Which system solution should be chosen at a future electrification of the roads for electric lorries?

Master’s thesis work summer/autumn 2011, part-time employment included






Department of Electric Power Systems

Which system solution should be chosen at a future electrification of the roads for electric lorries?


Aim:
The objective with this Master’s thesis work is to study the technical pros and cons using different types of possible power supply solutions to lorries trafficking the highways of Sweden in the future.

Pros with some systems are compatibilities to already existing standards, low cost, or a high degree of safety. Pros with others might be sustainability to heavy and congested traffic, cheaper maintenance, lower power losses, or similar.

The lorries should be able to drive also on non-electrified roads, that is, some kind of combustion engine or battery reserve is required on the vehicles. Therefore, a trade-off study between an electrically strong power supply system with a high investment cost, and a weaker one with lower investment cost could be done. In the latter case however, the lower power supply investment cost must be compensated either by bigger batteries or by higher fuel consumption.

Models for studying a typical Swedish (Railway Power Supply System) RPSS are available at the department of Electric Power Systems, modifications and possible simplifications of these will be needed. The need for simplifications are dependent on the availability of electrical lorry running resistance models, electrical lorry tractive force curves, regenerative braking, etc.

Comparisons of different power supply techniques should preferably be of mainly technical, but also of economical nature. The latter, because it is not only of interest showing that something is better, the price for an improvement is also an important issue – especially for a public good as the highway infrastructure.

Background:
Today’s trolley-buses use a low voltage DC power supply system where the power is led down to the vehicle from the overhead power line (from here on denoted catenary) by a current collector arm (from now on denoted pantograph) and the return current is led back from the vehicle through a pantograph to the return conductor. A pro with low voltage DC is that it is an existing standard for non-rail bound electric vehicles, whereas a con is that it is a weak power supply, which will need high density of power converter stations or thick conductors to compensate for the low voltage.

Today’s trams also use low voltage DC, but as for “normal” trains, the return current leaves the wheels to rail and soil. Since lorries have rubber wheels and run on normal roads this is not an option. A solution might however be some kind of return conductor partly buried in the road; there might however be safety-related cons with this. The solution shall also be reliable in the winter with snow on the road.

Today’s trains use high voltage AC either in a lower frequency than the public, i.e. 16.7 Hz in parts of Europe and 25 Hz in parts of America, or in the public frequency – 50 Hz or 60 Hz depending on the country’s standards. This is the strongest kind of power supply for vehicles used today, but since lorries are lighter and uses less power (Volvos strongest lorry today consumes 0,5 MW, and modern trains consume between 5 and 10 MW) it might not be needed – this is up to the study of this Master’s thesis to judge. A further alternative are systems with higher DC voltage, both on the catenary itself and/or on a parallel DC-supply system with DC/DC converters. Such a system, but for railway usage, is in fact studied in Master’s thesis project that will start up in August at KTH.

The PhD project “Future Feeding Systems for Railways” at KTH is about planning for future expansion of the railway power grid. Within this PhD project, a train power system simulator (TPSS) has been developed. The simulator is a Matlab program that for the solving of some equations calls upon GAMS (www.gams.com). In the Master’s Thesis project, “Direct generation of low frequency single phase AC for the Railway in Norway and Sweden”, a separate GAMS program was used for RPSS calculations. Depending of the details of the studies being made, either of them can be used, or something else, if the student comes up with good a suggestion.

Also driving the lorries with three-phase power has been discussed. There are however practical issues with three parallel pantographs or one pantograph with suitable number of electrically separated parts. This project shall although focus on the power supply system solution. There are other ongoing projects looking in to the pantograph issues. Power converters could be avoided between the public power grid and the power supply system for the lorries, when using a three-phase system. Other discussions include inductive charge/discharge of vehicles through inductors buried in the ground under the roads.

The Master’s thesis student is through this work given a broad insight into how to electrically driven vehicles on roads, but indirectly also how railway systems, work.

Examples of cases that could be studied:
• What are the pros and cons using different power supply technologies for lorries in a overall system approach that also includes the needed electric power equipment on the lorries?
• Can they be monetarily valued?
• In railways, where the traffic is centrally planned, very tight traffic is not allowed since the power supply would not cope with that for reasonable investment costs. On public roads however, congestions is not a rare situation. On the other hand, when the traffic is congested, only small amounts of tractive power is used, and in a non-planned traffic, the safety distances must be larger at higher speeds, when relying on the interactions of many human drivers.
• Can batteries onboard the vehicles be used actively in order to minimize the investment needs in power supply systems?
• Study what happens if all lorry traffic today goes electric? How will the road-traffic look in 30 years, how do we prepare for that situation?
• If combustion engines are started when the power supply is overexerted, how should that be valued as an environmental cost today? And how do we think that biofuels and petroleum based fuels are valued in 30 years?
• Etc.

Location:
The Master’s thesis work will be carried out at either the Swedish Transport Administration in Sundbyberg or Borlänge, or at KTH in Stockholm.

Suitable background:
A suitable background is a Master of Engineering in Electrical Engineering or Engineering Physics. An interest in Matlab programming, and railways is a plus

Literature:
If you have any explicit questions, contact us! Some literature might be in Swedish, be prepared for that!

Examiner:
Lennart Söder, KTH, [email protected]

Make a contact:
Are you interested in this Master’s thesis work? Then, please contact Lars Abrahamsson, [email protected], phone: +46 8-790 70 58 or cell phone +46 70 256 22 82. For specific questions about the electric lorries project, please contact Anders Bülund at the Swedish Transport Administration on [email protected] or on phone, +46101231824.