Dry Type Epoxy Resin Cast Transformer Structure

1. Iron Core

1.1 Core structure. The iron core of the dry-type transformer is the magnetic circuit part, which consists of an iron core column and an iron yoke. The windings are wrapped around the core legs, and the yoke is used to close the entire magnetic circuit. The structure of the iron core is generally divided into two types: core type and shell type. The core-type core is characterized in that the iron yoke rests on the top and bottom surfaces of the winding, but does not surround the sides of the winding; the shell-type core is characterized in that the iron yoke not only surrounds the top and bottom surfaces of the winding, but also surrounds the sides of the winding. Because the core-type iron core structure is relatively simple, the arrangement and insulation of the windings are also relatively easy, so the core-type iron core is mainly used in power dry-type transformers in my country, and the shell-type iron core is only used in some special dry-type transformers (such as electric furnace dry-type transformers).

1.2 Core material. Since the iron core is the magnetic circuit of the dry-type transformer, its material requires good magnetic permeability. Only with good magnetic permeability can the iron loss be small. Therefore, the iron core of the dry-type transformer is made of silicon steel sheets. There are two types of silicon steel sheets: hot-rolled and cold-rolled. Since the cold-rolled silicon steel sheet has higher magnetic permeability and smaller unit loss when magnetized along the rolling direction, its performance is better than that of the hot-rolled one. The domestic dry-type transformers all use cold-rolled silicon steel sheets. The thickness of domestic cold-rolled silicon steel sheet is 0.35, 0.30, 0.27mm and so on. The thicker the sheet, the larger the eddy current loss, and the thinner the sheet, the smaller the lamination factor, because the surface of the silicon steel sheet must be coated with a layer of insulating varnish to insulate the sheets.

2. Winding

The winding is the circuit part of the dry-type transformer, which is generally made of insulating enameled, paper-wrapped aluminum wire or copper wire.

According to the different arrangements of high and low voltage windings, the windings are divided into two types: concentric and diamond-stacked. For concentric windings, in order to facilitate the insulation of the winding and the core, the low-voltage winding is usually placed close to the core column; for the overlapped winding, in order to reduce the insulation distance, the low-voltage winding is usually placed close to the iron yoke.

3. Insulation

The main insulating materials inside the dry-type transformer are dry-type transformer oil, insulating cardboard, cable paper, crepe paper, etc.

4. Tap changer

In order to supply a stable voltage, control the power flow or adjust the load current, the dry-type transformer needs to be adjusted for voltage. At present, the method of adjusting the voltage of a dry-type transformer is to set a tap on one of its windings to cut or add a part of the turns of the winding to change the number of turns of the winding, so as to achieve a step-by-step voltage adjustment method that changes the voltage ratio. . The circuit in which this winding is tapped for voltage regulation is called a voltage regulating circuit; the switch used for changing and tapping for voltage regulation is called a tap changer. Generally, the appropriate tap is drawn on the high voltage winding. This is because the high-voltage winding is often sheathed outside, which is convenient to lead out and tap. In addition, the current on the high-voltage side is small, the cross-section of the tap lead and the current-carrying part of the tap changer is small, and the switch contact is also easier to manufacture.

The voltage regulation in which the secondary side of the dry-type transformer has no load and the primary side is also disconnected from the grid (without power supply excitation) is called non-excitation voltage regulation, and the voltage regulation in which the transformer winding taps with load is called on-load voltage regulation .

5. Oil tank

The oil tank is the outer shell of the dry-type transformer, the dry-type transformer body is placed in the oil tank, and the tank is filled with dry-type transformer oil. According to the size of the dry-type transformer, the oil tank structure is divided into two types: hanger body oil tank and hanging box shell oil tank.

(1) Lifter body type fuel tank. It is mostly used for dry-type transformers of 6300kVA and below. The box edge is set at the top and the box cover is flat. Due to the small capacity of dry-type transformers, the weight is light, and the body is easy to be lifted during maintenance.

(2) Hanging box shell type fuel tank. It is mostly used for dry-type transformers of 8000kVA and above. The tank edge is located in the lower part, and the upper part of the tank body is made into a hood shape, so it is also called a bell-type oil tank. There is no need for the hanger body during maintenance, only the upper box body can be hoisted.