Why should we measure the absorption ratio when measuring the insulation of power cables?

Some insulating objects (such as plastics, porcelain, etc.) can reach a stable value in an instant under the action of DC voltage. However, for electrical equipment such as generators, transformers, motors, cables, etc., their insulation is composed of composite media. Under the action of DC voltage, various polarization phenomena will occur. At the beginning of polarization, the current is very large. As the pressure increases, the current value decreases and the insulation resistance increases accordingly. This phenomenon is called absorption. In the absorption phenomenon, the current that decays the fastest is called capacitive current, the current that changes slowly over time is called absorption current, and the stable current that does not change with time is determined by the conductivity of the medium and is called conductivity current. Generally, the larger the capacity of the equipment, the more obvious this phenomenon is. Since the absorption current changes with time, the time must be specified when testing the insulation resistance and leakage current. When the insulation is damp or dirty, the leakage current increases and the absorption phenomenon is not obvious.

The factors that affect the insulation resistance measurement results are mainly temperature, humidity and discharge time. As the temperature rises, the polarization of the medium intensifies, resulting in an increase in conductivity and a decrease in resistance, so the insulation resistance decreases with the increase in temperature. The insulation resistance will be significantly reduced due to surface moisture absorption or the formation of a water film on the surface of the porcelain insulation. In addition, when the relative humidity is high, the insulation will absorb more moisture, which will increase the conductivity and reduce the insulation resistance. Testing the insulation resistance is equivalent to applying a DC high-voltage charge to the insulation, so the test piece is charged. After the test is completed, the test piece should be fully discharged, and the discharge time should be greater than the charging time, so that the residual charge cannot be fully discharged, and the charging current and absorption current obtained in the repeated measurement are smaller than the previous measurement value, thereby causing the absorption ratio to decrease and the insulation resistance value to increase.

During the absorption ratio test, release the test key according to the required test results (normal test, absorption ratio test, polarization index test). At this time, the intelligent dual-display insulation resistance tester stops high voltage output and automatically calculates and displays the test results (resistance value, absorption ratio, polarization index at each time state). Press the up or down key to display the test results (current resistance value, 15-second resistance value, 60-second resistance value, 10-minute resistance value, absorption ratio, polarization index) in a cycle. The DC2010 intelligent dual-display insulation resistance tester produced by Dingsheng Electric Power uses an embedded industrial single-chip microcomputer and a real-time operation software system. It has a high degree of automation and strong anti-interference ability. It can automatically calculate the absorption ratio and polarization index without manual intervention.
Note: The absorption ratio test time should be greater than 1 minute, and the polarization index test should be greater than 10 minutes. The maximum time for each measurement is 10 minutes. After 10 minutes, the instrument will automatically turn off the high voltage and automatically calculate and display the test results.