1. The effect of temperature

The temperature of the power equipment in operation changes with the surrounding environment, and its insulation resistance also changes with the temperature. Generally, the insulation resistance decreases with the increase of temperature. The reason is that the movement of ions and molecules inside the insulating medium increases when the temperature increases. , The moisture in the insulation and the impurities, salts and other substances contained in it also show a tendency to diffuse, which increases the conductivity and decreases the insulation resistance. This is different from the change in the resistance of the conductor with temperature. Different power equipment and different materials The insulation resistance varies with the temperature of the completed power equipment, and it is difficult to ensure that it is carried out at a completely approximate temperature. In order to compare the test results, the relevant units have given the temperature conversion coefficients of some equipment, but Due to the age of the equipment, the degree of dryness, and the temperature measurement method used, it is difficult to obtain an accurate conversion factor. Therefore, when actually measuring the insulation resistance, the test temperature (ambient temperature and equipment body temperature) must be recorded, and as much as possible It is possible to measure at similar temperatures to avoid errors caused by temperature conversion.

2. The influence of humidity and dirt on the surface of electrical equipment

The change of the environmental humidity around the power equipment and the surface contamination caused by air pollution have a great influence on the insulation resistance. When the relative humidity of the air increases, the surface of the insulation absorbs a lot of water, which increases the surface conductivity and decreases the insulation resistance. When a connected water film is formed, the insulation resistance is lower. For example, after the rain, the insulation resistance of a set of 220kV magnetic surge arresters is only 2000MΩ; when the surface current is shielded, the insulation resistance is more than 1000MΩ; When the surface is dry, the insulation resistance is also above 1000MΩ. The dirt on the surface of the power equipment greatly reduces the surface resistance of the equipment, and the insulation resistance is significantly reduced. According to the above two conditions, shielding rings must be used to eliminate the surface when measuring the insulation resistance on site. The effect of leakage current or drying and cleaning the surface of the equipment to obtain the true measurement value.

3. The effect of residual charge

The residual charge left in the operation of large-capacity equipment or the residual charge formed in the test is not completely discharged, which will cause the insulation resistance to be too large or too small, causing the measured insulation resistance to be untrue. The polarity of the residual charge is consistent with that of the megohmmeter. When the polarity is the same, the measured insulation resistance will be larger than the true value; when the polarity of the residual charge is opposite to the polarity of the megohmmeter, the measured insulation resistance will be lower than the true value, because when the polarity is the same , Due to the repulsion of the same sex, the megohmmeter outputs less charge; when the polarity is opposite, the megohmmeter should output more charges to neutralize the residual charge. In order to eliminate the influence of the residual charge, it must be fully grounded and discharged before measuring the insulation resistance. It should be fully discharged during the side measurement. Large-capacity equipment should be discharged for at least 5 minutes, such as a large-capacity transformer. After full discharge, the insulation resistance of its windings is measured to be 4000MΩ for the first time, and the same winding is tested for the second time (not fully discharged). ), the insulation resistance is 5000MΩ, and the third measurement after 10 minutes of discharge, the insulation resistance is 4000MΩ.