Contents I. Introduction
In recent years, doubly-fed wind turbines with 1.5MW/2MW or even higher capacity[1] have replaced the traditional squirrel cage wind turbines[2] and become the mainstream model of wind farms. With the development of offshore wind power, wind turbines with larger capacities have entered the stage of development and trial operation[3], [4]. At present, two main ways are used in high-voltage wind power generation systems: one uses the combination of high voltage wind turbine and high-voltage converter[5], [6], and the other uses the combination of low-voltage wind turbine, low-voltage converter, and transformer[7], [8]. As high voltage converter is expensive and has a complex structure and high control requirements, large capacity turbines generally use the second way. However, with the increase of the turbine capacity, the lower output voltage of the stator end of the turbine will lead to the increase of the output current of the machine end, which not only requires thicker cables and copper bars, but also brings considerable difficulties to the production process of the wind turbine. In addition, due to the low voltage of the stator end of the turbine, two or even more transformers are generally required to boost the voltage before being integrated into the grid, and a step-down transformer is also required to provide power support for the grid-side converter before it is connected to the grid[9], [10], which undoubtedly increases the manufacturing cost and operating cost of the system.
