Virtual Positive-Damping Reshaped Impedance Stability Control Method for the Offshore MVDC System

For the offshore medium-voltage direct-current (MVdc) system, the dc-side medium voltage can easily cause high-frequency oscillation and even instability owing to the complex impedance interactions. The virtual-resistance stability control method aiming at rectifier station is first introduced from low-voltage dc micro-grid application for mitigating its dc-side oscillation without affecting the load performance of the inverter station. Viewed from the dc input terminal, the small-signal dc impedance modeling of the overall system is established by considering the influences of dc cable, ac grid inductance, and input-parallel output-series structure of rectifier station. Then, the oscillation mechanism is analyzed by the impedance-based Nyquist stability criterion. It is found that only the virtual resistance deteriorates the stability of the MVdc system under the low switching-frequency condition, because the high-frequency oscillation peak may easily exceed the narrow control bandwidth of the rectifier station and fall into the negative-damping region, resulting in a poor robustness against the dc cable variation. To address this issue, the virtual positive-damping reshaped impedance stability control method is further proposed to maintain a larger positive damper in the actual oscillation frequency range regardless of the variation of dc cable length. Thus, the dc-side oscillation of the offshore MVdc system is effectively mitigated at the low switching frequency. Finally, simulation and experimental results validate the proposed control method.