Correct procedure for de-magnetizing transformer cores

[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.