DC power distribution has gained popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) design of a high-efficiency interface converter to connect dc microgrids with the existing low-voltage ac distributions, with a special focus on common-mode (CM) voltage attenuation.
Droop control has been used in dc microgrids to share loads among multiple sources. However, line resistance and sensor discrepancy deteriorate the performance. The quantitative relation between the droop voltage range and the load sharing accuracy is derived to help create droop design guidelines. DC system designers can use the guidelines to choose the minimum droop voltage range and guarantee that the sharing error is within a defined range even under the worst cases.
A nonlinear droop method is proposed to improve the performance of droop control. The droop resistance is a function of the output current and increases when the output current increases. Experiments demonstrate that the nonlinear droop achieves better load sharing under heavy load and tighter bus voltage regulation. The control needs only local information, so the advantages of droop control are preserved. The output impedances of the droop-controlled power converters are also modeled and measured for the system stability analysis.
Communication-based control is developed to further improve the performance of dc microgrids. A generic dc microgrid is modeled and the static power flow is solved. A secondary control system is presented to achieve the benefits of restored bus voltage, enhanced load sharing and high system efficiency. The considered method only needs the information from its adjacent node; hence system expendability is guaranteed.
A high-efficiency two-stage single-phase ac-dc converter is designed to connect a 380 V bipolar dc microgrid with a 240 V split-phase single-phase ac system. The converter efficiencies using different two-level and three-level topologies with state-of-the-art semiconductor devices are compared, based on which a two-level interleaved topology using silicon carbide (SiC) MOSFETs is chosen. The volt-second applied on each inductive component is analyzed and the interleaving angles are optimized. A 10 kW converter prototype is built and achieves an efficiency higher than 97% for the first time.
An active CM duty cycle injection method is proposed to control the dc and low-frequency CM voltage for grounded systems interconnected with power converters. Experiments with resistive and constant power loads in rectification and regeneration modes validate the performance and stability of the control method. The dc bus voltages are rendered symmetric with respect to ground, and the leakage current is reduced. The control method is generalized to three-phase ac-dc converters for larger power systems. / Ph. D. / DC power distribution gains popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) a high-efficiency converter design to connect dc microgrids with the existing low-voltage ac utility, with a special focus on controlling the dc bus to ground voltage.
An analytical model is established to solve the power flow and voltage distribution in a generic dc system. The impact from cable resistance and measurement error on droop control is quantitatively analyzed, based on which droop design guidelines are proposed. DC system designers can use the conclusion to choose a minimum droop voltage range and guarantee a predefined load sharing accuracy. A nonlinear droop control method and a communication-based control method are proposed to further improve the dc system performance. The benefits include better load sharing, tighter voltage regulation and higher system efficiency.
To connect dc grids with the low-voltage ac distribution, a high-efficiency bidirectional ac-dc interface converter is designed and built. Different converter topologies with stateof-the-art power semiconductor devices are evaluated. Based on the comparison, an interleaved converter is selected and achieves an efficiency higher than 97% with an optimized passive component design. This converter is also capable of generating symmetric dc bus to ground voltages using a dedicated common-mode voltage control system, and is thus suitable for bipolar dc distribution systems.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/77532 |
| Date | 27 April 2017 |
| Creators | Chen, Fang |
| Contributors | Electrical and Computer Engineering, Boroyevich, Dushan, Burgos, Rolando, Baumann, William T., Wicks, Alfred L., Centeno, Virgilio A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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