The so-called “partial discharge” refers to a kind of discharge in which only some areas in the insulation system discharge without forming a penetrating discharge channel under the action of an electric field. The main reason for partial discharge is that when the dielectric is uneven, the electric field strength borne by each area of the insulator is uneven. In some areas, the electric field strength reaches the breakdown field strength and discharge occurs, while other areas still maintain the characteristics of insulation. The insulation structure of large electrical equipment is complex, the materials used are diverse, and the electric field distribution of the whole insulation system is uneven.
Due to the imperfect design or manufacturing process, the insulation system contains air gap, or the insulation is damp during long-term operation, and the water decomposes under the action of electric field to produce gas and form bubbles. Because the dielectric constant of air is smaller than that of insulating materials, even if the insulating materials are under the action of a low electric field, the field strength of air gap bubbles will be very high, and partial discharge will occur when the field strength reaches a certain value. In addition, there are defects in the insulation or mixed with various impurities, or there are some poor electrical connections in the insulation structure, which will concentrate the local electric field. Where the electric field is concentrated, solid insulation surface discharge and suspended potential discharge may occur.
Causes of partial discharge
During the manufacturing process of the cable, if the insulation is mixed with metal impurities, pores and holes appear, or the uneven high voltage field strength is caused by the irregular protrusion of the inner and outer semiconductor layers, or the electrical tree in the insulation, partial discharge may occur at these parts. Because the dielectric constant of air is smaller than that of insulating materials, even if the insulating materials are under the action of a low electric field, the field strength of air gap bubbles will be very high, and partial discharge will occur when the field strength reaches a certain value.
Type of partial discharge
The common causes of internal partial discharge are air gap in insulator or bubble in liquid insulation. The mechanism of air gap discharge in insulation varies with the change of air pressure and electrode system. From the perspective of discharge process, it can be divided into electron impact ionization discharge and streamer discharge; In terms of discharge forms, it can be divided into pulse type (spark type) discharge and non pulse type (glow type) discharge.
At the high voltage end of electrical equipment, due to the concentration of electric field, the surface discharge field is strong and relatively low, which often produces surface partial discharge; The process and mechanism of insulator surface discharge is similar to that of air gap or bubble discharge in insulation, except that one end of the discharge space is insulating medium and the other end is electrode.
Corona discharge usually occurs when there is completely gas around the high-voltage conductor. Because the molecules in the gas move freely, the charged particles generated by the discharge will not be fixed at a certain position in space.
The above are the three most basic forms of partial discharge. In addition, the presence of water droplets in the insulator, conductive impurities and suspended potential in the electrical equipment will also cause partial discharge; Solid surface partial discharge and corona discharge may also occur inside the liquid insulation.
Hazards of partial discharge
Partial discharge has a corrosive effect on the insulation structure, and its damage mechanism to the insulation has the following aspects:
① Charged particles (electrons, ions, etc.) impact the insulation and destroy its molecular structure, such as fiber fragmentation, so the insulation is damaged.
② Due to the impact of charged ions, the local temperature of the insulation increases, which is easy to cause overheating of the insulation, and carbonization will occur in serious cases.
③ Ozone and nitrogen oxides produced by partial discharge will erode the insulation, and nitric acid will be produced in case of water, which will erode the insulation more violently.
④ During partial discharge, the oil is decomposed due to electrolysis and Schottky radiation effect of electrode. In addition, there are some impurities in the oil, so it is easy to agglutinate the oil sludge generated by polymerization at the paper layer (mostly at the oil wedge of turn insulation or other insulation). The generation of oil sludge will increase the dielectric damage angle of insulation, reduce the heat dissipation capacity, and even lead to the possibility of thermal breakdown. The continuous development of partial discharge will gradually expand the deterioration and damage of insulation, and finally shorten the normal life of insulation, reduce the short-term insulation strength, and even break down the whole insulation.
The root cause of cable partial discharge is the unqualified cable quality. The long-term use of such cables is extremely easy to cause accidents. To avoid this, the most basic way is to select the national standard cables with national certification and quality assurance: in order to avoid partial discharge of cables, Zhujiang cable actively introduces advanced wire and cable manufacturing technology at home and abroad Production equipment and testing equipment always adhere to the selection of high-quality raw materials at home and abroad, and implement a strict quality control system to ensure the quality of the company’s products from the source. Each wire and cable meets the national standards and the standards of the International Electrotechnical Commission.