Analysis of a seven-level inverter based on HB-NPC topology for PV transformerless applications

This paper presents a multilevel converter for a photovoltaic transformerless application. The proposed application consists of a power converter with asymmetrical DC-link voltage, referred to as 7L-HBNPC converter, able to inject active power into the grid, producing seven output voltage levels and mitigating the leakage-ground currents. The converter uses the conventional HBNPC connection to reach seven output voltage levels. The strategy to mitigate the leakage ground currents to the grid consists of a DC-link tied LC filter. This filter is formed of two output inductors with a couple of capacitors connected to the DC-link terminals. Thus, the model of the converter based on the common-mode and differential-mode variables is presented for this filter. Besides, based on the differential-mode model structure, a control scheme is proposed. This control scheme is aimed to guarantee active-power injection to the grid despite changes in the power of the reference. Besides, the common-mode model provides information on the behavior of the leakage–ground current compensation. The seven output voltage levels are obtained by a particular design of the modulation scheme structure which produces symmetrical seven output voltage levels in a specific DC voltage unbalance-value. Finally, the performance of the proposed application is assessed using numerical simulations in a 5 kW test converter.