Power flow control in future electric vehicles and dc microgrids with multiple energy sources

This research considers control of systems that contain several dc electric energy sources and an electric ac machine. It proposes utilisation of a multiphase machine (with a multitude of three-phase sub-windings) in such systems, with the idea of enabling arbitrary sharing of the energy between the sub-systems connected to the different sub-windings. The targeted applications are the future electric vehicles (EVs) and dc microgrid interconnection. The said machine is the propulsion motor in the former and the renewable energy generator in the latter case.

One way of overcoming the battery size problem in EVs is to design vehicles to use a multitude of different electric energy sources, such as batteries, fuel cells, flywheels, superconducting magnetic energy storage and photo-voltaic systems. If this is to be achieved, a suitable control strategy for the propulsion motor, which would rely on optimal utilisation of these sources, is required. The requirement is that the different sub-windings, connected to the different energy sources, can be controlled independently, so that simultaneous motoring and generating mode of operation of the different sub-windings can be realised. This will enable decoupled power flow control and hence lead to the optimal exploitation of the available energy resources, when observed from the overall system perspective. Independently controllable power sharing will enable transfer of energy from one source in the vehicle to the other in accordance with the external conditions and the driving regime (e.g. solar energy to charge the battery and/or a supercapacitor during vehicle's cruising, a supercapacitor to provide the energy boost during rapid accelerations and decelerations - thus reducing the required size of the battery).

Dc microgrids are foreseen as an important component of the future smart power systems. Commonly, microgrids contain a renewable energy generator, such as wind or hydro generator. Similarly to the EV scenario, the interconnection of dc microgrids, which will become possible through utilisation of the independent and decoupled power flow control of the renewable generator's three-phase winding sets, will eliminate the need to utilise additional power electronic converters (as the current state-of-the-art is) for this purpose. Controlled energy sharing enables simple "peak energy shaving" when the energy consumption peaks do not appear simultaneously in the interconnected microgrids. In simple words, using the proposed algorithms, a microgrid with a surplus of the energy may supply other microgirds that need more energy. Apart from power flow control, additional benefits of this solution are potential cost saving and existence of inherent galvanic isolation between different dc sub-systems.

The research will develop advanced control techniques for multiphase machines with multiple three-phase windings that will enable arbitrary circulation of the power through the machine's three-phase winding sets. This will be achieved by using two different electric machine modelling approaches. The first will use as the starting point a known approach, while the second one will be based on a new machine model transformation with power sharing coefficients that is to be developed in the project. Both approaches will yield models required to obtain subsequently high quality dynamic performance of the machine when used as a variable speed drive/generator. Once the two different approaches are fully developed and verified through the simulations, the final step will be experimental verification and comparison of the devised control strategies in laboratory conditions.

Low carbon technologies, including various means of electrified transportation and future smart grids with differing embedded forms of microgrids, will play a pivotal role in reducing the negative effects of the current carbonised economy on the climate change. The two main anticipated target application areas are:
a) future electric vehicles (with a multitude of electric energy sources) and
b) distributed electric energy utilisation in the form of stand-alone dc microgrids (that would benefit from a simple means for interconnection).
Targeting these two applications will have a significant impact on low carbon technologies in the future.

The research will not only advance the current knowledge in the area of multiphase drive/generation system control, but provide viable solutions for wider industrial use. In the first instance, it is expected that the anticipated research results will be immediately relevant to the academic community. In the medium to long term, the major beneficiaries are expected to be in the industrial sector, in particular the electric vehicle manufacturers and companies involved in the wider smart grid sector, where the project is expected to assist UK industry to lead future developments. Power flow control methods proposed in the project may provide potential step forward for the UK companies in the EV automotive sector.

The application of the new technology will strengthen the UK's global position as a leader in the automotive and smart distribution industry sectors. This will be beneficial for the UK economy, creating new job opportunities for engineers and considerable social and environmental benefits. For example, the overall economic and social benefit of EVs, connected and autonomous vehicles to the UK economy may be of the order of £ 51bn per annum by 2020. It is also anticipated that these technologies will provide a significant boost in employment opportunities, with 25,000 new jobs just in the automotive manufacturing by 2030 (J. Saker, "On the road to sustainable growth - Boosting electric vehicles in the UK," IMI Report, 2016).

The developments planned in the project will enable further penetration of pure electric vehicles into future transportation market, as well as deployment of stand-alone dc microgrids in future smart grids more likely. Multiple electric energy sources (batteries, supercapacitors, solar, fuel cells etc.) are widely available, but are at present rarely combined in electrified transportation and 'small-scale' power systems. Combining a multitude of such sources, in conjunction with an appropriate energy sharing control system, will enable improved energy harvesting and utilisation, thus leading towards a greener economy. The understanding of potential ways of controlling the multiple energy sources, when an electric machine is already present in the system (as a propulsion machine in electric vehicles, or as a renewable energy generator in dc microgrids), is important for researchers and industry alike. This aligns with several national priorities in the energy area. By combining a multitude of electric energy sources in an EV, it is likely the research will reduce the range anxiety problem, which is widely recognised as one of the main obstacles to wider penetration of EVs in the car market at present. The approach will reduce the overall cost of an EV (e.g. a battery can be of a smaller size if other energy sources are used as well). In relation to microgrids, the envisaged interconnection principle could lead to a better controllability and lower overall cost.