Scott-Tied Magnetically Linked Solar Multilevel Converter in Varying Environmental Conditions

Abstract

Recently, multilevel converters (MLCs) have given promising solutions for medium-power and high-power renewable energy systems. However, voltage level enhancement leads to a higher device count, especially for three-phase configurations. A megawatt-scale three-phase 27-level converter with a Scott-connected arrangement is used for medium-voltage applications to address such issues. The Scott transformer uses 1:3:9 turn ratios between the windings to have 27-level output converter voltage with the advantage of two-phase to three-phase conversion. Moreover, the transformers enable the converter output voltage to medium-voltage levels (typically 11 to 33 kV). Traditional three-phase systems have cascaded power converter modules in each phase. However, this system provides an alternative solution for solar power injection into the grid with minimum cascaded voltage source converters forming a two-phase network. The quadrature displaced nearest level modulation strategy fundamentally switches phase A and B legs. Moreover, the solar photovoltaic (SPV) system is modeled at a megawatt-scale and tested in various environmental conditions. Thermal, cost, and leakage-inductance models are presented for the Scott-tied solar MLC. Obtained results demonstrate the satisfactory observation and control of the SPV system for renewable energy applications.