IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 3, JUNE 2001 625 Control Strategies for Enhanced Power Smoothing in Wind Energy Systems Using a Flywheel Driven by a Vector-Controlled Induction Machine Roberto Cárdenas, Member, IEEE, Rubén Peña, Member, IEEE, Greg Asher, Member, IEEE, and Jon Clare, Member, IEEE Abstract—This paper presents a novel control strategy for power smoothing in wind energy applications, especially those feeding a stand-alone load. The system is based on a vector-con- trolled induction machine driving a flywheel and addresses the problem of regulating the dc-link system voltage against both input power surges/sags from a wind turbine or sudden changes in load demand. The control is based on a feedforward compensation scheme augmented by a nonlinear controller. Two feedforward compensation schemes are discussed and the limitations and performance of each scheme are analyzed. Experimental results are presented which verify the excellent performance of the feedforward compensation technique. Index Terms—Flywheels, fuzzy control, variable-speed drives, wind energy. I. INTRODUCTION I T IS well known that the mechanical shaft power obtained from a wind turbine may be approximated by [1] (1) where is the air density, is the the power coefficient, is the blade radius, is the blade pitch angle, is the tip speed ratio, and is the effective wind speed. Equation (1) shows that small variations in the wind speed produce large changes in the captured power. These power fluctuations propagate to the output of the wind energy conversion system (WECS), es- pecially if the system is fixed speed. For variable-speed systems, part of the power fluctuation is absorbed as inertial energy in the turbine. However, even for variable-speed systems, power fluc- tuations can still be problematic if the WECS is feeding a small grid or a stand-alone load [2]. For these applications, the wind turbine is augmented by an additional source, usually a diesel generator [3]. In such hybrid systems, wind speed fluctuations not only produce fluctuations in the generator output voltage, but also an unacceptable number of start/stop cycles of the diesel generator if a temporary energy buffer is not available. Manuscript received May 9, 2000; revised November 26, 2000. Abstract pub- lished on the Internet February 15, 2001.This work was supported by Fondecyt Chile under Contract 1980689 and the British Council. R. Cárdenas and R. Peña are with the Electrical Engineering Department, University of Magallanes, Punta Arenas, Chile (e-mail: rcd@ona.fi.umag.cl). G. Asher and J. Clare are with the Electrical and Electronics Engineering Department, University of Nottingham, Nottingham NG7 2RD, U.K. (e-mail: greg.asher@nottingham.ac.uk). Publisher Item Identifier S 0278-0046(01)03383-4. Fig. 1. Proposed compensation system. In this paper, the energy buffer takes the form of an induction machine feeding a flywheel as shown in Fig. 1. When the dc-link voltage decreases, the induction machine is controlled to operate as a generator, transforming the inertial energy stored in the flywheel into electrical energy supplied to the capacitors. When increases, the induction machine motors, transfer- ring energy from the capacitors to the flywheel. With reference to Fig. 1, the aim is to control the dc-link voltage against fluc- tuations in the generated current and the load current . In previous work using a structure similar to Fig. 1, scalar converters or slow dynamic methods have been used to control the speed of the machine–flywheel set [3]. However, in such a drive, the machine torque is not controlled directly and this results in slow dynamics and suboptimal voltage regulation. In [4], a vector-controlled induction machine, driving a flywheel, is used for power smoothing. However, no formal analysis of the dynamic of the system is presented, the controller designed is not discussed, and feedforward compensation is not consid- ered. Although a speed sensor will be used in this paper, this is not required in practice since sensorless techniques having the same torque dynamics as a sensored drive are well known [5], [6] and the flywheel will not be required to operate at zero speed. This paper addresses the nonlinear control problem of regu- lating using the current providing the analysis and inves- tigating the controller design. It is further shown that the regu- 0278–0046/01$10.00 © 2001 IEEE