. The current through the primary is measured. Other test sets are primary injection type and they can inject 400-800A of current through the primary and measure the secondary current. Is one recommended over the other? Is the primary injection method more accurate?

RE: Transformer Questions (Demag, Tests, etc)

I do not do that kind of car battery demagnetization and not on three-phase transformers. But it seems to be an entirely practical way of doing it.

An unconnected power transformer has a rather high winding inductance. And the resistance is low. That means that the L/R time constant is long. Probably in the several seconds range.

That means that current will not reach its final value immediately (never, actually) and you shall be able to watch current building up on an ammeter (I would prefer a recorder) so that you can cycle current from more than nominal current plus to, say, half nominal minus and then 1/4 plus and so on until you are down to a percent of nominal current or thereabout.

Taking the flux down exponentially will bring remanence down to very low levels. It corresponds to the oscillating deflux method where you connect a capacitor to the winding and let the decaying oscillation bring down the flux.

This method is being used in automatic demagnetizing plants for water filtration and probably also in other applications. The beauty of it is that it is simple and cheap. And, if you use a recorder to watch the current, you also get the process documented.

Remember that there will be very high inductive kick-backs and that you may hurt yourself if you touch the transformer terminals or the cables going to it. A high-powered varistor with around 50 V protection level will make it safer and produce less sparks. Kick-backs will still be felt if you touch the terminals - but it will probably not kill you.

A parallell free-wheeling diode could be used, but is not very practical because you need to change polarity all the time.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Transformer Questions (Demag, Tests, etc)

(OP)
Thanks! Excellent explanation.

RE: Transformer Questions (Demag, Tests, etc)

1) The magnitude of DC current to be used for demagnetisation is root2 times the AC HV excitation current at 100 % rated voltage( you can get it from test report of transformer) You can first try with middle phase.Then apply other phases too. When rated flux in one phase(say B phase), other phases will have only half the flux.

2)When the DC current as above is sent, the core is subjected to same B . When you repeat with reducing current, the residual flux will gradually disappear.

3) In service only the inrush current is issue, but that is not serious to warrant demagnetisation of core. Demagnetisation is done at site to check low voltage excitation current and winding resistance , to make sure transformer is healthy after a tripping on heavy line fault. To compare with bench mark values demagneisation is required.

4) Yes after a SFRA, leakage reactance test is superfluous.

5)It depends......The most critical tests are DGA and excitation current test which will make sure whether there is any internal fault in transformer.

6) I believe primary injection test is more accurate. But with internal bushing CTs in transformers, this is not possible. So manufacturers some time provide an additional test winding in CT.

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