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The principle of measuring dc leakage current of an insulator is basically the same as that of measuring insulation resistance.
The difference is: dc leak test voltage is generally higher than megohmmeter voltage, and can be adjusted, megohmmeter, otherwise, it is higher than the effectiveness of the defects found by megger, sensitive to reflect the crack porcelain insulation, the interior of the sandwich insulation be affected with damp be affected with damp and local fracture, loose insulating oil degradation, char along the surface of the insulation, etc.
Dc voltage test and leakage current measurement although the method is the same, but its role is different, the former is to test the insulation resistance strength, the test voltage is higher; The latter is used to check the insulation condition, the test voltage is relatively low. Therefore, dc voltage resistance is of special significance to find some local defects, and is widely used in the preventive test of high-voltage motors, cables and capacitors. It has the following characteristics compared with ac pressure test.

1. Test equipment is light and small

Dc withstand voltage test equipment is relatively light and convenient for preventive testing in the field. For example, for cable lines, if ac withstand voltage test, capacitance current per kilometer will be several amperes, requiring larger capacity test equipment. When the dc voltage test is done, only the insulation leakage current (up to milliampere level) is supplied after stabilization.

2. Can measure leakage current at the same time

Dc withstand voltage test can reflect concentration defects in insulation more effectively by measuring leakage current while gradually boosting the voltage. Figure 3-1 shows some typical leakage current curves of generator insulation during dc voltage withstand test. For good insulation, the leakage current increases linearly with voltage and the current value is small, as shown in curve 1. If the insulation is damp, the current value increases, as shown in curve 2. Curve 3 indicates the presence of concentration defects in insulation. When the leakage current exceeds a certain standard, the cause should be identified as far as possible to eliminate. If the leakage current around 0.5 times Ut has risen rapidly, as shown in curve 4, then the generator is at risk of breakdown during operation (excluding overvoltage).

When dc voltage withstand test is carried out on power cables, the reading of leakage current is usually used to find defects. For example, when the difference of three-phase leakage current is too large or the leakage current increases rapidly, the test voltage can be increased or the duration of voltage withstand can be extended to find defects according to the specific situation.

3. Less damage to insulation

Dc high voltage has little damage to the insulation of the tested product. When the dc acting voltage is so high that partial discharge occurs in the air gap, the counter electric field induced by the charge generated by the discharge will weaken the field strength in the air gap, thus inhibiting the partial discharge process in the air gap. If it is ac voltage test, due to the constant change of the direction of the voltage, such as air gap discharge, each half wave of partial discharge, this discharge will often promote the decomposition of organic insulating materials, aging deterioration, reduce the insulation performance, so that the local defects gradually expand. Therefore, dc withstand voltage test also has the nature of non-destructive test to a certain extent.

Compared with ac withstand voltage test, the disadvantage of DC withstand voltage test is: because of the different voltage distribution inside the insulation under AC and DC, the test of DC withstand voltage test is not as close to the reality as that under AC. Therefore, for xLPE cable, it is not advocated to use DC voltage test, DC voltage test discharge is not easy to put clean, easy to lead to charge retention, damage the test.
The selection of dc voltage withstand test voltage is also an important problem, it is a reference to insulation power frequency AC withstand voltage and ac, DC breakdown strength ratio, and mainly based on operating experience to develop. For example, the stator winding of the generator is 2-2.5 times the rated voltage; For 3, 6, 10kV cables, take 5~6 times the rated voltage, for 20, 35kV cables, take 4~5 times the rated voltage, and for cables above 35kV, take 3 times the rated voltage. The time of dc voltage withstand test can be longer than that of AC voltage withstand test, so the generator test is to increase the rated voltage by 0.5 times of each stage in stages, and stay for 1min in each stage to observe and read the leakage current value. During the cable test, the test voltage should be continued for 5min to observe and read the leakage current value.