IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 3, MAY/JUNE 2013 1299 Application of Electrical Variable Transmission in Wind Power Generation System Xikai Sun, Student Member, IEEE, Ming Cheng, Senior Member, IEEE, Ying Zhu, Student Member, IEEE, and Longya Xu, Fellow, IEEE Abstract—In the widely applied non-direct-driven wind power generation system, a gearbox is connected between the wind turbine and high-speed doubly fed induction generator so that the system can supply power with constant voltage and constant frequency as the wind turbine speed varies. However, the multi- level mechanical gearbox is rather vulnerable and poses many difficulties to maintain. Electrical variable transmission (EVT), an electrical continuous variable gearbox, is a very competitive alternative for the vulnerable constant speed-ratio mechanical gearbox. In this paper, the application of this machine in wind power generation system with its operational modes and control strategy is presented. The working principle of the EVT-based system is introduced, and the unique power conversion character- istics are investigated. A control algorithm with maximum power point tracking for the outer rotor is developed and implemented in a prototype machine. Both computer simulation and experimental results verify the working principle and the unique features of the EVT-based wind power generation system. Index Terms—Continuous variable gearbox (CVG), doubly fed induction generator (DFIG), electrical variable transmission (EVT), permanent-magnet generator (PMG), wind power. I. I NTRODUCTION W ITH EVER increasing concerns on environmental pro- tection and energy crisis in this new century, the devel- opment of sustainable energy resources has been taking on an accelerated pace, of which wind power is the most viable one. The capacity of the wind power generator has been increasing greatly from several kilowatts to nearly 10 MW, and the wind farms are developing from inner land to offshore area in the past 20 years. Manuscript received October 18, 2011; revised March 1, 2012 and June 21, 2012; accepted August 13, 2012. Date of publication March 18, 2013; date of current version May 15, 2013. Paper 2011-EMC-504.R2, presented at the 2010 IEEE Energy Conversion Congress and Exposition, Atlanta, GA, USA, September 12–16, and approved for publication in the IEEE TRANSACTIONS ON I NDUSTRY APPLICATIONS by the Electric Machines Committee of the IEEE Industry Applications Society. This work was supported in part by the National Natural Science Foundation of China under Grant 50777008 and Grant 50977011, in part by the National 863 Program of China under Grant 2007AA05Z457, in part by the Innovative Talent Program of Jiangsu Province, China, under Grant BK2010013, and in part by the Scientific Research Foun- dation of the Graduate School of Southeast University. X. Sun, M. Cheng, and Y. Zhu are with the School of Electrical Engineering, Southeast University, Nanjing 210096, China (e-mail: sunxikai@seu.edu.cn; mcheng@seu.edu.cn; yzhu1987@gmail.com). L. Xu is with the Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH 43210 USA (e-mail: xu.12@osu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2013.2253079 Currently, the most popular wind power generation system is one of the following two types: 1) high-speed generator [for example, doubly fed induction generator (DFIG)] system with a mechanical gearbox and 2) direct-driven permanent- magnet generator (PMG) system [1]–[3]. The main reason for the popularity of non-direct-driven DFIG connected to grid is their ability to supply power with constant voltage and constant frequency as the wind turbine speed varies. However, operating at high speed, DFIG has to work with a multilevel mechanical gearbox which is the most vulnerable part in the system and poses many difficulties to maintain, particularly for high-capacity offshore wind farms [4], [5]. On the other hand, the direct-driven PMG has got increasing acceptance in recent years due to high efficiency and less frequent maintenance. The generator with low speed, however, is huge and bulky which is a big issue for offshore wind farms. In order to reduce the size of the generator, a multilevel mechanical gearbox could be adopted. However, the full-capacity pulse-width modulation converters and the equipment to regulate the reactive power for the PMG system also increase the cost [6]. Recently, electrical variable transmission (EVT), an elec- trical continuous variable gearbox (CVG), is widely studied to replace the vulnerable mechanical gearbox in wind power application, hybrid electrical vehicles, and so forth [7]–[9]. These works mainly focus on the application of EVT machine in hybrid electrical vehicles. As a special electrical machine, the EVT machine operates with higher reliability than the multilevel mechanical gearbox and thus lessens the frequency as well as the cost of maintenance [5]. In [10], a novel wind power generation system based on EVT has been proposed, and the EVT-based wind power generation system configuration and working principle have been discussed generally in [11]. In this paper, the application of EVT machine in wind power generation system with its unique power conversion characteristics and control strategy is presented. First, the EVT machine structures and the working principles of the EVT- based wind power generation system are reviewed briefly after the introduction. Then, unique power conversion characteristics of the EVT-based system are investigated and compared with the conventional DFIG system and PMG system, including the improved power conversion coefficient, maximum power ac- quirement, and fault ride through (FRT) capability. A controller with maximum power point tracking (MPPT) for the EVT- based wind power generation system is developed and imple- mented in Matlab/Simulink. Computer simulation and system experiment results are finally provided to verify the working principle and the unique power conversion characteristics. 0093-9994/$31.00 © 2013 IEEE