Correct procedure for de-magnetizing transformer cores

[randywright]

What's the cause of a "magnetized" transformer and what's the correct procedure for de-magnetizing? I've been told you should de-magnetize each phase after doing winding resistance but another tech told me you can get away with doing a de-mag on only one phase. Also, what would happen if a magnetized transformer was re-energized?

[slts1991]

The magnetization is the current that satisfies the excitation condition of the transformer when the voltage is applied to the primary winding of the transformer with the secondary winding unloaded. You can calculate this current from the fundamental transformer equation.

The easiest and most efficient way to demagnetize a transformer is by applying voltage in reverse polarity. You should demagnetize each phase.

In practice, most Winding resistance testsets perform the demagnetization process automatically. I do not suggest grounding the transformer right after a test (in case you do not have a way to demagnetize the transformer) because it might cause eddy current.

What you need to do is to reverse the polarity by reversing the test leads.

There is this popular bad practice of grounding the transformer after winding resistance is performed which is not good.

I suggest to google the terms "transformer fundamental equation", "demagnetization", "magnetic flux" or "magnetization" for a better understanding.

[randywright]

Thanks, slts1991. Very informative post. Could you explain what would happen if you forget to demagnetize and then turn the transformer back on after testing?

[randywright]

Still haven't been able to find an answer on this one after many months. Has anybody ever energized a transformer without first de-magnatizing? Could the transformer be damaged?

[lester mcmanaway]

Still haven't been able to find an answer on this one after many months. Has anybody ever energized a transformer without first de-magnatizing? Could the transformer be damaged?

causes increased inrush currents (which can damage the unit if they exceed the fault current rating) that being said I have seen it done both ways many times without any noticeable ill effects. They do tend to be a touch louder upon re-energization however. This was in my earlier years, that being said we are currently practicing de-mag. on all units. I wouldn't advise energizing them without de-magnetizing first.

[randywright]

Thank you for the info. Do you de-mag on each phase or can you do it once all testing is complete?



causes increased inrush currents (which can damage the unit if they exceed the fault current rating) that being said I have seen it done both ways many times without any noticeable ill effects. They do tend to be a touch louder upon re-energization however. This was in my earlier years, that being said we are currently practicing de-mag. on all units. I wouldn't advise energizing them without de-magnetizing first.

[lester mcmanaway]

Thank you for the info. Do you de-mag on each phase or can you do it once all testing is complete?

We use a the tap box omicron makes. Hook up all 3 phases and run the demag when testing is complete

[randywright]

i am only familiar with the megger single phase sets. some guys say you only have to do it on the last winding, other guys say you have to do it after each winding.


We use a the tap box omicron makes. Hook up all 3 phases and run the demag when testing is complete

[lester mcmanaway]

i am only familiar with the megger single phase sets. some guys say you only have to do it on the last winding, other guys say you have to do it after each winding.

not sure why you would have to do it immediately after each test, I don't see how it would effect winding resistance on the remaining tests. we do it after all the tests are finished. you just want to do all your ac testing before the dc testing then demag.

[slts1991]

Still haven't been able to find an answer on this one after many months. Has anybody ever energized a transformer without first de-magnatizing? Could the transformer be damaged?

I am sorry Randy but for some reason my email filters notifications from this website into the spam so I did not know if you replied or not. If you do not demag the transformer those eddy currents might still be present under energized conditions which eventually will damage winding's insulation.

I am not saying that if you do not demag a xfmr it will blow up in a couple of years, but it is best to do so especially that the climate conditions wary in some cases and with the help of the eddy's it might create failure in time.
If you energize the xfmr when you do not demag it will not create immediate noticeable damage.

[roberts311]

What's the cause of a "magnetized" transformer and what's the correct procedure for de-magnetizing? I've been told you should de-magnetize each phase after doing winding resistance but another tech told me you can get away with doing a de-mag on only one phase. Also, what would happen if a magnetized transformer was re-energized?

At my company we were instructed to do the DC Transformer testing first (ie., Polarization Index, Winding Resistance) and the AC testing last (ie., TTR, Power Factor) because the AC changing polarity 120 times per second will wipe out any residual magnetic charge. Most Winding Resistance/Insulation Resistance test sets come with internal grounding to dissipate the magnetic charge anyway so for the most part it's a non-issue. Just remember to leave your test leads on until the test set has completed it's discharge.

[Shaw1903]

I thought the point in demagnetisations were to remove residual flux density from the core and discharge the winding? Surely the test unit you use such as a megger etc will only discharge the winding and leave residual flux density in the core?

[ElectTech]

Perform winding resistance measurements last as some residual magnetization may remain and influence other sensitive testing measurements. Always Demagnetize a Transformer after performing winding resistance measurements to protect your transformer from high in-rush currents.
The best way to Demagnetize with a winding resistance test set is to connect your Current and Potential leads across the center phase on the High side of the Transformer. The High side center phase has the most core material.
Run the Demagnetization sequence. We have found it is better to run the Demagnetization sequence twice (or Coarse and Fine). Do not Demagnetize Phase "A" then "B" then "C". It will not work.There will be an imbalance in the core material.

[SecondGen]

There is a good article in NETA World Summer 2019 titled Power Transformer Winding Resistance that is worth a read if you can get a copy. Here are some highlights regarding transformer magnetization:

The transformer core becomes magnetized from the saturation process during the winding resistance test. The core is saturated to remove the magnetic circuit, thereby isolating resistive components in the winding.

Magnetized transformers produce higher inrush currents upon energization. It's recommended that dc winding resistance is performed last to avoid contaminating other test results.

Saturated transformer cores can influence other diagnostic tests, such as sweep frequency response analysis (SFRA) and turns ratio (TTR). Excitation current tests and SFRA can be used to confirm and validate the presence or absence of magnetization.

Two techniques can be used to demag transformer cores:

1. Apply decreasing ac voltage. Equipment required is very cumbersome and expensive for field use.

2. Apply dc power to the transformer windings and reverse the polarity of the applied source a number of times while reducing the voltage, current, and applied time until the core is demagnetized. Modern winding resistance test sets have a de-mag function that perform this sequence automatically.