0-7803-9514-X/06/$20.00 ©2006 IEEE ICIEA 2006 An a-b-c Reference Frame-Based Compensation Strategy for Series Active Power Filter Control G. W. Chang *S. K. Chen Y. C. Chin W. C. Chen Department of Electrical Engineering *Department of Electrical Engineering National Chung Cheng University Chungchou Institute of Technology Chia-Yi 62105 Taiwan Chang-Hwa 51003 Taiwan Email: wchang@ee.ccu.edu.tw Email: tonych@dragon.ccut.edu.tw This paper presents a new control strategy to determine the reference compensation voltages of a series active power filter (APF) operated under non- ideal supply voltages in the steady state. With using the proposed control strategy, the load-side voltages can be maintained being sinusoidal and balanced. The effectiveness of the proposed control strategy is demonstrated through simulations by using Pspice. Results show that the expected series APF performances are achieved with adopting the new control strategy. Keywords: Series active power filter, harmonics, voltage distortion, imbalance 1. Introduction Among the power quality issues that have been consistently raised, the most concern is the occurrence of voltage disturbances such as harmonics and sags in industrial processes with critical loads [1]. Therefore, how to maintain ideal supply voltages to the disturbance-sensitive loads becomes a great concern. With the advanced development of power-electronic technologies, the active power filtering draws much attention in recent years. The applications of series APF for mitigating supply voltage distortions and imbalances are proved to be viable for the protection of many critical loads [2]-[7]. Fig. 1 shows the typical circuit scheme of a three-phase series APF. In this paper, the authors propose an a-b-c reference- frame based approach to determine the reference compensation voltages for series APF control. The proposed approach can keep load voltage being sinusoidal and balanced under non-ideal supply voltage and the series APF consumes minimum active power. 2. Proposed APF Reference Compensation Voltage Control Strategy The proposed compensation strategy of the series APF is based on the concept that the desired load voltages are required to be balanced, sinusoidal, and in phase with the positive-sequence load current. The goals of the series APF are 1) minimum active power consumed or supplied by the APF and 2) harmonic and imbalanced voltage compensations. Therefore, the APF must provide full compensation (harmonic and imbalanced voltages) for the non-linear load at positive- sequence fundamental frequency. To achieve these goals, the desired three-phase load voltages must be in phase with the positive-sequence fundamental load current components. That is, Fig. 1. Typical Scheme of a three-phase series APF. ) sin( 1 1 + + = φ ω t V v la (1) ) 120 sin( 1 1 + + − = φ ω o t V v lb (2) ) 120 sin( 1 1 + + + = φ ω o t V v lb (3) and ) sin( 1 1 1 1 + + + + = φ ω t I i m la (4) ) 120 sin( 1 1 1 1 + + + + − = φ ω o t I i m lb (5) ) 120 sin( 1 1 1 1 + + + + + = φ ω o t I i m lc (6) where + 1 m I and + 1 φ are the load current magnitude and the phase angle of the fundamental positive-sequence component, respectively. Under the conditions that the load active power is supplied by the source and the series APF does not provide or consume active power, it is required to find the voltage amplitude, V, from the sequential instantaneous current and real power components of the load. According to the symmetrical- component transformation for the three-phase rms currents at each harmonic order, the three-phase instantaneous load currents can be expressed by (7)-(9).