A transformer is a device that uses the principle of electromagnetic induction to convert alternating current energy into another type of electrical energy with different parameters such as voltage and current. The inner insulating gasket and bracket mostly use organic combustible materials such as paper, cotton yarn, cloth, wood, etc., and have a large amount of insulating oil. For example, a 1000 kW transformer uses about 0.012 cubic meters of wood, 40 kilograms of paper, and insulating oil 1 Ton. Therefore, its fire hazard is easy to burn. Once a serious overload or short circuit occurs inside the transformer, the flammable insulating material and insulating oil will be subjected to high temperature or arcing, decompose and burn, and generate a large amount of gas, causing the pressure inside the transformer to increase sharply. The shell is bursting, a large amount of fuel is injected, and the burning oil flow further expands the fire hazard and causes a large-scale power outage, which affects normal production and life. There are several reasons for the fire and explosion of the transformer in operation:
(1) Insulation damage 1. Insulation of coil insulation When the transformer is overloaded for a long time, it will cause the coil to heat up, causing the insulation to gradually age, causing short circuit between turns, short circuit between phases or short circuit to ground, causing the transformer to explode. Therefore, the transformer should be tested for insulation strength before installation and operation, and no overload is allowed during operation.
2. Poor oil quality, too little oil The transformer insulation oil is inadvertently stored, transported or operated to prevent moisture, impurities or other oils from being mixed into the oil, which will greatly reduce the insulation strength. A short circuit occurs when the dielectric strength is lowered to a certain value. Therefore, the insulating oil which has been placed for a long time must be tested before being put into operation, such as moisture, impurities, viscosity, breakdown strength, dielectric loss angle, dielectric constant and the like. Oil quality should also be tested regularly during operation. When problems are found, appropriate measures should be taken in a timely manner.
3. Iron core insulation aging damages the insulation aging between the silicon steel sheets, or the bolt sleeves of the clamped iron core are damaged, causing a large eddy current in the iron core, causing heat to increase the temperature, and also accelerating the aging of the insulation.
The transformer core should be tested regularly for its insulation strength (testing methods and requirements are the same as for the coil). When the insulation strength is lower than the standard, it is necessary to replace the bolt sleeve or insulate the iron core in time.
4. Inadvertent maintenance, damage insulation In the core inspection, often inadvertently damage the insulation of the coil and the porcelain casing. After the porcelain casing is damaged, if it continues to run, it will flash lightly and then short-circuit. Therefore, special care should be taken during maintenance to avoid damage to the insulation. After the overhaul, there should be a special person to check the tools (to prevent accidents in the fuel tank), check the various components, test the insulation, etc., to confirm that it is intact, safe and reliable before it can be put into operation. In addition, during the inspection, pay more attention to the safety distance of the lead wire to prevent flashover during operation due to insufficient distance, resulting in an accident.
(2) The joints inside the poorly connected coils, the joints between the coils and the joints leading to the high and low pressure porcelain sleeves and the joints on the tap changer. If the contact is poor, local overheating may occur, the coil insulation may be damaged, and a short circuit may occur. Open circuit. At this time, the high-temperature arc generated will also cause the insulating oil to rapidly decompose, generating a large amount of gas, causing a sudden increase in pressure, great destructive power, and serious consequences.
The wire contact is poor for the following reasons:
1. The bolts are loose. 2. Welding is not strong. 3. The tap changer contact is damaged.
For the above reasons, the following measures should be taken:
1. When the transformer is out of service, it should be inspected, and the bolts with poor contact must be tightened. For transformers that cannot be stopped, an external contact check must be performed.
2. The welding surface must be cleaned before welding, and the quality of the solder joint should be carefully checked after welding to prevent accidents caused by solder joints falling during operation.
3. The switch should be switched into position and the bolts should be tightened one by one to ensure that everything is correct and ready to operate.
(III) Load short circuit When the transformer load is short-circuited, the transformer will withstand a considerable short-circuit current. If the protection system fails or the setting value is too large, it may burn the transformer. Such an accident is not uncommon in the power supply system.
For this purpose, the transformer must be short-circuit protected. Medium and small transformers (especially for rural transformers), generally with drop fuses on the high voltage side, the melt should be selected to ensure that the short circuit accident occurs inside the transformer or the casing is blown; low voltage side with low voltage fuse Protection, the melt should also be able to be blown when a short circuit or overload occurs in each of the outlet circuits.
In addition, the high-voltage side of the transformer can also be used for short-circuit protection and overload protection through overcurrent relays. According to the transformer operation, capacity, voltage level, there should also be gas protection, differential protection, direction protection, temperature protection, low voltage protection, over voltage protection and other facilities.
(4) The grounding point of the secondary side (380/220 volts) of the oil-immersed power transformer shall be grounded. When the three-phase load is unbalanced, current will appear on the zero line. If the current is too large and the contact resistance of the grounding point is large, the grounding point will have a high temperature and ignite the combustibles. To do this, always check the grounding wire, whether the point is fully connected, and periodically test the grounding resistance.
In addition, in operation, you should also pay attention to the sound of the transformer, monitor the temperature rise changes at any time, check the oil level and oil color, and find abnormalities, and take timely measures to ensure safety.
(5) The current of the lightning-impacted power-immersed power transformer is mostly led by the overhead line, which is easily attacked by the overvoltage generated by the lightning strike, penetrates the insulation of the transformer, and even burns the transformer, causing a fire, so it is necessary to take corresponding Lightning protection measures.
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