Submodule Capacitor Voltage Balancing Through High-Frequency-Side Control for Multiport MMC-Based Solid-State Transformers

Abstract

This paper proposes a novel submodule capacitor voltage balancing control for multiport modular multilevel converter (MMC)-based solid-state transformers (SSTs). To balance each submodule capacitor voltage, the balancing control is usually applied to the MMC. However, the low switching frequency operation and bulky arm inductance of the MMC limit the controller bandwidth and stability margin. These issues are addressed by moving the balancing control to the back-end DC/DC converter which is operated at high switching frequency. The controller gains are designed based on the small-signal model. The superiority of the proposed controller over the conventional one has been verified through Bode plot, pole-zero map, and Nyquist path analyses. Experimental results for a 2.4 kW prototype system have also verified the accuracy of the model and effectiveness of the proposed balancing control for step load changes.INDEX TERMS Balancing control, modeling, modular multilevel converters, solid-state transformer. FIGURE 1. Basic structure and application of a multiport SST.