Insulation level of transformer

As an important electrical equipment in the power system, the insulation level of the transformer is directly related to the safe and stable operation of the power system. The insulation level is the ability of the transformer to withstand various overvoltages and long-term maximum working voltage during operation, and is a key factor that cannot be ignored in the design, manufacture, operation and maintenance of the transformer.

1. Definition of insulation level of transformer The insulation level refers to the ability of the insulation structure of the transformer to maintain integrity and safety when it withstands various overvoltages and long-term working voltages. This includes the voltage level that can be tolerated in conjunction with the protective lightning arrester and directly depends on the maximum voltage Um of the equipment. 

2. Insulation structure of transformer According to whether the insulation level of the winding line end and the neutral point is the same, the transformer can be divided into two insulation structures: full insulation and graded insulation. The transformer with full insulation structure has the same insulation level of the winding line end and the neutral point, has a higher insulation margin, and is suitable for transformers with high voltage levels and complex operating environments. The transformer with graded insulation structure sets different insulation levels between the winding line end and the neutral point according to actual needs to optimize the insulation design and reduce costs.

3. Testing of the insulation level of the transformer To ensure that the insulation level of the transformer meets the design requirements, a series of insulation tests are required. For transformers with a voltage level of 220kV and below, a 1-minute power frequency withstand voltage test and an impulse voltage test are usually carried out to assess their insulation strength. For transformers with higher voltage levels, more complex impulse tests are also required. In factory tests, a withstand voltage test is often carried out at more than twice the rated voltage to simultaneously assess the insulation performance of the main insulation and longitudinal insulation.

In addition, measuring the insulation resistance, absorption ratio and polarization index of the winding together with the bushing is also an important means to evaluate the overall insulation condition of the transformer. These measurements can effectively detect the overall moisture of the transformer insulation, the moisture or dirt on the surface of the components, and the concentrated defects of penetration.

4. Factors affecting the insulation level of the transformer During the operation of the transformer, the factors that affect the insulation level mainly include temperature, humidity, oil protection method and overvoltage effect. 1) Temperature: Temperature is a key factor affecting the insulation performance of the transformer. The insulation performance of the insulating material decreases with the increase of temperature, and the presence of moisture in the oil will also accelerate the aging of the insulation. Therefore, controlling the operating temperature of the transformer and maintainingthe good condition of the insulating material are important measures to improve the insulation level.

2) Humidity: The presence of humidity will accelerate the aging of the insulating material and reduce its insulation performance. Therefore, during the operation of the transformer, the ambient humidity should be strictly controlled to prevent the insulating material from getting damp.

3) Oil protection method: Different oil protection methods have different effects on the insulation performance. Since the oil surface of the sealed transformer is insulated from the air, it can effectively prevent the volatilization and diffusion of CO and CO2 in the oil, thereby maintaining the good performance of the insulating oil.

4) Overvoltage effect: Overvoltage effect is another important factor affecting the insulation level of the transformer. Both lightning overvoltage and working overvoltage may cause damage to the insulation structure of the transformer. Therefore, when designing and operating the transformer, the impact of overvoltage must be fully considered and corresponding protection measures must be taken.