Individual Module Power Control of Solid-State Transformer for Multiport Configuration in Railway Traction Systems

This article proposes an individual module power control (IMPC) method for multiseparated winding (MSW)-solid-state transformer (SST) to realize multiport configuration in railway traction systems. In general, SST for railway traction systems features an input-series-output-parallel (ISOP) structure composed of cascaded H-bridge (CHB) multilevel converters and isolated dc-dc converters. This structure allows all rear stages, including the propulsion inverters, to be powered from a common dc output, necessitating additional dc-dc converters for interfacing with dc loads and batteries. Therefore, this article proposes the IMPC method for independently controlling module power in CHB, enabling multiport configuration. This approach allows direct connection of dc loads and batteries to SST modules without additional dc-dc converters while maintaining the characteristics of modularity. As a result, a small bulk and low construction cost can be achieved. To realize the IMPC method, the following steps were taken. First, the modulation method of CHB is suggested to synthesize the switching reference voltages individually considering the maximum power delivery. Second, the reactive current injection strategy is explained for full-range operation regardless of load conditions. Third, the linear modulation condition of IMPC is analyzed. Finally, the control block diagram implementing IMPC is presented. The validity of IMPC is demonstrated through experimental results.