Thesis

Enhancing offshore wind farm transmission efficiency : HVDC converters and DC collection systems

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Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17229
Person Identifier (Local)
  • 202291225
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Offshore wind energy is one of the fastest-growing renewable energy sources, driven by consistently higher wind speeds and the availability of large installation areas. However, challenges persist, including the substantial size and weight of collection systems and the need for efficient long-distance power transmission to shore. This thesis aims to develop offshore wind power transmission systems that deliver higher efficiency, reduced costs, and minimized size and weight. The research begins with the investigation of onshore voltage source converter (VSC)-based high-voltage direct current (HVDC) systems. Three new HVDC converter topologies are proposed, combining half-bridge submodules (HB-SMs), a minority of full-bridge submodules (FB-SMs), and antiparallel thyristors. The inclusion of thyristors improves converter efficiency and provides DC fault-blocking capability with the support of FB-SMs. Additionally, the proposed designs reduce the number of semiconductors required, resulting in cost-effective solutions. The first topology uses thyristors in series with both HB-SMs and FB-SMs to enhance efficiency. However, its DC fault-blocking performance is relatively slow as it relies on thyristor commutation to block the AC component of the DC fault current from the grid. The second topology addresses this limitation by employing additional thyristor branches parallel to the submodules. These branches bypass the AC fault component swiftly, isolating the DC side and enabling fast fault-blocking action. The third topology further improves efficiency by leveraging these parallel thyristor branches during normal operation. Known as power groups, each group consists of SMs in parallel with thyristor branches. The thyristors bypass the SMs when generating zero voltage, reducing conduction losses and enhancing overall efficiency. The thesis then focuses on the DC collection system, proposing a novel series-isolated parallel (SIP) DC collection architecture. This architecture comprises parallel wind turbine groups (WTGs), each consisting of series-connected wind turbines (WTs). The series-connected WTs utilize three-phase uncontrolled rectifiers on their DC sides, significantly improving system efficiency. Each WTG integrates a DC-DC converter that steps up the group voltage to the HVDC transmission level while isolating faults within the series string. The proposed SIP architecture enhances reliability by isolating faults at the WTG level without requiring costly DC circuit breakers. Compared to conventional DC parallel architectures, the SIP approach achieves lower costs, reduced size and weight, and improved reliability, providing a more efficient and practical solution for offshore wind energy collection and transmission.
Advisor / supervisor
  • Williams, B. W.
  • Ahmed, Khaled
Resource Type
Note
  • This thesis was previously held under moratorium from 24th February 2025 until 24th February 2026.
DOI
Date Created
  • 2024

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