I. Introduction
Wind energy is attracting more and more attention and has been widely developed around the world. Many countries are actively promoting the development of wind power generation. In different countries, the development mode of wind power is also different. For instance, wind power is mostly developed with distributed and small-capacity wind farm which to be integrated to low voltage level grid and local consumed in Europe, while the major development mode is to build large-scale and centralized wind farms which will be integrated to high voltage level grid and their power will be transmitted over long distances in china [1]. There are planned to build eight million kilowatts level wind power bases, such as Jilin, Mengdong, Mengxi, Hebei, Gansu, Xinjiang, Jiangsu and Shandong in china [2], and the total wind power will reach 100 million kilowatts in 2015.