In the ‘MS-Tankstelle’ project, Fraunhofer ISE worked with industrial partners to develop the medium-voltage system technology for future fast-charging stations that will enable peak loads of several megawatts. The result is a system that is not only more flexible, but also cheaper.
The abbreviation ‘MS’ in the project name stands for medium voltage. Raising the voltage level on the charging station side is one of the levers for reducing the use of materials and the costs of high-power charging stations. Although today’s high-power chargers are already connected to the medium-voltage grid, current systems work with lower voltages from the transformer station via the charging station to the vehicle.
This is also a suitable solution for individual charging stations and smaller charging parks. However, the team at Fraunhofer ISE is focussing on larger systems. As charging takes longer than a typical refuelling process, even with vehicle charging capacities of between 150 and 350 kW, ‘in future, petrol stations will need around 15 to 25 charging points instead of eight pumps to serve the same number of vehicles in the same amount of time’, according to the institute – you can see where the name ‘MS petrol station’ comes from.
If fast charging is carried out at so many charging points in parallel, a power requirement of 1.5 to 3.5 megawatts is required. ‘This means that future fast-charging stations can no longer be supplied via the low-voltage grid – even if the filling station is not fully utilised, the required power would exceed 300 kW. Distribution within the petrol station or car park should also not take place in the low-voltage grid, as the long cable runs (100 metres and more for 25 charging points) and the high power lead to high installation costs and high losses in the cables,’ the press release states.
The system for charging stations developed in the project therefore relies on a medium-voltage network that is operated with a rectifier at a voltage of 1,500 VDC. The logic behind this is the same as the change from 400 to 800 volts on the vehicle side: at a higher voltage, more power can be transmitted at the same amperage without the need to install thicker cables. This saves on expensive copper and also helps to protect the environment and conserve resources. Incidentally, the 1,500 volts were chosen because this is the limit of low voltage and other standards apply above this value. According to Fraunhofer ISE, there are plans to increase the voltage beyond this in follow-up projects.
Specifically, Sumida Components & Modules GmbH, Infineon Technologies AG and AEG Powersolutions GmbH, together with Fraunhofer ISE, have developed an electrically isolated converter that couples the direct current distribution network of the charging stations (the 1,500-volt network) to the vehicle battery and controls the charging process. This converter has an output of 175 kW and is designed so that two units can be connected in parallel as a system – to enable charging outputs of up to 350 kW. The stations are compatible with the CCS1 and CCS2 standards, i.e. on the vehicle side for currents of up to 500 amps and voltages of up to 1,000 volts. The Megawatt Charging System (MCS) for electric trucks is also planned for the future. A corresponding converter module still needs to be developed for this, for which some components need to be adapted. As in modern electric cars, silicon carbide semiconductors are also used in the converter to increase efficiency.
Another advantage of the solution cited by Fraunhofer ISE in the press release is that the concept of the central rectifier and a 1500 V DC distribution developed in the project allows the grid connection components (transformer and rectifier) to be dimensioned and scaled more independently of the charging electronics. In view of the high demand for power electronics and components such as cables and transformers, this should ‘significantly’ reduce material requirements compared to current solutions.
“The topology we developed in this project finds applications not only in charging stations but also for its integration in renewable hybrid power plants or stationary battery storage systems,” explained Andreas Hensel, Group Manager High Power Electronics and System Technology at Fraunhofer ISE.
