Digital Resonant Current Controller for LCL-Filtered Inverter Based on Modified Current Sampling and Delay Modeling

In this article, a digital resonant current controller is proposed for pulsewidth modulation (PWM) control of an LCL-filtered inverter based on a modified current sampling method and time delay modeling. Since the current sampling technique employed here uses a freely tunable sampling instant within the PWM signal period, a corresponding LCL filter model discretization technique is used based on a modified Z-transform. This is applied to model the fractional delay introduced by the current sampling technique, which uses sampling instants from within the PWM signal period. Modified Z-transform-based delay modeling has not previously been employed for LCL-filtered inverters, although it has been used in inverters with an RL load. The proposed technique is implemented with a fifth-order current controller, which directly controls the power converter ac output current without the need for active damping, as is typical for converters with LCL filters. A method based on pole placement is adopted to tune the controller parameters, in which the positions of all the closed-loop poles of the control system are determined. The responses of the controller to reference and disturbance signals are analyzed through simulation and experimental testing for different sampling instants within the PWM signal period.