Factors affecting insulation resistance

1. Influence of temperature

The temperature of the running power equipment will change with the surrounding environment, and its insulation resistance will also change with temperature. In general, the insulation resistance decreases with the increase of temperature. Because when the temperature rises, the movement of ions and molecules inside the insulating medium intensifies, and the moisture in the insulation and the impurities, salts and other substances contained therein also tend to diffuse, which increases the conductivity and reduces the insulation resistance. This is different from the change of the resistance of the conductor with temperature.

Different power equipment and the same power equipment made of different materials have different insulation resistance changes with temperature. It is difficult to ensure that the on-site measurement is carried out at a completely similar temperature. In order to compare the test results, the relevant units have given the temperature conversion coefficients of some equipment, but due to the influence of the obsolescence, dryness, and temperature measurement methods used, it is difficult to obtain an accurate conversion coefficient.

Therefore, when measuring the insulation resistance, the test temperature (ambient temperature and equipment body temperature) must be recorded, and the measurement should be carried out at a similar temperature as much as possible to avoid errors caused by temperature conversion.

2. Influence of humidity and surface dirt on power equipment

The changes in humidity around power equipment and surface dirt caused by air pollution have a great influence on insulation resistance. When the relative humidity of the air increases, the surface of the insulation will absorb a lot of water, which will increase the surface conductivity and reduce the insulation resistance. When a connected water film is formed on the surface of the insulation, the insulation resistance is lower. For example, after the rain, the insulation resistance of a group of 220kV magnetic blow arresters was only 2000MΩ: when the surface current was shielded and measured again, the insulation resistance was more than 10,000MΩ; the next afternoon, it was sunny, and the insulation resistance was also more than 10,000MΩ when the surface was dry. Dirt on the surface of power equipment will also greatly reduce the surface resistance of the equipment and significantly reduce the insulation resistance. Therefore, when measuring the insulation resistance on site, a shielding ring must be used to eliminate the influence of the surface leakage current, or dry or clean the surface of the equipment to obtain the true measurement value. 3. The influence of residual charge The residual charge left over from the operation of large-capacity equipment or the residual charge formed during the test is not completely discharged, which will cause the insulation resistance to be too large or too small. This causes the measured insulation resistance to be untrue. When the polarity of the residual charge is the same as that of the insulation resistance meter, the measured insulation resistance will be larger than the true value; when the polarity of the residual charge is opposite to that of the insulation resistance meter, the measured insulation resistance will be smaller than the true value. Because when the polarity is the same, the insulation resistance meter outputs less charge due to like charges repelling each other; when the polarity is opposite, the insulation resistance meter needs to output more charge to neutralize the residual charge.

In order to eliminate the influence of residual charge, the insulation resistance must be fully grounded and discharged before measuring the insulation resistance. It should also be fully discharged when repeated measurements are made. Large-capacity equipment should be virtually discharged for at least 5 minutes. For example, a large-capacity transformer has a winding insulation resistance of 4000MΩ for the first time after full discharge. The same winding is measured again for the second time (not fully discharged), and the insulation resistance is 5000MΩ. After 10 minutes of full discharge, the insulation resistance is 4000MΩ for the third measurement.

IV. Influence of induced voltage

In the on-site preventive test, due to the capacitive coupling between the live equipment and the power-off equipment, the power-off equipment carries an induced voltage of a certain voltage level.

Induced voltage has a great influence on insulation resistance measurement. When the induced voltage is strong, the insulation resistance meter may be damaged or the pointer may swing randomly, and the true measurement value cannot be obtained.

For example, a 220kV metal oxide arrester composed of two sections, the insulation resistance of the upper section is measured to be 50,000MΩ, and the insulation resistance of the lower section is 20,000MΩ. The end of the upper section is grounded, and the voltage is measured from the middle (measuring the insulation resistance value of the upper and lower sections in parallel). Due to the reduction of the induced voltage, the measured insulation resistance is 100,000MΩ. For another example, the induced voltage of a certain phase of a 220kV current transformer is strong. When measuring the insulation resistance of the primary to the end screen, the pointer swings around 500MΩ. When the high-voltage lead is grounded, the insulation resistance of the end screen to the primary and the ground is measured with the same insulation resistance meter, and the insulation resistance is 2000MΩ.

It can be seen that the influence of induced voltage on insulation resistance is great. When measuring insulation resistance, measures such as electric field shielding should be taken if necessary to overcome the influence of induced voltage.

5. Influence of the maximum output current value of the insulation resistance meter

The maximum output current value of the insulation resistance meter (measured by short-circuiting the output end of the milliammeter) has a certain influence on the measurement of the absorption ratio and polarization index. Therefore, a large-capacity insulation resistance meter should be used to measure the absorption ratio and polarization index, that is, a meter with a maximum output current of 1mA or more should be selected. For large power transformers, a meter with a maximum output current of 3mA or more should be selected.