A model-based controller of a three-level stacked-cell grid connected converter

This paper presents a model-based controller for a single-phase grid-tied transformerless multilevel PV inverter. The selected topology is known as single-phase three-level stacked-cell (3L-SC). It is based on the conventional H-bridge, where a multilevel output voltage is achieved by using a split DC-link with an active bidirectional switch. The proposed controller comprises a current control loop aimed to guarantee active power injection, and a voltage balance loop aimed to balance the capacitor voltages in the split DC-link. In the inner loop, a sinusoidal current in phase with the grid voltage is injected in spite of harmonic distortion present on the voltage signal. For this, the inner loop includes a particular modification of a repetitive scheme, which is able to handle a fractional delay issue. A pulse width based modulation strategy is also proposed to generate proper switching sequences for the switching devices in the multilevel inverter. Numerical results are presented to show the performance of the proposed solution.