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Power transformer operation and maintenance


Power transformer operation and maintenance

Industry news
Release time:
2019/02/26 10:03
Rated operation mode of the transformer
1. Allowable range of transformer voltage change
When the voltage is lower than the rated voltage of the transformer, it has no major influence 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 the transformer insulation has a certain influence.
The voltage deviation is in the range of 5%. The primary voltage of the transformer is within 5% of the rated value, and the secondary side can be operated with rated output.
2. Heat and cooling of the transformer
The power loss in the windings and the iron core is all converted into heat, so that the temperature of each part of the transformer rises, and the temperature rise of each part is not uniform.
Heat dissipation process: Heat is transferred from the windings and 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, the high voltage winding and the low voltage winding is all transmitted to the oil; in the whole 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. The forced oil circulation air cooling or forced oil circulation water cooling, the larger capacity of the use of guide cooling, set a certain oil path inside the high and low voltage windings, can greatly improve the heat dissipation efficiency.
Power transformer operation and maintenance
3, the transformer allows temperature and temperature rise
The allowable temperature of the transformer depends on the insulating material. Oil-immersed transformers are insulated with paper and oil.
The transformer winding can be used for a period of 20-30 years at a maximum temperature of 98 °C. In actual operation, the maximum 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 top oil temperature.
The transformer over-temperature protection configuration sends an alarm signal when the temperature is lower than the fixed value, and trips when the temperature exceeds the high temperature setting.
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, the specified service life can still be obtained as long as the life of the transformer during the overload period and the life lost during the underload can be compensated for each other. The normal overload capacity of the transformer is based on the principle of not sacrificing 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 hot spot of the winding shall not exceed 140 °C, and the temperature of the upper 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 first task is to ensure uninterrupted power supply, and the aging acceleration of the transformer insulation is secondary, and the accident overload is at the expense of the life of the transformer. The insulation aging rate allows for much higher than normal overload. Accident overload is also called emergency load. It is to put some load on the transformer for emergency use in a short period of time. In order to ensure the reliability, in determining the allowable value of the transformer accident overload, the temperature of the hot spot of the winding during the general accident overload shall not exceed 140 °C, and the load current shall not exceed twice the rated value.
Precautions when the transformer is overloaded
(1) Transformers operating at full load throughout the day should not be overloaded.
(2) The transformer is operated at low load, and the overload condition and duration allowed by the peak load of the transformer are strictly limited.
(3) Low load in summer, suitable for overload operation in winter, within 15% overload.
(4) Under normal circumstances, the overload of oil-immersed self-cooling and oil-immersed air-cooled transformers shall not exceed 30%; the forced oil circulation air-cooled or water-cooled transformer overload shall not exceed 20%.
(5) When the overload is running, the coolers are all put into operation.
Allowable value of transformer insulation resistance
Power transformer operation and maintenance
After the overhaul, long-term deactivation or standby transformer, the insulation resistance should be measured before commissioning.
The specification of the insulation resistance value of the transformer:
(1) Compared with the previous measurement results, it shall not be less than 70%.
(2) Below 500V, the insulation resistance shall not be less than 0.5MΩ; above 500V, not less 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) Double winding measurement items: primary side to ground, secondary side to ground, and secondary side to secondary side insulation resistance.
Power transformer operation and maintenance
Transformer inspection
1. Inspection before power transmission
The work ticket is terminated and the safety measures before the inspection are restored.
Open the gas relay bleed valve, let go of the gas, and close the valve.
The cooler is powered and the cooler is operating normally. The transformer oil level is normal.
The transformer has no oil leakage.
2, running inspection
The transformer oil pillow oil level and oil color are normal and there is no oil leakage.
The casing has no oil leakage, no damage, no discharge.
The transformer sounds normal and the silica gel is dry.
The transformer body has no oil leakage.
The transformer oil temperature and temperature controller are normal.
The oil system valve is opened and closed correctly, the fan and the oil pump operate normally without gas in the gas relay, and the valve between the relay and the oil pillow is opened.
The dry transformer temperature indication is correct and the cooling fan is working properly.
When the ambient temperature is too high or too low, check if the oil level of the transformer is in the normal range.
In the rain and snow, check for discharge.
Increase the number of inspections within 72 hours of commissioning.
Power transformer operation and maintenance
Gas protection and operation of the tap changer
1. Operation of gas protection device
During normal operation, the heavy gas protects the pressure plate from the trip position and the light gas protects the signal position. In special cases, the heavy gas protection pressure plate needs to change the signal position.
The most common situation in the work is that after the transformer is repaired and changed oil or filtered oil, it will be put into operation for a period of time, usually 2-3 days, after the gas is released without gas or air bubbles, then the heavy gas protection is thrown to the trip. position.
2. Operation and maintenance of the tap changer
1. No-load pressure regulation: When changing the tap position, turn a few turns in the forward and reverse directions to eliminate the oxide film and oil stain on the contact, and pay attention to the correctness of the position. After changing the tap position, measure the DC resistance.
2. On-load pressure regulation: Take the oil sample every three months for testing; the electric operating mechanism checks to maintain a good state; after 5000 changes, the inspection is taken out. (If the tap is changed in the case of outage, it is also necessary to measure whether the DC resistance is acceptable)
Respirator and silicone operation
The respirator allows the gas in the oil pillow to breathe through it, remove impurities, prevent the oil in the oil pillow from coming into direct contact with the atmosphere, and bring moisture into the transformer.
When the silica gel in the respirator becomes wet, it needs to be replaced with a new silica gel or a silica gel.
Grease cleaner: equivalent to oil filter, eliminating moisture, impurities and so on. (less frequently used)
Power transformer operation and maintenance
Transformer oil operation
1. The role of oil in the transformer: insulation and cooling. (The role of oil in the circuit breaker: insulation and arc extinguishing)
2. After the transformer is loaded, the temperature of the oil will gradually increase. The increase of the oil temperature will accelerate the oxidation rate of the oil. In severe cases, the insulating oil will deteriorate and the insulation strength of the oil will drop rapidly.
3. Under normal circumstances, the oil aging process is very slow at 60-70 °C, and the normal operating transformer oil temperature is basically in this range. When the temperature reaches 120 ° C, the oxidation of the oil begins to increase.
Sample the oil regularly or as needed.