Leakage current and insulation resistance

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

    (1) The test voltage is high and can be adjusted at will. When measuring the leakage current, a corresponding test voltage is applied to the tested 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 weakness of the insulation itself. Because certain defects or weaknesses in the insulation can only be exposed under higher electric field strength.

    (2) Leakage current can be monitored at any time by a microammeter, with high sensitivity and good measurement repeatability. For example, for a certain station

    VMNT-220 oil-less circuit breaker, the insulation resistance of each phase measured with a megger is above 10000MΩ, when the 40kV DC leakage current is measured, the three-phase current is significantly asymmetrical, two of which are 2μA, the other The phase is 60μA. Finally, it is found that the supporting porcelain sleeve of this phase has cracks.

    (3) The insulation resistance value can be converted according to the measured value of the leakage current, while the insulation resistance value measured by the megger cannot be converted into the leakage current value. Because the conversion must first know the voltage applied to the tested equipment. Although the megohmmeter has the specified voltage value engraved on the nameplate, the actual voltage applied to the tested equipment is not necessarily this value. The insulation resistance of the test equipment is related. When the insulation resistance of the tested equipment is very low, the voltage applied to the tested equipment is also very low. Only when the insulation resistance tends to infinity, the voltage applied to the tested equipment is close to the nameplate value. This is because when the insulation resistance of the device under test is too low, the internal resistance voltage drop of the megohmmeter causes the voltage on the "line" terminals to drop significantly.

    (4) The relationship curve of i=f(u) or i=f(t) can be used and the absorption ratio can be measured to judge insulation defects. The relationship curve between leakage current and pressurization 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 time, and the final steady-state value is also larger, that is, the insulation resistance is smaller.

Leakage current vs. pressurization time curve

1-good; 2-damp or defective

(Huiputech)