Design of a 5.6 MW Three-Level Neutral-Point-Clamped Converter for the CRAFT NNBI Power Supply

Neutral beam injection (NNBI) is an auxiliary heating method for magnetically confined fusion, in which the accelerating power supply must maintain stringent dynamic stability. This work focuses on a 5.6 MW three-level neutral-point-clamped (NPC) converter developed for the CRAFT NNBI acceleration power supply, which operates on the low-voltage side of the HV accelerating stage. The system consists of a phase-controlled rectifier, an LCR passive filter, and an NPC inverter. To achieve precise regulation of the adjustable output voltage, a two-stage control strategy is adopted. First, the DC-bus voltage is controlled; then, the NPC duty cycle is modulated. By coordinating these two stages, the ripple in the final high-voltage DC output is maintained within the required envelope. The transfer function is derived, and the overall scheme is verified in MATLAB/Simulink. Experimental results over 20–100% of rated operation show an 80 ms rise time, approximately ±3.75% ripple (15 kV peak-to-peak at 200 kV), and a 17 µs device-level fault-protection time. These results demonstrate that the converter meets the CRAFT NNBI stability and dynamic-response requirements.