Low-Frequency Scheduler for Optimal Conduction Loss in Series/Parallel Modular Multilevel Converters

Abstract

Various advantages are directing the attention to modular multilevel converters (MMCs). Simple ways to balance the voltage difference of MMC modules are strongly desired but an open problem. The modular multilevel series/parallel converter (MMSPC) can be an efficient solution for this problem. However, implementing an effective control method is essential for close-to-optimum operation of MMSPCs. Generally, on the higher level of control, various modulation techniques can control the output, while a scheduler balances the module voltages on a lower control level. This article proposes a simple yet powerful module scheduling method for MMSPCs. This scheduling method can be combined with many modulation techniques, such as nearest-level modulation (NLM). It reduces the control complexity using the sensorless balancing capability, improves efficiency by minimizing the conduction loss, and achieves a compromise between balancing the module voltages and switching loss, alhough applicable to other modulation methods, simulation, and experiments combine the proposed method with NLM for a comparison with the prior art of scheduling. Based on the presented results and analysis, the algorithm is suited particularly for MMSPCs with a low to medium number of modules. The proposed technique achieves 18%–55% reduction in power loss and 7.5% improvement in averaged capacitor currents compared to other algorithms.