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Demagnetization of Transformer

Introduction

After a DC current test, such as a winding resistance measurement, the magnetic core of a power or instrument transformer may be magnetized (remanent magnetism or remanence). Also, when disconnecting a transformer from service, some amount of magnetic flux trapped in the core could be present. That practically means that, although there is no presence of an external magnetic field, magnetic induction in the transformer core (marked Br on the B-H curve, Figure 1) is not equal to zero. B

Figure 1 – B-H curve

The remanent magnetism can cause various problems such as erroneous diagnostic electrical measurements on a transformer, or an inrush and asymmetrical currents at start-up of power transformer, or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, the demagnetization of the transformer should be performed.

How to Detect Remanence

When suspecting remanent magnetism, various tests like magnetization/excitation current measurement test or FRA (Frequency Response Analysis) can be performed, in order to check the transformer core magnetization.

An easy and highly effective method for confirmation that the magnetic core is magnetized or demagnetized is excitation current measurement, which can be effectively performed with our TRT series devices.

The excitation current is the current that magnetizes the transformer core. The more energy is needed to magnetize the transformer, the higher this current will be. This principle is used when determining if the transformer core is successfully demagnetized. Excitation currents are measured before and after demagnetization and compared. If the measured excitation currents follow a symmetrical pattern as expected for that particular construction, then the transformer is demagnetized. Otherwise, demagnetization needs to be performed.

Different magnetization level of the transformer core legs can easily be detected with FRA tests. FRA testing results are highly influenced by inductance value, which is dependent on the magnetization level.

FRA Demagnetization Graph
Figure 2 – FRA test results of transformer core legs

Demagnetization Using DEM60C

If a user suspects possible remanence, or when the remanent magnetism is confirmed by TRT instruments or in some other tests, demagnetization needs to be performed.

DEM60C instrument performs fast and reliable fully automatic demagnetization. High output currents of up to 60 A provides an efficient demagnetization independently of the transformer core state.

Transformer core demagnetization is performed by applying alternating DC current with decreasing magnitude down to zero, following a proprietary developed program. By reducing the magnitude of the applied current to the zero value, the total magnetic flux, or remanent magnetism, is also annulled.

Demagnetization process
Figure 3 – Demagnetization process

In order to perform an effective demagnetization, one needs to be very careful in defining a starting demagnetization current for the test. It is recommended that the starting current has the starting value of at least 20% above the value of the transformer saturation current.

The demagnetization process duration depends on the total inductance of the transformer. The higher the inductance of the transformer under test is, the duration of the demagnetization process is increased.

Example

The 100 MVA transformer, 110/36,75 kV, YNyn0, was tested. After the winding resistance test, the DEM60C device was used to demagnetize the transformer.

The demagnetization current waveform is presented in Figure 5. A three-phase demagnetization was performed using the starting current of 5 A.

Demagnetization Current Graphs
Figure 4 – The demagnetization current waveform

The efficiency of the demagnetization process was confirmed by measuring excitation currents using a TRT device and comparing values taken before and after the demagnetization.

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December 22, 2020

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