The main factors affecting insulation resistance are as follows:
1. Influence of Temperature
The temperature of electrical equipment in operation changes with the surrounding environment, and its insulation resistance also changes with temperature. Generally, insulation resistance decreases as temperature rises. The reason is that when temperature increases, the movement of ions and molecules inside the insulating medium intensifies, and the moisture, impurities, salts, and other substances contained in the insulation also tend to diffuse, increasing conductivity and reducing insulation resistance. This is different from how the resistance of conductors changes with temperature. Different electrical equipment and equipment made of different materials have different variations in insulation resistance with temperature, and it is difficult to ensure measurements are taken at nearly identical temperatures on site. To compare test results, some organizations have provided temperature conversion coefficients for certain equipment, but due to many influencing factors such as the degree of aging, dryness, and the temperature measurement methods used, it is difficult to obtain an accurate conversion coefficient. Therefore, when actually measuring insulation resistance, the test temperature (ambient temperature and equipment body temperature) must be recorded, and measurements should be taken at similar temperatures as much as possible to avoid errors caused by temperature conversion.
2. Influence of Humidity and Surface Contamination of Electrical Equipment
Changes in ambient humidity around electrical equipment and surface contamination caused by air pollution have a significant impact on insulation resistance. When the relative humidity of the air increases, the surface of the insulation adsorbs a lot of moisture, increasing surface conductivity and reducing insulation resistance. When a continuous water film forms on the surface of the insulation, the insulation resistance becomes even lower. For example, after rain, the insulation resistance of a set of 220kV magnetic blow surge arresters was measured to be only 2000MΩ; when the surface current was shielded, the insulation resistance was above 1000MΩ; the next afternoon in sunny weather, with the surface dry, the insulation resistance was also above 1000MΩ. Surface contamination of electrical equipment also greatly reduces the surface resistance, causing a significant drop in insulation resistance. According to the above two situations, when measuring insulation resistance on site, it is necessary to use a shielding ring to eliminate the influence of surface leakage current or to dry and clean the equipment surface to obtain true measurement values.
3. Influence of Residual Charge
Residual charges left from operation of large-capacity equipment or residual charges formed during tests that are not completely discharged can cause the insulation resistance to be too high or too low, resulting in inaccurate measured values. When the polarity of the residual charge is the same as that of the megohmmeter, the measured insulation resistance will be higher than the true value; when the polarity is opposite, the measured insulation resistance will be lower than the true value. The reason is that when polarities are the same, due to repulsion of like charges, the megohmmeter outputs fewer charges; when polarities are opposite, the megohmmeter must output more charges to neutralize the residual charge. To eliminate the influence of residual charge, it is necessary to fully ground and discharge before measuring insulation resistance, and also fully discharge during repeated measurements. For large-capacity equipment, discharge should last at least 5 minutes. For example, for a large-capacity transformer, after full discharge, the first measurement of the insulation resistance of a certain winding was 4000MΩ; the second measurement of the same winding (without full discharge) gave 5000MΩ; after full discharge for 10 minutes, the third measurement gave 4000MΩ.
