Investigation of three-level NPC-qZS inverter-based grid-connected renewable energy system

The neutral point clamped quasi-Z-source (NPC-qZS) inverter is poised to become a potential candidate for renewable energy applications because it yields a continuous input current and voltage boost. In this study, the closed-loop control of grid-tied three-phase (3-phi) NPC-qZS inverter, fed with renewable energy sources, is proposed for the first time. A dynamic model has been developed to accurately design the control strategy. The proposed strategy includes the control of grid-tied current and the peak dc-link voltage (PDV). The control of grid-tied current is achieved through a damped-second-order-generalised integrator. The PDV is estimated indirectly from the voltages of qZS network capacitors and is regulated by an integral-double-lead controller. Two modified modulation techniques based on phase-opposite disposition and in-phase disposition are proposed to yield shoot through by injecting 3rd harmonics for maximum constant boost control. A comparison is drawn between the performance of the proposed controller and sliding-mode controller on the dc side. The controller design and system performance are validated through real-time simulation and experimentation on a practical setup in the laboratory.