Stability Analysis and Controller Synthesis for Single-Loop Voltage-Controlled VSIs

This paper analyzes the stability of digitally voltage-controlled voltage-source inverters (VSIs) with the linear voltage regulators. It is revealed that the phase lags, caused by using the resonant controller and the time delay of a digital control system, can stabilize the single-loop voltage control without damping of the LC-filter resonance. The stability region for the digital single-loop resonant voltage control is then identified, considering the effects of different discretization methods for the resonant controller. An enhanced voltage control approach with a widened stability region is subsequently proposed, and a step-by-step design method of the proposed controller is developed based on the root contours in the discrete z-domain. Simulations and experimental tests of a 400-Hz VSI system validate the stability analysis and the performance of the proposed control approach.