Exploration of Multilevel Current Source Converters

Power electronic conversion is a central element of energy conversion systems, acting as the interface between different forms of electrical energy and is an enabling technology for low-carbon economy. By 2030, it is expected that as much as 80% of all electric power will use Power Electronics somewhere within the energy supply chain between generation and consumption.

Multilevel converter is one type of power electronics converter and can offer substantial benefits over the established two-level converter counterparts. These include reduced harmonic distortion, less voltage/current stress (dv/dt, di/dt), and mitigated electro-magnetic interference (EMI).

Multilevel converters can be classified into multilevel voltage source converters (MVSCs) and multilevel current source converters (MCSCs). The MCSC exhibits several unique advantageous characteristics that may favour its adoption in the low voltage converters used in aircraft, hybrid/electric vehicles and micro renewable power generation. For example, inductors are used as the main energy storage elements in MCSCs, which are more reliable than capacitors in MVSCs. The MCSC has intrinsic current limiting capability, which can be used to constrain fault currents as well as leaving more time for device over-current protection. It may prevent further damage or failure of other components and avoid fire due to over-current.

The research into MCSCs is at its infancy. The project therefore will explore the MCSCs through converter topology derivation, modulation techniques, inductor current balancing, loss and efficiency evaluation, etc. The successful investigations of these challenges will reveal the benefits of the MCSCs and facilitate the wide application of the converter. The research will be carried out through modelling, control, simulation and experimental verification.

The study of power converter topologies is valued as enabling research in power electronic systems. A breakthrough in MCSCs will benefit the UK world-leading aerospace industry and renewable energy OEMs. Successful development of MCSCs will also feed into the component or sub-system supply chain, in exploiting new power semiconductor technologies (reverse blocking IGBT, wide-bandgap devices) and in requiring new power module configurations and new wound components.

Power electronics is a key enabling technology for low-energy/low-carbon economy. The proposed fundamental research on the topology and control of multilevel current source converters (MCSCs) will explore the full potential of a high power density, high reliability power conversion system. This underpinning research into converter topologies may have significant application potential for the aerospace and renewable energy industry, with the UK taking an international lead in this field.

Non-academic Beneficiaries
Industry
One of the main beneficiaries of the proposed power conversion research will be the aerospace sector where reduction of mass of components and high reliability is of primary concern. The Advisory Council for Aeronautics Research in Europe (ACARE) defines goals for research in terms of environmental impacts, efficiency and maintainability of systems to be used in the next generation of aircraft. Specifically, the aim of the resulting targeted research is to achieve a 50% reduction in fuel burn and CO2 emissions alongside an 80% reduction in NOx emissions by 2020, enabling the industry to meet its own targets set by EC. More electric aircraft (MEA) is one of the critical technologies to achieve this and the proposed research aligns well to this direction.
Safran Power UK as an industrial partner will directly benefit in gaining a significant competitive advantage through the adoption of new power converter technologies on future generations of aircraft. The research will guide and influence future research and development programmes. Communication between Bristol University and Safran will be carried out through scheduled quarterly technical meetings, engagement of engineers and technical report, etc.
Society
There will also be societal impact because weight reduction by using multilevel converters, e.g. for aerospace sector, will reduce the fossil fuel usage and emissions. In renewable energy sector, the proposed research may enable transformer-less structures for solar power generation, which is more efficient and reliable. This will reduce the cost and maintenance of the system and facilitate wider usage of solar energy and benefit the society in terms of reduced emission and low-carbon economy. By 2020, 15% of electrical power generation will come from renewable sources. High efficient and reliable power converter technology is a key element in interfacing wind and solar energy to the grid. The renewable energy sector could create 500,000 new jobs in UK by 2020. It is also critical to achieve the UK's ambitions for low-carbon economy with government targets for a 34% cut in 1990 CO2 emission levels by 2020 and a greater than 80% cut by 2050.
People
Output of trained people will also be an important outcome of the project. The RA, engineers and students will develop the skills in the modelling, design and control of advanced power converters, through the participation of the project and attending related courses taught by PI (e.g. current source converters in Advanced Power Electronics Design), which help addressing the shortage of trained people in the field of power electronics in the BIS report. Such expertise underpins the value-added manufacturing and knowledge economy that directly benefits the UK. The converter demonstration can also be used in University Open Days to stimulate the school students' interest in engineering area.
Academic Career Impact
Given that the proposal is for a First Grant Application, a critical aspect of its impact is its ability to provide the applicant (Dr. Yuan) with a framework for development of independent research career. The knowledge, expertise, industrial participation, equipment and current relevance contained within the proposed project will be essential for the applicant's career, strengthening the research capability in multilevel converters and building up national and international links with leading research institutes and the academic community.