Supervisor:
Zhe Chen

Co-Supervisor:
Yanbo Wang, Dong Liu

Assessment Committee:
Associate Professor Sanjay Kumar Chaudhary (Chair)
Professor Christoph Michael Hackl, Institute for Sustainable Energy Systems, Munich Uni-versity of Applied Sciences
Prof. Francesco Castelli-Dezza, Department of Mechanical Engineering, Politecnico di Milano

Moderator:
Sanjay Kumar Chaudhary

Abstract:

Bidirectional DC/DC converters interfacing the distributed DC power generations, loads and energy storage facilities are being employed as a key part for energy conversion and system coordination in the future energy system. Dual-active-bridge (DAB) converters are widely regarded as one of the most effective bidirectional DC/DC converters due to their ability to support bidirectional power flow and provide galvanic isolation. Nevertheless, conventional DAB converters face challenges related to voltage stresses when interconnecting low voltage (LV) and medium voltage (MV) DC buses. In applications that requires high input or output voltage levels, such as PV systems with Maximum Power Point Tracking (MPPT) and batteries during charging and discharging process, the operating volage gain of conventional DAB converters is limited. Furthermore, achieving more compact designs with higher power density and lower cost is crucial in commercial applications.

This thesis focuses on hybrid-bridge (HB) based bidirectional DC/DC converters, addressing the aforementioned limitations. In this thesis, both the structures and control strategies have been investigated and further optimized to address alternative challenges. To achieve higher compactness, an HB converter solution with a DC blocking capacitor is proposed by eliminating two clamping diodes from the existing neutral-point-clamped (NPC) type converters, thereby reducing the cost and improving overall energy efficiency. To extend the operation range, a pulse width modulation and single phase shifting (PWM-SPS) hybrid control strategy is proposed for HB converters. An asymmetric active-neutral-point-clamped (A-ANPC) structure is employed to maintain controllability of the operating voltage gain, exceeding the capabilities of traditional NPC converters. To realize seamless transition between forward mode and backward modes, a sinusoidal-wave modulation method is proposed for HB based series-resonant DC/DC converters as a specialized modulation strategy with fixed switching frequency. The proposed method allows the voltage conversion gain to remain independent of the load.

Overall, this work significantly advances the performance, compactness, and cost-effectiveness of bidirectional DC/DC converter for future energy system.