Rated operation mode of transformer

　　1. Allowable range of transformer voltage change

　　When the voltage is lower than the rated voltage of the transformer, it has no major impact on the transformer, and only reduces the output of the transformer;

　　When the voltage is higher than the rated voltage of the transformer, the transformer core loss and reactive power loss increase, and have a certain impact on the transformer insulation.

　　Voltage deviation ±5% range. If the primary voltage of the transformer is within ±5% of the rated value, the secondary side can be operated with rated output.

　　2. Heating and cooling of the transformer

　　The electrical energy loss in the winding and the iron core is all converted into heat, which causes the temperature of each part of the transformer to rise, and the temperature rise of each part is not uniform.

　　Heat dissipation process: The heat is transferred from the windings and iron core to the transformer oil, and the heat of the hot oil is transferred to the surrounding air through the radiator.

　　The characteristics of the heat dissipation process: the heat generated by the iron core, high-voltage winding, and low-voltage winding is all transferred to the oil; during the entire heat and heat dissipation process, the temperature difference of each part is large, and the temperature of the winding is the highest; the large-capacity transformer has large loss and large heat generation, which requires It adopts forced oil circulation air cooling or forced oil circulation water cooling. The larger capacity adopts guided cooling. A certain oil circuit is set inside the high and low voltage windings, which can greatly improve the heat dissipation efficiency.

　　3. The allowable temperature and temperature rise of the transformer

　　The allowable temperature of the transformer depends on the insulating material. Oil-immersed transformers are insulated by paper and oil.

　　When the highest temperature of the transformer winding is 98℃, the service life is 20-30 years. In actual operation, the highest temperature of the winding temperature is generally less than 98°C, and the service life of the transformer can exceed 30 years.

　　When the transformer is running, focus on monitoring the winding temperature and the top oil temperature.

　　Transformer over-temperature protection configuration, it will send an alarm signal when it exceeds the low-temperature fixed value, and trip when the temperature exceeds the high-temperature fixed value.

　　4, transformer overload capacity

　　When the transformer is in normal operation, the load rate of the daily load curve is mostly less than 1. According to the principle of equivalent aging, as long as the longer life of the transformer during the overload period and the life less loss during the underload period can compensate each other, the specified service life can still be obtained. The normal overload capacity of the transformer is formulated on the principle of not sacrificing the normal life. That is, in the entire time interval, as long as the transformer insulation aging rate is less than or equal to 1, and the following conditions are met:

　　(1) During the overload period, the temperature of the hottest spot of the winding shall not exceed 140°C, and the temperature of the upper oil layer shall not exceed 95°C;

　　(2) The maximum overload of the transformer shall not exceed 50% of the rated load

　　When the system fails, the primary task is to try to ensure uninterrupted power supply, while the acceleration of the aging of the transformer insulation is secondary. The accident overload is at the expense of the life of the transformer. The insulation aging rate is allowed to be much higher than the normal overload. Accident overload is also called emergency load, which is to allow the transformer to carry more load for emergency use in a relatively short period of time. In order to ensure reliability, when determining the allowable value of transformer accident overload, the temperature of the hottest winding of the general accident overload should not exceed 140°C, and the load current should not exceed twice the rated value.

　　two

　　Matters needing attention when the transformer is overloaded

　　(1) Transformers that operate at full load all day should not be overloaded.

　　(2) When the transformer is running at low load, the overload condition and duration allowed by the peak load of the transformer are strictly limited.

　　(3) Low load in summer, proper overload operation in winter, overload within 15%.

　　(4) Under normal circumstances, the overload of oil-immersed self-cooling and oil-immersed air-cooled transformers shall not exceed 30%; the overload of forced oil-circulating air-cooled or water-cooled transformers shall not exceed 20%.

　　(5) When overloading, the coolers are all put into operation.

　　three

　　Allowable value of transformer insulation resistance

　　The insulation resistance of transformers that have been out of service for a long period of time or are spared after being overhauled should be measured before being put into operation.

　　The requirements of transformer insulation resistance value:

　　(1) Compared with the last measurement result, it shall not be less than 70%.

　　(2) Below 500V, the insulation resistance shall not be lower than 0.5MΩ; above 500V, the insulation resistance shall not be lower than 1MΩ per kilovolt.

　　(3) The absorption ratio of the high and low voltage windings to the ground is not less than 1.3.

　　(4) Dual winding measurement items: primary side to ground, secondary side to ground, and secondary side to secondary side insulation resistance.

　　Four

　　Transformer inspection

　　1. Inspection before power transmission

　　The work ticket ends, and the safety measures before the overhaul are restored.

　　Open the gas release valve of the Buchholz relay, close the valve after releasing the gas.

　　The cooler is powered on and the cooler is operating normally. The transformer oil level is normal.

　　The transformer has no oil leakage.

　　2, inspection during operation

　　The oil level and oil color of the transformer oil pillow are normal and there is no oil leakage.

　　The casing has no oil leakage, no damage, no discharge phenomenon.

　　The sound of the transformer is normal, and the silica gel is dry.

　　The transformer body has no oil leakage.

　　Transformer oil temperature and thermostat are normal.

　　The oil system valve is opened and closed correctly, the fan and oil pump are operating normally, there is no gas in the gas relay, and the valve between the relay and the oil pillow is open.

　　Whether the temperature indication of the dry-type transformer is correct and whether the cooling fan is working normally.

　　When the ambient temperature is too high or too low, check whether the transformer oil level is within the normal range

　　Rain and snow weather, check for discharge.

　　Within 72 hours of operation, increase the number of inspections.

　　Fives

　　Gas protection and tap changer operation

　　1. Operation of gas protection device

　　During normal operation, the heavy gas protection pressure plate is thrown in the trip position, and the light gas protection is thrown in the signal position. Under special circumstances, the heavy gas protection pressure plate needs to be changed to the signal position.

　　The most common situation in work is that after the transformer is overhauled and oil changed or filtered, the heavy gas protection is turned on to trip again within a period of time, usually 2-3 days, after the transformer is put into operation again. Location.

　　2. Operation and maintenance of tap changer

　　1. No-load voltage regulation: When changing the position of the tap, make a few turns in the forward and reverse directions to eliminate the oxide film and oil stains on the contacts, and pay attention to the correctness of the position. After changing the tap position, measure whether the DC resistance is qualified.

　　2. On-load voltage regulation: Take oil samples for laboratory tests every three months; check the electric operating mechanism and keep it in good condition; after changing 5000 times, take it out for inspection. (If you change the tap under the condition of shutdown, you also need to measure whether the DC resistance is qualified)

　　six

　　The operation of respirator and silica gel

　　The respirator allows the gas in the oil pillow to breathe through it, removes impurities, prevents the oil in the oil pillow from directly contacting the atmosphere, and brings moisture into the transformer.

　　When the silica gel inside the respirator becomes pink due to dampness, it needs to be replaced with a new silica gel or the silica gel should be damp-dried.

　　Oil purifier: It is equivalent to an oil filter, which eliminates moisture and impurities in the oil. (Rarely adopted)

　　seven

　　Transformer oil operation

　　1. The role of oil in transformers: insulation and cooling. (The role of oil in circuit breakers: insulation and arc extinguishing)

　　2. After the transformer is loaded, the temperature of the oil will gradually increase, and the increase of the oil temperature will accelerate the oxidation rate of the oil. In severe cases, the insulating oil will deteriorate, and the insulating strength of the oil will also drop rapidly.

　　3. Under normal circumstances, when 60-70℃, the oil aging process is very slow, and the temperature of the transformer oil in normal operation is basically in this range. When the temperature reaches 120°C, the oxidation of the oil begins to intensify.

　　Sampling and testing the oil regularly or as needed.