Dielectric stress, design and validation of MV switchgear

Abstract

Medium voltage switchgear is a key component within electrical power distribution networks. It is designed and tested for safe and reliable operation under various operating conditions. Medium voltage air-insulated switchgear (AIS) and gas-insulated switchgear (GIS) have to be validated according to the requirements of international standards like IEC 62271-1 and IEEE C37.100.1 for instance. The dielectric design is optimized with an extensive use of simulation and experimental test validations required by standards, customers specifications or defined by the manufacturers of switchgear for its own quality. That ensures insulation reliability in various applications and operating environments. The first part of this paper concerns dielectric stresses of switchgear during its operational life under real network conditions. The appearance of overvoltages through lightning strokes or switching operations is analysed. The second part of this paper concerns dielectric design and validation of MV AIS switchgear depending of environmental conditions (humidity, salt, dust…) and in particular the impact on insulation ageing and potential degradation. Insulation techniques (GIS and AIS) and testing procedures are discussed. The degradation of the external insulating of AIS is demonstrated and can be mitigated by the choice of an higher value of the dielectric BIL or power frequency withstand (example for 12 kV rating: 95 kV BIL instead of 75 kV BIL or 42 kV instead of 28 kV 50 Hz). On the other hand, no additional margin on the dielectric withstand is necessary for protected insulation (GIS).