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

Lead Research Organisation: Liverpool John Moores University
Department Name: Engineering Tech and Maritime Operations

Abstract

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.

Planned Impact

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.
 
Description Different types of energy sources (e.g. battery, fuel-cell, super-capacitor) are meant to be used in different applications. A battery is a good choice for a continuous supply of medium electrical power, whereas, super-capacitors are intended for short bursts of power. An application that demand variety of operating modes including both continuous supply of medium power and short bursts of power are electric vehicles. At present the standard is utilization of a battery as a single type of electrical energy source in electric vehicles. This project allowed utilization of multiple energy sources in electric vehicles, by providing means of power transfer from one energy source to another through machine itself. A manner of the said power transfer is developed within the project. The findings and claims are proven by experimental results which yield great match with developed simulations.
Exploitation Route The findings from the project are communicated to industrial partners, and received a very positive feed-back. This further boosted probability that the industry will adopt the developed concept and implement it in mass production.
Sectors Energy

Transport

 
Description The project has proven viability of simultaneous utilization of different energy sources in electric vehicles coupled through the multiphase machine directly. It has provided viable solutions for wider industrial use. This opened up a new area in development of modern automotive drivetrain topologies and generated a need for additional jobs. It is expected that it will strengthen the UK's global position as a leader in the automotive sector. This sector is of high importance to the UK considering that 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. The sector is anticipated to 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).
Sector Energy,Transport
 
Description Bayerische Motoren Werke AG (BMW), Germany 
Organisation Bayerische Motoren Werke (BMW)
Country Germany 
Sector Academic/University 
PI Contribution The first project report was submitted to the industrial partner in November 2017. As a part of the project plan, on 21 Feb 2018 the work was presented to the BMW team via online meeting. The second and final project was submitted to the industrial partner in May 2018. The final meeting took place at BMW, Munich, Germany on 17 May 2018.
Collaborator Contribution Based on the presented report and the discussion during the meeting, the partners from BMW directed us for the further steps in the project. The partner directed the project towards the real-life challenging problems that appear in electric vehicle practice.
Impact See the list of Publications under the relevant section. In addition Internal report 1 and report 2, as specified in the grant proposal. This collaboration is not multidisciplinary, it is in the area of power electronics and drives.
Start Year 2017
 
Description Infineon Technologies AG, Germany 
Organisation Infineon Technologies
Country Germany 
Sector Private 
PI Contribution I have collaboration with Infineon Technologies. I shared the data and the approach how we do the modelling of the electrical machines. I was supporting their internal project, by doing machine's parameters estimation based on the nameplate data.
Collaborator Contribution Infineon Technologies contributed with discussions and by setting up the problem from the industrial perspective. They shared the final findings about the developed machine model with us.
Impact See the list of Publications under the relevant section. In addition Internal report 1 and report 2, as specified on the grant proposal. This collaboration is not multidisciplinary, it is in the area of power electronics and drives.
Start Year 2017