Design and Control of A DC Grid for Offshore Wind Farms

Fujin Deng

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling

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Wind power is growing rapidly around the world, and the offshore wind farm is currently seen as a promising solution to satisfy the growing demand for renewable energy source. Along with the increase in the capacity of offshore wind farms and the distance between offshore wind farms and land, the high-voltage direct current (HVDC) is attractive. In addition, the DC grid may also be interested for interconnecting the wind turbines in the collection level. As a consequence, a DC grid can be established for the offshore wind farm, where the wind power collection system and power transmission system both adopt DC technology. S far, the existing grid codes for wind turbines are mainly focused on AC system. Therefore, the faults analysis in the DC grid and the appropriate fault protections are required for the DC grid.
This thesis focuses on the design and control of the DC grid for offshore wind farms. The DC grid layout for offshore wind farm is introduced. The wind turbine configurations in DC grid are studied. A DC/DC converter is proposed for the wind turbine directly integrating to the DC grid. The operation principle of the DC/DC converter and the control of the wind turbine in DC grid are proposed as well. The HVDC transmission system configuration is studied. A DC/DC converter is proposed as the offshore converter to step up the collection level voltage to the transmission level voltage. The control strategies for the offshore converter and the onshore converter are proposed. The control of the DC grid under AC grid faults is discussed, and the improved control is presented to ride-through the AC grid faults for the DC grid. The cable fault in the HVDC transmission system is discussed. The performances of the HVDC system under faults are analyzed. And then, the protective inductor is proposed and designed for the HVDC system. Afterwards, the redundancy of the HVDC system under cable faults is studied, and a fault ride-through strategy is proposed for the DC grid under cable faults.
Throughout the thesis, the DC grid for offshore wind farms examples are modeled with the professional tool PSCAD/EMTDC, and the DC/DC converter prototype was built and tested in the laboratory, and the results verify the theoretical analysis.
ForlagDepartment of Energy Technology, Aalborg University
Antal sider132
StatusUdgivet - 2012

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