I. Introduction
WITH the continuous development of the grid, long-distance and large-capability electricity transmission has become a tendency of modern power system[1]. Developing and utilizing energy around the world as well as generalizing clean energy substitution and electrical energy substitution basing on the extreme high voltage grid are the inherent demands of sustainable development. Extreme high voltage transmission technique can optimize the allocation of resources and improve economic and environmental benefits in the long run. It is inevitable to erect several extreme high voltage AC and DC transmission lines in one transmission corridor due to the shortage of corridor resources which makes the extreme high voltage AC and DC transmission lines an electromagnetic interference source. With the enhancement of environmental protection consciousness the safety of transmission equipment and maintenance staff can not be ignored.