Modular multilevel converter based supercapacitor integration strategies and their comparative evaluation for railway traction drive systems

Modular multilevel converters are an emerging class of converters for traction applications. Supercapacitor energy storage integration within the sub-module provides multiple advantages compared to the traditional two-level design in terms of reliability, power quality, drive efficiency and recovery of regenerative energy. This paper explores different topologies of modular multilevel converters with integrated supercapacitor energy storage system for the typical voltages (750 V, 1.5 kV and 3 kV) of the railway traction system. The paper first derives different ways of integrating supercapacitor modules using a modular multilevel configuration. Thereafter, a detailed comparison in terms of converter cost/VA rating, reliability and suitability for traction drive systems have been presented to identify the most suitable modular converter scheme with integrated energy storage.