Split-Multistring 3-Level NPC Photovoltaic Inverter Suitable for Partially Shaded Conditions

To meet the continuous increase in demand of green energy, large solar photovoltaic (PV) plants are being installed worldwide. However, in most of the cases, the large PV plants are underperforming due to the impact of partial shading and/or panel mismatch. To address this, a split-multistring 3-Level NPC photovoltaic inverter is presented that promises to supply good quality of power to the grid with maximum efficiency under partially shaded conditions. In this configuration, each PV string with individual boost converter and MPP tracker is connected across DC link capacitor of NPC inverter, producing higher yield. However, in partial shaded conditions, the voltage unbalance across DC link capacitors due to unequal power generated in individual string produces distortion and affects the quality of output power supplied to the grid. To compensate this, a Carrier Based Pulse Width Modulation (CB-PWM) based voltage balancing scheme is adopted here that reduces the unbalance quickly without introducing low frequency oscillations. The dynamic responses of the system including power control, grid synchronization, Total Harmonic Distortion (THD), unbalance compensation etc. at different irradiation are simulated in MATLAB/Simulink. The simulation results prove the effectiveness of the proposed system at partial shaded conditions and/panel mismatch, which are very common occurrences in medium to large PV plants.