Overvoltage protectors, as crucial devices for suppressing transient and transient overvoltages in power systems, rely heavily on the electrical characteristics, thermal stability, and durability of their core materials. Different materials offer advantages in response speed, current carrying capacity, energy absorption characteristics, and service life. Appropriate material selection and composite application are key to improving the overall protection level of the protector.
Currently, the main materials used in overvoltage protectors include metal oxide varistors (MOVs), silicon carbide (SiC) varistors, gas discharge tube (GDT) filling media, discharge gap electrode materials, and composite functional layers. Among these, MOVs are the most widely used core component material, primarily composed of zinc oxide (ZnO) with small amounts of bismuth, cobalt, manganese, and other metal oxides sintered together. ZnO-based varistors exhibit excellent nonlinear current-voltage characteristics, displaying high resistance and minimal leakage current under normal operating voltage. When the voltage exceeds the threshold, the resistance rapidly decreases, forming a low-resistance channel, enabling nanosecond-level response and absorption of large current energy. Its advantages include high current capacity, fast response, and long lifespan, but it has certain requirements for thermal stability under sustained overvoltage, necessitating thermal tripping or heat dissipation design.
Silicon carbide (SiC) varistors were a common material in early overvoltage protectors, formed by pressing SiC particles with a ceramic binder and then sintering at high temperatures. SiC has a high critical electric field strength and good high-temperature performance, allowing it to operate at higher voltages. However, its nonlinear coefficient is lower than that of MOVs, resulting in a relatively slow response speed and larger leakage current. It is now mostly used in secondary protection combined with spark gaps or in special high-temperature environments.
Gas discharge tubes are made of inert gas (such as argon, neon, or a mixture of gases) and metal electrodes. Their working principle utilizes the breakdown of gas under high voltage to form an arc discharge, discharging overvoltage energy to ground. Gas discharge tubes have extremely high current capacity, capable of withstanding surge currents of tens of kiloamperes, but their response time is relatively long, and they may experience follow current problems in DC or low-frequency circuits. Therefore, they are often used in conjunction with MOVs or current-limiting resistors to compensate for the shortcomings of a single material.
The electrodes of the discharge gap are mostly made of copper-tungsten alloy, stainless steel, or silver-plated copper, aiming to balance high melting point, high conductivity, and resistance to arc erosion. The gap structure can quickly break down under high voltage to form a conductive path, offering advantages such as simple structure, low cost, and resistance to repeated impacts. However, its low-voltage protection accuracy is inferior to MOVs, and it is susceptible to aging due to environmental factors.
Composite functional layers have become a trend in recent years. By coating the surface of MOVs or SiC with high-temperature resistant, moisture-resistant, and flame-retardant polymer or ceramic coatings, or by adding a thermally conductive insulating substrate internally, mechanical strength, environmental adaptability, and heat dissipation performance are improved. This combination of materials can significantly improve the long-term reliability of the device in outdoor, chemical, and marine environments while maintaining the original electrical characteristics.
Overall, the material system of overvoltage protectors is dominated by MOVs, supplemented by SiC, gas discharge dielectrics, and metal electrodes, achieving the goals of rapid response, high-energy discharge, and durable operation through single or composite structures. The appropriate selection of different materials and process optimization enable the protector to cope with diverse overvoltage threats ranging from low-voltage civil facilities to high-voltage transmission systems, thus building a solid insulation protection barrier for electrical equipment.
