52 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 28, NO. 1, MARCH 2013 Two-Time-Scale Coordination Control for a Battery Energy Storage System to Mitigate Wind Power Fluctuations Quanyuan Jiang, Member, IEEE, and Haijiao Wang Abstract—In this paper, a two-time-scale coordination control method to mitigate wind power fluctuations using a battery energy storage system (BESS) is proposed. Two-time-scale maximal power fluctuation restrictions (MPFRs) are set for the combined output of the wind farm and the BESS: the maximal fluctuation of the combined power in any 1- and 30-min time window must be kept within γ 1min % and γ 30min %, respectively, of the rated power of the wind farm. A flexible first-order low-pass filter (FLF) with an optimization of time constant T f is developed to limit the power fluctuation under restriction with smaller BESS capacity. Then, a coordination control method is developed on base of the FLF, which contains: 1) PSO-based time constant real-time optimizing algorithm; 2) remaining energy level feedback control; 3) two-time- scale coordination control strategy. Finally, an estimation of BESS capacity and power rating for a wind farm to achieve the MPFRs is presented, and the whole method is tested in a time-domain simulation system. Index Terms—Battery energy storage system (BESS), first-order low-pass filter (FLF), fluctuation mitigation, power fluctuation, wind generation. NOMENCLATURE P W Wind generation power output. P W, pre Predicted wind power output. P W, rated Rated power of the wind generation. P O, exp Expected combined power output obtained with an FLF algorithm. P O Combined power output of the wind generation and the BESS. ΔP t O,p min Maximal fluctuation of P O during the previous p-minute period. P B, ord Power order for the BESS. P B Power output of the BESS. P B, Lim Power output limit of the BESS. REL Remaining energy level of the BESS. Manuscript received February 10, 2012; revised May 18, 2012 and August 9, 2012; accepted October 9, 2012. Date of publication November 29, 2012; date of current version February 7, 2013. This work was supported by the Na- tional Key Basic Research and Development Program (“973” Program) of China under Grant 2012CB215106, and the National High Technology Research and Development Program (“863” Program) of China under Grant 2011AA05A113. Paper no. TEC-00068-2012. The authors are with the College of Electrical Engineering, Zhejiang University, Hangzhou, 310027, China (e-mail: jqy@zju.edu.cn; unique1988@ zju.edu.cn). Digital Object Identifier 10.1109/TEC.2012.2226463 Capacity Minimal BESS capacity required for power fluctuation mitigation during a time period. Power Minimal BESS power rating required for power fluctuation mitigation during a time period. T f Time constant of the FLF. α Variable to be optimized, associated with T f . γ 1min One-minute fluctuation restriction, given as a percentage value of P W, rated . γ 30 min Thirty-minute fluctuation restriction, given as a percentage value of P W, rated . Δt Step size for the coordination control method. I. INTRODUCTION W IND power generation has recently shown rapid devel- opment throughout the world and is regarded as a clean and renewable energy resource. However, due to strong fluctu- ation of wind generation power, the integration of wind power at high level of penetration creates significant challenges for power system operation [1]–[4]. Power fluctuations at different time-scales influence different aspects of power system oper- ation [2]. Fluctuations from several minutes to several hours influence generation reserves and the energy dispatch of power system [1], while fluctuations from several seconds to several minutes impact power system frequency control [4]. The prob- lem of “hard to integrate” appears as a barrier to wind energy adoption in varying degrees for different countries. As a result, some electric power companies have announced technical re- quirements for integrated wind farms, and the maximal power fluctuation of different time-scales should be mitigated within different restrictions [5], [6]. Some improved methods based on pitch angle control and converter control of wind turbine generator were developed to smooth the fluctuations of wind power. Rawn et al. devel- oped a new control topology for converter-interfaced wind tur- bines, which causes the power injected into the grid is a low- pass filtered version of the incident wind power [7]. Uehara et al. presented a coordinated control of dc-link voltage and pitch angle to smooth output power for permanent magnet syn- chronous generator [8]. Changling et al. introduced P ref versus ω m functions into decoupled PQ control of double-fed induc- tion wind turbines (DFIGs), which could smooth the wind power fluctuations [9]. With the recent rapid development of novel energy storage techniques such as batteries, electric double-layer capacitors 0885-8969/$31.00 © 2012 IEEE