Oil-immersed transformer structure
The core part of the three-phase oil-immersed transformer is composed of a closed iron core and windings sheathed on the iron core column. In addition, there are oil tanks, oil conservator, casing, breathing apparatus, explosion-proof tube, radiator, tap switch, Buchholz relay, thermometer, oil purifier, etc.
(1) Iron core
The iron core is the magnetic circuit part of the transformer. In order to reduce the hysteresis and eddy current loss in the iron core, the iron core is made of silicon steel sheets 0.35mm～0.5mm thick. The surface of the silicon steel sheets is coated with insulating paint or the surface oxide film is used to insulate the sheets from each other. According to the arrangement of windings in the iron core, there are iron core type and iron shell type. The vertical part of the iron core of the three-phase transformer is called the iron core column, and the low voltage winding and the high voltage winding of the transformer are sleeved on the column; the horizontal part is called the iron yoke, which is used to form a closed magnetic circuit.
In large-capacity transformers, in order to enable the heat generated by the core loss to be fully taken away by the insulating oil during circulation, so as to achieve a good cooling effect, a cooling oil passage is often provided in the core.
(2) Winding
The winding is also called the coil, which is the circuit part of the transformer, which is divided into the primary and secondary windings. The winding connected to the power supply is called the primary winding, and the winding connected to the load is called the secondary winding. The primary and secondary windings are made of copper wire or aluminum wire wrapped with high-strength insulation.
The primary and secondary windings of each phase of the three-phase transformer are cylindrically sleeved on the same iron core column, the low-voltage winding with fewer turns is sleeved inside and close to the iron core, and the high-voltage winding with more turns is sleeved outside the low-voltage winding. This placement is because it is easier for the low-voltage winding to insulate the iron core. Between the low-voltage winding and the iron core and between the high-voltage winding and the low-voltage winding, sleeves made of insulating materials are used to isolate them reliably. In order to facilitate heat dissipation, a certain gap is left as an oil passage between the high and low windings to allow the transformer oil to circulate.
The main faults of transformer windings are short-circuit between turns and short-circuit to the shell. Turn-to-turn short circuits are mainly caused by insulation aging, or due to the overload of the transformer and mechanical damage to the insulation during short-circuit. When the oil level in the transformer drops, when the winding is exposed to the oil level, a short circuit between turns can also occur; in addition, when there is a through short circuit, the winding is deformed due to the overcurrent, which causes mechanical damage to the insulation, and a short circuit between turns will also occur.
In the case of a short circuit between turns, the current in the short-circuit winding may exceed the rated value, but the entire winding current may not exceed the rated value. In this case, the gas protection is activated, and the differential protection device will also activate when the situation is serious.
The cause of short circuit to the shell is also caused by insulation aging or oil damp, oil level drop, or due to lightning and operating overvoltage. In addition, when a through short circuit occurs, the winding is deformed due to overcurrent, which will also cause a short circuit to the housing. When short-circuiting the shell, the gas protection device and the grounding protection are generally activated.
(3) Fuel tank
The oil tank is the outer shell of the transformer, and the iron core and windings are installed in it and filled with transformer oil. For transformers with relatively large capacity, radiating fins or radiating pipes are installed outside the oil tank. Oil leakage is a common problem with fuel tanks.
Transformer oil is a kind of mineral oil with good insulating properties. It has two functions:
One is insulation. Transformer oil has better insulation performance than air. Soaking the windings in the oil can improve the insulation performance of all parts and avoid contact with air to prevent the windings from getting damp;
The second is the role of heat dissipation, using the convection of the oil to dissipate the heat generated by the iron core and windings to the outside through the box wall and the heat dissipation pipe. Transformer oil is divided into three specifications of No. 10, No. 25, and No. 45 based on its freezing point. Their freezing points are -10 ℃, -25 ℃, and -45 ℃ respectively, which are generally selected according to local climatic conditions.
(4) Oil conservator (oil pillow)
The oil conservator, commonly known as the oil pillow, is a cylindrical container, placed horizontally above the oil tank, and connected to the oil tank of the transformer by pipes. The volume of the oil conservator is generally about 10% of the volume of the oil tank. The oil conservator is a capsule oil conservator, and the capsule isolates the oil in the oil conservator from the outside air. When the transformer oil heats up, the oil flows from the oil tank to the oil conservator; when the transformer oil shrinks, the oil flows from the oil conservator to the oil tank. The oil conservator has two functions: First, when the volume of transformer oil expands or shrinks with the change of oil temperature, the oil conservator plays the role of oil storage and replenishment, ensuring that the oil tank is full of oil and the iron core and windings are immersed. In the oil; second, it can reduce the contact area between the oil surface and the air, and prevent the transformer oil from being damp and deteriorated.
The oil level indicator of the oil conservator adopts the connecting rod type ferromagnetic oil level gauge to observe the oil level. When the oil is insufficient due to leakage or other reasons, it should be filled with oil in time. The oil level gauge is engraved with the oil level height standard line when the oil temperature is -30℃, +20℃ and +40℃, as the standard for oil filling. The oil level marked with +40°C indicates the maximum limit line of the oil level during full load operation when the maximum ambient temperature of the transformer at the installation site is +40°C, and the oil level must not exceed this line; +20°C indicates the oil level during full load operation when the annual average temperature is +20°C Height: -30°C means the lowest oil level line of the no-load transformer when the environment is -30°C, which should not be lower than this line. If the oil level is too low, oil should be added. The oil pillow is equipped with breathing holes to make the upper space of the oil pillow communicate with the atmosphere. When the transformer oil expands and contracts with heat, the air in the upper part of the oil pillow flows in and out through the breathing hole, and the oil level can rise or fall to prevent deformation or damage of the oil tank.
(5) Casing
The lead wire of the transformer winding is connected to the external circuit through a guide rod. The bushing is an insulator between the guide rod and the tank cover, and it plays the role of insulating and fixing the guide rod. There are two types of bushings: high-pressure bushing and low-pressure bushing.
Insulating sleeve
The lead wire of the transformer winding must pass through an insulating sleeve when it passes through the oil tank from the tank to insulate the live lead. The insulating sleeve is mainly composed of a central conductive rod and a magnetic sleeve. One end of the conductive rod in the fuel tank is connected with the winding, and the other end is connected with the external circuit. It is a fault-prone part of the transformer.
The structure of the insulating sleeve mainly depends on the voltage level. Generally, simple solid magnetic bushings are used for low voltage. When the voltage is high, in order to strengthen the insulation capacity, an oil-filled layer is left between the porcelain sleeve and the conductive rod. This kind of casing is called an oil-filled casing. For voltages above 110kV, capacitive charging bushings, referred to as capacitive bushings for short, are used. In addition to filling the inner cavity of the ceramic sleeve with oil, the capacitive insulator also covers the conductive rod between the central conductive rod (hollow copper tube) and the flange, which serves as the main line between the flange and the conductive rod. insulation.
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