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Title: Control and protection of HVDC grids
Author: Wang, Sheng
ISNI:       0000 0004 6059 0575
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2016
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The decarbonisation of Europe’s energy sector is a key driver for the development of integrated HVDC networks or DC grids. A multi-terminal HVDC grid will enable a more reliable power transfer from offshore wind farms and will facilitate the cross-border exchange of energy between different countries. However, the widespread deployment of DC grids is prevented by technical challenges, including the control and protection of DC grids. In order to close the gap, this thesis aims to contribute to three aspects (1): developing a control method for DC grids operation; (2): developing a method for optimising wind power delivery using DC grids; (3): developing a protection method for fast DC fault current interruption. The control of a DC grid demands the regulation of DC voltage and hence keeps the power into and out from the DC grid balanced. It is also important to keep the accuracy of regulating the converter DC current. In this thesis, the Autonomous Converter Control (ACC) is developed to meet this requirement. With this method, alternative droop control characteristics can be used for individual converters to share the responsibility of regulation of DC voltage while precisely controlling the converter DC current. The control algorithms of alterative droop characteristics are developed and interactions of different control characteristics are analysed. Furthermore, the potential risk of having multiple cross-over in control characteristics is uncovered. The method for designing droop characteristics is provided to avoid the multiple cross-over. The ACC is demonstrated on different simulation platforms including the PSCAD/EMTDC and a real-time hardware 4-terminal HVDC test rig. It is found that the proper use of alternative droop characteristics can achieve better current control performance. The adverse impact of having multiple cross-over in control characteristics is also studied using both simulation platforms. The effect of the control of both converters and DC power flow controllers (DC-PFCs) on DC power flow in steady state is also investigated. A method for re-dispatching control orders to optimise the wind power delivery is developed. Case studies are undertaken and it is found that both the DC line power loss and wind power curtailment can be reduced by redispatching the control orders of converters and DC-PFCs. The protection of a DC grid demands a very fast speed for fault current interruption. Conventional methods proposed for HVDC grid protection take delays of several milliseconds to discriminate a faulted circuit to healthy circuits and then allow the DC circuit breakers (DC-CBs) to open at the faulted circuits. The fault current will keep rising during Control and Protection of HVDC Grids iv the delayed time caused by fault discrimination. The Open Grid protection method is thus developed to interrupt fault current before fault discrimination. With this method, multiple DC-CBs open to interrupt the fault current based on local measurements of voltage (and current) and the DC-CBs on healthy circuits will reclose to achieve discrimination afterwards. This will reduce the delay for fault current interruption and hence the fault current can be interrupted with a much smaller magnitude. The developed Open Grid method is tested via simulation models developed in PSCAD/EMTDC. The results show that the Open Grid can detect very quickly and discriminate various faults under different fault conditions in a meshed HVDC grid.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering