A multi-mode sub-synchronous damping controller for renewable-thermal-bundled power transmission by LCC-HVDC system

With the increase of renewable power generation, multiple poorly-damped or unstable sub-synchronous oscillation (SSO) modes induced by various devices may coexist in the renewable-thermal-bundled power transmission by LCC-HVDC system, including SG-based torsional SSOs and converter-based electromagnetic SSOs. Due to the complex dynamic interactions, it is still challenging to develop the consistent interpretation of the control interactive mechanism for different SSO modes, and to design a supplementary controller for multi-mode SSO suppression. To address these, this paper establishes the system equivalent single-input single-output (SISO) transfer function model for each SSO mode, by retaining its dominant energy storage element dynamic of the correlated device, and then their respective SSO stability is evaluated from a unified damping torque perspective. Subsequently, the interaction paths of key control loops contributing to the damping of each SSO mode are derived. On this basis, a damping enhancement capability index is proposed to identify the preferred control loop for the damping controller placement. Based on phase compensation and gain adjustment of the key control loop interaction path, a multi-mode sub-synchronous damping controller (MSDC) is proposed to realize the targeted damping compensation for different modes. Finally, the proposed MSDC approach is validated on a PVthermal-bundled power transmission by LCC-HVDC system, which effectively suppresses the PV-induced, LCC-induced or SG-induced SSOs.