Operation Principle of Power Transformer_ Rated Operation Mode of Power Transformer

Operation principle of power transformer1. Operating temperature of transformer

During the operation of the transformer, copper loss and iron loss will be generated, and these two parts of loss will finally be transformed into heat to raise the temperature of the transformer. Most power transformers in China adopt class a insulation. In the process of heat transmission during transformer operation, the temperature of each part varies greatly. The temperature of winding is the highest, followed by the temperature of iron core, and then the temperature of insulating oil. Moreover, the oil temperature of the upper layer is higher than that of the lower layer. The allowable temperature during transformer operation is determined by the upper oil temperature. Class a insulated transformer shall be adopted. In normal operation, when the surrounding temperature is 40 , it is required that the maximum upper oil temperature of the transformer shall not exceed 85 .

2. Temperature rise during transformer operationThe difference between the transformer temperature and the surrounding medium temperature is called the temperature rise of the transformer. Because the temperature of each part of the transformer varies greatly, it will affect the insulation of the transformer. Moreover, when the temperature of the transformer increases, the loss of the winding will increase. Therefore, it is necessary to specify the temperature rise of each part of the transformer at rated load. For transformers with class a insulation, when the ambient temperature is 40 , the allowable temperature rise of upper oil is 55 and that of winding is 65 .3. Voltage variation range during transformer operation

In the power system, due to the voltage fluctuation of the power grid, the voltage added to the transformer winding will also change. When the voltage of the power grid is less than the rated voltage of the tap used by the transformer, there is no damage to the transformer; When the voltage of the power grid is higher than the rated voltage of the tap of the transformer, the temperature of the transformer winding will rise, the reactive power consumed by the transformer will increase, and the waveform of the secondary coil will be distorted. Therefore, it is generally appropriate that the power supply voltage of the transformer does not exceed 5% of the rated voltage of the tap.

4. Requirements for parallel operation of transformersThe primary windings of two or more transformers are connected in parallel to the common power supply, and the secondary windings are also connected in parallel to supply power to the load. This method is called transformer parallel operation. In the current power system, with the increase of system capacity, the parallel operation of transformers is very necessary.Rated operation mode of power transformer

1. Allowable range of transformer voltage variationWhen the voltage is lower than the rated voltage of the transformer, it has no great 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.

The voltage deviation is within 5%. When the primary voltage of the transformer is within 5% of the rated value, the secondary side can operate with rated output.2. Heating and cooling of transformerAll the electric energy losses in the winding and iron core are converted into heat, which increases the temperature of each part of the transformer, and the temperature rise of each part is not uniform.

Heat dissipation process: the heat is transferred from the winding and iron core to the transformer oil, and the heat of the hot oil is transferred to the surrounding air through the radiator.Characteristics of heat dissipation process: all heat generated by iron core, high-voltage winding and low-voltage winding is transmitted to oil; In the whole heating and heat dissipation process, the temperature difference of each part is large, and the winding temperature is the highest; The large capacity transformer has large loss and heat generation. Forced oil circulation air cooling or forced oil circulation water cooling shall be adopted. The larger capacity transformer adopts guided cooling. A certain oil circuit shall be set inside the high and low voltage winding, which can greatly improve the heat dissipation efficiency.3. Allowable temperature and temperature rise of transformer

The allowable temperature of the transformer depends on the insulating material. Oil immersed transformers are paper and oil insulated.When the maximum temperature of transformer winding is 98 , the service life is 20-30 years. During actual operation, the temperature at the highest point of winding temperature is generally less than 98 , and the service life of transformer can exceed 30 years.During transformer operation, the winding temperature and top oil temperature shall be monitored.

The transformer overtemperature protection configuration sends an alarm signal when the temperature exceeds the low temperature setting value, and trips when the temperature exceeds the high temperature setting value.4. Transformer overload capacity

During normal operation of transformer, the load rate of 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 service life of transformer with more loss during overload and less loss during under load can compensate each other. The normal overload capacity of transformer is formulated on the principle of not sacrificing normal service life. That is, in the whole time interval, as long as the insulation aging rate of transformer is less than or equal to 1, and the following conditions are met:

(1) During overload, the temperature of the hottest spot of the winding shall not exceed 140 , and the upper oil temperature shall not exceed 95 ;(2) The maximum overload of the transformer shall not exceed 50% of the rated load

When the system fails, the primary task is to ensure uninterrupted power supply, and the aging acceleration of transformer insulation is secondary. Accident overload is at the expense of transformer life. The allowable insulation aging rate is much higher than that under normal overload. Accident overload, also known as emergency load, is to make the transformer carry more load in a short time for emergency use. In order to ensure reliability, when determining the allowable value of transformer accident overload, the temperature of the hottest spot of winding in general accident overload shall not exceed 140 , and the load current shall not exceed twice the rated value.