Application studies of the modular multilevel converter topology to the acceleration grid power supply of the DEMO neutral beam injector

The present conceptual design of the Neutral Beam Injectors (NBIs) for DEMO foresees an acceleration voltage of 1 MV, subdivided in 5 steps of 200 kV each, as for the ITER NBI. This voltage is provided by the Acceleration Grid Power Supply (AGPS), which shall also guarantee the capability of handling repetitive grid breakdowns and provide the required high dynamic response, low ripple and high accuracy. These requirements addressed the ITER AGPS design towards a challenging solution based on multiple conversion stages, where the last rectification and filtering stage is gas-insulated. The research here presented aims at evaluating the possible utilization of Modular Multilevel Converters (MMCs) for the AGPS of future NBIs. A preliminary design of the DEMO AGPS has been carried out, where each 200-kV voltage step is generated by an air-insulated MMC stage operating as a rectifier. The main circuit parameters of the MMC have been determined, considering normal and anomalous working conditions. A numerical model has been developed to verify the converter performances in stationary and dynamic conditions and to simulate breakdown and beam-off events. The area occupied by the proposed MMC-based AGPS has been tentatively estimated and compared to that of the ITER solution. The results of the analysis are shown in this paper. Finally, strengths and criticalities of the MMC solution with respect to the ITER-like approach are discussed.