I. Introduction
The renewable energy is becoming more and more important for the development of the world, especially for the countries, which lacks of primary energy. With the increase of the electric power demand in China, the power grid keeps extending and many ultrahigh-voltage (UHV) transmission lines have been built, which can satisfy the increasing electric power demand. However, new problems for the expanding power grid rise up, such as the high operation cost, great difficulty in operation, which also decreases the reliability and stability of the power grid [1].