Simulation analysis on reducing the electric field stress at the triple junctions & on the insulator surface of the high voltage vacuum interrupters

Surface flashovers that are initiated from the triple junctions in vacuum interrupters can restrict their overall performance of vacuum´s solid insulation especially for the voltage ranges of 72.5 kV and above. This has to be taken into account for the insulation coordination of vacuum interrupters. A basic model of a 72.5 kV vacuum interrupter was simulated in order to study the electric field stress at the outside edge of the metal shield, at the triple junctions and on the surface of the insulator. The high field stress at the outer edge of the metal grading shields can be reduced by using external field grading rings. But when it comes to triple junctions where the initiating event of inner surface flashover takes place and the insulator surface where the secondary electron emission takes place, there are many factors that influence the field stress at these regions. In this paper, some of those factors are simulated in ANSYS Maxwell 2D and observed the electric field distribution at the critical areas. One of those factors that influence the field stress at the triple junctions is the shape of the insulator. Different shapes of the ceramic insulator especially at the point of contact with metal shield are simulated. And another factor that influences the field distribution at the triple junction area is the implementation of metal inserts with different heights at the both ends of the insulator. Their influences on the field distribution are simulated. On the other hand, spraying of electrons from the metal shields under high field stress on to the insulator surface may lead to charge accumulation and cause secondary electron emission. So the metal shield curvature is optimized in order to reduce the field stress on its surface.