Complex Vector Admittance Modeling and Stability Analysis of FSC-VSPSUs Under Rapid Speed Mode

Full-size converter variable-speed pumped storage units (FSC-VSPSUs) feature bidirectional power flow. They serve as critical equipment for enhancing regulation margins and promoting renewable energy integration. However, bidirectional energy flow elevates oscillations and instability risks. To address this issue, this paper establishes a complex-vector model of the FSC-VSPSU based on dc-bus decoupling and reveals the underlying mechanism of the internal frequency coupling effect. Comparing the coupling admittance of the FSC-VSPSU with that of a traditional grid-following converter (GFL) reveals that a pair of coupling peaks, absent in traditional converters, appear near the fundamental frequency of the FSC-VSPSU, with the admittance magnitude reaching twice that of the grid-following converter. The influencing factors of system stability under generating and pumping modes are comparatively analyzed using the impedance criterion. This indicates that the machine-side parameters exert a relatively minor impact on grid-connected stability. But the system presents a greater instability risk in pumping mode than in generating mode. Finally, the simulation results validate the correctness of the theoretical analysis presented in this paper.