The DC-side floating capacitors in the floating power modules of power converters are subject to high voltage fluctuation, due to the presence of reactive harmonic components. Utilizing passive capacitors, as done in traditional methods, helps reduce the DC-bus voltage ripple but makes the system bulky. An active capacitor can be integrated with the floating H-bridge power modules to remove the effect of the ripple powers on the DC bus. The auxiliary circuit, which is much smaller in size compared to an equivalent passive capacitor, helps increase the power density of the system. This work focuses on the analysis of power components, and the extension of the active capacitor to the Perturbation Injection Unit (PIU), in which the DC side is highly distorted by multiple harmonic components. A control scheme is proposed to compensate for these multiple harmonics and balance the DC-link voltage in the active capacitor. Also, an equivalent DC-bus impedance model is introduced, which is more accurate than that in existing works. Simulation studies and evaluation of the design have verified the effectiveness of the active capacitor solution. / Single-phase power converters have been widely used in many applications such as electric vehicles, photovoltaic (PV) systems, and grid integration. Due to their popular application, there is a need to reduce the sizes and volumes while still maintaining good performances of the systems.
One of the most effective methods, which is a subject in many research works, is to replace the bulky passive capacitor bank in a system by an active capacitor. The active capacitor is designed to absorb the ripple components in the DC side of the converters, which results in a constant DC-link voltage. In comparison to the passive capacitor solution, the active capacitor is much smaller in size but can give a better DC-bus ripple performance. Therefore, the active capacitor has become an attractive solution for the single-phase converters.
The active capacitor for the traditional rectifier, where the DC side is directly connected to a load, has been intensively investigated in the past decade. However, there is limited research regarding the active capacitor for rectifiers with floating H-bridge power modules.
This work extends the application of the active capacitor to the Perturbation Injection Unit (PIU), which is a grid-connected single-phase rectifier with floating H-bridge power modules. The selection of a suitable active capacitor for the PIU is based on the evaluation of various active capacitor banks. Limits in existing control schemes, which prevent the extension of the active capacitor to the PIU, are thoroughly studied. An effective voltage-mode control scheme is then proposed for the selected active capacitor, which makes it an attractive solution for the PIU. Moreover, limits of the DC-bus impedance analysis using traditional assumptions in existing works are investigated, and an improved DC-bus impedance model is proposed. Based on the operation conditions of the PIU and the proposed impedance model, the active capacitor's components can be properly designed, and improved configurations in terms of the equivalent impedance can be analyzed. Simulation results, as well as the design and evaluation of the active capacitor, demonstrate great improvements in terms of volume and weight over the traditional passive capacitor bank.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/104466 |
| Date | 07 February 2020 |
| Creators | Nguyen, Tam Khanh Tu |
| Contributors | Electrical Engineering |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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