Operating Principles of Isolated Cascaded-Choppers DC-DC Converter Based on Variable Duty Ratios

This paper focuses on an isolated cascaded-choppers (CC) DC-DC converter for energy storage systems (ESS) in low-voltage direct-current (LVDC) grids. The CC DC-DC converter presents good controllability, low peak and RMS currents, and high efficiency compared with the conventional three-phase dual-active-bridge (TP-DAB) converter. Despite of its advantages, the CC DC-DC converter still faces the challenges of large peak current caused by zero-sequence DC current and power capacity drop caused by overmodulation, particularly when the product of transformer windings turns ratio and voltage conversion ratio is significantly far from one. To solve these issues, this paper proposes new operating principles based on variable duty ratios in the three-phase bridges of the CC DC-DC converter, which can achieve DC-current elimination and increase power transfer capability simultaneously. The validity and effectiveness of the new operating principes and the effect of DC-current elimination are verified experimentally using a 150 V, 2.5 kW downscaled model.