Transformers are very common in our daily life. The alternating current we use is stepped down through transformers. A transformer is composed of an iron core (or magnetic core) and coils. The coils have two or more windings, among which the winding connected to the power supply is called the primary coil, and the remaining windings are called secondary coils. It can transform alternating voltage, current and impedance. So, do you know how to maintain a transformer? Generally speaking, the temperature rise, temperature, capacity and parameters of a transformer should all be maintained within a reasonable range. Let's take a look at the detailed knowledge together next.
Key points for transformer maintenance
1. Allowable temperature
When a transformer is in operation, copper and iron losses occur in its coils and core. These losses are converted into heat energy, causing the temperature of the transformer's core and coils to rise. If the temperature exceeds the allowable value for a long time, the insulation will gradually lose its mechanical elasticity and age.
When a transformer is in operation, the temperatures of different parts are not the same. The temperature of the coil is the highest, followed by that of the core. The temperature of the insulating oil is lower than that of the coil and the core. The oil temperature at the top of the transformer is higher than that at the bottom. The allowable temperature during the operation of the transformer is checked based on the temperature of the upper oil layer. For transformers with Class A insulation during normal operation, when the ambient air temperature reaches a maximum of 40 ° C, the ultimate operating temperature of the transformer windings is 1050 ° C. As the temperature of the winding is 100 ° C higher than that of the oil, to prevent the deterioration of the oil quality, it is stipulated that the upper layer oil temperature of the transformer should not exceed 950 ° C. Under normal circumstances, to prevent the insulating oil from oxidizing too quickly, the upper layer oil temperature should not exceed 850 ° C. For transformers that adopt forced oil circulation water cooling and air cooling, the upper oil temperature should not frequently exceed 750 ° C.
2. Allowable temperature rise
Only monitoring the upper oil temperature of the transformer during operation cannot ensure its safe operation. It is also necessary to monitor the temperature difference between the upper oil temperature and the cooling air - that is, the temperature rise. The difference between the temperature of a transformer and that of the surrounding air is called the temperature rise of the transformer. For transformers with Class A insulation, when the maximum ambient temperature is 40 ° C, the national standard stipulates that the temperature rise of the windings is 650 ° C, and the allowable temperature rise of the upper oil layer is 550 ° C. As long as the temperature rise of the transformer does not exceed the specified value, it can ensure the safe operation of the transformer within the specified operating years under the rated load.
3. Reasonable capacity
During normal operation, the power load that the transformer bears should be around 75% to 90% of its rated capacity.
4. The maximum unbalanced current at low voltage of the transformer shall not exceed 25% of the rated value. The allowable range of voltage variation for the transformer power supply is ± 5% of the rated voltage
If the voltage exceeds this range, a tap changer should be used for adjustment to bring it within the specified range. When making adjustments, power should be cut off. Usually, voltage regulation is achieved by changing the position of the tap of the primary winding. The device that connects and switches the position of the tap is called a tap changer. It adjusts the transformation ratio by changing the number of turns of the high-voltage winding of the transformer. Low voltage has no effect on the transformer itself, only reducing the output to some extent, but it has an impact on the electrical equipment. When the voltage rises, the magnetic flux increases, the core becomes saturated, the core loss increases, and the transformer temperature rises.
5. Overload
Overload can be divided into two situations: normal overload and accident overload. Normal overload is caused by an increase in users' electricity consumption under normal power supply conditions. It will cause the transformer's temperature to rise, leading to accelerated aging of the transformer's insulation and a reduction in its service life. Therefore, under normal circumstances, overloading operation is not allowed. Under special circumstances, transformers can operate under overload for a short period of time, but in winter, they must not exceed 30% of the rated load, and in summer, they must not exceed 15% of the rated load. In addition, the over-load capacity of the transformer should be determined based on the temperature rise of the transformer and the manufacturer's regulations.
When an accident occurs in the power system or a user's substation, to ensure continuous power supply to important equipment, it is allowed for the transformer to operate under overload for a short period of time, that is, accident overload. When an accident overload occurs, the coil temperature will exceed the allowable value, thus causing the insulation to age faster than under normal conditions. However, the chance of an accident overloading is rare. Under normal circumstances, transformers operate under underload, so short-term overloading is the ultimate risk of damaging the transformer. The time and multiple of the accident overload shall be implemented in accordance with the regulations of the manufacturer.