Towards the overvoltage protection by modem ZnO surge arresters

Polymeric materials are used for the insulation of metal oxide (MO) surge arresters for medium and high voltage systems. The advantage of the MO-material and the polymeric materials, especially silicon, can be combined to get new apparatus and solutions for overvoltage protection. Starting with the established well-known design of the directly in silicon moulded MO-surge arresters further developments to improve the overvoltage protection are introduced. One solution is the use of line arresters (combination of arrester and support insulator) in medium and high voltage systems. Another solution could be the combination of transformer bushings (medium voltage transformers) with MO-surge arresters. In this case the MO-material (sintered ceramic) is arranged concentric around the transformer bushing, in one silicon housing. This provides optimal protection, because the distance between the arrester and the bushing is practically zero. Arrester designs adapted to specific equipment give optimized protection as well. Examples are plug-in arresters for encapsulated medium voltage substations and special MO-surge arresters for cable and cable sheath protection. Completely new solutions are possible using sintered microvaristors, which are mixed into the silicon. This composite material can be used for overvoltage protection as well as for field grading purposes, for instance in cable end bushings. Examples and prototypes of the mentioned solutions are shown.