Overhead transmission lines (OTL) have always been the main solutions to the large capacity power transmission over long distances. However, since the towers are growing significantly in size with the increase of voltage level, it takes a great deal of steel and land source for the construction of transmission lines. As a consequence, cross arms were introduced and have been used in various countries. Considering that the flashover characteristics over long distances are usually hard to predict from the results of short distances, full scale tests are of great significance for both laboratory research and engineering application. Therefore, a full-scale AC 500 kV model tower with composite cross arms is manufactured for this research. Switching overvoltages of different polarities were applied, and the flashover characteristics were measured. The developing process of the streamer-leader discharge was recorded, and the major path of the flashover was identified. The measured flashover results were corrected to standard conditions and clearly confirmed the polarity effect of lightning flashover. The feasibility of composite cross arms was thus confirmed, with a structural optimization being proposed accordingly.

operation overvoltage,cross arm,composite insulator,Transmission Tower