Simulation Study of 4 kV, 5A Modular Multilevel Converter as a Rectifier for Neutral Beam Injectors

The Modular Multilevel Converter (MMC) is a multilevel converter topology which is majorly in use for medium/high-power applications, specifically for high-voltage direct current (HVDC) transmission systems. The MMC is well known for its modularity, scalability, controllability, low expense for redundancy and utilization of conventional components. This paper conducts a comprehensive examination of a 4 kV, 5 A three-phase Modular Multilevel Converter (MMC) operating as a rectifier, with specific relevance to its application in a Neutral Beam Injector (NBI) system. This paper addresses the primary technical challenges in implementing MMC, namely, minimizing the dominant second harmonic circulating current, balancing the voltage across isolated capacitors, smooth and fast transient response under open loop operation. Additionally, the study scrutinizes the impact of switching frequency on the MMC rectifier's output response, considering waveform shape, mean value, and rise time, while holding all other parameters constant. These findings offer valuable guidance for designing and operating MMC rectifiers in practical applications.