Impact of controller parameters on stability of MMC-based HVDC systems for offshore wind farms

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.