Design of a Grid-Forming, Multi-Loop Control Scheme for Parallel Connected, Three-Phase Quasi-Z-Source Inverters

The quasi-Z-source inverter has been the subject of numerous power electronics publications since its invention in the early 2000s. While often applied as an interface for renewable energies such as wind and PV, there is a lack of literature where the qZSI functions in a grid-forming role. The goal of this work is to design and implement a control scheme for a 3-phase qZSI in order to enable it to operate in a grid-forming role. The qZSI is analyzed in great detail and compared to a conventional voltage source inverter scheme that is commonly used in renewable energy interfaces. Literature is presented to demonstrate the need for current programmed mode (CPM) control for the dc side of the qZSI, and the control scheme is compared to a common control scheme used for boost converters. Then, recent literature is presented to support why the universal droop control (UDC) scheme was chosen for the ac-side control for this work. After contextualizing the design of the overall qZSI control scheme, the final control scheme is presented, which utilizes CPM indirect control on the dc-side, and UDC on the ac side. The stability and dynamic response of the system is analyzed in detail for the chosen gains and component values. Through PLECS simulations, the qZSI system presented in this work demonstrated its ability to operate in a grid-forming role and potentially superior performance when compared to conventional VSI systems. While a much more optimized design approach is needed for both the qZSI and VSI to truly compare the two systems, this work demonstrates that the qZSI is more than capable of operating in a grid-forming role. It handles large step changes in load and input voltage with quick rise times and good damping, and exhibits quick and stable responses when operating in parallel with other inverters. This work concludes with some considerations for future work on this topic