A new multilevel converter for Megawatt scale solar photovoltaic utility integration

Abstract

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.

Application of solid state transformers in utility scale solar power plants

“…The usage of SiC switches reflects directly on higher switching frequency and passive components size reduction. The high temperature capability of SiC devices also allows a reduction of the forced cooling requirement that further contributes to increase the overall conversion efficiency [15]. Currently SiC devices are still quite expensive, but prices are rapidly going down as production increases.…”

“…Cascaded Multilevel and SST topologies have many similarities, such as the multilevel output to the MV grid, a greater number of MPP trackers than the central inverters, and also correlated control strategies [12] [15]. A DC-DC-AC SST topology is proposed for application in central inverters.…”

“…A DC-DC-AC SST topology is proposed for application in central inverters. It differs from the DC-AC-AC topology, already proposed in the literature [15], and it respects the Brazilian energy code, where the power factor is restricted to 0.95 (inductive or capacitive) [16].…”

“…The use of this technology in solar farms is of great interest, as it would provide a rise in the total efficiency, both in conversion and in power generation, thanks to the increased number of MPP trackers [15] ( fig. 2(b)).…”

Cascaded multilevel SST medium voltage converter for solar applications

“…Developed originally for AC-AC applications with the same input and output frequencies [5][6][7], the Solid State Transformer -SST -is based on a high-frequency AC link and a high-frequency transformer. In solar farms, the application of this technology has attracted attention, since it could provide a total efficiency increase, both in conversion and in power generation, thanks to the increased number of MPP trackers [8] (fig. 4).…”