Small-signal modeling of digitally controlled grid-connected inverters with LCL filters

When LCL filters are applied to digitally controlled grid-connected inverters, the design of controllers is usually implemented using classic average models. The accuracy of these models in s-domain is only guaranteed in low-frequency range. In order to predict the dynamic behaviors, new small-signal z-domain models are deduced for digitally controlled grid-connected inverters with converter current control scheme and converter current plus grid current control scheme. The proposed methods model the inverters including different delay effects under most possible circumstances, which allows direct design for controllers in z-domain. The stability boundaries obtained from the root loci of the classic average models and the proposed z-domain models and the discrete state-space models are compared to the simulation results, showing that the proposed z-domain models are more effective in predicting instabilities. Experimental results are presented and compared to the average models predictions and z-domain models predictions, which shows the proposed models are capable of predicting the values of control variables at the true sampling instants.