Factors affecting insulation resistance

    1. The influence of temperature

    The temperature of the power equipment in operation will change with the surrounding environment, and its insulation resistance will also change with the temperature. In general, the insulation resistance decreases with increasing temperature. Because when the temperature rises, the movement of ions and molecules inside the insulating medium intensifies, and the moisture in the insulating material and the impurities and salts contained in it also show a tendency to diffuse, which increases the conductance and reduces the insulation resistance. This is not the same as the change in the resistance of a 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 approximate temperature. In order to compare the test results, the relevant units have given some equipment temperature conversion factors, but due to the age of the equipment, the degree of dryness, the temperature measurement method used, etc., it is difficult to get an accurate conversion factor.

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

    2, the impact of humidity and dirty power equipment

    The changes in the humidity of the environment 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 insulator will adsorb a lot of moisture, which will increase the surface conductivity and decrease the insulation resistance. When insulated

    When a connected water film is formed on the surface of the object, the insulation resistance is lower.

    For example, the insulation resistance of a set of 220kV magnetic surge arresters measured after rain is only 2000MΩ: when the surface current is shielded and then measured, the insulation resistance is more than 10 000MΩ; the next day in the afternoon on a sunny day, the surface is measured in a dry state The insulation resistance is also above 10 000MΩ.

    The dirt on the surface of power equipment will also greatly reduce the surface resistance of the equipment, and the insulation resistance will drop significantly. Therefore, shielding rings must be used to eliminate the effect of surface leakage current when measuring insulation resistance on site, or to dry or clean the surface of the equipment to obtain the true measurement value.

    3. The influence of residual charge

    The residual charge left in the operation of large-capacity equipment or the residual charge in the test is not completely discharged, which will cause the insulation resistance to be too large or too small. Cause the measured insulation resistance to be untrue. When the polarity of the residual charge is the same as the polarity of the insulation resistance meter, the measured insulation resistance will be greater than the true value; when the polarity of the residual charge is opposite to the polarity of the insulation resistance meter, the measured insulation resistance will be greater than the true value. The value decreases. Because when the polarity is the same, the insulation resistance meter will output less charge due to the repulsion of the same sex; when the polarity is opposite, the insulation resistance meter will 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. When repeating the measurement, it must be fully discharged. The large-capacity equipment shall be discharged for at least 5 minutes, such as a large-capacity transformer. The insulation of one of its windings is measured for the first time after full discharge. The resistance is 4000MΩ, the same winding is tested again for the second time (not fully discharged), the insulation resistance is 5000MΩ, and the third time after the full discharge is 10min, the insulation resistance is 4000MΩ.

    4, the influence of induced voltage

    In the field preventive test, due to the capacitive coupling between the live equipment and the power-off equipment, the power-off equipment has a certain voltage level of induced voltage.

    The induced voltage has a great influence on the insulation resistance measurement. When the induced voltage is strong, it may damage the insulation resistance meter or cause the pointer to oscillate, and the true measurement value cannot be obtained.

    For example, a 220kV metal oxide arrester composed of two sections, measuring the insulation resistance of the upper section is 50 000MΩ, and the insulation resistance of the lower section is 20 000MΩ, connect the end of the upper section to ground, and measure from the middle section (measure the upper and lower sections in parallel. Resistance value), due to the reduced induced voltage, the measured insulation resistance is 100 000 MΩ. Another example is a 220kV current transformer with a strong induced voltage on the high-voltage lead of a certain phase. When measuring the insulation resistance of the last screen, the pointer swings around 500MΩ, and the high-voltage lead is grounded. Use the same insulation resistance meter to measure the insulation of the last screen to the primary and ground. In the case of resistance, the insulation resistance is 2000MΩ.

    It can be seen that the induced voltage has a great influence on the insulation resistance. Measure the insulation resistance, and if necessary, take measures such as electric field shielding to overcome the influence of the induced voltage.

    5, the influence of the maximum output current value of the insulation resistance meter

    The maximum output current value of the insulation resistance meter (the output terminal is measured by a milliamp meter short-circuit) has a certain influence on the absorption ratio and polarization index measurement. 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 and above should be used, and a meter with a maximum output current of 3mA and above should be used for large power transformers.

(Huiputech)