Key Technologies Research on AC Excitation System for Large Variable-Speed Units

Addressing the high-voltage, high-capacity, and compactness requirements of 300 MW-class large variable-speed pumped storage units, this paper systematically investigates key technologies of AC excitation systems, including power branch topology, IGBT voltage balancing, compact design methods, and control device architecture. Through comparative analysis of electrical characteristics and engineering applicability of various topologies such as two-level, NPC three-level, and MMC, the diode-clamped three-level (NPC) topology is selected as the optimal solution. To address the series voltage balancing issue of devices in medium-voltage high-power converters, an active voltage balancing strategy based on collector-emitter voltage integration is proposed, combined with RC passive damping snubber circuits, effectively suppressing voltage imbalance. In terms of compact design, a two-column three-level direct-series module structure and laminated busbar design scheme are proposed, significantly reducing parasitic inductance. Finally, the feasibility and reliability of the proposed solutions are verified through pulse tests, power-frequency back-to-back tests, and low-frequency back-to-back tests. The research results can provide technical support for the engineering application of AC excitation systems for large variable-speed pumped storage units.