A frequency domain model for beat frequency oscillation analysis in microgrid

Power electronic converters are typical nonlinear systems in frequency domain. With a perturbation frequency excitation, the current and voltage of power converters in steady state contain not only perturbation but also many sidebands. In a Microgrid containing a large amount of power converters, one converter's switching frequency ripples are other converters' perturbations and beat frequency components will be generated if switching frequencies of those power converters are different. As the control loops always take high gains in low frequency regions and the low pass filters almost have no effect on low-frequency disturbances, if the beat frequency component has certain values, they may be magnified and presented as oscillations. This paper develops a frequency domain model to describe the characteristics of power converter around switching frequency range and to analyze the high frequency interactions of power converters in Micorgrid. The voltage mode controlled boost converters are illustrated as demonstration. The proposed model indicates that switching frequency affects the output characteristics of power converters significantly. In a system consists of two boost converters in parallel, the beat frequency oscillation that traditional output impedance models fail to explain could be accurately predicted by the proposed model. In addition, based on this model, design guideline is proposed to avoid the potential beat frequency oscillation in parallel system. Simulation results validate the accuracy and effectiveness of the proposed method.