1. The cause of partial discharge

The so-called "partial discharge" refers to a discharge in which only a partial area of the insulation system is discharged under the action of an electric field without forming a through discharge channel. The main reason for partial discharge is that when the dielectric is not uniform, the electric field strength of each area of the insulator is uneven. In some areas, the electric field strength reaches the breakdown field strength and discharge occurs, while the other areas still maintain insulation characteristics. The insulation structure of large-scale electrical equipment is relatively complicated, the materials used are various, and the electric field distribution of the entire insulation system is very uneven. Due to the imperfect design or manufacturing process, the insulation system contains air gaps, or the insulation is damp during long-term operation, and the water decomposes under the action of the electric field to generate gas and form bubbles. Because the dielectric constant of air is smaller than that of insulating material, even if the insulating material is under the action of not too high electric field, the field strength of the air gap bubble 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 various impurities mixed in, or some electrical connections in the insulation structure are poor, which will cause the local electric field to concentrate, and the solid insulation surface discharge and floating potential may occur in the place where the electric field is concentrated.

2. Types of partial discharge

From the point of view of the location, discharge process and phenomenon of partial discharge, partial discharge can be divided into three types: internal discharge, surface discharge and corona discharge:

① Internal discharge

The common cause of internal partial discharge is the existence of air gaps in solid insulators or air bubbles in liquid insulation. The mechanism of the discharge in the internal air gap of the insulation varies with the change of the gas pressure and the electrode system. From the perspective of the discharge process, it can be divided into two types: electron impact ionization discharge and streamer discharge; in the form of discharge, it can be divided into pulse type (spark type) ) Discharge and non-pulse type (glow type) discharge are two basic forms. Generally, partial discharges are pulsed discharges, and a single separated discharge pulse can be observed at a certain phase of the external machining frequency voltage. In theory, the discharge pattern of internal discharge is symmetrical in the positive and negative half-waves of the power frequency, but the insulation resistance of the insulating material around the air gap or bubble is not infinite under ideal conditions, and the discharge may occur along the air gap. Or because of the surface discharge of the bubble wall, the actual positive and negative power frequency periodic discharge patterns are not completely symmetrical, and have a great relationship with the form of the electrode system: the more symmetrical the electrode system structure, the positive and negative power frequency periodic discharge patterns The more symmetrical.

② Surface discharge

At the high-voltage end of electrical equipment, due to the concentration of electric field and the relatively low creeping discharge field strength, surface partial discharges are often generated; the process and mechanism of surface discharge of insulators are similar to those of the internal air gap or bubble discharge of the insulation, but they are different. One end of the discharge space is an insulating medium, and the other end is an electrode. If the electrode system is asymmetric, the discharge patterns that occur in the positive and negative half waves of the power frequency are also asymmetric. When one end of the discharge is a high-voltage electrode, and the non-discharged electrode is grounded, the positive half-cycle discharge is large but the number of times is small, and the negative half-cycle discharge number is large but the discharge volume is small. If the electrode system is reversed, the discharge pattern is also reversed.

③ Corona discharge

Corona discharge usually occurs when the high-voltage conductor is completely surrounded by gas. 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. For the needle-plate electrode system, the field strength near the tip of the needle is the highest and discharge occurs. Because the negative polarity is easy to emit electrons, and the positive ions collide with the cathode to cause secondary electron emission, making the discharge appear first in the negative polarity. When the applied voltage is low, the corona discharge pulse appears near the 90° phase of the negative half cycle of the applied voltage, and is almost symmetrical to 90°; when the voltage rises, a small number of discharge pulses with large amplitude and small number appear in the positive half cycle.

The above are the three most basic forms of partial discharge. In addition, the presence of water droplets in the insulator, conductive impurities, and the presence of suspended potential bodies in electrical equipment can also cause partial discharges; solid surface partial discharges and corona discharges may also occur inside liquid insulation.

3. The harm 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, destroying its molecular structure, such as fiber fragmentation, and thus the insulation is damaged;

② Due to the impact of charged ions, the insulation will have a local temperature increase, which will easily cause overheating of the insulation, and carbonization will occur in severe cases;

③ Ozone (O3) and nitrogen oxides (NO, NO2) produced by partial discharge will corrode the insulation, and when exposed to moisture, it will produce nitric acid, which will erode the insulation more severely;

④ During partial discharge, the oil decomposes due to electrolysis and the Schottky radiation effect of the electrode, and there are some impurities in the oil, so it is easy to cause the sludge generated by polymerization to agglomerate at the paper layer (mostly in the winding insulation or At other insulating oil wedges), the formation of sludge will cause the dielectric damage angle tgδ of the insulation to increase sharply, reduce the heat dissipation capacity, and even cause the possibility of thermal breakdown. The continuous development of partial discharge will gradually expand the deterioration and damage of the insulation, which will eventually shorten the normal life of the insulation, reduce the short-term insulation strength, and may even cause the entire insulation breakdown.