IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 8, AUGUST 2010 2761 Cascaded Nine-Level Inverter for Hybrid-Series Active Power Filter, Using Industrial Controller Alexander Varschavsky, Juan Dixon, Senior Member, IEEE, Mauricio Rotella, and Luis Morán, Fellow, IEEE Abstract—An industrial controller, specifically designed for two- and three-level converters, was adapted to work on an asymmetrical nine-level active power filter (APF). The controller is now able to make all required tasks for the correct operation of the APF, such as current-harmonic elimination and removal of high-frequency noise. The low switching-frequency operation of the nine-level converter was an important advantage in the appli- cation of the industrial controller. In addition, with the nine-level filter, switching losses were significantly reduced. The filter was designed to work as voltage source and operates as harmonic isolator, improving the filtering characteristics of the passive filter. The control strategy for detecting current harmonics is based on the “p-q theory” and the phase-tracking system in a synchronous reference frame phase-locked loop. The dc-link voltage control is analyzed together with the effect of controller gain and delay time in the system’s stability. Simulations for this application are displayed and experiments in a 1-kVA prototype, using the aforementioned industrial controller, were tested, validating the effectiveness of this new application. Index Terms—Active filters, harmonic distortion, multilevel systems, power quality. I. I NTRODUCTION T HE constant increase in power electronic devices, used by industrial and commercial consumers, has deteriorated seriously electric power systems. More transmission losses, power-transformer and neutral-conductor overheating, power- factor correction-capacitor overloading, and induced noise in control systems are only a few of the problems that harmonic distortion may bring into home and industrial installations [1], generating considerable economic losses to distribution companies and end users [2]. During many years, the solution used to minimize harmonic pollution has been tuned passive filters. However, they have quite a few disadvantages, like fixed compensating charac- teristic (given only by the tuned frequencies), parallel and series resonance with source-voltage harmonics, and filtering Manuscript received October 22, 2008; revised August 19, 2009; accepted September 26, 2009. Date of publication October 20, 2009; date of current version July 14, 2010. This work was supported by the Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) through Project Fondecyt 1070751, ABB Chile, and Millenium Project P-04-048-F. A. Varschavsky is with the CGE Distribución S.A., Santiago 8340434, Chile (e-mail: auvarsch@uc.cl). J. Dixon is with the Department of Electrical Engineering, Pontificia Univer- sidad Catolica de Chile, Santiago 7820436, Chile (e-mail: jdixon@ing.puc.cl). M. Rotella is with the ABB S.A., Santiago 7780006, Chile (e-mail: mauricio. rotella@cl.abb.com). L. Morán is with the Department of Electrical Engineering, Universidad de Concepción, Concepción 53-C, Chile (e-mail: luis.moran@udec.cl). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2009.2034185 characteristics strongly affected by source impedance. They are also bulky, and they lost their effectiveness with the passage of time [3], [4]. Several topologies of active power filters (APFs) have been proposed [5]–[7] as a solution to passive-filter problems. Most of the APFs that have been implemented until now are of shunt type [8]–[11], but they are comparatively more expensive due to its large rating of about 30%–60% of the load [12]. Even more, they cannot compensate correctly for harmonic voltage produced by power rectifiers with large dc-link capacitor [13]. Numerous series APF have been proposed [3], [4], [12], [14], [15], most of them operating as hybrid, in conjunction with shunt passive filters. The advantages of hybrid topologies are quite significant since the series active filter can be very low rated, between 3% and 10% [12], and the disadvantages of the passive filters are mitigated. Another advantage of this hybrid topology is that harmonic voltage and current-producing loads can be effectively compensated. However, series active filters proposed until now had been implemented in two-level PWM based inverters, with the known disadvantages that they present, such as high-order harmonic noise and additional switching losses due to high-frequency commutation [16]. Multilevel inverters have become very popular in the last few years, due to their capability to generate cleaner voltage waves and lower switching losses [17], [18]. If the cascaded H-bridge topology scaled in powers of three is utilized, the number of sources and semiconductors is minimized [19]–[24]. With this topology, each H bridge operates at a lower frequency, decreasing switching losses and permitting the use of slower semiconductors. This paper shows that lower frequency opera- tion of the asymmetrical converter has permitted the adaptation of industrial controllers for filtering purposes. II. SYSTEM DESCRIPTION A. System Configuration The circuit of Fig. 1 shows the basic topology of the system, which is composed by three 9-level inverters connected in series between the source and the load and a shunt passive filter tuned at fifth and seventh harmonics. The passive filter presents a low-impedance path to load-current harmonics and also helps to partially correct the power factor. B. Multilevel Inverter Each phase of the nine-level series APF comprises two H bridges connected at the same dc-link capacitor. The two bridges are connected to the ac line using independent 0278-0046/$26.00 © 2010 IEEE