A Multilevel Medium-Voltage Inverter for Step-Up-Transformer-Less Grid Connection of Photovoltaic Power Plants

Recently, medium (0.1-5 MW) and large (> 5 MW) scale photovoltaic (PV) power plants have attracted great attention, where medium-voltage grid connection (typically 6-36 kV) is essential for efficient power transmission and distribution. A power frequency transformer operated at 50 or 60 Hz is generally used to step up the traditional inverter's low output voltage (usually <= 400 V) to the medium-voltage level. Because of the heavy weight and large size of the power frequency transformer, the PV inverter system can be expensive and complex for installation and maintenance. As an alternative approach to achieve a compact and lightweight direct grid connection, this paper proposes a three-phase medium-voltage PV inverter system. The 11-kV and 33-kV PV inverter systems are designed. A scaled down three-phase 1.2-kV test rig has been constructed to validate the proposed PV inverter. The experimental results are analyzed and discussed, taking into account the switching schemes and filter circuits. The experimental results demonstrate the excellent feature of the proposed PV inverter system.