20 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 1, MARCH 2014 Flicker Mitigation by Individual Pitch Control of Variable Speed Wind Turbines With DFIG Yunqian Zhang, Zhe Chen, Senior Member, IEEE, Weihao Hu, Member, IEEE, and Ming Cheng, Senior Member, IEEE Abstract—Due to the wind speed variation, wind shear and tower shadow effects, grid connected wind turbines are the sources of power fluctuations which may produce flicker during continuous operation. This paper presents a model of an MW-level variable- speed wind turbine with a doubly fed induction generatorto investi- gate the flicker emission and mitigation issues. An individual pitch control (IPC) strategy is proposed to reduce the flicker emission at different wind speed conditions. The IPC scheme is proposed and the individual pitch controller is designed according to the gener- ator active power and the azimuth angle of the wind turbine. The simulations are performed on the NREL (National Renewable En- ergy Laboratory) 1.5-MW upwind reference wind turbine model. Simulation results show that damping the generator active power by IPC is an effective means for flicker mitigation of variable speed wind turbines during continuous operation. Index Terms—Flicker, flicker mitigation, individual pitch con- trol (IPC), variable speed wind turbine. I. INTRODUCTION D URING the last few decades, with the growing concerns about energy shortage and environmental pollution, great efforts have been taken around the world to implement renew- able energy projects, especially wind power projects. With the increase of wind power penetration into the grid, the power qual- ity becomes an important issue. One important aspect of power quality is flicker since it could become a limiting factor for in- tegrating wind turbines into weak grids, and even into relatively strong grids if the wind power penetration levels are high [1]. Flicker is defined as “an impression of unsteadiness of visual sensation induced by a light stimulus, whose luminance or spec- tral distribution fluctuates with time” [2]. Flicker is induced by voltage fluctuations, which are caused by load flow changes in Manuscript received October 8, 2012; revised January 15, 2013, February 19, 2013, and November 16, 2013; accepted December 3, 2013. Date of publication January 9, 2014; date of current version February 14, 2014. This work was sup- ported by the Danish Agency for Science, Technology and Innovation, under “Dynamic wind turbine model—from wind to grid,” DSF-09-071588, and also supported by the National Natural Science Foundation of China under Grant 51320105002. Paper no. TEC-00528-2012. Y. Zhang is with the Department of Energy Technology, Aalborg University, Aalborg DK-9220, Denmark. He is also with the School of Electrical Engineer- ing, Southeast University, Nanjing 210096, China (e-mail: yqz@et.aau.dk). Z. Chen and W. Hu are with the Department of Energy Technology, Aalborg University, Aalborg DK-9220, Denmark (e-mail: zch@et.aau.dk; whu@et.aau.dk). M. Cheng is with the School of Electrical Engineering, Southeast University, Nanjing 210096, China (e-mail: mcheng@seu.edu.cn). 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/TEC.2013.2294992 the grid. Grid-connected variable speed wind turbines are fluc- tuating power sources during continuous operation. The power fluctuations caused by wind speed variation, wind shear, tower shadow, yaw errors, etc., lead to the voltage fluctuations in the network, which may produce flicker [3]. Apart from the wind power source conditions, the power system characteristics also have impact on flicker emission of grid-connected wind turbines, such as short-circuit capacity and grid impedance angle [4], [5]. The flicker emission with different types of wind turbines is quite different. Though variable-speed wind turbines have better per- formance with regard to the flicker emission than fixed-speed wind turbines, with the large increase of wind power penetration level, the flicker study on variable speed wind turbines becomes necessary and imperative. A number of solutions have been presented to mitigate the flicker emission of grid-connected wind turbines. The most commonly adopted technique is the reactive power compen- sation [6]. However, the flicker mitigation technique shows its limits in some distribution networks where the grid impedance angle is low [7]. When the wind speed is high and the grid impedance angle is 10 ◦ , the reactive power needed for flicker mitigation is 3.26 per unit [8]. It is difficult for a grid-side converter (GSC) to generate this amount of reactive power, es- pecially for the doubly fed induction generator (DFIG) system, of which the converter capacity is only around 0.3 per unit. The STATCOM which receives much attention is also adopted to reduce flicker emission. However, it is unlikely to be financially viable for distributed generation applications. Active power con- trol by varying the dc-link voltage of the back-to-back converter is presented to attenuate the flicker emission [8]. However, a big dc-link capacitor is required, and the lifetime of the capacitor will be shortened to store of the fluctuation power in the dc link. An open-loop pitch control is used in [6] and [8] to investigate the flicker emission in high wind speeds, however, the pitch actuation system (PAS) is not taken into account. Because the pitch rate and the time delay of the PAS make great contributions to the results of the flicker emission of variable-speed wind turbines, it is necessary to take these factors into consideration. In recent years, IPC which is a promising way for loads re- duction has been proposed [9]–[11], from which it is notable that the IPC for structural load reduction has little impact on the electrical power. However in this paper, an IPC scheme is proposed for flicker mitigation of grid-connected wind turbines. The power oscillations are attenuated by individual pitch angle adjustment according to the generator active power feedback and the wind turbine azimuth angle in such a way that the volt- age fluctuations are smoothed prominently, leading to the flicker 0885-8969 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.