Abstract-- This paper proposes an adaptive variable-structure control scheme for a three-phase PWM converter under unbalanced conditions. The controller regulates the DC output of PWM converter in presence of unknown disturbance and uncertainty in model parameters of the converter. Using differential geometric tools, the system is transformed to a normal form. Then an adaptive control scheme is designed to regulate the output of the transformed system. The stability of the proposed controller is proved using the Lyapunov theory. Simulation results illustrate the effectiveness of the proposed method. Index Terms-- PWM VSC rectifier, unbalanced voltage source, adaptive control. I. INTRODUCTION urrent and voltage converters are two types of three- phase ac/dc converters. Each one is comprised of a three-phase ac side and a dc side. The current converters have three voltage sources in parallel with the ac side and a current source in series with the dc side. The voltage converters have three current sources in series with the ac side and a voltage source on the dc side [17]. A type of ac/dc converters are pulse width modulation converters which are widely used in industrial applications of power conversion, such as uninterruptible power supply (UPS) systems, variable- speed motor drives, electric traction and induction heating. The main features of PWM are constant dc-bus voltage, low harmonic distortion of the utility currents, bidirectional power flow, and controllable power factor [1-6]. Due to common usage of PWM converters, many investigations have been focused on the control of PWM Behnam Khaki is with the SAZAB Consultant Engineering Company, Ahvaz Iran (e-mail: behnam_khaki@aut.ac.ir). Adel M. Sharaf is with the University of Trinidad & Tobacco, Arima Trinidad W. I (phone: +1-86-642-888; fax: +1-868-643-1617; e-mail: adel.sharaf@utt.edu.tt). Seyyed Hossein Mousavi is with the Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB, CANADA (e- mail: mousavi3@ualberta.ca). Navid Noroozi is with School of Electrical & Computer Engineering, Shiraz University, Shiraz, IRAN (e-mail: noroozi@shirazu.ac.ir). Alireza Seifi is with School of Electrical & Computer Engineering, Shiraz University, Shiraz, IRAN (e-mail: seifi@shirazu.ac.ir). 978-1-4673-0136-7/11/$26.00 ©2011 IEEE converters [4], [7-10]. Reference [7] analyzed design methods and the performance of the voltage and current proportional plus integral (PI) controllers, which are as usually made up of inner current control loops and an outer voltage control loop in a cascade structure. The cascade control structure is not considerably effective [4]. In [8] and [9], the PWM converters have been modeled in a single nonlinear system using a power balance between the input and output sides. The nonlinear system was analyzed and the controllers were designed using small-signal analysis, which is acceptable only around particular operating points, not in the whole operating range. In [10], an adaptive backstepping control strategy for a three-phase PWM converter was represented. In the mentioned paper, nonlinear system modeling was derived with state variables of the input current and the output voltage by using power balance of the input and output. However, this technique only considered the load resistance deviation as the system uncertainty. In practice, the source voltage becomes often distorted and unbalanced, which in turn, leads to appearance of abnormal harmonics of low frequencies at both the input and output converter terminals [11,12]. In this regard, [13] proposed a double current controller to eliminate the ripple components in the DC-link voltage under unbalanced source voltage, where the positive-sequence and negative-sequence components are controlled separately. However, the source voltage distortion was not considered. In [14], a resonant current controller was presented to solve the current distortion problem due to the source voltage harmonics. Since signal processing is not required, this method has an advantage of fast response. Nevertheless, the resonant controller may amplify the control gain of other frequency components. In addition, the effect of unbalanced source voltage has not been considered. Most of the work cited before assumed either system parameters are known or only a few parameters are not given. Moreover, some of them have not considered the source voltage distortion. In this paper an adaptive variable-structure controller is proposed for a three-phase PWM converter while all converter model parameters are unknown. The controller regulates the DC output of PWM converter in presence of unknown disturbance (i.e. unbalanced operation) and uncertainty in model parameters of the converter. In this regard, since the nonlinear model of PWM converter is input- Adaptive Control of DC Link Voltage of PWM VSC Rectifier under Unbalanced Voltage Source and Uncertain Parameters B. Khaki, A. M. Sharaf, Senior Member, IEEE, S. H. Mousavi, N. Noroozi, and A. Seifi C