Substitution of silver/cadmium oxide in high voltage disconnectors

In high voltage substations with multiple busbar configuration, a load transfer without load interruption from one busbar to another is a common operation. For this task, disconnectors with an interruption capability up to 0.5 MVA are used which have to manage switching arcs at the break as well as at the make operation. Pantograph disconnectors are well known as reliable and space-saving solutions. There are two possibilities for the contact systems. The combination of current-carrying contacts and switching contacts has a considerable advantage in comparison to the use of additional low erosion pretravel contacts. Up to now, silver cadmium oxide (Ag/CdO) is used as an arc resistant contact material for pantograph disconnectors without pretravel contacts. Due to an increasing environmental awareness the Ag/CdO should be replaced by a more environmentally friendly material. It is shown that an optimized silver tin oxide (Ag/SnO/sub 2/) material with additives fulfills the requirements. In regard to the switching behavior, Ag/SnO/sub 2/ is superior to Ag/CdO and even in comparison to pure silver contacts it shows an equal current carrying behavior. On the basis of the results Ag/CdO can be replaced in pantograph disconnectors.