[Transformer core] Transformer core material Transformer core function Transformer core grounding

1. The silicon steel sheet itself is also a conductor. Under the action of AC magnetic flux, it is inevitable that an induced current will be generated in the iron core.

2. If the iron core is a whole, it is equivalent to a conductor with a large area. The circuit is short and its resistance value is very small. Although the induced voltage is not high, the current is very large. . This produces huge eddy current (circulation current) losses. This is very detrimental to the transformer. In severe cases, it causes the metal to heat up rapidly and partially melt the iron core, which is called iron core “misfire”.

3. After each piece is coated with insulating paint, the iron core is divided into many small-section conductors at the resulting cross-section. In this way its resistance becomes larger. Therefore, although the iron core coated with insulating paint also has eddy current losses (part of the iron losses), it is much smaller than before.

4. Therefore, the thinner the silicon steel sheet, the smaller its loss. Normally, the insulating paint on the surface of cold-rolled silicon steel sheets has been painted before leaving the factory. As long as we don’t damage it, we don’t need to paint it anymore.

Therefore, this problem can be avoided by stacking thin silicon steel sheets coated with insulating paint so that each silicon steel sheet forms an independent conductor

The role of the transformer core

A transformer is a device that converts AC voltage, current and impedance. When AC current flows through the primary coil, AC magnetic flux is generated in the iron core (or magnetic core), causing the secondary coil to A voltage (or current) is induced in it. A transformer consists of an iron core (or magnetic core) and a coil. Among them, the transformer core – is the main magnetic circuit for coupling magnetic flux in the transformer. The function of the iron core of the transformer is to form a magnetic circuit of coupling flux with very small reluctance. Because the reluctance is very small, the working efficiency of the transformer is greatly improved. Broadly speaking, transformers are classified according to the coupling material between coils, including air core transformers, magnetic core transformers, and iron core transformers. Air-core transformers and magnetic-core transformers are mostly used in high-frequency electronic circuits. Transformers in power circuits must be iron-core transformers. Commonly used transformer cores are generally made of silicon steel sheets. Silicon steel is a ferrosilicon alloy with a silicon content of about 3% and the rest being mainly iron. Silicon steel sheets are widely used in medium and low frequency transformers and motor cores, especially power frequency transformers. Because silicon steel itself is a material with strong magnetic permeability, it can produce greater magnetic induction intensity in the energized coil, which can reduce the size of the transformer and improve the working efficiency of the transformer. Silicon steel is characterized by having the highest saturation magnetic induction intensity (above 2.0T) among commonly used soft magnetic materials. Therefore, when used as a transformer core, it can work at a very high operating point (such as an operating magnetic induction value of 1.5T). However, silicon steel also has the largest iron loss among commonly used soft magnetic materials. In order to preventThe iron core generates heat due to too much loss. Its frequency of use is not high and it can generally only work below 20KHz. Therefore, the frequency of power circuits is mostly around 50Hz. According to the above introduction, it can be seen that transformers in high-frequency circuits do not require iron cores, while transformers in power-frequency circuits or circuits with lower frequencies require iron cores. Otherwise they won’t work properly.

The transformer core is grounded

The reason why the transformer core is grounded:

Electricity When the transformer is operating normally, the iron core must be reliably grounded at one point. If there is no grounding, the floating voltage of the iron core to the ground will cause intermittent breakdown discharge of the iron core to the ground. After the iron core is grounded at one point, the possibility of forming a floating potential of the iron core is eliminated. However, when the iron core is grounded at more than two points, the uneven potential between the iron cores will form circulating currents between the grounding points, causing multi-point grounding heating failures in the iron core. A grounding fault in the core of the transformer will cause local overheating of the core. In severe cases, the local temperature rise of the core will increase, causing light gas operation, or even causing a tripping accident due to heavy gas operation.

The melted partial iron core causes a short-circuit fault between the iron sheets, which increases the iron loss and seriously affects the performance and normal operation of the transformer, so that the iron core silicon steel sheets must be replaced for repair. Therefore, the transformer does not allow multiple points of grounding and can only have one and only one point of grounding.

The scope includes:

1) Multi-phase short circuit inside the transformer.

2) Short circuit between turns, short circuit between winding and core or casing.

3) Iron core failure.

4) Oil may leak below the oil level.

5) The tap changer has poor contact or the wire welding is not firm.

There are differences in the functions of main transformer differential and gas protection

1. Main transformer differential protection is designed and manufactured based on the principle of circulating current, while gas protection is based on the internal fault of the transformer. It is designed and manufactured with the characteristic of producing or decomposing gas.

2. Differential protection is the main protection of the transformer, and gas protection is the main protection in case of internal fault of the transformer.

3. Different protection scopes: A differential protection:

1) A multi-phase short circuit occurs in the main transformer lead wire and transformer coil.

2) Severe single-phase inter-turn short circuit.

3) Protect coils and ground faults on lead wires in large current grounding systems. B gas protection:

1) Multi-phase short circuit inside the transformer.

2) Short circuit between turns, short circuit between turns and iron core or outside.

3) Iron core failure (heating and burning).

4) Oil may leak below the oil level.

5) The tap changer has poor contact or the wire is not welded

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