Three-phase low-frequency commutation inverter for renewables

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 the IEC61000-3-4 technical report. The justification for this approach is that, from the resulting line voltage degradation, there is no difference between injected and absorbed current. This paper presents a three-phase inverter using low-frequency commutation. An auxiliary circuit is added to the inverter topology in order to reduce the output voltage distortion, thus improving the current waveform. The main advantages of this approach are the minimization of the switching losses (i.e. high efficiency) and the elimination of the EMI (which avoids high-frequency filters necessary in high-frequency commutation inverters).