Contents I. Introduction
Recently, with the problems of global warming and coal, oil, natural gas and other fossil energy depletion worsening more and more, countries of the world, especially developed countries invested a lot of funds to study on the non-polluting renewable energy. Because the wind energy has the characteristic of wide distribution, large reserves, non-polluting, it is took the attention of the governments [1]. The wind turbine can be divided into level-axis wind turbines and vertical-axis wind turbines. Because the vertical wind turbine has low wind energy utilization efficiency and poor startup performance, it has not been paid attention. However, with the large-scale design and manufacture of the wind turbine, the generating cost of vertical wind turbine is gradually decreased unit kw [2]. The new vertical wind turbine which is discussed in this paper not only overcomes the shortcomings of low wind energy utilization efficiency and poor startup performance of the traditional vertical wind turbine, but also inherits the advantages of high reliability, low maintenance costs, simple design and manufacture of the blades. According to the development trends of the wind turbine, a new vertical wind turbine system which is discussed in this paper adopts permanent magnet synchronous generators in order to simplify the structure of the system and reduce maintenance costs [3]. The studies on the constant speed control of new vertical wind turbine have great significance in which it is applied in a small and medium off-grid wind power generation system.
