1. Monitoring instruments

The instruments of the transformer control panel, such as ammeters, voltmeters, power meters, etc., should be read every 1-2 hours and the daily load curve should be drawn. When operating under overload, the meter should be read every 0.5 hours. When the meter is not in the control room, it should be recorded at least twice per shift.

2. Monitor the voltage of the transformer power supply

The variation range of power supply voltage should be within ± 5% of the rated voltage. If the voltage is too high or too low for a long time, the tap changer of the transformer should be adjusted to make the secondary voltage tend to be normal.

3. Measure whether the three-phase current is balanced

For transformers with Y and Yn0 connections, the line current should not exceed 25% of the rated current on the low voltage side. If it exceeds this limit, the load on each phase should be adjusted to balance the loads as much as possible.

4. Allowable temperature and temperature rise of transformers

(1) Allowable temperature

During the operation of transformers, copper and iron losses occur, causing the coils and iron cores to heat up. The allowable temperature of a transformer is determined by the heat resistance strength of the insulation material used in the transformer. The insulation of oil immersed power transformers belongs to Class A, and the insulation is made of impregnated organic materials such as paper, wood, and cotton yarn, with an allowable temperature of 105 ℃. The component with the highest temperature in a transformer is the coil, followed by the iron core, and the transformer oil temperature is the lowest. The insulation between coil turns is made of cable paper, and the average temperature of the coil can be measured. Therefore, the temperature of the coil during operation should be ≤ 95 ℃.

The operating temperature of power transformers directly affects their output capacity and service life. If the temperature exceeds the allowable value for a long time, the insulation of the transformer is easily damaged and the service life is reduced. The reduction of the service life of transformers can generally be calculated according to the "eight degree rule", that is, when the temperature rises by 8 ℃, the service life is reduced by half. The experiment shows that if the temperature of the hottest point of the transformer winding is maintained at 95 ℃, the transformer can operate continuously for 20 years. If the winding temperature rises to 105 ℃, the service life decreases to 7.5 years. If the winding temperature rises to 120 ℃, the service life decreases to 2.3 years. It can be seen that the service life of a transformer is mainly determined by the operating temperature of the winding.

The temperature of transformer winding is related to the load size and ambient temperature. The difference between the temperature of the transformer and the ambient temperature is called the temperature rise of the transformer. For transformers with Class A insulation, when the ambient temperature is 40 ℃ (the highest ambient temperature), the national standard stipulates that the temperature rise of the winding is 65 ℃, and the allowable temperature rise of the upper layer oil temperature is 45 ℃. As long as the upper layer oil temperature and temperature rise do not exceed the specified value, the safe operation of the transformer within the specified service life can be guaranteed.

Allowable temperature=Allowable temperature rise+40 ℃

When the ambient temperature is above 40 ℃ and heat dissipation is difficult, the transformer is not allowed to operate at full load. When the ambient temperature is less than 40 ℃, although it is beneficial for heat dissipation, the heat dissipation capacity of the coil is limited by structural parameters and cannot be improved, so overloading operation is not allowed. When the ambient temperature is below zero degrees, the transformer is overloaded and the upper oil temperature is maintained below 90 ℃, not exceeding the allowable value of 95 ℃. However, due to the inability to improve the heat dissipation capacity of the coil, the coil temperature rises and heats up, exceeding the allowable value.

For example, in an oil immersed self cooling transformer, when the ambient temperature is 32 ℃, the upper oil temperature is 60 ℃, which does not exceed 95 ℃. The temperature rise of the upper oil is 60 ℃ -32 ℃=28 ℃, which is less than the allowable temperature rise of 45 ℃. The transformer can operate normally. If the ambient temperature is 44 ℃ and the upper oil temperature is 99 ℃, although the temperature rise of the upper oil is 99 ℃ -44 ℃=55 ℃, which does not exceed the temperature rise limit, the upper oil temperature exceeds the allowable value, so operation is not allowed. If the ambient temperature is -20 ℃ and the upper oil temperature is 45 ℃, although it is less than 95 ℃, the temperature rise of the upper oil is 45 ℃ - (-20) ℃=65 ℃, which has exceeded the temperature rise limit and is not allowed to operate.

Therefore, only when the upper oil temperature and temperature rise value do not exceed the allowable values, can the safe operation of the transformer be guaranteed.

5. Operation of Transformer Oil

Check the height of the oil level inside the oil pillow and oil filling sleeve, and whether there is any oil leakage at the sealing point. The oil level indicator should generally be at 1/4 to 3/4. Excessive oil level is generally caused by abnormal operation of the cooling device or internal faults in the transformer, resulting in high oil temperature. If the oil level is too low, check whether there is serious oil leakage at each sealing point of the transformer and whether the oil drain valve is tightly closed. The oil color inside the oil gauge tube should be transparent with a slight yellow tint. If it appears reddish brown, it may be caused by dirt on the oil level gauge or by the transformer oil running for too long and the oil temperature being high, resulting in deterioration of the oil quality.