Online Optimization of Decision Control Variables Ensuring Loss-Minima Tracking in a TAB-Based DC–AC–DC Switching Network

This article presents the formulation, implementation, and execution of the switching network loss optimal hybrid phase-duty control strategy for a three-port dc-dc-ac converter realized by triple-active-bridge (TAB) topology. Keeping energy storage integrated solar inverter in hybrid microgrid application as a design target, the converter under study has a 40V dc input coming from a solar panel, while its dc output port is connected to a 28V battery, and the ac port is connected to a 120Vac 60Hz grid. The work aims at providing an efficient and easy-to-implement solution for incorporating the dynamically varying optimal TAB bridge voltage duty cycle parameters based on the operating load and ac output phase angle condition that ensures the maximum efficiency operation of the high-frequency (HF) switching network. With this purpose, the power losses in the HF switching network (in TAB) are precisely quantified by employing a generalized-harmonic-approximation (GHA) based circuit model. Following that, a detailed mathematical framework is formulated and presented in the paper that synthesizes the polynomial regression models for the optimal duty variables as a nonlinear function of the operating load and ac voltage phase condition. A comparative study on the implementation of different possible polynomial models of the duty cycle parameters is also presented. These developed models are implemented inside the digital controller along with the decoupled PI controllers that control the phase-shifts. With the implementation of the proposed optimal hybrid phase-duty control, the experimental result concludes an average efficiency increment up to 2% compared to the conventional phase shift modulation-oriented TAB control while maintaining an ac voltage THD of 1.78%.