Views: 2 Author: Site Editor Publish Time: 2018-04-09 Origin: Site
The varistor has a large stray capacitance, when it is used in the protection of AC power system, it often forms a significant leakage current test in the normal start-up condition. For example, a varistor with a stray capacitance of 2000pf is placed in a 220 V / V / 50 Hz AC power system with a leakage current of up to 0.14 Ma. Such a large leakage current will often have an effect on the normal operation of the system. When the varistor and ceramic gas discharge wire are connected in series, the total capacitance of the whole series branch can be reduced to several μ F because the stray capacitance of the gas discharge tube is very small.
In this series patchwork branch, the discharge tube acts on a switch, and when there is no transient overvoltage, it can separate the varistor from the system, making the varistor almost free of leakage current. This reduces the reference unstructured voltage of the varistor. . The selection of varistor should be able to ensure the cut off of the arc area of the discharge current. When the discharge tube is on in the arc region, the voltage on both sides of the discharge tube is very low, only about 15 V, so the peak voltage of the whole series branch can be regarded as falling on the varistor.
It can be concluded that a conservative method is that the limited operating voltage of the system should be reduced to the varistor, and the current in the current varistor should be slightly smaller in the discharge tube arc area. This helps to effectively stop the freewheeling of the arc zone during the transient overvoltage.