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Title: An investigation of an energy diverting converter for HVDC applications
Author: Maneiro, Jose
ISNI:       0000 0004 7972 2240
Awarding Body: Staffordshire University
Current Institution: Staffordshire University
Date of Award: 2019
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Wind power generation in Europe has experienced an unprecedented expansion fuelled by a very favourable regulatory framework promoted to fight climate change. It is currently the second largest power generation source accounting for 17% of the total energy mix and in 2016 it covered an impressive 10.4% of the total energy demand. With faster wind speeds and better availability, offshore wind farm developments have also experienced a surge in recent years. There are 12.7 GW of cumulative installed capacity with the hot spot located in the North Sea. The grid integration of offshore wind farms has evolved to meet the requirements of recent projects, much larger in power capacity and located farther offshore. High voltage direct current (HVDC) connections using state of the art multilevel voltage source converters are now the industry standard for distant wind farms, with transmission capacities of up to 1 GW. The scale of the projects and frequent grid weakness at the onshore locations challenge transmission system operators which need to ensure the entire grid stability. Grid codes have evolved to regulate such interconnections, with a set of well specified requirements which need to be fulfilled. One such requirement is the fault ride-through capability, which defines the need for the HVDC interconnector to remain connected during onshore grid faults. A Dynamic Braking System (DBS) is a power electronics device that provides fault ride-through capability to the HVDC interconnector by absorbing the excess energy injected to the link for the duration of the fault. This energy is commonly dissipated in a resistive element. In this way the DC over-voltage is avoided and the operation of the connected wind farms is kept undisturbed. There is a lack of knowledge in the design and implementation of such devices. Therefore four concepts put forward by industry and other researchers are studied in this work. The rating of the different components in each circuit is investigated as the basis for the comparison. Taking into account the modular structure of AC/DC converters in HVDC stations it makes commercial sense to reuse the same modules as building blocks for the DBS. With modular structures, a good balancing of the total energy stored in the converter and its distribution among the different modules is one of the key elements. Modular DBS circuits can synthesize multilevel voltage waveforms, allowing for advanced power modulation strategies. Two novel strategies are developed in the thesis and an accurate mathematical modelling is performed to ensure that the energy balance conditions are met for all points of operation. An overall control strategy for each of the four circuits is also developed and presented in the thesis. A good coordination of the protective actions of the DBS and the main HVDC converters is important to ensure that no negative interactions occur. An operation strategy based on over-voltage thresholds is developed in the thesis. Accurate simulation models of the HVDC link integrating the DBS and controls are also implemented to give the required degree of confidence in the overall system behaviour. These are finally validated by a laboratory scaled-down test platform, where the control actions and the different converters are implemented in real hardware, and the correct coordination of all the elements during a fault event is experimentally tested. The main drawback of the DBS solution usually highlighted in literature is its cost. The option of adding some extra functionality to better justify the economic investment is explored in this thesis, resulting on a multifunctional circuit named Energy Diverting Converter (EDC). Two proposals including active filtering and HVDC tapping are developed in this thesis, for which two patent applications have been filed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available