The construction of a gapless zinc oxide lightning arrester is quite simple. The zinc oxide lightning arrester comes in the family of metal oxide lightning arresters. A specific number of zinc oxide cylindrical discs stacked together in a column forms the main structure of the arrester. A polymer or porcelain cylinder holds the stack of blocks inside it. The metal covers fitted at both ends of the porcelain or polymer cylinder keep the blocks in place with spring pressure. The remaining part inside the cylinder is filled with fiber-reinforced composite material. The studs projected from the top and bottom cover of the zinc oxide lightning arrester facilitate the connections for the device with line and earth.
Zinc oxide blocks used in the device have very high nonlinear v-i characteristics. The nonlinear resistance is an inherent characteristic of metal oxide substances. The zinc oxide block possesses this property very prominently. Further, 10 to 5 % of other materials like alumina, antimony, trioxide bismuth oxide, cobalt oxide, zirconium improves the nonlinearity of resistance of zinc oxide blocks.
The current carrying capacity of a zinc oxide block depends on its cross-sectional area. The v-i characteristics of zinc oxide gapless zinc oxide lightning arrester can be represented with the following equation.
Where I and V are the reference current and reference voltage respectively. The value of α is 30 to 40 for zinc oxide blocks.
From the above characteristics curve, we can see that in the normal voltage range the current through the arrester changes linearly. Also, within the normal voltage limit, the current through the device changes in the milliampere range with a significant change in the voltage. But in the nonlinear region of the carve, the current changes in kiloampere range with a small change in voltage. Therefore it proves that due to this nonlinearity in the resistance of the device, zinc oxide blocks provide effective protection against an electric surge for overvoltage.
Effect of Temperature on the Performance of Gapless Zinc Oxide Lightning Arrester
The nonlinear characteristic of metal oxide highly depends on the temperature. Zinc oxide like other metal oxides has a negative temperature coefficient of resistance. Therefore the current through the device for a certain given voltage increases with increasing temperature. Even at normal operating voltage, the continuous leakage current through the lightning arrester increases with increasing temperature. This is the reason the thermal design of the arrester is so important for the proper dissipation of heat generated inside.