An Additional Adaptive Active Damping-Based Strategy for Suppressing Oscillations in Islanded Hydropower HVDC Sending Systems

In islanded hydropower high voltage DC (HVDC) sending systems based on modular multilevel converters, the sending-end system is usually characterized by long electrical distance, weak system strength, and insufficient inertia support, which may easily induce oscillations between the converter station and the grid-side AC system. To address the medium- and high-frequency oscillation problem between the converter station and the AC system in islanded hydropower HVDC sending systems, this paper first establishes an MMC AC-side impedance model suitable for medium- and high-frequency oscillation analysis based on harmonic linearization. A dual-resonance-point model is adopted to equivalently represent the sending-end AC system impedance, and the impedance method is used to analyze the stability of the interconnected system. Then, by comparing the effects of three conventional feedforward additional active filtering methods on MMC impedance improvement, the limitations of conventional methods in oscillation suppression are identified. Furthermore, based on the conventional methods, a multi-link adaptive active damping strategy is proposed from the perspectives of the power loop, voltage feedforward, and phase compensation control links, which consists of power loop filtering, feedforward cascaded notch filtering, and adaptive phase compensation. Finally, an equivalent simulation model of the islanded hydropower HVDC sending system is built in PSCAD, and the effectiveness of the proposed strategy in suppressing oscillations between the converter station and the AC system is verified.