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

Lead Research Organisation: University of Nottingham
Department Name: Faculty of Engineering


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.

Planned Impact

The outcomes of the project will facilitate integration of renewable wind energy into the electrical system, by reducing the cost of the power converter station, thanks to the reduced weight and volume. Installation and maintenance will also be simplified. Overall, this will result in a lower cost for the infrastructure, thus supporting the feasibility of new HVDC links for offshore wind farms. In the short term, lower cost will attract new investments, positively reflecting on the productivity of the entire wind energy supply chain. In the longer term, impact will be reflected on a reduction of electricity bills for the end users. Compact converters will also benefit the onshore side, minimising land consumption.

Consolidation of the amount of renewable energy integrated into the grid using more compact and cost effective power converters has a positive impact on the UK electrical system not only from an environmental perspective, but also from a resilience point of view. Diversification of the energy sources, and exploitation of local resources such as wind in the North Sea, improves the capability of the country to adapt to changes in the international energy market, maintaining a dominant role in the international scene. A key element of a resilient energy network is the cost of the infrastructure: the lower the cost the faster and more sustainable growth will be. Also, developing cost effective power conversion solutions will enhance competitiveness of the UK stakeholders, impacting on export of UK HVDC technology in growing markets such as those of the developing countries.

In the short term, this project will develop a new family of compact power converters suitable for HVDC applications with size and weight constraints, and demonstrate their competiveness if compared with state of art solutions. In the medium term the technology will be consolidated and in the longer term it should ideally be receipted by industry and turned into commercial products. HVDC companies based in the UK, such as GE Grid Solutions are the ideal interlocutor for the exploitation of the technology. The company could access new knowledge, and propose a new generation of products on the market that could overtake what competitors currently offer. Having GE or other large companies involved will also benefit the UK economy as a whole, considering the extensive supply chain needed for manufacturing HVDC converters. This will enhance productivity, competitiveness and creativity of the UK economy. In addition, knowledge transfer to industry will result in new skilled workforce capable of leading and guiding technology, anticipating future needs and challenges.

All the aspects discussed above respond to specific HM Government's strategies such as "Offshore Wind Industry Strategy: Business and Government Action" (2013), "Our Electricity Transmission Network: a Vision for 2020" (2012), "Power Electronics: a Strategy for Success" (2011) and "Towards a smart energy system" (2015). This project will contribute to the government's commitment to meet carbon reduction targets and to support the development of new skills for the power electronics industry. In addition, a recent report from National Grid "UK Electricity Interconnection: Driving competition and innovation in the HVDC supply chain" (2016) highlights the importance of the HVDC supply chain in the UK economy and calls for innovative solutions to sustain growth in the sector and productivity of the nation. Resilience has been identified as a crucial aspect by a recent report from the IET, "Resilience of Electricity Infrastructure" (2015).

Finally, this research project can have positive impact on education in Power Electronics as the results can be presented and discussed with students during lectures or seminars, thus improving their skills and awareness of present and future challenges in HVDC power conversion and renewable energies.


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