Three-level NPC inverter with novel voltage equalization for PV grid interface suitable for partially shaded conditions

Abstract

In a conventional grid connection of Photovoltaic (PV) system, a number of PV modules are connected in series and integrated to grid through a conventional two level inverter. Due to the impact of partial shading and module mismatch, such systems have a much lower yield compared to their maximum potential. To address this problem, direct connection of PV source to a three-level diode clamped inverter is considered in this paper, resulting in a single stage power conversion. A novel auxiliary converter is proposed that allows the two split halves of the PV source to be directly connected to the two dc-link capacitors, with each half operating at it's respective Maximum Power Point (MPP). This auxiliary converter can be rated for less power converter compare to the inverter capacity. The unequal MPP voltage of these split PV panels may cause detrimental effect on output current of inverter. To address this power quality issue, a modified switching algorithm is discussed to achieve voltage equalization across the split DC-Links of the multilevel inverter. Though the case of a split PV source (with two halves) has been considered, the proposed scheme is equally valid for a PV array split into more than two parts that may be suitable for a higher level Neutral Point Clamped (NPC) inverter. The proposed auxiliary converter and algorithm are evaluated and verified by simulation in MATLAB-Simulink.