Modelling of three-phase grid-connected inverters

Grid-connected inverters are key elements that interface the distributed energy sources with the grid. In this paper, a graphic modelling method based on switching flow graph is presented for modelling inverters, with a one-cycle controlled inverter as an example. This method features great simplicity and accuracy, and is compatible with dynamic system-simulation software, such as MATLAB. PSPICE simulations were conducted to verify the switching flow graph model in three cases: steady-state, step response and unbalanced voltage conditions. The results show that the discrepancy between the model and the PSPICE output is within 1.5%. In addition, experiments were conducted for the inverter under unbalanced conditions and the test results are very close to the prediction of the switching flow graph model. The inverter models can be incorporated into a power system simulation package to perform large system dynamic and stability analysis for systems containing inverters.