Power main circuit design of the flexible excitation systems for large hydropower units

In response to the system voltage stability and damping issues caused by insufficient dynamic reactive power support capability of renewable energy units， this paper proposes a novel power main circuit topology for flexible excitation systems suitable for large hydropower units. Based on the characteristics of large hydropower units， a main circuit consisting of a front-end active neutral-point clamped （ANPC） three-level converter and a novel rear-end three-level chopper is designed. Key parameters such as DC-side voltage， voltage and current withstand capability of power device， DC-side capacitance， and grid-side inductance are analyzed. Through the Simulink simulation platform， the performance of the flexible excitation system is compared with that of a conventional thyristor excitation system. The results demonstrate that the flexible excitation system exhibits significant advantages in terms of strong excitation performance and support capability under fault disturbances， effectively reducing oscillation time and enhancing power system stability. This research provides a theoretical basis for the optimal design of excitation systems for large hydropower units and lays the foundation for the engineering application of flexible excitation systems.