Analysis of an LCL-Isolated Modular Multilevel DC-DC Converter

One of the key elements of realizing DC grids is efficient DC-DC converters for medium voltage applications. The Isolated Modular Multilevel DC-DC (IMM DC-DC) converter is a promising solution for interconnection of the Medium Voltage DC (MVDC) grids. However, the main drawback of this converter is its high reactive power and conduction losses, when it operates at its maximum power. An LCL resonant filter can substantially reduce the reactive power on the High Frequency Transformer (HFT) terminals of the converter and consequently, reduce the conduction losses of the converter. This paper presents a comprehensive analysis of the LCL-IMM DC-DC converter. In this paper, first, the design of an LCL resonant filter is presented. Second, the impact of the LCL resonant filter on the reactive power, the HFT primary- and secondary-side currents, upper- and lower-arm currents, and soft-switching operation of the converter is investigated. The presented analysis is validated by time-domain simulation studies of a test converter in the Matlab/Simulink software environment.