The specific reasons for causing a transformer to explode are generally the following:
1. Winding insulation damage causes a short circuit, causing a fire or explosion accident
(1) Winding insulation aging, deterioration, and insulation strength are reduced. In severe cases, the insulation is lost, causing winding turns and inter-layer short-circuits to cause the windings to burn. When a short-circuit fault occurs in the winding, a discharge arc is generated, and the temperature thereof is above 3000 ° C. Under the action of high temperature, the insulating oil decomposes a large amount of flammable gas (hydrogen gas and acetylene gas, etc.), and is mixed with air to a certain concentration to form an explosive mixture. Burning or exploding in the event of a discharge spark. The causes of insulation insulation aging are:
1) Long-term overload of the transformer causes insulation aging. From the previous description of the load capacity of the transformer and the deterioration of the insulation, it can be known that the life of the transformer insulation mainly depends on the time during which the winding temperature phase is continuously applied. The temperature of the winding is extremely high, and the longer the action time, the faster the transformer insulation degradation speed and the shorter the service life. Because the insulation of the windings of the currently operating transformers is nothing more than the insulation of the cable paper, cotton yarn and thread, etc., this type of insulation is Class A insulation, its maximum operating temperature is 105 ° C (ie when the ambient temperature is 40 ° C) For the oil-cooled transformer, the oil temperature rises by 35 °C, and the average temperature difference between the winding and the oil is 30 °C). If this temperature limit is exceeded, the Class A insulation material will quickly lose its mechanical strength and become brittle, so that the transformer will be damaged by vibration, electric power and current under the action of normal voltage, causing short circuit between winding layers and turns.
2) The insulating corrosion of the insulating oil deteriorates the insulation aging. The insulating oil of the transformer can oxidize the oil due to the small amount of air remaining in the oil or the oxygen generated by the thermal decomposition of the insulating oil of the transformer. The higher the operating temperature of the transformer, the more air is conditioned in the seals of the transformer, and the more oxidized (aged) the oil is. When the oil temperature is below 80 °C, the oxidation is very weak. When the oil temperature exceeds 95 °C, the oxidation rate increases sharply. When the oil temperature exceeds 115 to 120 °C, the oil begins to crack. After oxidation of the insulating oil, the acid value increases and tgδ increases. After the acid value is increased, the insulation will be corroded, which will deteriorate the insulation and reduce the insulation strength. When the insulating oil is seriously oxidized, sludge and moisture are also precipitated. The sludge precipitates on the surface of the solid medium, affecting heat dissipation, causing the local temperature to rise, and further accelerating the aging of the insulation. After the sludge is deposited on the line, the effective creepage distance of the insulator is greatly shortened, and the surface flashover discharge is easily formed, which also accelerates the oxidation and insulation aging and deterioration of the oil.
(2) An inter-turn short circuit accident is caused by a decrease in insulation strength due to water ingress. The inlet water of the transformer enters from the bottom of the terminal cap of the high-voltage bushing, because the bottom seal of the terminal cap is aging and deteriorated (brittle cracking), causing water seepage in the background or water in the rain along the high-voltage lead, causing the high-voltage line near the root of the line to be trapped or interlayer. Short circuit. In the past few years, such accidents have occurred continuously in large-capacity transformers, as can be seen from the examples of accidents in the future. Forced oil circulation water Cooling transformer, because the cooler leaks into the oil, causing a short circuit due to a decrease in insulation strength. For example, a 90MVA (220kV) main transformer of a hydropower plant, a water cooler that is mistakenly thrown into the water, causes heavy gas action during the temperature rise and pressure test, and short circuit between the high voltage windings.
In addition, the top of the oil pillow of the transformer, the explosion-proof membrane, the respirator, the sealing plate of the oil inlet valve stem of the submersible pump are not tightly sealed, and the water, moisture or air is introduced, so that the insulation is lowered, causing a short circuit between the turns or the layers. For example, a substation SFSL2-20000/110 main transformer, because the oil pillow respirator has no oil seal, poor operation management, water accumulation in the oil pillow, along the gas relay communication pipe to the line, resulting in 110kVC phase line from top to bottom 21 Break through the short circuit between 28 turns.
(3) Welding debris, ferromagnetic substances and other impurities (such as filter mesh and activated alumina) enter the transformer, which not only blocks the oil passage, but also causes the inter-turn short circuit caused by the carbonization of the insulation. Adsorption of ferromagnetic material at the winding causes a decrease in insulation strength and causes winding failure.