Internal causes Existence of internal wear and tear Because the hysteresis loss of iron core, eddy current loss and copper loss of coil are converted into heat in operation of high frequency transformer, the temperature rises, and heat diffuses to the surrounding by radiation and conduction. When the heating and heat dissipation reach equilibrium, the temperature of each part tends to be stable. Iron loss match (hysteresis loss and eddy current loss) is basically unchanged loss, which is related to the structure of high frequency transformer, so it can not be reduced or eliminated in operation; while copper loss (line loss) varies with load. Poor contact of tap changer In the operation of high frequency transformer, the contact resistance of the tap-changer increases due to insufficient spring pressure, small contact, oil film and dirt. The contact overheating is the most common cause, and the temperature increases continuously. Especially when switching tap-changers or high frequency
transformers are overloaded, it is easy to make tap-changers contact badly and overheat. The main reasons for poor contact of tap changer are insufficient pressure of contact point, oil mud film between static and dynamic contacts, burns on contact surface or location indication which does not correspond to the contact position of switch. Interturn Short Circuit of Winding Several adjacent turns of high frequency transformer windings will appear a closed short-circuit circulating current due to insulation damage or aging, which will reduce the number of windings. High heat generated by short-circuit circulating current will increase the temperature of transformer and burn high frequency transformer in serious cases. When the windings of high frequency transformer are short-circuit between turns, arc light appears at the short-circuit point to heat all parts of the transformer and cooling oil. When the cooling oil is overheated and boiling, the sound of "grunting" can be heard, which makes the temperature of transformer rise sharply. Local overheating of iron core Iron core is made up of insulated silicon steel sheets. In the operation of high frequency transformer, the insulation between silicon steel sheets is damaged due to external force damage or insulation aging, as well as aging damage of core-piercing screw insulation. The eddy current increases, which causes local heating. Generally, the temperature of transformer oil can not be observed to rise in light cases. In serious cases, the iron core is heated severely. Temperature increases, no-load loss increases and insulation decreases. High frequency transformer oil shortage or heat sink blockage High-frequency transformer oil is the main insulation in transformer, which plays the role of insulation, cooling and isolation. If oil is short or the heat sink is blocked, the circulating cooling speed of oil decreases, resulting in high-frequency transformer operating temperature. External causes Faults in Cooling Circulation System of High Frequency Transformer In addition to cooling and heat dissipation by heat sinks, power high frequency transformers also have forced oil circulation, air cooling, water circulation and other heat dissipation modes. Once the cooling and heat dissipation system fails or the heat dissipation conditions are poor, the temperature of high frequency transformers in operation will rise (especially in hot summer season). High frequency transformer chamber with blocked air inlet and outlet or serious dust accumulation The air inlet and outlet of the high frequency transformer room is the passage of air convection in the operation of the transformer. Once the blockage or dust accumulation is serious, the heating condition of the high frequency transformer remains unchanged and the heat dissipation condition becomes worse. It can not heat the surrounding air in time, which leads to the temperature rise in the operation of the high frequency transformer.
