Electromagnetic Transient Modeling, Simulation, and Characteristic Analysis of a High-Head Hydropower Unit Islanded via Flexible DC Transmission System
This study addresses the electromagnetic transient (EMT) modeling and operational stability of high-head hydropower units connected to a modular multilevel converter-based high-voltage direct current (MMC-HVDC) islanded transmission system, a configuration critical for developing hydropower resources in Tibet. A refined model of the high-head unit incorporating dynamic penstock characteristics and a detailed MMC-HVDC system model are established. An initialization method tailored for hydro-DFACTS EMT simulation is proposed, and a complete model is implemented on the CloudPSS platform. Simulation results demonstrate that the dynamic characteristics of high-head units degrade short-term stability compared to conventional units. A pronounced hydraulic-electrical coupling between the unit and the MMC-HVDC system can induce ultra-low-frequency oscillations in the sending-end system. Furthermore, the system exhibits elevated subsynchronous oscillation risk in the [30, 50] Hz band, with phase differences exceeding 180°. The study concludes that high-head characteristics are a key factor influencing small-signal stability, necessitating refined modeling of the penstock and water turbine for accurate stability assessment.