When the DC high-voltage generator is tested, there are doubts why it is necessary to test by adding DC negative high voltage.

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 same rod-to-board gap, the breakdown voltage when the rod is negatively charged is more than twice as high as when it is positively charged. 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 the internal insulation, and the internal insulation has more defects and failures than the external insulation. Do a DC withstand voltage test on the equipment. Although both the internal and external insulation are tested, the main thing is to check the internal insulation. Liquid and solid materials, and the use of inter-electrode shielding measures, the positive and negative voltages have little effect on insulation breakdown. For example, the cable insulation is only 10% lower than the negative polarity in the positive polarity breakdown. When a DC high voltage test voltage is applied to the equipment, it is generally not expected that the external insulation will flashover. For this reason, a negative 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 insulation quality. Due to the electrodialysis phenomenon of moisture (that is, the water molecules have the nature of positive ions in the electric field). During the test, the high voltage is applied to the live parts. Because the high voltage is of negative polarity, the moisture is easily adsorbed near the high voltage pole, which makes it easy to detect the defects of the test object.