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

For the offshore 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 aiming at rectifier station is introduced from low-voltage DC micro-grid application for mitigating its DC-side oscillation without affecting the load performance. Viewed from the dc input terminal, the small-signal DC impedance modeling of the overall system is established with considering the influences of DC cable, AC grid inductance and IPOS 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 might easily exceed the narrow control bandwidth of the rectifier station and fall into the negative-damping region, resulting in 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 DC cable variation. Thus the DC-side oscillation is effectively mitigated at the low switching frequency. Finally, simulation and experimental results validate the proposed control method.