Zinc oxide varistor discs are the core components of a surge arrester; their non-linear voltage-current characteristics determine their protective performance. Under normal operating voltage, the discs remain in a high-resistance state, conducting virtually no current. When an overvoltage occurs in the system, the resistance drops sharply, allowing for the rapid discharge of energy.
This characteristic stems from the unique structure of zinc oxide grains and their intergranular boundaries: the grains themselves are highly conductive, while the intergranular layers act as insulators under low electric fields but undergo "tunneling breakdown" when subjected to high electric fields.
The insulating structure must be capable of withstanding both the system's operating voltage and lightning-induced surges. Common types include:
• Porcelain housings: These offer high mechanical strength but are heavy; they are typically used in fixed installations such as substations.
• Composite insulators: Featuring a silicone rubber sheath, these are lightweight and resistant to pollution-induced flashover, making them well-suited for use on transmission lines.
Regardless of the type, internal grading rings (voltage-equalizing rings) are required to optimize the electric field distribution and prevent partial discharge.
Metal components include the top and bottom flanges, connecting bolts, and current-discharge paths. The flanges are generally cast from aluminum alloy, which ensures mechanical strength while keeping weight low. Connecting hardware is galvanized to prevent corrosion, while the current-discharge paths are designed to ensure that the flow of lightning current does not result in an excessive temperature rise.