[Measurement transformer] Annex 1 – What is remanence?

What is residual magnetism?

The magnetic circuit is made up of magnetic walls, inside are magnetic molecules (magnetic moments) oriented parallel to each other.

– In the normal state, when no current is applied, the magnetic field intensity or magnetic force H = 0 The magnetic moment in these magnetic walls is arranged chaotically, so the magnetic flux density total B = 0.

– When applied to an electric current, the magnetic force gradually increases, the magnetic moments gradually align in the same direction. The magnetic flux density also increases.

– When all magnetic moments are arranged in the same direction, the magnetic flux density reaches its maximum value. This state is called magnetic circuit saturation state.

– When reducing the magnetic force, the magnetic flux density does not decrease according to the old rule but decreases according to the rule of the residual magnetic curve as shown below. If no residual demagnetization measures are used   , it will take a long time for the magnetic moments to be arranged in their original order (B = 0). Therefore, this phenomenon is called remanent magnetism.

[Transformer] Residual demagnetization

Purpose

Before learning about the purpose of demagnetization, we need to know the effects of residual magnetism in the transformer.

·        Effect on shock current: When the transformer is energized, the shock current can be many times larger than the rated current. If the transformer magnetic core still has residual magnetism, the maximum current can reach the level of short circuit current. This current can cause unwanted effects such as: Mechanical deformation of the coil, false start-up of protective devices, increased pressure on the connection and voltage drop in the grid.

·        Influence on other measurements: Exciting current measurement, the magnetic balance test, sweep frequency response analysis measurement (SFRA)

Therefore, it is necessary to demagnetize the residual magnet before performing experimental items related to the MBA magnetic circuit.

Method

1.     Vibration demagnetization:

2.     Demagnetization through heating to the Curie temperature.

3.     Electrical demagnetization.

Because when the first two methods cannot be used for transformers, the electrical method is the only choice. There are many ways to demagnetize using electrical methods. The following is the most common method used in laboratory equipment today.

·        Apply DC current to the transformer (this current should not exceed 10 - 15% of the transformer's nominal current).

·        In the first step, the transformer's magnetic circuit is saturated to measure the initial residual magnetism.

·        During the residual demagnetization cycle, the current amplitude is gradually reduced and continuously reversed until the residual magnetism in the transformer is completely eliminated (less than the set threshold value).

Figure 1: Remanent demagnetization cycle

Remanent demagnetization experiment diagram:

Conclude

Remanent demagnetization is an important test performed to prevent the effects of high shock currents and increase the reliability of diagnostic testing. Previously, demagnetization was often done manually by using a polarity-reversing battery based on the experimenter's feelings. Nowadays, advanced laboratory equipment allows demagnetizing transformers reliably without any difficulty. The amount of residual magnetism initially and after complete reduction can be measured.