New backstepping controllers with enhanced stability for neutral point clamped converters interfacing photovoltaics and AC microgrids

Abstract

This work presents a new approach to obtain pulse width modulation (PWM) backstepping controllers with enhanced stability for neutral point clamped (NPC) multilevel converters to deliver energy from photovoltaic (PV) panels into AC microgrids. Stability enhanced backstepping non-linear controllers are obtained from the equations of the dq frame converter model to regulate the PV voltage to track the power point, and to balance the capacitor voltages through DC biasing of the PWM carriers, using a novel dynamic equation of the capacitors incremental unbalance voltage, while controlling the grid injected AC currents. Besides, the proposed controllers can change the PV panels operating point to curtail inject power for AC voltage / frequency regulation. The NPC converter and AC microgrid are simulated in MATLAB / Simulink and implemented in the laboratory to evaluate the performance of the PV energy conversion using the new stability enhanced backstepping PWM control. Simulation and experimental results show that, regarding predictive controllers, the novel stability enhanced backstepping requires lower microprocessor power than predictive controllers while presenting a similar behavior in PV voltage and power point tracking regulation, or for voltage / frequency regulation of the microgrid. AC injected currents show low levels of total harmonic distortion similar to predictive controllers and can operate at near unity power factor.