Small-Signal Modeling and Fixed-Gain PI Voltage Control of a Triple-Active-Bridge Converter Under Different Power Flow Conditions

Triple-Active-Bridge (TAB) converters provide bidirectional isolated power transfer among three DC ports and are well suited to electrified-vehicle power architectures that must coordinate a traction battery, auxiliary energy-storage or generation units, and regulated DC buses. In such systems, changes in vehicle operating mode or energy-management strategy can redistribute power among the ports and thereby modify the local converter dynamics seen by the voltage controllers. This paper presents an operating-point-dependent small-signal modeling and fixed-gain proportional–integral (PI) voltage control study of a voltage-fed TAB converter under two representative power flow configurations: single-source dual-output operation and dual-source operation. A reduced harmonic state-space model is linearized around selected equilibrium conditions to obtain a 2 × 2 control-to-output transfer function matrix that retains both direct and cross-coupled dynamics. Fixed-gain PI voltage control is investigated in single-source dual-output operation. Additional dual-source simulations evaluate Port-3 voltage regulation with the Port-2 generator command held constant, while independent fixed-command perturbations quantify output voltage sensitivity and power redistribution. Closed-loop voltage responses of the nonlinear fundamental-harmonic model and the detailed switched simulation are compared using a 110 V reference and identical PI gains. This comparison illustrates behavior under the selected controller settings but does not establish quantitative validation of the linearized model. Root-locus and closed-loop transient analyses are used as loop-level diagnostics of the fixed PI tuning. A laboratory prototype based on three silicon-carbide active bridges, a three-winding high-frequency transformer, and a TI F28379D digital controller further demonstrates reconfigurable operation under the two source/load assignments. The ideal model analysis quantifies local voltage-channel interaction, while the additional dual-source simulations demonstrate how generator-command changes affect output voltage and source power allocation. For multi-source EV and charging power architectures, this finding indicates that operating-point variation should be considered when applying conventional fixed-gain PI regulation to isolated multiport converters.