Research on Air Clearance of Tower for AC 750kV Single Circuit Transmission Line

The contents related to the 750kV voltage grade must be added in the revised power industry standard DL/T620-1997, i.e., Overvoltage Protection and Insulation Coordination for AC Electrical Installations. For this reason, based on the research results in the field of insulation coordination principle for extra- and ultra-high voltage transmission lines and discharge characteristics of external insulation for towers, the author researches and analyzes the air clearance of the towers for AC 750kV single circuit transmission line, and points out that the switching overvoltage should not be regarded as the controlled condition for the length of tower cross-arm for side phases. In this paper the recommended values of air clearance are given, that is, the air clearance should be 4.8m for the middle phase with V-type of insulator strings within the tower window; the air clearance should be 3.5m for side phase with I-type of insulator strings within the tower window. Finally, it is suggested to fully utilize the resources in test base for ultra-voltage transmission to carry out the experimental studies on the extra-and ultra-voltage transmission lines with wave-head lengths of 1000μs and above as well as the studies on the switching impulse of substation simulation electrodes to meet the demand of the development of extra- and ultra-voltage transmission projects.