Control of Direct AC/AC Modular Multilevel Converter in Railway Power Supply System

In the railway power supply system, the direct three-to-single phase AC/AC Modular Multilevel Converter (MMC) has received lots of attention due to its salient features compared to the traditional traction transformers. Recent researches mainly focus on the single-phase side 16.7Hz situation, and the single-phase side 50Hz case has not been detailed discussed. In this paper, the authors firstly establish the steady-state model of MMC, and derive the expression of capacitor voltage variation in Submodules (SMs). The severe voltage imbalance between upper-and lower-arms is observed in the analysis when MMC operates at single-phase side 50Hz condition. To eliminate this threat and ensure safe operation of MMC, the high-frequency voltage and current injection method is applied, which effectively balance the voltage between upper-and lower-arms. The overall control and different levels of capacitor voltage control are presented in details. Simulation result in PSCAD/EMTDC software environment verifies the effectiveness of arm voltage balance method.