Hybrid AC/DC hubs for network connection and integration of renewables

Student thesis: Doctoral Thesis

Abstract

High-voltage direct current (HVDC) technology has been identified as a preferred choice for long-distance power transmission, especially offshore. With the rapid development of wind energy, many point-to-point HVDC systems with different voltage levels have been built. For increased operation flexibility and reliability, and better use of the existing assets, there is a need to interconnect different AC and DC networks as part of the future transmission network infrastructure development. To address the demands of connecting wind farm converter stations with other AC/DC systems, different hybrid HVDC converters for network connection and integration of renewables are proposed and evaluated in this thesis with the consideration of converter power rating, cost, efficiency and operation flexibility including response during faults. A hybrid LCC-MMC AC/DC hub (LCC-MMC Hub) is proposed in this research, where a modular multilevel converter (MMC) and a line-commutated converter (LCC) are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. Compared to the "conventional" DC network interconnection based on a DC/DC converter, the proposed hybrid LCC-MMC Hub requires the lower power rating of a MMC with large part of the power handled by a LCC, potentially leading to higher overall efficiency and lower cost. Coordinated controls of the LCC and MMC are developed to ensure stable system operation and system safety. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection method for the hybrid AC/DC hub is proposed and studied. This hybrid hub uses large AC side filters, which might be the disadvantage for certain applications. Considering the future development of offshore production platforms (e.g. oil/gas and hydrogen production plants), a diode rectifier-modular multilevel converter AC/DC hub (DR-MMC Hub) is proposed to integrate offshore wind power to onshore DC network and offshore production platforms with different DC voltage levels. In this design, the DR and MMCs are connected in parallel at the offshore AC collection network to integrate offshore wind power, and in series at the DC terminals of the offshore production platform and the onshore DC network. Compared to the parallel operation of the DR-MMC HVDC system, the required MMC power rating in the proposed DR-MMC Hub can be reduced due to the series connection, potentially leading to lower investment cost and power loss. System control of the DR-MMC AC/DC hub is designed for different operating scenarios. System behaviours and requirements during AC and DC faults are investigated. The hybrid MMCs with halfbridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operation and protection during DC faults. Power regulation of series-connected configuration might be problematic in certain applications. To address the needs for increased DC network interconnection and the high cost of the existing F2F DC/DC converter design, a hybrid F2F DC/DC converter, as a potential option, is proposed for unidirectional applications. In the proposed DC/DC converter, the internal AC grid is established by a small MMC based STATCOM, and the active power is transferred through the DR and LCC. Compared to the conventional F2F DC/DC converters in terms of topological features and operation efficiency, the proposed DC/DC converter could offer higher power capability, higher converter efficiency and lower investment cost than those of the MMC based F2F DC/DC converters. The operation and control of the LC
Date of Award15 Sep 2021
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorLie Xu (Supervisor) & Hong Yue (Supervisor)

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