When electric buses are charged along the bus line, the charging should be completed as fast as possible – typically in just a few minutes. This is why the charging current with fast chargers is very high and the peak load for the power grid is several hundred kilowatts. Grid capacity needs to be sized according to these peaks so that the stability of the power grid will not be sacrificed. Increasing number of e-buses and fast chargers can cause major challenges for power grid management, especially when e-bus routes extend to suburbs where the power grid is not sized for such high loads. How can the power grid sustain the increasing number of e-buses? Turku University of Applied Sciences (TUAS) is finding solutions to these challenges in an ongoing ERDF funded SeBNet project (Smart Electric Bus Network Integration).
The city of Turku has an ambitious aim to electrify the majority of public transportation by 2030. In autumn of 2015, the city started this transition with a pilot project where they electrified bus line 1 with 6 Linkker buses and two fast-charging stations. Bus line 1 operates between the airport and the port and the end-of-line fast chargers are thus located in industrial areas with strong power grid. In cooperation with Turku Energia sähköverkot (local DSO), TUAS started simulating what kind of challenges might be met with wider deployment of e-buses. The simulations were carried out by developing an actual lab-scale model of a charging station and using simulation software like MATLAB and PowerFactory.
TUAS has developed a Hardware-In-the-loop (HIL) type laboratory model of a next-generation fast charger. The HIL simulator is based in the chargers used in Turku using the same product line components when possible while scaling down 1:100 in charging power. The main benefit of the HIL simulator is that it can be used to safely test different scenarios or validate software simulations in a close-to-reality simulator where all parameters can be controlled. These tests would also not be possible on actual charging stations without causing serious disturbance to the e-bus operation.
The hardware-in-a-loop simulator at the Turku University of Applied Sciences’ DES Lab*
Photo by Samuli Ranta
In comparison to the charging stations, The HIL simulator has been upgraded with a 1 kWh LTO battery and a solar array simulator emulating solar PV production. The battery operates as a buffer between the power grid and the bus levelling the load peaks. It also makes it possible to easily integrate local PV generation and thus helps to decarbonize the electricity used by e-buses. The HIL simulator can be programmed to run an endless variety of scenarios for different routes, bus types and load profiles. It can as well be used for simulating scenarios where the battery capacity would be utilized for grid stabilization in-between the bus charging events.
Along with HIL simulations, TUAS has carried out software simulations using MATLAB and PowerFactory. PowerFactory is used to simulate parts of the power grid and what kind of impact charging infrastructure would have when integrated into different parts of the power grid. These simulations are important especially when planning to introduce fast charging in weaker parts of the grid. MATLAB simulations are used for simulating scenarios of wider e-bus deployment and how charging of the e-bus fleet can be handled without compromising grid stability or causing traffic blocking at the charging stations. Currently, TUAS is running scenario simulations about the marketplace, the primal transport hub, which is outlined to have up to 16 fast chargers in the future.
SeBNet project will end in June 2020 and after that, there will still be plenty of room for further research. Simulations made in the project about wider e-bus deployment are just preliminary. Simulations models should be developed towards a holistic model, which would simulate a fully electrified public transportation system.