UHV Disconnectors: Switching Surge Design and Testing of External Insulation

The switching surge insulation characteristics of a disconnector (formerly "switch") are presented in a generalized form, based on a series of tests performed on UHV disconnectors with various heights H and clearances between contacts D. The knowledge of these characteristics and of the distribution of the expected overvoltages in service allows to determine the total risk of discharge RT and the risk of discharge across the longitudinal insulation RL. A general method for the design of a disconnector that meets prescribed maximum tolerable risks is presented. The statistical significance of a type test is then investigated, and the procedures recommended by IEC and IEEE are criticized. Some changes in the testing procedure are finally proposed, in order to ameliorate the tests and to reduce the differences between their requirements on the design of a disconnector.