Oil-immersed transformers can be divided into three-phase transformers and single-phase transformers according to the number of phases. In the three-phase power system, the general application of three-phase transformer, when the capacity is too large and subject to transport conditions, in the three-phase power system can also be applied to three single-phase transformer transformer group.Brief Introduction of Oil-immersed TransformerDistribution transformer is an important equipment in the power supply and distribution system of industrial and mining enterprises and civil buildings. It reduces the 10(6)kV or 35kV network voltage to the 230/400V bus voltage used by the user. These products are suitable for AC 50(60)Hz, three-phase maximum rated capacity of 2500kVA (single-phase maximum rated capacity of 833kVA, generally do not recommend the use of single-phase transformers), can be used in indoor (outside), capacity of 315kVA and below can be installed on the pole.Classification of oil-immersed transformers1. According to the structure, it can be divided into iron core type transformer and iron shell type transformer. If the winding is wrapped in the periphery of the core, it is an iron core type transformer; if the core is wrapped in the periphery of the winding, it is an iron shell type transformer. However, the two are slightly different in structure, and there is no essential difference in principle. Power transformers are iron core type.2. According to the number of phases can be divided into three-phase transformer and single-phase transformer. In the three-phase power system, the general application of three-phase transformer, when the capacity is too large and subject to transport conditions, in the three-phase power system can also be applied to three single-phase transformer transformer group.3. According to the winding can be divided into double winding transformer and three winding transformer. Usually the transformer is a two-winding transformer, that is, there are two windings on the core, one for the primary winding, one for the secondary winding. A three-winding transformer is a large capacity transformer (above 5600 kVA) used to connect three different voltage transmission lines. In special cases, there are also Satons transformers with more windings.　The principle of oil immersed transformerOil-immersed transformer components include body (iron core, winding, insulation, lead), oil-immersed transformer oil, oil tank and cooling device, pressure regulating device, protection device (moisture absorber, safety air passage, gas relay, oil storage cabinet and temperature measuring device, etc.) and outlet casing.Oil-immersed transformer is an important equipment used to change voltage and transfer electric energy in power system, and it is the basis of safe and economic operation of power grid. Oil-immersed transformer is mainly based on the principle of electromagnetic induction work. On the closed iron core, there are two mutually insulated windings. Among them, the side connected to the power supply is called the primary winding, and the side that outputs the electric energy is called the secondary winding.When an AC supply voltage is applied to the primary winding, an AC current is passed through the winding to produce an alternating magnetic flux in the core. This alternating magnetic flux passes not only through the primary side winding, but also through the secondary side winding, and the induced potentials E1 and E2 are generated in each of the two windings. At this time, if the secondary side winding is connected to the load of the external circuit, a current flows into the load, that is, the secondary side winding has electric energy output.　Structure of oil-immersed transformerThe transformer is mainly composed of iron core, winding, oil tank, oil conservator, insulating sleeve, tap changer and gas relay.1. The iron core is the magnetic circuit part of the transformer. During operation, hysteresis loss and eddy current loss are generated and heat is generated. In order to reduce the heat loss and reduce the volume and weight, the iron core is composed of cold rolled grain oriented silicon steel sheet with high magnetic permeability coefficient less than 0.35mm. According to the arrangement of windings in the iron core, there are iron core type and iron shell type.In large-capacity transformers, in order to make the heat generated by the core loss can be fully taken away by the insulating oil during circulation to achieve a good cooling effect, a cooling oil channel is often provided in the core.2. Winding and iron core are the core components of the transformer. Since the winding itself has resistance or contact resistance at the joint, heat is generated by I2Rt. Therefore, the winding cannot pass a current higher than the rated current for a long time. In addition, a large electromagnetic force is generated on the winding when the short-circuit current is passed, and the transformer is damaged. Its basic winding has two kinds of concentric and overlapping.3. Transformer winding main fault is inter-turn short circuit and short circuit to the shell. Inter-turn short circuit is mainly due to the insulation aging, or due to the transformer overload and through the short circuit insulation by mechanical damage. The oil level in the transformer drops, causing the winding to expose the oil level, can also occur inter-turn short circuit; In addition, when there is a cross-circuit, the winding is deformed due to overcurrent, causing mechanical damage to the insulation, and inter-turn short circuit will also occur. In case of inter-turn short circuit, the current in the short-circuit winding may exceed the rated value, but the current of the whole winding may not exceed the rated value. In this case, the gas protection action, the situation is serious, the differential protection device will also operate. The cause of short circuit to the shell is also due to insulation aging or oil moisture, oil level drop, or due to lightning and operating overvoltage. In addition, when a through short circuit occurs, the winding is deformed due to an overcurrent and a short circuit to the case occurs. When the shell is short-circuited, it is generally the gas protection device action and grounding protection action.4. The body (winding and iron core) of the oil-immersed transformer is installed in an oil tank filled with transformer oil, and the oil tank is welded with steel plates. The oil tank of medium and small transformers consists of a box shell and a box cover. The body of the transformer is placed in the box shell, and the body can be lifted out for maintenance by opening the box cover.Characteristics of oil-immersed transformerThe oil-immersed transformer adopts a fully sealed structure, and the oil of the oil-immersed transformer is isolated from the air, which slows down the change of the oil. Surface electrostatic spray, beautiful and corrosion resistant. Oil-immersed transformer in the case of severe overload, the coil insulation will not aging, thermal breakdown, in the use of the period to maintain excellent electrical and mechanical properties. Redesigned coil oil channel reduces hot spot humidity. The redesigned fuel tank increases the heat dissipation surface to ensure that severe overload conditions will not cause aging of the oil.