Definition: The arrester which uses zinc oxide semiconductor as a resistor material, such type of arrester is known as a metal oxide surge arrester or ZnO Diverter. This arrester provides protection against all types of AC and DC over voltages. It is mainly used for overvoltage protection at all voltage levels in a power system.
Construction & Working of Metal Oxide Surge Arrester
The zinc oxide is a semiconducting material of N-type. It is pulverised and finely grained. More than ten doping materials are added in the form of fine powders of insulating oxides such as Bismuth (Bi2O3), Antimony Trioxide (Sb2O3), Cobalt Oxide(CoO), Manganese Oxide (MnO2), Chromium oxide (Cr2O3). The powder is treated with some processes, and the mixture is spray dried to obtain a dry powder.
The dry powder is compressed into disc-shaped blocks. The blocks are sintered to obtain a dense poly- crystalline ceramic.The metal oxide resistor disc is coated with a conducting compound to protect the disc from undesirable environmental effect.
The conducting coating also provides proper contacts and uniform current distribution. The disc then enclosed in a porcelain housing filled with nitrogen gas or SF6 gas. Silicon rubber is used to keep the disc in a position. It also helps in heat transfer from disc to the porcelain housing. The disc is held under pressure using suitable springs.
The ZnO element eliminates series sparks gaps in the diverter. The voltage drop in ZnO diverter takes place at the grain boundaries. There is a potential barrier at the boundary of the each grain of ZnO and this potential barrier control the flow of current from one grain to the next.
At normal voltage, the potential barrier does not allow the current to flow through it. At over voltage the barrier collapse and sharp transition of current from insulating to conducting state take place. The current start flowing and the surge is diverted to ground.
After the travelling of the surge, the voltage across the diverters falls, and the current is reduced to the negligible value of the resistor units, and there is no power follow current.
Advantages of Metal Oxide Surge Arrester
The metal oxide surge arrester has the following merits:
- It eliminates the risk of spark over and also the risk of shock to the system when the gaps break down.
- It eliminates the need of voltage grading system.
- At the normal operating condition, the leakage current in the ZnO is very low as compared to other diverters.
- There is no power follow current in ZnO diverter.
- It has high energy absorbing capability.
- ZnO diverters possess high stability during and after prolonged discharge.
- In ZnO diverter, it is possible to control the dynamic overvoltages in addition to switching surges. This results in economic insulation coordination.
Note: Sinter is the process of making a solid mass of material either by heating or by applying the pressure on the material without melting it