1522 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006 Three-Phase Low-Frequency Commutation Inverter for Renewable Energy Systems Geomar Machado Martins, José Antenor Pomilio, Senior Member, IEEE, Simone Buso, Member, IEEE, and Giorgio Spiazzi, Member, IEEE Abstract—The connection of distributed power sources with the utility grid generally needs an electronic power converter for processing the locally generated power and injecting current into the system. If the source provides a dc voltage, the converter must be able to produce a low-distortion high-power-factor ac current. The same aspects related with the voltage and current distortion produced by nonlinear loads can be considered for the injection of power into the grid. In the absence of a specific standard, this paper takes as a reference the limits for current harmonics given by international standards. The justification for this approach is that, from the resulting line voltage degradation, there is no difference between injected and absorbed currents. This paper presents a three-phase inverter using low-frequency commutation. An auxiliary circuit is added to the inverter topol- ogy to reduce the output voltage distortion, thus improving the current waveform. The main advantages of this approach are the minimization of the switching losses and the elimination of the electromagnetic interference, which avoids high-frequency filters necessary in high-frequency commutation inverters. Index Terms—DC–AC power conversion, energy conversion, fuel cells, standby generators. I. I NTRODUCTION T HE CONNECTION of distributed power sources with the utility grid generally needs an electronic power converter for processing the locally generated power and injecting cur- rent into the system. If the source provides a dc voltage, the converter must be able to produce a low-distortion high-power- factor ac current. The same aspects related with the voltage and current dis- tortion produced by nonlinear loads can be considered for the injection of power into the grid. In the absence of a specific standard, this paper takes as a reference the limits for current harmonics given by the IEC 61000-3-4 technical report [1] and the EN61000-3-2 [2]. The justification for this approach is that, from the resulting line voltage degradation, there is no difference between injected and absorbed currents. The rec- ommendations proposed in [1] are applicable to low-voltage single-phase and three-phase systems with input current greater than 16 A per phase. The harmonic current limits are a per- Manuscript received December 23, 2004; revised May 17, 2005. Abstract published on the Internet July 14, 2006. G. M. Martins and J. A. Pomilio are with the School of Electrical and Computer Engineering, State University of Campinas, 13081-970 Campinas, Brazil (e-mail: antenor@dsce.fee.unicamp.br). S. Buso and G. Spiazzi are with the Department of Electronics and Infor- matics, University of Padova, 35131 Padova, Italy (e-mail: giorgio.spiazzi@ dei.unipd.it). Digital Object Identifier 10.1109/TIE.2006.882023 Fig. 1. Three-level PWM waveform and filtered resulting voltage. centage of the fundamental component, measured at the rated power. For lower current [2], the current harmonic limits are constant (see the Appendix). If other standard should be con- sidered, the only modification is to make adequate the harmonic limits for the converter parameter calculations. For low-power applications like photovoltaic panels and small fuel cells, a single-phase inverter (dc–ac converter) can be used for connecting the alternative source with the grid [3]. However, above a few kilowatts, it is more convenient to use three-phase inverters. Pulsewidth-modulated (PWM) converters can be used to produce any voltage or current waveforms. This modulation technique has been used for many applications, including the connection of dc sources to the grid [4]. The main advantage of PWM inverters is the possibility of producing any current waveform, as shown in Fig. 1. Nev- ertheless, these converters present some drawbacks especially related with the electromagnetic interference (EMI) generation, due to the high-frequency commutation, and a relatively low efficiency, due to the power switch losses [5]. A low-pass filter is necessary to attenuate the high-frequency components due to the switching process. There are other converter topologies presented for grid inter- face applications [6], but they also use high-frequency commu- tation; thus, EMI filters are still necessary if one is interested in injecting a low-distortion current into the line. This paper presents a three-phase inverter using low- frequency commutation. The converter is based on the low- frequency commutation three-phase rectifier [7]. An auxiliary circuit is added to the inverter topology to reduce the output voltage distortion, thus improving the current waveform. This circuit is composed by three bidirectional switches (S a1 , S a2 , and S a3 ), two capacitors (C 1 and C 2 ), and the input inductors. 0278-0046/$20.00 © 2006 IEEE