Control of grid-tied photovoltaic systems with neutral point clamped inverters

This paper presents the operation of a grid-tied photovoltaic power generator considering a two stages configuration for active power injection. A Neutral Point Clamped Inverter (NPC Inverter) was used for the DC-AC conversion and a DC-DC Boost converter complemented the system configuration. The control system includes the following loops: a maximum power point tracking algorithm, a DC bus voltage control, a reference generator, a capacitors voltage balancing loop, a current controller and a Level Shifted Pulse Width Modulation (LS-PWM) technique. The work is mainly focused in balancing the voltages in the DC bus capacitors by adding an offset level to the modulation signal which ensures the good performance of the NPC inverter. The operation of the system was evaluated in a PSIM® software environment for different irradiance conditions. The results show the outstanding performance of the photovoltaic system specifically in the Maximum Power Point Tracking (MPPT), the DC side voltages balance in the NPC inverter and the active power control.