I. INTRODUCTION
With the development of wind power generation technology, the proportion of wind power generation in the new energy field continues to increase. According to a research report released by the Global Wind Energy Council, by 2023, the global wind power installed capacity have reached 1000 GW and will grow well. At present, wind turbines are mainly divided into two types: doubly fed-and direct-driven. Since direct-driven wind turbines are based on permanent magnet synchronous motors, they are significantly better than doubly fed ones in terms of efficiency and reliability and, therefore, have attracted much attention from the wind power industry.