DC-DC CONVERTER FOR POWER COLLECTION IN WIND FARMS

Offshore wind farms have grown rapidly in number in recent years. Several large-scale offshore wind farms are planned to be built at further than 100 km from the United Kingdom coast. While high-voltage high-power installations have addressed the technical issues associated with reactive power flow in AC transmission, reactive power can be avoided by using High-Voltage Direct Current transmission (HVDC). Reactive power causes problems when transmission distances are long, therefore, HVDC transmission is now being considered for wind farm grid connection. However, as wind farms constitute weak systems Line Commutated Converter (LCC) based HVDC is not viable and newer Modular Multilevel Converter (MMC) based Voltage Source Converters (VSC) are needed for the AC-DC conversion. One of the key components in such systems is the DC-DC converter, which is required to act as the interface between the generation, transmission, and distribution voltage levels, and reduces the power conversion stages, avoiding transformers typically used in AC grid integration systems.
In addition, there is no high-power Medium-Voltage MV DC-DC converter available for offshore wind farm energy systems at present. The specification requirements of high-power MV DC-DC converters can be set once the output characteristics of the wind turbine generators have been reviewed. An offshore wind farm with MVDC-grid collection does not exist today, but it is a promising alternative, although specification analysis of high power MV DC-DC converters is necessary.
The work reported in this thesis aims to introduce two types of high power MV DC-DC converter topologies, for offshore wind farm energy systems, termed single-stage, and multi-stage converters. Ways of reducing losses by soft switching and reduction in the number of components are considered. Both topologies are based on the Marx principle where capacitors are charged in parallel and discharged in series to achieve the step-up voltage transformation. During doldrums, light and calm wind, and for maintenance work, it is necessary to supply the offshore wind farm with auxiliary power. This thesis proposes a novel Bidirectional Modular DC-DC converter (BMDC) and evaluates its performance. The simulation results show that the proposed BMDC allows up to 5% of the wind farm’s power rating to be drown from the onshore substation. This means that the proposed DC-DC converter is capable to provide bidirectional power flow.
For offshore wind farm application, BMDC can be inserted between the offshore wind farm and onshore substation. The studies, in this thesis, are based on an input DC collection at 6 kV with the DC to DC converter stepping up the voltage to 30 kV. The proposed system is integrated and simulated with the DC offshore wind farm and a Voltage Source Converter (VSC) in the onshore station.
The steady-state simulation results, to transmit the power between two different voltage levels, and the dynamic performance of the proposed converter were investigated. The advantages of the proposed converter include its simple design and that it does not require an AC transformer; hence can easily be implemented in an offshore wind farm since it requires less weight and size on the platform in the sea, which ultimately results in minimal cost. Furthermore, the proposed converter can ride through a fault which complies with the UK Grid code. However, in this case, it is necessary to provide protection systems such as a large chopper resistor for energy absorption or de-loading the wind turbine. Finally, the proposed integrated BMDC converter showed its suitability for offshore wind farms as well as improving their reliability.