Power-Flow Decoupling Method for the Modular Magnetic-Coupled Converter (MMCC)

Abstract

The modular magnetic-coupled converter (MMCC) contributes to the significant reductions in dc capacitance and transformer volume in medium-voltage ac–ac applications (Du et al., 2024). However, the internal magnetic-coupled dc–dc stage with none-resonance operation suffers from serious power-flow cross-coupling issue, which prevents MMCC from hybrid ac–dc and pure dc–dc applications. To solve this problem, this article proposes a power-flow decoupling method. For the first time, it decouples the complex power-flow relationships into a concisely linear relationship via mathematical model simplification. The simplification is realized by defining coordinate with real-axis aligning to the sum average phasor and by quantifying the sum average phasor amplitude with triangular approximation. As compared to the prior software and hardware solutions, 1) the proposal decouples the power flows by simplifying the mathematical model rather than by arbitrarily resolving the complex relationships, leading to more than 99.5% reduction in calculation burden in multiple port applications; and 2) it secures the reliable decoupling operations even under dc port fault condition because it does not rely on the hardware of any dominant active bridge. Nevertheless, the proposed method imposes extra requirement on the LC-tank inductance, which has to be specially designed. The validation of the proposed method is fully confirmed by the 200-V 3.0-kW experiments.