IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 1, JANUARY/FEBRUARY 2001 81 A Closed-Loop Selective Harmonic Compensation for Active Filters Paolo Mattavelli, Member, IEEE Abstract—This paper proposes a control algorithm for parallel active power filters, based on current-controlled pulsewidth-mod- ulated converters, which allows precise compensation of selected harmonic currents produced by distorting loads. The approach is based on the measurement of line currents and performs the com- pensation of the selected harmonics using closed-loop synchronous frame controllers. Thanks to the closed-loop operation, full com- pensation of the desired harmonics is achieved even in the presence of a significant delay in the current control. Thanks to the selective approach, active filter interactions with possible dynamic compo- nents of the load are minimized. Moreover, the complexity of the synchronous frame controllers is overcome using equivalent sta- tionary frame controllers. Experimental results confirm the theo- retical expectations. Index Terms—Active filters, current control, harmonic filtering, pulsewidth-modulated power converters. I. INTRODUCTION A CTIVE power filters (APFs) are powerful tools for the compensation not only of current harmonics produced by distorting loads, but also of reactive power and unbalance of nonlinear and fluctuating loads. Design and control of active filters have been deeply investigated in several papers [1]–[13], [17]–[25], covering different issues such as selection of com- pensation strategies, modulation techniques, electromagnetic interference (EMI) filters, and active filter topologies. In conventional load current detection methods [1], [2], the generation of the active filter current reference is usually based on the harmonic detection of load currents, using the well-known instantaneous power theory [1], time-domain correlation techniques [5], etc. Using this solution, however, due to the time-domain approach, the delay of the APF current control causes incorrect compensation and unwanted remaining harmonics in the line currents. This effect is dramatic, espe- cially when a fully digital control implementation is used, since the achievable performance may decrease below an acceptable level [6], [7]. For this reason, the high-performance hysteresis control [7] has been almost a mandatory choice for the achievement of a satisfactory harmonic compensation. Paper IPCSD 00–053, presented at the 2000 IEEE Applied Power Electronics Conference and Exposition, New Orleans, LA, February 6–10, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial Power Converter Committee of the IEEE Industry Applications So- ciety. Manuscript submitted for review August 1, 2000 and released for publi- cation September 28, 2000. This work was supported in part by Ricerca-Area Distribuzione e Sistemi di Utenza-ENEL, Italy. The author is with the Department of Electrical Engineering, University of Padova, 35131 Padova, Italy (e-mail: mattavelli@light.dei.unipd.it). Publisher Item Identifier S 0093-9994(01)00891-X. Besides the current control delay, conventional supply current detection and load current detection methods [8] are also prone to instability when the load includes capacitive and/or resonant elements, as reported in many papers [8]–[13]. Instead, com- pensation strategies based only on voltage detection methods [10]–[12] are not affected by the aforementioned instability, but their application implies an increased interaction with the supply voltage harmonics. In order to cope with the delay of the voltage-source inverter (VSI) current loop, it is possible to use a selective compensation achieved by means of bandpass filters on load currents whose frequency response has a leading phase equal to the delay introduced by the VSI current loop. For such purpose, several solutions have been proposed, such as low-pass filters in the harmonic reference frames [18], [19], discrete-Fourier-trans- form-based algorithms [17], and so on. With these solutions the delay of the VSI current control is theoretically compensated, but the approach, essentially based on feedforward open-loop compensation, is sensitive to parameter mismatches and relies on the ability to accurately predict the VSI current control performance. In this paper, instead, a closed-loop compensation of selected line current harmonics is proposed. As in any selective ap- proach, it is assumed that the harmonics produced by distorting loads are slowly varying and, thus, also the active filter control can be realized so as to perform only a slow, but very precise, harmonic compensation. This concept has been applied using a closed-loop regulation of line current harmonics which includes a synchronous reference frame controller for each harmonic (both for the positive-sequence and negative-sequence compo- nents) and uses the detection of line currents instead of load currents. A related approach applied to hybrid shunt filters can be found in [21]–[23], where only specific negative-sequence harmonics (i.e., the 5th) and positive-sequence harmonics (i.e., the 7th) are considered. The proposed solution, which is particularly suited for a fully digital implementation, presents the following advantages. • The control is selective for the harmonic compensa- tion, giving numerous benefits, since the filter rating and bandwidth requirements can be strongly reduced. • The control is insensitive to the delay of the VSI cur- rent control and each selected harmonic can be completely eliminated (almost 100% compensation) because of the closed-loop operation; moreover, the control is not sensi- tive to parameter mismatches. • Oscillations and interactions, which may occur be- tween the active filter and load [8]–[13] due to presence of 0093–9994/01$10.00 © 2001 IEEE