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Improve the Flexibility of Power Distribution Network by Using Back-to-back Voltage Source Converter
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Wu, Ruizhu (2018) Improve the Flexibility of Power Distribution Network by Using Back-to-back Voltage Source Converter. PhD thesis, University of Warwick.
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WRAP_Theses_Wu_2018.pdf - Submitted Version - Requires a PDF viewer. Download (6Mb) | Preview |
Official URL: http://webcat.warwick.ac.uk/record=b3438563~S15
Abstract
Employing increasing distributed generations (DGs) into existing distribution networks is an inevitable trend of the development of modern electric power systems because of the benefits including the environmentally friendly generation, higher efficiency and improved flexibility and reliability. However, high DG penetration level could pose various issues among which the voltage violation and fault level increase are the most concerned. According to the current situation of UK distribution networks, voltage violation is likely to be the first constraint to be met when DG penetration level is increased to certain level. Therefore, compensators are considered to be implemented to regulate the voltage. The reactive power compensators that widely used in transmission systems appear less effective in distribution networks thus active power compensation is desired. Soft-open points (SOPs) are power-electronic devices used replacing the normally-open points which can control active power transfer between two feeders and/or provide reactive power compensation. The back-to-back voltage source converter (B2B-VSC) is preferred as the SOP because of its capability of restricting fault current despite that it has higher power loss and associated capital cost. Two types of controller are developed for the B2B-VSC-based SOP: one is based on the PI control theory and the other is based on the concept of synchronverters. For the former type, the controller design is introduced comprehensively including system modelling and parameters selection. The precise selection of the damping ratio for nonstandard second-order system is derived, and a technique of resetting integrator in output voltage controller loop to achieve fast and smooth islanding transition is proposed. For the latter type, modifications are made to adapt the synchronverter idea to the application of an SOP. Simulations and experiments are carried out to validate the controller designs and both the controllers are verified to be able to provide sufficient performance on voltage regulation, fault current restriction and independent load supply in island mode. In general, the controller based on PI control theory has better performance in fault condition thanks to the current control loop, and the controller based on synchronverter owns better reliability because it does not require additional detections and signal switches inside the controller. At last, the use of an SOP in a dynamic load dominated network after the loss of mains is further investigated. Torque-speed characteristic is used to analyse the influence of the VSC’s filter impedance on the stability margin of an induction motor. Though the filter impedance can significantly decrease the stability margin, the output impedance of the VSC can be mitigated by properly designing the output voltage controller. Simulation and experiment are carried out to validate the analyses and controller design. The results show that the VSC is capable of supplying an induction motor in island mode.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > T Technology (General) | ||||
Library of Congress Subject Headings (LCSH): | Power electronics, Electric controllers, Electric power systems | ||||
Official Date: | October 2018 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Ran, Li | ||||
Format of File: | |||||
Extent: | xviii, 168 leaves: illustrations, charts | ||||
Language: | eng |
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