A Pseudobinary Cascaded H-Bridge Converter for the Front-End Stage of Solid-State Transformers

A converter based on the binary cascaded H-bridge (BCHB) regarded as the pseudobinary CHB (PBCHB) is proposed for the ac–dc front-end stage in solid-state transformers (SSTs). The hybrid modulator of the BCHB produces step-like sinusoidal waveforms with near-line-frequency commutations in the high-voltage cells and high-frequency commutations in the low-voltage cell. The power stage design of high-voltage cells transferring the active power can be modularized. However, the power transfer among them is unequal, and the issue has not been investigated. This article proposes a 15-level PBCHB that maintains a modular design while ensuring balanced power transfer and negligible switching losses among the high-voltage cells, regarded as active cells. The low-voltage high-frequency cell, which is regarded as the floating capacitor cell, is used as a power quality enhancer without real power consumption. Consequently, the original BCHB architecture is slightly modified along with the controller design to become PBCHB, balancing the power transfer among the active cells while maintaining equal voltage levels at their dc side. In this article, the design and analysis of the modulator and its controller for the PBCHB are presented in detail. The effectiveness of the proposed front-end SST with the proposed modulation and control technique is verified in a 1.2 kV/3 kW single-phase prototype.