The cause of partial discharge in high-voltage switchgear

In the operation of power equipment, when partial discharge occurs, various charged particles such as electrons, positive and negative ions, generally have more energy than the bond energy of polymers under the stimulation of the electric field. These charged particles hit the dielectric air gap wall, breaking the insulator's stable chemical bonds. At the same time, the medium heating at the partial discharge point will produce extremely high temperatures, which will cause thermal cracking of the insulator or promote oxidative cracking, which will destroy the molecular structure of the insulator.


High-voltage switchgear is a combined electrical device consisting of a variety of materials, such as epoxy resin, sulfur hexafluoride, engineering plastics, etc. There are many reasons for this to cause partial discharge, and the common ones are as follows:


(1) Defects in the manufacturing process of insulation materials.


Due to the influence of various factors such as insulation structure and process, residual bubbles or impurities may exist in the insulator, and the dielectric constant of the gas is often smaller than the dielectric constant of liquid or solid materials. Under the alternating electric field, the distribution of the electric field intensity is inversely proportional to the dielectric constant. Therefore, if bubbles are present in a liquid or solid medium, the electric field intensity within the bubble is higher than the electric field strength of the surrounding medium, but the breakdown electric field strength is much lower than the electric field strength of the liquid or solid medium. In this way, the bubbles are first broken down and discharged, while the other dielectrics remain insulated, creating a typical partial discharge.


(2) The electric field distribution is uneven.


Burrs or sharp corners on the surface of the conductor can concentrate the electric field, resulting in partial discharge. However, metal particles can be carried into the manufacturing and assembly of switchgear. These particles inside the cabinet have a strong ability to accumulate charges, resulting in electric field distortion and discharge in this area; If the surface of the switchgear insulator is too dusty or not smooth enough, it will lead to insufficient creepage distance, resulting in partial discharge; Even on grounded shells, if the weld is rough, the surface has pores or slag, and the electric field is unevenly distributed, a partial discharge will occur.


(3) The dielectric is uneven.


With the development of insulation technology, high-voltage metal-enclosed switchgear with atmospheric insulation and composite insulation are the most widely used products. However, it is the use of composite insulation technology that leads to differences in the dielectric properties of the various parts of the insulation material. The bubbles and impurities introduced during the manufacture of insulating materials further distribute the medium unevenly, causing discharge in high-pressure environments.


In addition, busbar burrs, voltage, temperature, humidity, insulation conditions and the external environment can all affect partial discharge.


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