Overvoltages due to vacuum switches and their reduction in MV networks

Vacuum switches are applied more and more widely in industrial and power-station auxiliary medium voltage (MV) power networks. In the first place this concerns the supply of high power motors and reactive power compensation capacitor systems in which an operating endurance and capability is required. Vacuum switches are distinguished by many operating qualities but they are liable to produce overvoltages, in particular, during the commutation of inductive loads such as motors, transformers and of capacitive circuits such as single or parallel capacitor banks. Various solutions are applied for reducing overvoltages due to the operation of vacuum switches. However, the application of metal oxide surge arresters (MOA) is most common. The problem of these overvoltages and of their reduction in MV networks has been subject to research and discussions in many countries over many years. The authors authors have carried out ample research work on these overvoltages taking into account vacuum switches and MOA of different types. Tests were made both in actual industrial and power station 6 kV networks and, in laboratory conditions, on a physical model. The report presents the main results of the said tests.