Why do you use negative polarity pressure for DC voltage test?

When customers and friends make DC withstand voltage tests on electrical equipment, the use of negative pressure has been questioned. The analysis for this is as follows:

In the DC withstand voltage test of electrical equipment, the negative pressure is often used? To understand from two aspects:

1. In a very inhomogeneous electric field, the breakdown voltage of a gas has a great relationship with the polarity of the charge carried by the electrode. In the same rod-to-plate gap, the breakdown voltage when the rod is negatively charged is more than double that when it is positively charged. That is, in a non-uniform electric field, the relationship between the discharge voltage of the rod-to-plate gap and the polarity of the rod voltage is such that the discharge voltage is high when the rod is negatively charged.

The external insulation of electrical equipment is close to this extremely non-uniform electric field. The general external insulation level of electrical equipment is higher than that of internal insulation. The defects and failures of internal insulation are also more than external insulation. DC withstand voltage test is performed on the equipment. Although both internal and external insulation are tested, the main problem is to check the internal insulation. The internal insulation is mainly Liquid, solid materials, and the use of inter-electrode shielding measures, the positive and negative polarity of the voltage on the insulation breakdown of the impact. If the cable insulation is only 10% lower than the negative polarity in positive polarity breakdown. When the DC high voltage test voltage is applied to the device, the flashover of the external insulation is generally not desired. For this purpose, a negative DC voltage is used.

2. In the DC high voltage test, the insulation of the test equipment is mainly based on the moisture and moisture contained in the equipment is the key to affect the quality of the insulation, due to the phenomenon of water electrodialysis (water molecules have positive ions in the electric field), During the test, the high pressure is applied to the live parts. Since the high pressure is the negative polarity, the water is easily adsorbed near the high voltage electrode, so that the defect of the test object can be easily detected.