Leakage Current and Insulation Resistance

The principle of measuring leakage current is essentially the same as that of measuring insulation resistance, and the nature of the defects that can be detected is also roughly the same. However, since the power supply used in leakage current measurement is generally supplied by high-voltage rectifier equipment, and the leakage current is directly read by a microammeter. Therefore, compared with insulation resistance measurement, it has its own characteristics as follows:

(1) The test voltage is high and can be adjusted at will. When measuring leakage current, the corresponding test voltage is applied to the test equipment of a certain voltage level. This test voltage is much higher than the rated voltage of the megohmmeter, so it is easy to expose the weaknesses of the insulation itself. Because some defects or weaknesses in insulation can only be exposed under higher electric field strength.

(2) The leakage current can be monitored at any time by the microammeter, with high sensitivity and good measurement repeatability. For example, for a VMNT-220 oil-free circuit breaker, the insulation resistance of each phase measured by a megohmmeter was above 10000MΩ. When measuring the 40kV DC leakage current, the three-phase current was significantly asymmetric, with two phases being 2μA and the other phase being 60μA. Finally, it was found that the porcelain sleeve supporting this phase had cracks. (3) The insulation resistance value can be converted based on the leakage current measurement value, while the insulation resistance value measured by a megohmmeter cannot be converted into the leakage current value. Because the conversion first requires the voltage applied to the device under test. Although the megohmmeter has a specified voltage value engraved on the nameplate, the actual voltage applied to the device under test is not necessarily this value, but is related to the insulation resistance of the device under test. When the insulation resistance of the device under test is very low, the voltage applied to the device under test is also very low. Only when the insulation resistance tends to infinity, the voltage applied to the device under test is close to the nameplate value. This is because when the insulation resistance of the equipment under test is too low, the internal resistance voltage drop of the megohmmeter causes the voltage on the "line" terminal to drop significantly.

(4) The insulation defect can be judged by using the relationship curve of i=f(u) or i=f(t) and measuring the absorption ratio. The relationship curve between leakage current and pressure application time is shown in the figure. Under the action of DC voltage, when the insulation is damp or defective, the current decreases slowly with the pressure application time, and the final steady-state value is also larger, that is, the insulation resistance is smaller.