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, or due to DC component of high fault currents, 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) may not be equal to zero.
The remanent magnetism can cause various problems such as erroneous diagnostic electrical measurements on a transformer (excitation current and frequency response analysis), 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, demagnetization should be performed.
How to Detect Remanence
When suspecting remnant magnetism, various tests like excitation current measurement or FRA (Frequency Response Analysis) can be performed 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. Excitation current is the current that magnetizes transformer core. The more energy is needed to magnetize the transformer, the higher this current will be. This principle is used when determining if transformer core is successfully demagnetized. Excitation currents before and after demagnetization are measured and compared. Measured excitation currents after demagnetization should be symmetrical and lower than currents before demagnetization in case excitation current is inductive and not capacitive. In cases when excitation currents are capacitive, they can be even higher after demagnetization, but they should be symmetrical.
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 of the magnetization level.
Demagnetization with DEM60R Device
If a customer suspects possible remanence, or when the remanent magnetism is confirmed by excitation current or FRA measurement, demagnetization needs to be performed.
The DEM60R 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 and transformer size.
Transformer core demagnetization is performed by applying alternating 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 decreased to a level where its influence becomes insignificant.
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.
Calculating Remanent Magnetism
DEM60R has the option to calculate remanent magnetism. When this option is enabled, DEM60R will calculate initial flux, maximum flux, and remanent flux, all in Volt-seconds. Initial flux represents the flux present in the transformer core before the demagnetization. Maximum flux is the highest value to which the tested core or core leg can be magnetized. Remanent flux represents the flux that remains in the transformer core or core leg after the demagnetization. Both initial and remanent flux are also displayed as the percentage of maximum flux.
In the following example, a 150/150/50 MVA autotransformer, 220/110/10,5 kV, YNa0d5, was tested. After winding resistance test, DEM60R device was used to demagnetize the transformer. Three-phase demagnetization was performed on the transformer HV side, in the following order: A-N, C-N, B-N. The results are given in the table below.
|Phase||Initial flux||Maximum flux||Remanent flux|
|A-N||292.56 Vs 99.09%||295.24 Vs 100.00%||1.48 Vs 0.50%|
|B-N||20.10 Vs 6.89%||291.54 Vs 100.00%||1.47 Vs 0.50%|
|C-N||19.46 Vs 5.08%||383.42 Vs 100.00%||2.48 Vs 0.65%|
It can be noticed that initial flux in phases C and B is much lower than in phase A. This is because demagnetization of one phase partially demagnetizes other two phases, but not to acceptable level. Therefore, demagnetization of all phases is necessary.
The calculation of remanent magnetism prolongs the demagnetization process twice. For this reason, it is possible to disable this feature, to speed up the test.
June 3, 2021