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The resonance or instability phenomena can happen due to the impedance interactions between the modular multilevel converter (MMC) and the wind power inverter in an MMC-HVDC system connected with offshore wind farms which consist of full-power converters. To analyse the stability of the interconnected system, the impedance-based Nyquist stability analysis method is employed. The analytical ac-side small-signal impedance model of a three-phase MMC is first developed in this paper. The analytical impedance model is verified by means of injecting a small perturbation voltage/current signal in a nonlinear time-domain simulation model. The impacts of the controller parameters on the stability of the interconnected system are then examined by using frequency-domain analysis approach, which can offer guidance for controller parameters design. Finally, the time-domain simulation results validate the theoretical analysis.
Lyu et al. (2015) studied this question.