Electric strength of combined insulation

    There are many requirements for the insulation of high-voltage electrical equipment. In addition to excellent electrical properties, it also requires good thermal, mechanical and other physical and chemical properties. It is often difficult for a single type of dielectric to meet these requirements at the same time, so the actual The insulation structure is generally not a single insulating material, but a combination of multiple dielectrics. For example, the outer insulation of a transformer is composed of the outer porcelain sleeve of the bushing and the surrounding air, while the inner insulation is composed of a combination of solid and liquid media such as paper, cloth, bakelite cylinder, polymer, and transformer oil.

    The electrical strength of the combined insulation structure not only depends on the electrical characteristics of the various media used, but also has a lot to do with the proper coordination of the characteristics of the various media.

    The common form of combined insulation is laminated insulation composed of multiple dielectrics. Under the action of the applied voltage, the withstand voltage of each layer of the medium must be an important factor affecting the electrical strength of the combined insulation. The most ideal distribution principle of the voltage of each layer is to make the electric field strength of each layer of the combined insulation be proportional to its electric strength. In this case, the electrical strength of the entire composite insulation is the highest, and the use of various insulating materials is the most reasonable and full.

    The voltage that each layer of insulation bears is related to the characteristics of the insulating material and the type of applied voltage. For example, under DC voltage, the voltage shared by the insulation of each layer is proportional to its insulation resistance, that is, the electric field strength in each layer is inversely proportional to its conductivity; but under the action of power frequency AC and impulse voltage, the voltage shared by each layer It is inversely proportional to the capacitance of each layer, that is, the intensity of the electric field in each layer is inversely proportional to its dielectric constant. It can be seen that under DC voltage, materials with high electrical strength and high conductivity should be used where the electric field is the strongest; while under power frequency AC voltage, materials with high electrical strength and high dielectric constant should be used in the The place where the electric field is strongest.

    When using multiple media in combination, one important principle should also be noted, that is, to make their respective advantages and disadvantages complement each other, maximize their strengths and avoid weaknesses, and complement each other.

    However, the actual insulation structure is often very complicated, and the above-mentioned principles are often difficult to achieve at the same time. For example, in cables with impregnated paper as insulation, the dielectric constant and electrical strength of the cable paper are both greater than that of the mineral oil impregnant. Under the action of AC voltage, the electric field intensity in the paper layer is smaller than the electric field intensity in the oil layer, which is unreasonable and unfavorable. However, because the impregnation treatment can eliminate the voids and bubbles in the paper, this process must be adopted. Measures. On the contrary, under the action of DC voltage, since the voltage distribution at this time depends on the conductivity of the medium (inversely proportional), the conductivity of paper is much smaller than that of oil, so the electric field strength shared by the paper layer with higher electric strength It is also larger, which is obviously a reasonable and favorable voltage distribution condition.

    It should also be pointed out that in the composite insulation structure, the temperature of each part may have a large difference, so when discussing the voltage distribution in the composite insulation, we must also pay attention to the influence of temperature differences on the electrical characteristics and voltage distribution of various insulating materials .

(Huiputech)