Investigation of single unit flashovers in HVDC insulator strings

In HVDC transmission lines, a special phenomenon, so-called single unit flashover, is observed under some special conditions. Only one or two insulator units repeat flashovers, not resulting in an overall flashover along the string. However, higher magnitudes of audible noise, radio and television interferences may give a serious problem. Based on an investigation on ±500 kV full scale insulator strings, the higher occurrence probability of single unit flashovers under cold-wet-switch-on conditions was clarified compared with normal continuous operating voltage conditions. The effectiveness of the authors´ newly proposed countermeasures to prevent single unit flashovers was demonstrated by experiments in laboratory and in the field