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DC collection systems for offshore wind farms

Abeynayake, Perumbada Arachchige 2021. DC collection systems for offshore wind farms. PhD Thesis, Cardiff University.
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Abstract

Power generation through natural resources has found to be one of the best options to minimise climate change and global warming concerns. Among the naturally replenish sources, power generation from offshore wind accounts for a larger share. This has been showcased by the rapid development of offshore wind farms (OWF)s especial in the North sea. At the OWF collection system level, only alternating current (ac) technology is being used at present. Conversely, the use of direct current (dc) technology could provide additional benefits in terms of control flexibility, minimising system losses, and increasing power density of components. However, there are still a number of technical challenges that require addressing. One of the major aspects is the reliability of this concept as a whole. The research work presented in this thesis is aimed to address the existing challenges, in particular, from the component level to the system level from the perspective of reliability. The main contributions of this research work comprise of four parts, namely, (1) reliability analysis of semiconductors of dc-wind turbine machine side converter, (2) propose a new selection guideline based on reliability and costs to identify the most suitable multi-level converter topology for offshore wind power dc collection systems at different voltage levels and power levels, (3) identification of the most suitable dc collection system topology in terms of reliability and other economic factors, and (4) development of an analytical methodology to asses the availability of offshore wind farms considering the cable network dependency. One of the key building blocks of a dc collection system is the dc wind turbine (dcWT). The lifespan of a wind power system is highly influenced by the reliable operation of its power converter. A mission-profile based reliability assessment technique considering long-term and short-term thermal cycles are used to evaluate the lifetime of power electronic components of a dual active bridge based dcWT. Further, to ensure an effective lifetime evaluation of the entire converter system, a Monte Carlo method is used to generate the lifetime distributions and entire unreliability functions for power semiconductors. To utilise the full capacity of the dc technology in the context of the OWF collection system, the selection of a suitable power electronic converter topology is a key aspect. iii iv A selection criterion based on the optimal redundancy level with the consideration of the converter reliability, preventive maintenance interval, operational efficiency, the total cost of ownership and return on investment is proposed. The primary motivation of this work is to investigate the feasibility of utilising suitable multi-level voltage source converter topologies at different medium voltage dc levels and power levels. To select a suitable dc collection system topology, a comprehensive analytical reliability evaluation method based on Universal Generating Function (UGF) is proposed with associated economic factors. This strategy combines the stochasticity of wind with multiple power output states of a single wind turbine (WT). Subsequently, the relationship between the output states and corresponding state probabilities of WTs are combined using the UGF technique considering the network structure. To identify the best topology, the investment- and operating- costs (which includes network losses) are incorporated. The OWF collection system is made up of a considerable number of inter-array cables. The effectiveness of the OWF to export energy to the grid depends on the availability of that network. Therefore, it is imperative to include the reliability of the collection system in the overall availability assessment. However, this increases the number of components significantly, introducing the dimension curse. This combined with wind turbine output dependence makes the inclusion of the collection system in OWF availability assessment computationally intractable. An analytical reliability model based on the UGF technique is proposed accounting for the cable network dependency. Further, the impact of modelling wind farm components using a binary Markov model rather than a multi-state one is also investigated

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: DC Collection System, Offshore Wind, Reliability, Power Electronics, Economic Analysis, Availability
Date of First Compliant Deposit: 25 October 2021
Last Modified: 25 Oct 2021 11:49
URI: https://orca.cardiff.ac.uk/id/eprint/145019

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