Explore the Operating Limit of Cascaded H-Bridge Converters with Uneven Power Distribution

Abstract

When different H-bridge cells in a cascaded H-bridge (CHB) converter distribute uneven powers, the individual DC voltage balancing control is vital for the CHB converter to maintain stability. Hence, the limited ability of voltage balancing control specifics the operating limit of such a CHB converter. Existing analysis on the voltage balancing limits is based on the limitation of the duty-ratio. However, it usually has a large error, especially in low switching frequency conditions. This paper first reveals another important influencing factor on the voltage balancing limits. It is the harmonic-power-unbalance (HPU) caused by the voltage balancing control. The mechanism of the HPU is that the voltage balancing control-induced uneven regulations of duty-ratios break the symmetry of CPS-PWM, leading to low-order switching harmonic currents in the CHB; and then the harmonic current multiplies the corresponding harmonic voltage of each cell, resulting in uneven active powers which can intensify the DC voltage unbalance. A comprehensive dual-loop system is built to depict the interaction between the voltage balancing control and the HPU, and thereafter, be used to analyze the voltage balancing limits of a CHB converter. The results show that the voltage balancing limits are sensitive not only to the duty-ratio but also to the filter inductance and the switching frequency. These theory and results give a good guide to the analysis, design, and operation of CHB-based converters.