Many testers wondered why they should test by adding DC negative high voltage when doing DC high voltage generator test. We analyze the reasons for the following two points:

1. In an extremely uneven electric field, the breakdown voltage of the gas has a great relationship with the polarity of the charge carried by the electrode. In the gap between the same rod and the plate, the breakdown voltage when the rod is negatively charged is higher than that of the positive electrode. Time is more than twice as high, that is, in an uneven electric field, the relationship between the discharge voltage of the rod-to-board gap and the polarity of the rod voltage: the discharge voltage is high when the rod is negative.

The external insulation of electrical equipment is close to this extremely uneven electric field. Generally, the external insulation level of electrical equipment is higher than that of internal insulation, and internal insulation has more defects and failures than external insulation. DC withstand voltage test is performed on the equipment, although both internal and external insulation It has been tested, but the main thing is to check the internal insulation. The internal insulation is mainly liquid and solid materials, and inter-electrode shielding measures are adopted. The positive and negative voltages have little effect on the insulation breakdown. For example, the cable insulation is in the positive polarity. The breakdown is only 10% lower than the negative polarity. When a DC high voltage test voltage is applied to the equipment, it is generally not desirable to flashover on the external insulation. For this reason, a negative polarity DC voltage is used.

2. In the DC high voltage test, the moisture and moisture contained in the insulation of the inspection equipment are the key to the quality of the insulation. Due to the electrodialysis phenomenon of moisture (that is, water molecules have the nature of positive ions in the electric field). During the test, high voltage is applied to the live parts. Because the high voltage is of negative polarity, moisture is easily adsorbed near the high voltage pole, which makes it easy to detect defects in the test object.