Factors influencing the breakdown of solid media and measures to improve its electrical strength:
1) Voltage action time: the action time of external voltageBreakdown voltageTaking the commonly used electrical cardboard (size mm) as an example, as shown in Figure 2-19, it is in a wide range (area b) in the figureBreakdown voltageIt is independent of the voltage action time, only when the time is less than the microsecond level (area A).Breakdown voltageonly elevated. This is similar to the volt-second characteristic of gas discharges, and region B is in the electrical breakdown range, because heat and chemistry do not work at a very short time. In the C area, it belongs to the thermal breakdown range, because the breakdown when the AC voltage action time is longer, it is often the thermal process that plays a decisive role, and the longer the voltage action time, the lower the breakdown value.
2) Temperature: As shown in Figure 2-20, when the temperature is below the t0 valueBreakdown field strengthVery high, and almost independent of temperature, it is an electrical breakdown; When above the t0 value, the higher the ambient temperature, the worse the heat dissipation conditionsBreakdown voltagethe lower. The t0 value of this transition temperature is different for different materials, and even if the same material is thicker and heat dissipation is difficult, the t0 value may appear at lower temperatures. That is, thermal breakdown occurs at lower temperatures.
3) Uniformity of electric field: uniform and dense material, in a uniform electric field, the dielectric withstand voltage is higherBreakdown voltageThere is a linear relationship with the thickness; In an inhomogeneous electric field,Breakdown voltageIt decreases greatly, and increases very slowly with the increase of thickness, as shown in Figure 2-21, when the thickness increases, heat dissipation is difficult, and thermal breakdown may occur, so it is of little significance to increase the thickness. The commonly used solid medium is often very uneven and dense, even in the uniform exit, due to the porosity or other defects will distort the electric field, the stomata will be free first, which is harmful to the solid medium. The insulating material that has been dried and impregnated can significantly improve itElectrical strength
4) Voltage type: under the same circumstances, the medium is under AC, DC and impulseBreakdown voltageoften differs in its impact coefficient (shockBreakdown voltagewith power frequencyBreakdown electricityPressThe ratio of amplitude) is often greater than 1, at direct currentBreakdown voltageIt is also often much higher than the power frequency, because the loss in the solid medium under DC is small, and the partial discharge is weak
When the applied voltage frequency is high, the partial discharge and loss are large, and the heating is serious, which can lead to thermal breakdown, or the chemical change brought about by partial discharge, so that the insulation is damaged, deteriorated, and electrochemical breakdown occurs.
5) Cumulative effect: In the inhomogeneous electric field, when the external voltage is higher, although a strong partial discharge has occurred, but because the voltage action time is very short, no breakdown is formed, and local damage is formed in the medium, so when the voltage is applied next time, the breakdown will be formed, so when the medium increases with the external impact or power frequency test voltage, itBreakdown voltagewill go down.
6) Dampness: The dampness of the medium will make itBreakdown voltagedecline, which is related to the performance of the medium itself. Medium that is easy to absorb moisture, after moisture absorptionBreakdown voltageThe drop is very large, so the insulation used under high pressure should pay attention to removing moisture in the manufacture and pay attention to moisture prevention in operation.
7) Mechanical load: Solid materials can sometimes encounter a large mechanical load in use, so that the material cracks, itsBreakdown electricityPressSignificantly reduced. For example, suspension insulators are subject to mechanical and electrical action when working, so they must undergo a joint electromechanical load test before leaving the factory.
In addition, organic solid materials may become brittle, crack and lose their elasticity due to heat and chemistry during operation, and can no longer be used as insulating materials.