Multi-terminal dc transmission micro-grid system for interfacing multiple wind turbines

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Moves to reduce carbon emissions and improve efficiency has primed interest in new technologies for the generation of electrical power. Unlike conventional plant, new generating technologies are not naturally suited to direct connection to the fixed frequency grid supply. Furthermore, in the case of renewable generation, restrictions on geographical location pose problems for electrical connection. Power electronic conversion thus plays a significant role in efficiently capturing and distributing the generated energy. This proposal addresses one important aspect of this research area: the efficient, robust and low-cost capture and transmission of renewable energy (RE) from multiple renewable resources. The use of DC networks to aggregate and transmit power from has been identified as a solution to such problems; to date work in this area has be concentrated at concept study and simulation level. Our collaborative proposal seeks to develop a novel and innovative DC current link system. The research will investigate the academic research aspects of realising a DC current link technology for the capture of renewable energy and other forms of low-carbon-derived electrical energy. Traditional wind turbine interfacing to the AC grid has been based on AC concepts. Recently ABB have installed the first offshore interconnect based on dc transmission. The system uses their standard HVDC Light technology, which offers bidirectional power flow control. Embedded renewable generation whether wind or wave, onshore or offshore, generally does not require the bidirectional power flow capability of HVDC Light (and similar techniques) but does require efficient, low-cost multi-source control. Existing techniques, e.g.HVDC Light, are not suitable. The proposed system departs from existing DC transmission technology. The proposed system is based on the concept that paralleling energy sources should always be based on paralleling current sources - not voltage sources as currently exemplified by HVDC systems. In our proposed system the single-ended step-up converter, operated with an outer current control loop, is the basic building block. The topology is scaleable, reliable and low-cost compared with existing AC and DC converter technologies used in distribution. Connection of additional sources is simple and low-cost thus the system lends itself to community-based schemes. Additionally, the majority of lower power RE systems utilise permanent magnet generators therefore require only unidirectional power flow from the RE source to the grid. The unidirectional nature of the power flow results in significant simplification of the DC system that is not realised in AC systems and existing bidirectional DC technology. The technology that will be developed by this project is a key enabler for the integration of multi-source low-carbon energy. The academic research team will investigate detailed modelling, simulation, design and experimentation on a demonstrator DC link system. Two PhD themes have been identified. The first will have a PhD student investigating the conversion electronics required to buffer and transform generates electrical energy onto the novel network. The second PhD student will research and address the important issue of regulating the flow of power from the low carbon energy source to the centralised grid interfacing converter. A post-doctoral research fellow will provide overall project management, liaise with the industry partner during development of the six-turbine demonstrator site, and assess and evaluate the performance of the demonstrator. On completion of this project, there will be a six 15kW turbine array that demonstrates the novel conversion technologies and innovative control algorithms developed through this important research. The demonstrator will be an exemplar of the synthesis between internationally-leading academic research, industrial experience and exploitation, and entrepreneurial skill.