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The main factors that affect the measurement of the insulation resistance

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1, Effect of temperature

For the running equipment, the temperature changes with environment, the insulation resistance also change with temperature. Under normal circumstances, the insulation resistance decreases with temperature increasing. The reason is that, once the temperature has risen, the inner ion and molecular of insulating medium will move fast, and the moisture, impurities, salt and other substances which inside of insulator will in diffuse trend, the conductivity will increase, and insulation resistance will decrease too. This conductor resistance variation with changing temperature is not likely. As different electrical equipment with different materials, that the insulation resistance with changing temperature are also different, it is difficult to ensure live measurement in similar temperature. In order to compare the testing results, some temperature conversion factor was given by related units, however as there are many influence factors such as obsolescence degree of equipment, dry state and different temperature measurement methods, so it’s difficult to get an accurate conversion factor. Therefore when measuring insulation resistance, should record the testing temperature (ambient temperature and equipment body temperature), and measure at similar temperatures to avoid errors due to temperature conversion.

2, The effects of humidity and surface dirt

The ambient humidity changes and air pollution have great effect on the insulation resistance. The insulator surface will adsorb moisture with increasing humidity, the surface conductivity increase, and the insulation resistance decrease. When the insulator surface form a water film, the insulation resistance will be more lower. After raining, the measured insulation resistance of 220kV magnetic blowout arrester is only 2000MΩ; when the surface current shielded, the insulation resistance is more than 1000MΩ; in the next sunny afternoon, under dry condition, the measured insulation resistance is more than 1000MΩ. The dirty surfaces of electrical equipment also can reduce the surface resistance and insulation resistance greatly. In these two cases, when measuring the insulation resistance, should shield the leaked current and clean the surface of device, in order to get the true measurement value.

3, The effects of residual charge

The remaining residual charge will make the insulation resistance is too large or too small, and lead to untruthful value of measured insulation resistance. When the polarity of residual charge is same as megger polarity, the measured insulation resistance value is large than the real value; while when the polarity of residual charge is opposite with megger polarity, the measured insulation resistance value is smaller than real value. The reason is that, when in same polarity, like poles repel, the megger will output less charge; when in opposite polarity, the megger need to output more charge to neutralize the residual charge. To eliminate the effects of residual charge, it should be fully grounding repeatedly before measure the insulation resistance, and for the mass storage equipment, it should be charged for 5 minutes at least. For large-capacity transformer, the first measured resistance value is 4000MΩ after fully discharged, then make the second measurement to the same winding (not fully discharged), the measured insulation resistance is 5000MΩ; at last, make the third measurement with fully discharge for 10 minutes, the insulation resistance is 4000MΩ.
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