A new multilevel converter for Megawatt scale solar photovoltaic utility integration

This paper presents a new multi-level DC-AC-AC converter topology for medium voltage grid integration of Megawatt (MW) scale utility photovoltaic (PV) plants. It is envisioned that a large PV field is divided into many zones, each comprising of two PV arrays. The number of zones depends on the voltage of the grid with which it is interfaced. In the proposed approach, zonal power balancing is achieved by employing a current-sharing technique. The power conversion architecture consists of an IGBT based full-bridge inverter feeding a medium frequency (MF) transformer with three secondary windings. The voltages at the transformer secondaries are then converted to three phase line frequency AC by three, full-bridge AC-AC converters. This also eliminates the 2
^nd
harmonic power from the DC bus, thereby reducing the capacitor size. By stacking several such modules in series, a high quality multilevel medium voltage output is generated. Further, the bulky line frequency utility interface transformer is eliminated. A new control method is proposed for the series connected modules during partial shading while minimizing the switch ratings. This paper presents the analysis, design example and simulation of a 10 MW PV system with preliminary experimental results on a laboratory prototype.