Selection of flow

As a widely used varistor, surge protection varistors are usually used to protect thyristors. Among them, 3KA varistor model is usually used for overvoltage absorption of electrical equipment, and 5ka resistance model is mainly used for lightning overvoltage absorption and electronic equipment. 10kA varistor will be used for lightning protection. Here we will take lightning protection as an example. Generally, during the test, we will select the common composite wave (1.2/50, when the generator is open circuit output μ Voltage wave s; 8 / 20 at short circuit output μ Current wave s; The internal resistance of the generator is 2 Ω) to evaluate the on-line capability of the equipment. In the 4KV test, the maximum current absorbed by the surge protection rheostat can reach 2kA, while in the 6kV test, the maximum current absorbed is 3KA. However, in actual selection, the current capacity of the selected surge protection rheostat shall be increased accordingly.

Inherent parasitic capacitance

Like other resistance elements, the surge protection varistor also requires technicians to consider its inherent parasitic capacity in the actual application process. According to the size and nominal voltage, the value of the inherent parasitic capacitance is hundreds to thousands of PF, which directly determines that it is not suitable for use at high frequency, otherwise it will affect the normal operation of the system.

Selection of varistor voltage

In the process of circuit design and component selection, the selection of varistor voltage also needs to be taken seriously by engineers. In the current application, the deviation between the actual voltage sensitive to the pressure of the surge protection rheostat and the nominal voltage should be considered as 1.1.1.2 times of the nominal voltage. However, the possible range of voltage fluctuation in the AC circuit should be considered as about 1.4.1.5 times of the rated voltage. The relationship between the AC peak value and the effective value is usually maintained at about 1.4 times of the pressure-sensitive surge protection (2.2.2)