One-cycle-controlled three-phase grid-connected inverters and their parallel operation

Grid-connected inverters are necessary for converting the dc power generated by photovoltaic or fuel cells to the ac power of the utility grid. Parallel operation of these inverters extends the power range to a much higher level and allows modular design. This paper studies current sharing and circulating current among paralleled inverter modules and proposes a new parallel operation method based on one-cycle control (OCC) with combined vector operation and bipolar operation. With some minor additions to the original OCC circuit and a simple add-on communication path among the individual modules, the current is properly shared among all the inverters and the circulating current is limited to an acceptable range, while OCC advantages such as constant switching frequency, no multipliers, and simple circuitry are preserved. Two OCC inverters of 1.5 kW each were built and tied together using the proposed parallel operation. Experiments were conducted to demonstrate the simplicity and effectiveness of this method.