Research on control techniques for fuel cell power conditioning system with Low DC bus voltage ripple

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Research on control techniques for fuel cell power conditioning system with Low DC bus voltage ripple


Author: Cao, Lingling
Title: Research on control techniques for fuel cell power conditioning system with Low DC bus voltage ripple
Degree: Ph.D.
Year: 2015
Subject: Proton exchange membrane fuel cells.
Fuel cells.
Direct energy conversion.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electronic and Information Engineering
Pages: xxvii, 156 pages : color illustrations
Language: English
InnoPac Record:
Abstract: Proton-exchange membrane(PEM) fuel cells have been widely used as clean energy sources such as in fuel-cell vehicles and stand-alone power generators. Due to the intrinsic load-dependent output-voltage characteristic of fuel cells, the use of power conditioning systems is mandatory to provide regulated output voltage or power to their loads. In addition, the slow dynamic response of fuel cells and their balance-of-plants also leads to the need of fast-response energy storages for meeting transient load power changes. In the case where regenerative loads are present, these energy storages must also be configured to absorb the energy returned by the loads, which requires that the power conditioning systems have the capability to handle bidirectional power flows. As a result, multi-input bidirectional power converter systems have become a standard configuration of fuel cell energy systems. In this thesis, a new multi-input bidirectional dual active bridge (DAB) dc-dc converter is developed where the power flows of the fuel cell branch and the energy storage branch can be controlled individually using phase-shift modulation. In accordance with their physical characteristics, the fuel cell branch is configured to deliver the average load power and the energy storage branch is configured to regulate the dc bus voltage, which demands that it responds to transient load power changes. It has been reported that the lifetime of fuel cells can be degraded if their output power contains a significant amount of low-frequency component in the proximity of 100 Hz, which is found when they are used to drive inverter loads at 50 or 60 Hz. Interestingly, evidence shows that this does not occur for very-low-frequency and high-frequency output power variations. In view of these findings, in this thesis, the author embarks on an in-depth research on the optimization of the output impedance of the energy storage branch for effective absorption of low-frequency harmonic current generated by inverter loads. Initially, the closed-loop output impedance of the energy storage branch under single-voltage-loop and dual-loop controls are derived and compared. It is shown that both control strategies can effectively reduce the energy storage branch’s output impedance, thus favoring the flow of harmonic current and preventing it from being drawn from either the fuel cell branch or the dc-link capacitor. The use of conventional PI control alone is not sufficient to suppress the voltage ripple on the dc bus due to harmonic current being drawn from the dc-link capacitor. To enhance the harmonic current absorption capability of the energy storage branch, proportional-resonant(PR) control is proposed to generate an extremely low-impedance path for the flow of harmonic current at specific frequency. A low-cost,analogue-based frequency-adaptive PR controller is further proposed to adjust the resonant frequency to account for variations of inverter frequency. Subsequently, as inferred from the Mason’s gain formula, a systematic derivation of a family of four basic modes of output-impedance shaping methods, namely the load-current feed-forward, virtual resistor, virtual capacitor, and virtual inductor, are derived. Based on these basic modes, more complex output impedances can be derived by combining them in appropriate ways. To gain a better understanding of the different characteristics of the four basic modes, thorough mathematical analysis and experimental verification are performed for each of them. This is followed by the application of these basic modes in optimizing the closed-loop output impedance of the energy storage branch for low-frequency harmonic current absorption. Specifically, it is found that the virtual-inductor method is capable of generating the lowest output impedance but it also results in unwanted resonance peak in the output impedance of the energy storage branch, which can destabilize the system. The problem is remedied by introducing damping in the form of virtual resistor. The combination of these methods are shown to be very effective in low-frequency harmonic current absorption and for realizing near-ripple-free dc bus voltage regulation.

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