Virtual Friction for Oscillation Damping and Inertia Sharing from Multi-Terminal VSC-HVDC Grids

This paper proposes a control scheme for multi-terminal HVDC (MTDC) interconnections that introduces an effect equivalent to a mechanical coupling between asynchronous ac networks providing damping of frequency oscillations and frequency support. From the control systems perspective this virtual mechanical coupling is equivalent to a mechanical friction interconnecting the ac networks (modelled as equivalent rotating masses). Also, the control system provides an inertia sharing effect. These two properties can be used to effectively damp frequency oscillations, and provide frequency and inertia support to any ac network connected simultaneously. It is shown that the controller can effectively attenuate poorly damped oscillations that are observed at the MTDC terminals. The dynamic properties of the proposed controller are analysed and its impact of the stability of the three ac networks is evaluated by using a simplified model. The controller was validated by using detailed simulations.