EPSRC 1st Grant: "Compact Modular Multilevel Converters for Offshore Wind Integration"

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The global electricity network has recently been experiencing a large scale integration of renewable energy sources, mainly photovoltaic and wind. This transformation is driven by the need for a reduction of carbon dioxide emissions, to limit greenhouse effect and mitigate global warming at the same time improving security of the supply. Large coal power plants are the main contributors of CO2 emissions, and electricity demand is constantly growing, especially in large urban/industrial areas. In this scenario, renewable energies are the only viable alternative to reduced environmental impact and carbon footprint of the electrical system. The main drawback of renewable energies is that they are usually generated far from where the energy is consumed. Typical examples in the UK are offshore wind farms, harvesting energy in the North Sea and delivering it to the mainland. Installing wind farms offshore gives higher wind speed and minimises the environmental impact, but might result in hundreds of kilometres separating the generator and the users. When distance increases, traditional and well-established AC transmission technology becomes unsustainable for its high energy loss.High Voltage DC (HVDC) is the technology enabling bulk power transmission over long distances (>600km for overhead cables, >40km for submarine cables), thanks to its higher efficiency and lower cost. Compared to AC power transmission, DC transmission is more complex, relying on Power Converter stations to transform from AC to DC at the wind farm side and back to AC when power is delivered to the mainland. Major issues in the design of converter stations for HVDC are size, weight, cost, efficiency, and manufacturing/maintenance. The basic problem is that these converters, when based on conventional technology, can be as large as a medium-sized industrial building and as heavy as 10000 tons for a typical 1GW installation. This poses two main challenges, at both ends of the HVDC link:1.Offshore challenge: installing large and bulk converters offshore increases the cost of the platform, and reduces competitiveness of offshore wind. In addition, construction, commissioning and maintenance of the converter are both complex and expensive.2.Onshore challenge: the converter onshore is often located in densely populated areas where energy is needed but land is expensive and limited. Also, environmental and visual landscape impact are a concern. This project will propose compact power conversion topologies for offshore and urban stations that have reduced size, weight, cost and environmental impact while maintaining adequate performances. In addition, the commissioning phase will be taken into account in the explored topologies, in order to increase modularity at system level and reduce construction efforts. The topologies will be discussed with key industry stakeholders and compared to standard state of art solutions, to identify the most attractive option, and the result of this trade off will feed into three work packages: design of the proposed converter, computer simulation and construction of a laboratory demonstrator to prove the feasibility and functionality of the proposed technology.