Thesis
Hybrid AC/DC hubs for network connection and integration of renewables
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2021
- Thesis identifier
- T16059
- Person Identifier (Local)
- 201752379
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- High-voltage direct current (HVDC) technology has been identified as a preferredchoice for long-distance power transmission, especially offshore. With the rapiddevelopment of wind energy, many point-to-point HVDC systems with differentvoltage levels have been built. For increased operation flexibility and reliability, andbetter use of the existing assets, there is a need to interconnect different AC and DCnetworks as part of the future transmission network infrastructure development. Toaddress the demands of connecting wind farm converter stations with other AC/DCsystems, different hybrid HVDC converters for network connection and integration ofrenewables are proposed and evaluated in this thesis with the consideration ofconverter power rating, cost, efficiency and operation flexibility including responseduring 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 seriesat 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 paralleloperation of the DR-MMC HVDC system, the required MMC power rating in theproposed DR-MMC Hub can be reduced due to the series connection, potentiallyleading to lower investment cost and power loss. System control of the DR-MMCAC/DC hub is designed for different operating scenarios. System behaviours andrequirements during AC and DC faults are investigated. The hybrid MMCs with halfbridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operationand protection during DC faults. Power regulation of series-connected configurationmight be problematic in certain applications.To address the needs for increased DC network interconnection and the high costof the existing F2F DC/DC converter design, a hybrid F2F DC/DC converter, as apotential option, is proposed for unidirectional applications. In the proposed DC/DCconverter, the internal AC grid is established by a small MMC based STATCOM, andthe active power is transferred through the DR and LCC. Compared to the conventionalF2F DC/DC converters in terms of topological features and operation efficiency, theproposed DC/DC converter could offer higher power capability, higher converterefficiency and lower investment cost than those of the MMC based F2F DC/DCconverters. The operation and control of the LCC and MMC-STATCOM is designed,and the system start-up procedure is presented. Detailed analysis of the behaviours andprotection methods during DC faults is demonstrated. It needs to acknowledge that theconverter requires large amount of passive AC filters which may lead to large footprint.In addition, the proposed DC/DC converter only support unidirectional power flow.For the three proposed topologies, extensive time-domain simulation results based on PSCAD/EMTDC software have been provided to verify the feasibilities andeffectiveness (including steady state and dynamic performance) in normal operationand various fault scenarios.
- Advisor / supervisor
- Xu, Lie
- Resource Type
- DOI
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PDF of thesis T16059 | 2021-10-28 | Público | Descargar |