Field calculations on epoxy resin insulated vacuum interrupters

Electrical components in medium voltage switchgear systems, which were up to now mainly gas (SF₆), oil or air-insulated, can nowadays also be insulated by epoxy resin. Encapsulating vacuum interrupters with epoxy resin allows shorter interrupters by increased external flashover voltage, and by doing so decreasing the overall switchgear size. The latest development is to encapsulate all main components, i.e. busbars, vacuum interrupters and cable terminations, in such a way that a uniform insulation level is maintained over the entire assembly, thus preventing the development of open arcs. It is shown that the geometry of the epoxy resin is designed in such a way that an optimal field distribution is achieved around the vacuum interrupter. This is done by presenting results of numerical calculations. These calculations show the positive influence that epoxy resin can have on the electrical field distribution, especially along the aluminium oxide ceramic of the vacuum interrupter and at triple-junctions. Some discussion is made concerning the manufacturing methods of epoxy resin insulated components, which ensure that there are no inhomogeneities in the solid insulating material, so all primary insulation parts are free of partial discharge. Due to the progress made in the fundamental understanding of materials and newly developed processes, a high reproducibility and continuous quality can be ensured. Typical applications of epoxy resin-insulated vacuum interrupters are given as examples