Fully discrete-time domain model and damping characteristics accurate analysis for grid-connected inverter with LCL filter

Grid-connected inverter with LCL filter, which is regarded as a common interface, plays a significant role in distributed generation system. In order to promise the steady-state and dynamic performance, large damping ratio is expected due to weak damping ability of grid-connected inverter with LCL filter. However, it is still unclear to calculate the damping ratio accurately and analyze maximal damping performance in fully discretetime domain. To address this issue, this paper presents an improved damping characteristic analysis strategy which can realize the maximal damping ability. Furthermore, the proposed analysis strategy can adapt to both single-phase and three-phase systems well. Firstly, an accurate damping ratio calculation expression of damped plant model is derived in fully discrete-time domain. Then, based on the expression, the damping characteristics is accurately analyzed by considering three influence factors simultaneously, i.e., digital control delay time, the ratio of resonance angular frequency to sampling frequency and feedback algorithm. Thirdly, to realize the largest damping ability, the maximal damping ratio (MDR) curve based on damping characteristics is presented, which also provides a guideline to design corresponding controller algorithm in practical application. Finally, the proposed analysis strategy is verified by experimental results and deep explanations are also given in detail.