Structure of Power Transformer
Structure of Power Transformer
Power transformer is mainly composed of magnetic core and conductive coil. The coil and iron core are installed in the oil tank filled with transformer oil, and the end of the coil is led to the outside of the oil tank through the insulating sleeve to connect with the external circuit. In addition to coils and cores, transformers also have accessories such as fuel tanks, oil pillows, tap changers, safety airways, gas relays and insulating bushings.
1. Iron Core
Iron core is the magnetic circuit part of transformer and the framework of transformer structure. The primary and secondary windings of the transformer are all wound on the core. Cores are usually made of 0.35-0.5mm thick silicon steel sheets overlapped and coated with insulating paint.
The coil is the circuit part of the transformer. It is usually made of insulated copper or aluminium wires. The outer part of the copper wire is insulated by cable paper. The coils of power transformer are wound in a cylindrical way, that is, the high and low voltage coils are coiled together on a concentric circle, the low voltage coils are coiled near the core, and the high voltage coils are coiled outside the low voltage coils. High and low voltage coils are separated by insulated cardboard, and oil passages are left, so that transformer oil can flow freely between the two coils. Figure 11-2 shows the outline of high and low voltage coils.
3. Fuel tank
The oil tank is the outer shell of the transformer, which is welded by steel plate. In addition to the components of the core and coil, there is also transformer oil. The core and coil are immersed in transformer oil. The oil tank protects the core and coil from the erosion of external force and moisture. At the same time, the heat generated by the core and coil in operation is transmitted to the wall of the box through the convection of transformer oil. The outer side of the box wall is provided with a heat sink. The hot oil in the box passes through the box wall and the heat sink to disperse heat into the surrounding air and cool the oil.
4. Transformer Oil
Transformer oil is a kind of superior insulating oil. It can not only cool coils and cores, but also enhance the insulation performance between coils and cores. Transformer oil circulates in the gap between coils, transfers the heat inside to the tank shell, and emits it into the air through the surface of the shell.
Transformer oil has the characteristics of high insulation strength, low viscosity, stable chemical performance, high ignition point, no impurities and good arc extinguishing performance. According to its solidification point, it can be divided into three specifications: No. 10 (solidification point is 10 C), No. 25 (solidification point is 25 C) and No. 45 (solidification point is 45 C). When supplementing and replacing transformer oil, it must be noted that the oil number should be the same.
5. Oil pillow
The oil pillow, also known as the oil storage tank, is a cylindrical container, which is mounted on the top of the top cover of the tank and connected with the oil tank of the transformer through the pipeline. Transformer oil fills the tank and reaches half of the oil pillow. When the transformer is running, the temperature rises and the oil expands, and the oil in the tank flows into the pillow. When the oil temperature drops, the oil flows into the tank again, which makes the tank full of oil all the time. The pillow is equipped with an oil meter, through which the height of the oil surface can be monitored.
6. Insulation bushing
The insulating sleeve consists of an external porcelain sleeve and a conductive rod in the center. When the outlet of transformer leads from the inside of tank to the outside of tank, it must pass through the insulating sleeve to insulate the live wire from the grounded tank. Insulating bushing is usually made into multi-stage umbrella to increase the surface discharge distance. The shape of high-voltage bushing is tall and the shape of low-voltage bushing is short.
7. Safe airway
Safety airway, also known as explosion-proof tube, is installed on the top of the top cover of the tank. It is a long steel cylinder with explosion-proof film on the top. The function of explosion-proof tube is to protect transformer. When the transformer fails, the temperature rises and the oil decomposes sharply, resulting in a large number of explosive gases. When the pressure inside the tank exceeds a certain value, the oil and gas will break through the explosion-proof film of the explosion-proof tube and eject outward, so that the explosion or deformation of the tank can be avoided. Small transformers generally do not have explosion-proof tubes. Generally speaking, only transformers above 100KA are equipped with oil pillows, and transformers above 1000KA are equipped with explosion-proof tubes.
8. Gas relay
Gas relay is commonly known as gas relay. It is installed in the pipeline between the tank and the pillow. Gas relay is an important protection device reflecting the internal fault of transformer tank. When there are winding turn-to-turn short circuit, partial discharge and other faults in the transformer tank, the surrounding transformer oil will be decomposed to produce gas. The gas from the tank passes through the gas relay into the oil pillow and runs into the air. At the same time, the air flow drives the action mechanism of the gas relay. When the internal fault of transformer is serious to a certain extent, the gas relay acts and gives an alarm signal. When the internal fault of transformer is serious enough to damage the equipment and affect the safe operation, the gas relay action mechanism automatically cuts off the power supply and stops the transformer operation, which ensures the safety of the transformer.
Type of Transformer
The meaning of transformer type is as follows:
Main parameters of transformer
1. Rated capacity
The rated capacity of transformer refers to the maximum apparent power output of transformer under rated voltage, rated current, rated frequency and rated service conditions, in units of volt-ampere (VA) and kilovolt-ampere (KVA).
2. Rated voltage
Rated voltage refers to the no-load voltage value of the secondary side (also called secondary) when the primary side (also known as primary) of the transformer is applied with rated voltage, expressed in units of V or KV.
3. Rated Current
The rated primary and secondary currents (I1N and I2N) of transformers refer to the line currents allowed for long-term passage of primary and secondary windings of transformers under rated voltage and rated ambient temperature in units of An (A).
4. Variable Ratio
Variable ratio refers to the ratio of rated voltage of one winding to rated voltage of another winding with lower or equal rated voltage. The ratio is not less than 1.
5. Impedance Voltage
The purpose of impact test of power transformer before commissioning
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