Impedance Modeling and Stability Analysis of DR-MMC Based Hybrid HVDC for Offshore Wind Farm Integration

The diode rectifier- modular multilevel converter (DR-MMC) based hybrid HVDC system is an attractive topology for integrating large-scale offshore wind power. To investigate the interaction stability of the cascaded DR-MMC converter integrating with offshore wind farms, this article proposes the sequence impedance model considering frequency coupling among DR, MMC, controllers and DR commutation process. Firstly, the frequency interaction between DRs and MMC at the DC side is disclosed. Then, using the equivalent impedance method, frequency-coupling terms that mainly affect impedance characteristics are analyzed. The impacts of wind power curtailments and two reactive power compensation methods on wideband impedance characteristics are revealed. It is shown that the decrease of wind power will weaken the system phase margin. And the reactive power compensation via MMC is more prone to instability than that via AC filters. Besides, the impedance characteristics and stability between DR-MMC and classic MMC systems are compared. The correctness of stability analysis based on the proposed sequence impedance models is validated by simulations in PSCAD/EMTDC.