A Novel DC-Side-Port Impedance Modeling of Modular Multilevel Converters Based on Harmonic State Space Method

With the rapid development of modular multilevel converter (MMC) based DC grids, the resonance and instability issues of these DC grids are getting more and more attention. Though some models of the dc-side-port (DSP) impedance of the MMC have been reported to help the above stability analysis, they always ignore the internal harmonic dynamics of the MMC. Actually, these ignored internal harmonic dynamics are very important things to the MMC, such as the capacitor voltage fluctuations and harmonic circulating currents. Therefore, this paper will further model the DSP impedance of the MMC with the consideration of its internal harmonic dynamics via the harmonic state space (HSS) method. The derived DSP impedance is verified by comparing with the measured one through a nonlinear time-domain model. Furthermore, the impact of the number of harmonics considered in the HSS model on the accuracy of the analytical DSP impedance model is also discussed. The test results show that the proposed DSP impedance model not only covers all the existing DSP impedance models with setting the harmonic order h=0, but also can accurately reflect the internal harmonic effects with setting the suitable harmonic order h.