The wiring and principle of the DC leakage current test are the same as those of the DC withstand voltage test, and they are often carried out simultaneously. The principle of leakage current measurement is basically the same as that of insulation resistance measurement, except that the power supply used in leakage current measurement is generally an adjustable DC high-voltage device, and a microammeter is used to directly measure the current flowing through the test object. Compared with insulation resistance measurement, leakage current measurement has the following advantages:
(1) The test voltage is higher and can be adjusted at will. According to the different voltage levels of the test object, a corresponding DC test voltage is applied, which is much higher than the voltage of a megohmmeter. For example, for the primary winding of a 110kV transformer, a DC high voltage of 40kV needs to be applied. Therefore, measuring leakage current is more likely to detect certain insulation defects (such as porcelain insulation cracks, local damage, insulation oil deterioration, insulation surface carbonization, etc.) than using a megohmmeter to measure insulation resistance.
(2) The use of a microammeter to monitor leakage current has high sensitivity and can be repeatedly compared multiple times.
(3) The insulation resistance value can be converted from the measured leakage current value, but the insulation resistance value measured by a megohmmeter generally cannot be converted into a leakage current value. This is because, according to the load characteristics of the megohmmeter, the output terminal voltage of the megohmmeter is related to the insulation resistance value of the test object and may not necessarily be the standard voltage indicated on the megohmmeter nameplate.
(4) During the leakage current test, the relationship curve between leakage current and pressurization time and the relationship curve between leakage current and applied voltage can be plotted, and the insulation condition can be judged through these curves.
