Harmonic analysis model for voltage source converter under unbalanced conditions

Voltage source converters (VSCs) generate harmonics under unbalanced conditions. With the wider applications of VSCs, the harmonics produced by VSCs will provide new challenges of harmonic problems in the modern power system. The harmonics generated by VSCs are caused by the switching characteristics of power electronic devices and the interaction between the ac and dc sides, which is a non-linear and complex problem. Dynamic phasor sequence components (DPSCs) are applied into the analysis of the harmonic produced by VSC in this study. With selecting the DPSCs of current harmonics on ac side and voltage harmonics on dc side as the state variables, a harmonic analysis model of VSC is presented as a linear time invariable system in the state space. The model considers both the detailed switching dynamics and the interactions between the ac and dc sides of VSC. The model also shows the analytical expression of the relationship between harmonics and sequence components of supply voltages. By solving the model, the harmonics generated by VSC under unbalanced conditions can be calculated mathematically. The comparison with simulation and experiment results can testify that the analysis model is very useful to analyse the harmonics of VSCs directly.