Damping Low-Frequency Oscillations Through VSC-HVdc Stations Operated as Virtual Synchronous Machines

With proper control methods, grid-connected power converters can mimic the dynamics of conventional synchronous generators (SGs), i.e., acting as virtual synchronous machines (VSMs), for improving the grid frequency stability and ensuring a smooth transition toward converter-dominated power systems. However, the integration of VSMs will inevitably influence the low-frequency oscillations (LFOs) in power grids which are usually caused by the interactions among SGs. This influence should be of concern especially when the VSM's capacity is similar to that of SG, e.g., operating voltage-source converter (VSC)-HVdc stations as VSMs. This paper investigates how the VSM affects LFOs in power systems by analyzing its equivalent damping torque. We show that with proper control design, VSMs can provide considerable positive damping torque to effectively damp LFOs in power grids. Particularly, using the damping torque analysis, the influences of virtual impedance and grid frequency detector (i.e., phase-locked loop) are elaborately studied so as to provide design guidelines for better damping LFOs. Finally, case studies of a two-area test system with integration of a VSM-based VSC-HVdc station verify the validity of the analysis.