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Title: Topologies and modulation schemes for immittance based three-phase dual-active-bridge DC-DC converter to achieve wide-range high-efficiency performance
Authors: Khan, Akif Zia
Degree: Ph.D.
Issue Date: 2020
Abstract: Three-phase dual-active-bridge (3p-DAB) converter is an attractive choice for bidirectional power conversion in high-power applications, however, conduction loss caused by circulating current and shrinkage of soft-switching region under varying input-to-output voltage ratios and different load conditions are the main bottlenecks that severely affect its efficiency performance. To address these issues, three different immittance based topologies and modulation schemes have been proposed in this work for the 3p-DAB converter. The proposed topologies and modulation schemes are targeted to minimize the switching loss and conduction loss simultaneously over the entire operating range to achieve wide-range high-efficiency performance. Based on this backdrop, the thesis has been classified in six chapters. A brief description of each chapter is presented below: Chapter 1 discusses about the background, importance, some emerging applications and functional overview of isolated bidirectional dc-dc converters (IB-DCs). Through detailed discussions, 3p-DAB converter is identified as a preferred IBDC topology for high-power applications and its limitations are presented. Moreover, the main objectives of the thesis are presented in this chapter that includes overcoming the limitations of conventional 3p-DAB converter to achieve wide-range high efficiency performance under varying input-to-output voltage ratios. Finally, an outline of the thesis concludes chapter 1. Chapter 2 gives an overview of the conventional 3p-DAB converter from the topology and control perspective. A comprehensive literature review is also presented in this chapter to update the readers about the work done on the 3p-DAB converter to improve its efficiency performance. Moreover, the research gaps in the existing literature are also highlighted in this chapter that the current research work has attempted to bridge. Chapter 3 proposes a 3p-DAB resonant immittance (3p-DAB-RI) converter that can achieve unity-power-factor operation at all of its ac ports leading to reduced RMS port current, lower conduction loss and complete elimination of reactive power. Moreover, it can also achieve full-range zero-voltage-switching (ZVS) irrespective of variations in input-to-output voltage ratios to diminish the switching loss. However, for unity-power-factor operation, 33 % of the switches are hard-switched leading to increased switching loss whereas for full-range ZVS operation, 3p-DAB-RI converter suffers from high conduction loss due to increased circulating current. Moreover, as the duty cycle modulation is employed to modulate output power for both modes, 3p-DAB-RI converter suffers from high circulating current under light-load conditions (when the duty cycle of voltage waveforms is small) leading to poor-light load efficiency.
Chapter 4 proposes a 3p-DAB reconfigurable resonant (3p-DAB-RR) converter to overcome the limitations of 3p-DAB-RI converter proposed in chapter 3. The 3p-DAB-RR converter proposed in this chapter can transform between 3p-DAB-RI converter and a 3p-DAB series resonant (3p-DAB-SR) converter to offer additional degree-of-freedom in shaping the efficiency performance of 3p-DAB converter and enhance its light-load efficiency. The 3p-DAB-RR converter offers the flexibility to operate with unity power factor, full-range ZVS and enhanced light-load efficiency under varying operating conditions by appropriate selection of the converters operation mode. Based on a careful loss analysis and verification of the loss model by experimental measurements, it has been proposed to operate the converter as 3p-DAB-SR converter with single phase-shift (SPS) modulation for low-medium power levels and operate as 3p-DAB-RI converter with unity-power-factor operation for medium-high power levels to achieve overall wide-range high-efficiency performance. However, the drawbacks of hard-switching of 33 % switches in the unity-power-factor operation of 3p-DAB-RI converter and high circulating current, hard-switching of switches in 3p-DAB-SR converter under wide-range variations in input-to-output voltage ratios still remain unsolved. Chapter 5 proposes a 3p-DAB reconfigurable and tunable resonant (3p-DAB-RTR) converter to achieve wide-range zero circulating current and full-range ZVS operation irrespective of wide-range variations in input-to-output voltage ratios. For low-medium power levels, the converter operates as a tunable 3p-DAB-SR converter with impedance modulation method. Under this mode of operation, the output power is controlled by modulating the impedance of series LC resonant network with the aid of SCC while keeping the switching frequency and phase-shift constant. For medium-high power levels, the converter operates as a tunable 3p-DAB-RI converter with dynamic frequency matching (DFM) modulation. Under this mode of operation, the output power is controlled by synchronously varying the resonance frequency of the immittance network with the switching frequency by using switch-controlled capacitor (SCC). The combination of both operation modes jointly leads to wide-range zero circulating current and full-range ZVS operation for all the switches simultaneously yielding wide-range high-efficiency performance. Chapter 6 concludes the thesis and highlights the contributions of the work. Moreover, suggested applications and power levels for the proposed topologies and potential future research directions are also presented in this chapter. Finally, a comprehensive comparison of the proposed topologies are presented in this chapter to conclude the thesis.
Subjects: Electric current converters
DC-to-DC converters
Hong Kong Polytechnic University -- Dissertations
Pages: xxxi, 175 pages : color illustrations
Appears in Collections:Thesis

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