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Title: Development of LCL DC/DC transformer and fault current limiting LCL VSC converter for high power DC networks
Author: Zhang, Lu
ISNI:       0000 0004 5361 695X
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2014
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In order to satisfy the huge demand energy transmission in future, the DC grid concept is proposed based on voltage sourced converter (VSC) HVDC and modular multilevel converter (MMC) HVDC technologies. It provides an attractive approach for long distance power transmission such as offshore renewable energy transmission in Europe. However, there are two main obstacles in the DC grid development. The first obstacle is the DC fault detection and selective isolation. Under severe fault condition, the DC grid is desired to isolate the healthy and faulty part which implies the whole grid system will operate normally during the fault. The second obstacle is the voltage stepping in DC grid system. The high power converter is desired to achieve high voltage stepping ratio yet must be cost-effective. In this thesis, an IGBT-based DC/DC converter employing an internal inductor-capacitor-inductor (LCL) passive circuit is presented to overcome above two obstacles. The proposed converter can achieve high voltage stepping without internal AC transformer implying smaller converter size and it is also designed to have reasonable efficiency in high power application. In addition, the converter has good response even under extreme fault conditions. The IGBT-based LCL DC/DC converter design procedure and performance under fault condition is investigated based on the theoretical studies initially. The converter is modelled on PSCAD platform under normal/fault operation and the simulation results are used for converter efficiency calculation and fault analysis. The advantages of IGBT-based LCL DC/DC converter are demonstrated by comparing with other two high power DC/DC converter topologies. A low power level prototype of LCL DC/DC converter is built following the design principle. The hardware results are used to verify the theoretical conclusions. The VSC converter is defenceless to DC faults in DC grid application. In order to overco The VSC converter is defenceless to DC faults in DC grid application. In order to overcome this major drawback, a fault tolerant VSC converter employing LCL passive circuit is studied in this thesis. The LCL VSC converter design principle is presented by analysing the converter equations. The converter model is developed on PSCAD platform under normal/fault operation. An advanced control method is designed based on developed MATLAB analytical model to improve the LCL VSC converter stability. The advantages of LCL VSC converter are presented by comparing with its performance with conventional L-VSC converter considering efficiency and fault response. A fault tolerant DC grid topology employing LCL VSCs and using low speed protection is also investigated in this thesis. The simple mechanical DC circuit breakers are used at DC bus bars and at connecting points of each DC cable. A comprehensive protection scenario including DC cable differential protection, DC bus bar protection and back up protection is employed to protect the whole DC grid against any probable DC faults. An accurate DC cable model is adopted for a four-terminal DC grid which is modelled on PSCAD platform. The advantage and feasibility of this method in DC fault protection is investigated based on the developed grid model.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council (EPSRC) ; Scottish Funding Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Electric power distribution