Switching and lightning surge dielectric strength of UHV insulating suspension sets

The authors report principal results of dielectric strength studies of UHV line suspension insulator strings exposed to switching and lightning surges. The strings were assembled of insulator units of different types. The studies revealed effects of the design of the suspension set on its flashover characteristics. When positive polarity lightning impulses are applied to insulating suspension sets, flashover voltages increase linearly with the length of the insulator string. The average flashover field intensity is 0.5 MV/m. When negative polarity impulses are applied to long strings of cap-and-pin insulators, flashover voltages are substantially lower than in the positive polarity case. The rate of increase of the flashover voltage diminishes with larger string lengths. An efficient means of enhancing the impulse strength of suspension sets is to screen the bottom end of the insulator string or to penetrate the bundle by lowermost insulator units. The result can be a 25-40% increase of the negative polarity flashover voltage