AC Voltage Sensorless-Based Natural Frame Control of Cascaded H-Bridge Converter Based on Virtual Flux Observer

In this study, a novel natural frame control method without AC voltage sensors is proposed for single-phase cascaded H-bridge (CHB) multilevel converter and implemented in the three-cell CHB converter platform. An improved virtual flux observer (VFO) is introduced to convert the estimated AC voltage signal into the virtual flux (VF) signal and then to vectorize it. The relationship between the instantaneous power and flux linkage is used to achieve independent control of the active and reactive currents. Compared with the conventional single-phase dq control, phase-locked loop (PLL) and coordinate transformation are unnecessary for natural frame control. Furthermore, only orthogonal voltages are required in the proposed control scheme, thus eliminating the time delay caused by constructing orthogonal current signals to improve dynamic performance. The simplification of the entire system is achieved by the omission of AC voltage sensors, the PLL, and coordinate transformations without deteriorating the steady-state and dynamic performances of the entire system. Simulation and experimental results verify the effectiveness of proposed control method.