Concept design of the control method for the NBI acceleration grid power supply-conversion system of CFETR

Abstract The China Fusion Engineering Test Reactor (CFETR) uses a negative-ion neutral beam injection (N-NBI) system that requires a powerful acceleration grid power supply (AGPS). The AGPS is rated at -1000 kV/80 A and feeds one injector. AGPS-conversion system, which uses a three-level neutral-point-clamped active front-end rectifier as well as a five-level neutral-point-clamped / cascaded H-bridge inverter, was investigated in the reference design. Because the control problem of the conversion system is crucial for the AGPS, a well-designed control method can improve the steady-state performance during operation substantially. To achieve the full potential of the proposed design and meet the challenging demands of the CFETR N-NBI system, a new control method is proposed and investigated in this paper. Considering the requirement to provide a wide range output voltage, the new control method provides an adjustable DC-link voltage for the rectifier with unity power factor and a suitable modulation factor for the inverter, while maintaining a low ripple voltage of output voltage. The steady performance of the AGPS is successfully improved using phase-shifted carrier-pulse-width modulation. Simulation results are performed that validate the new control method.