Power oscillation analysis and control of three-phase grid-connected voltage source converters under unbalanced grid faults

Control of three-phase grid-connected voltage source converter under unbalanced grid faults greatly depends on the active and reactive powers processed by the converter. The instantaneous active power theory with sequence decomposition is employed to analyse the instantaneous power components, especially the second-order oscillation power. Study shows that the second-order oscillation power comprises two quadrature components, the cosine and sine terms, which are contributed by the average active power and the average reactive power, respectively. This finding sheds insight on the regulation of oscillation power under unbalanced grid conditions. Based on this observation, the authors propose a positive and negative sequence conductance and susceptance control scheme, which enables simple regulation of the active power or reactive power oscillation with the average active power and reactive power control. In addition, the authors investigate the relationship between the positive/negative sequence conductance and susceptance distribution factors with power oscillation and peak current. A maximum current limitation scheme is embedded into the current reference generation block for overcurrent protection. Numerical simulations and prototype measurements verify the accuracy of the analysis and the effectiveness of the control scheme.