Insulation material electrical properties

The electrical properties of the insulating material are mainly manifested in the electrical conductivity, dielectric properties and dielectric strength of the material under the action of an electric field. They are represented by four parameters: insulation resistivity ρ (or conductivity γ), relative permittivity εr, dielectric loss angle tan δ, and breakdown strength EB.

1. Insulation Resistivity and Insulation Resistance Any dielectric can not be an absolute insulator, there are always some charged particles, mainly intrinsic ions and impurity ions. Under the action of the electric field, they can make a directional movement, forming a leakage conduction current, which is also often referred to as a leakage current. The equivalent circuit of the insulating material under applied voltage is shown in Figure 2-1a; the current under the action of DC voltage is shown in Figure 2-1b. In the figure, the current Ii of the resistance branch is the leakage current; the current Ia flowing through the series branch of the capacitor and resistance is called the absorption current, which is the current formed by the slow polarization and the ion volume charge; the capacitance branch current IC This is called charging current and is a current composed of geometrical capacitance effects.

(1) During normal operation (steady state), the leakage current determines the conductivity of the insulating material. Therefore, the larger the leakage resistance branch, the better the insulation performance of the material.

(2) The increase in temperature, humidity, impurity content, and electromagnetic field strength will decrease the resistivity of the dielectric material.

2. Permittivity The dielectric constant is a performance parameter that indicates the dielectric polarization characteristics. The larger the dielectric constant, the stronger the polarization of the dielectric, and the more bound charges generated. The bound electric charge also generates an electric field, and the electric field always weakens the external electric field. Capacitors are now used to illustrate the physical meaning of permittivity. When the capacitor plate is vacuumed, its capacitance is Co, and when the plate is filled with some kind of dielectric, its capacitance becomes C, then the ratio of C to Co is the dielectric constant of the dielectric in the filled dielectric Later, due to the polarization of the dielectric, a bound charge appears close to the surface of the dielectric. Corresponding to this, the free charge on the plate is also correspondingly increased. After the dielectric is filled, more free charges are contained on the plate, indicating the capacitance. Was increased. Therefore, it can be seen that the relative dielectric constant is always greater than one. The dielectric constant of an insulating material varies depending on factors such as power supply frequency, temperature, and humidity. As the frequency increases, the dielectric constant decreases. As the temperature increases, the dielectric constant increases. However, when the temperature exceeds a certain limit, the polarization becomes more difficult due to the increased thermal motion, and the dielectric constant decreases. As the humidity increases, the dielectric constant of the dielectric increases significantly. Therefore, by measuring the dielectric constant, the degree of dielectric moisture can be judged. Atmospheric pressure has a significant influence on the dielectric constant of the gas material. As the pressure increases, the density increases and the relative dielectric increases.

3. Dielectric loss Under AC voltage, part of the electric energy in the dielectric is irreversibly transformed into heat energy, which is called dielectric loss. The energy consumed per unit of time is called dielectric loss power. Dielectric loss causes the medium to generate heat, which is the source of dielectric thermal breakdown. The phasor relationship between current and voltage when AC voltage is applied.

The phase difference φ of total current and voltage is the power factor angle of the dielectric. The angle δ of the power factor angle is called the dielectric loss angle.