There are many reasons for excessively high transformer temperatures, which could be due to internal transformer malfunctions or external factors.
One: Internal Transformer Malfunctions
When a transformer operates, the temperature of each part tends to stabilize when heat generation and dissipation reach equilibrium. If, under the same conditions, the oil temperature is more than 10°C higher than usual, or the temperature continues to rise despite a constant load, it can be considered that an internal transformer malfunction has occurred.
1. Poor Tap Changer Contact
During operation, insufficient contact pressure or contamination at the tap changer’s contact points can increase contact resistance. Increased contact resistance leads to increased contact temperature and overheating. This is especially true after tap changing and during transformer overload operation, where poor tap changer contact and overheating are more likely. Poor tap changer contact can be identified by frequent operation of the light gas valve. Oil sample analysis can reveal that poor tap changer contact causes a rapid drop in the oil flash point. Additionally, the contact condition of the tap changer can be determined by measuring the DC resistance of the coil.
2. Short Circuit Between Winding Turns
Due to insulation damage between adjacent turns of the winding, a closed short-circuit current will form. Simultaneously, the reduced number of turns in this phase’s coil leads to high heat generated by the short-circuit circulating current, causing excessive temperature rise in the transformer and potentially burning it out.
Many factors can cause inter-turn short circuits, such as mechanical damage to the insulation due to rough manufacturing processes; insulation aging caused by high temperatures; and axial displacement of the turns under electrodynamic forces, leading to insulation wear. However, the main causes of inter-turn short circuits are overvoltage and overcurrent. Severe inter-turn short circuits cause the oil temperature to rise, and the oil at the short-circuited turn appears to boil, producing a gurgling sound. Oil samples taken for testing show deterioration in oil quality, progressing from a light gas trip to a heavy gas trip. At this point, inter-turn short circuits can also be detected by measuring DC resistance.
3. Short Circuit Between Silicon Steel Sheets in the Core
Due to external damage or insulation aging, the insulation between the silicon steel sheets is damaged, increasing eddy currents and causing localized overheating. In addition, damaged insulation of the through-core screw is also a cause of eddy currents. In mild cases, it causes localized heating, and the rise in transformer oil temperature is generally not observable; in severe cases, it causes the core to overheat, the oil temperature to rise, frequent activation of the light gas system, and a decrease in the oil’s flash point; in extreme cases, the heavy gas system will activate.
4. Oil shortage or blockage in the cooling pipes: Transformer oil is the main insulation inside the transformer, playing a role in insulation, cooling, and arc extinguishing. If there is a shortage of oil or blockage in the cooling pipes, the oil’s circulation and cooling rate decreases, leading to an increase in transformer temperature during operation.
V. External causes of excessive transformer temperature
1. Severe overload: During transformer operation, the hysteresis loss, eddy current loss, and copper loss of the coils are all converted into heat, causing the temperature to rise. Iron loss is a relatively constant loss, related to the transformer structure, and therefore cannot be reduced or eliminated during operation. However, copper loss varies with the load; under severe overload, the copper loss increases, leading to an increase in transformer temperature.
2. Blocked or Severely Dust-Accumulated Air Inlets and Outlets in the Transformer Room: The air inlets and outlets of the transformer room are the channels for air convection during transformer operation. If they are blocked or severely dust-accumulated, the transformer’s heat dissipation conditions deteriorate while its heating conditions remain unchanged, preventing timely heat dissipation to the surrounding environment and causing the transformer’s operating temperature to rise. Measure the air temperature at a distance of 1.5m to 2m from the transformer tank, at a height half the height of the tank. If the measured temperature is 8-10℃ higher than the ambient air temperature, the ventilation in the transformer room should be improved.
3. Transformer Cooling Circulation System Failure: In addition to heat dissipation pipes, power transformers also use forced air cooling, forced oil circulation, and water circulation for cooling. Failure of the cooling system or poor heat dissipation conditions will cause the transformer’s operating temperature to rise.