Analysis and controller design for stand-alone VSIs in synchronous reference frame

The common practice for controlling the stand-alone voltage source inverters (VSIs) is to transform abc voltage and current signals to DC signals using the dq transformation, which makes it possible to control the new DC voltage and current signals just using simple proportional-integral controllers with a zero steady-state tracking error. However, the transformation causes coupling terms between d- and q-axis components in both current control loops and voltage control loops, which deteriorate the dynamic and steady-state performance of the control system. This study presents a step-by-step graphical analysis and design approach for a three-phase stand-alone VSI system in the dq reference frame, which provides a clear systematic decoupling process to design the inner- and outer-loop current and voltage controllers, respectively. The closed-loop d- and q-axis output impedances of the stand-alone VSI system by considering the coupling effects between axes are also formulated, modelled, and evaluated. The study investigates how the decoupling and compensating terms added in the current and voltage controllers affect the dynamic performance and output dq impedances of the VSI system. Simulation and hardware results verify the effectiveness of the graphic design and analysis strategy.