Models and effects of different updating and sampling concepts to the control of grid-connected PWM converters — A study based on discrete time domain analysis

PI-based current control of voltage source pulsewidth-modulation (PWM) converters with L- and LCL-filters is addressed. The design and analysis of these systems require proper system modeling. Computational and sampling delays are most important in this context. Three widely used PWM and sampling concepts are considered: single-update PWM with sampling at the start of the switching period, single-update PWM with sampling in the middle of the switching period, and double-update PWM with updating and sampling twice per switching period. For each, the resulting delay is accurately modeled. Moreover, this paper presents a design method, in all three cases, of a PI-based current-control using discrete-time analysis. The derived models are analyzed by root-locus and step response analysis and thoroughly verified by measurements. Additionally, the impact of updating and sampling strategy on an LCL filter system with different characteristic resonance frequencies is analyzed.