Control of four leg inverter for hybrid power system applications with unbalanced load I. Vechiu a, * , H. Camblong a , G. Tapia b , B. Dakyo c , O. Curea a a LIPSI-ESTIA, Bidart, Technopo ˆle Izarbel, 64 210 Bidart, France b University of Basque Country (UPV-EHU), Europa Plaza 1, E-20018 Donostia-San Sebastia ´ n, Spain c GREAH – University of Le Havre, 25 rue Philippe Lebon, BP 540, France Received 6 June 2006; accepted 24 December 2006 Available online 27 February 2007 Abstract This paper proposes an improved control strategy for a three phase, four leg inverter used for the simultaneous supply of three phase and single phase AC loads in a transformerless hybrid power system (HPS) application. First, the mathematical model of the inverter is obtained by using the average technique, which allows converter systems to be rapidly and accurately simulated. Then, two control strat- egies are described. The first one uses the conventional dq0 frame rotating with the angular frequency x (x = 314 rad/s) and PI (propor- tional integral) controllers to ensure voltage and current regulation. The second one is an improved control strategy based on the decomposition of the supply three phase voltage and current into instantaneous positive, negative and homopolar sequence components using phasor representation. These three sequences are controlled independently in their own reference frames as DC signals. The posi- tive sequence is regulated by the PI controllers in a positive reference frame, which rotates counterclockwise, while the negative sequence is regulated by the PI controllers in a negative reference frame, which rotates clockwise at the same angular frequency. Since the proposed strategy uses phasor representation, the actually superposed phasors of the homopolar sequence are spatially displaced at 120°. Then, using a negative reference frame system, the obtained DC signals are regulated by the PI controllers. Simulation results show the validity of the innovative control strategy. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Converters; Current control; Hybrid power system; Power quality; Symmetrical components; Voltage unbalance 1. Introduction In many of the remote areas, where the cost of the grid extension is prohibitive and the fuel price increases drastically with the remoteness of the location, the asso- ciation of renewable energy sources with conventional Diesel generators (DG) has demonstrated substantial reduction of fuel consumption and increased efficiency of the overall system [1]. In order to achieve fuel saving, the DG should operate intermittently. Intermittent Diesel operation means that the DG should be brought into operation with high efficiency and switched off when unsatisfactory operating conditions occur. Considering an application with the interconnection of AC power sources and DC power sources allowing intermittent DG operation (Fig. 1), an autonomous inverter is required to supply the AC loads with constant voltage magnitude and frequency. A major drawback in such HPS supplying three phase loads is voltage unbalance. Unbalanced loading condi- tions can occur in HPS for a variety of reasons. In gen- eral, small loads (relative to the power range of the system) are configured to draw power from only one phase. When several single phase loads are placed on a distribution system, the fluctuating power required from each of these loads can cause unbalance in the power sys- tem. To supply both three and single phase AC loads, a 0196-8904/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.12.019 * Corresponding author. Tel.: +33 5 59 43 84 74; fax: +33 5 59 43 84 05. E-mail address: i.vechiu@estia.fr (I. Vechiu). www.elsevier.com/locate/enconman Energy Conversion and Management 48 (2007) 2119–2128