The new generation of high power chargers allows compact designs for charging stations with small footprints and high efficiency. The higher power density facilitates the installation of fast chargers in high numbers. However, the increasing demand on power challenges available grid connections. This article investigates options for installing high power chargers on existing, low power grid connections. The proposed solution remains valid when a high power grid connection becomes available.
High power charging systems (HPC systems) provide power levels of 150 kW up to 450 kW per vehicle according to the current standards. With the new generation, designs are moving from separate entities (such as transformers, power converters and charging stations) to highly compact solutions, i.e. charging stations with integrated power converters, as illustrated in figure 1.
The major benefit of the integrated design is the small footprint, which allows installing multiple charging stations without sacrificing much floor space in places such as parking lots, at shops, workshops or restaurants. The higher power density of compact designs provides a significantly improved efficiency, i.e. lower operational costs and less noise for cooling systems. The charging stations galvanically isolate vehicles from the grid and from each other by integrated mid-frequency transformers. However, high power presents new challenges: Demands on the grid connection are high.
High power chargers in significant numbers demand a medium voltage grid connection with a suitable transformer at 10 kVAC to 30 kVAC. This way, the charging stations connect to voltage levels of 690 VAC or higher at the low voltage terminal of the transformer. A transformer of an overall power 1600 kVA allows the connection of 10 to 20 HPC stations. The installed charging power may well exceed the transformer power by a factor of 2 to 4, depending on the concurrency and mean power consumption of vehicles connected to the charging stations.
If a grid connection with high power is not available, existing low voltage connections represent a severe bottleneck: The provision of 450 kW of charging power at a 400 VAC terminal generates currents of 650 A per phase, which easily overload existing power supplies. In summary, 400 VAC terminals cannot release the full potential of high power charging.
Existing 400 VAC or 690VAC terminals are not likely to support more than 150 kW of continuous power. However, in many cases, there is a limited number of busy hours with high power demand, and periods of low power demand, such as in shops, car shops, workshops or restaurants. In such cases, more charging power may be provides by an energy storage system, such as a battery. As further option, the energy storage system may have an extra supply such as solar power. Figure 2 illustrates this case.
The key feature of such a system is an extra DC power supply connecting the charging stations to the battery backup, and connecting the charging stations to each other. The voltage level of the DC system needs to significantly higher than the level of the AC-connections in order to avoid the same bottlenecks. This way, the energy storage provides a high power flow to the charging stations in busy hours, and is replenished by a low power flow from the grid, respectively by solar power.
A drawback of this solution is the space consumption of the energy storage system. However, the higher voltage level of the DC supply allows covering bigger distances. This way, the energy storage system may be placed at a suitable place, e.g. around the corner in a separate container, or within the building. Table 1 shows a comparison of cable losses at different voltage levels and different power demand. The comparison is based on a length of 100 m of copper conductors with the cross sections indicated in the table.
At a voltage level of ± 750 VDC cable losses just represent about one tenth in comparison to the losses of a 3-phase AC-system at 400 VAC (at equal power and at equal cable cross sections). The implication is that 400 VAC is not suited to cover any significant distance (i.e. beyond 20m). On the other hand, 400 VAC is well suited to provide continuous power of about 150 kW.
The charging stations indicated in figure 2 provide both AC and DC connections. Depending on the AC power level, the DC connections allow overcoming bottlenecks in AC power. The higher level of DC voltage facilitates placing the energy storage system away from the charging stations. Figure 3 illustrates a possible scenario.
The charging stations are located next to the shop, workshop or restaurant, using existing AC terminals. One charging station may load two or more vehicles. Each charging station provides an AC terminal and a DC terminal. The battery storage rests in a suitable place outside the parking section. With a low power grid connection, the energy storage provides high charging power.
The installation is ready for a transition from low power to high power grid connections: the charging stations provide 690 VAC terminals. Also, the high power DC terminals may be used for a supply power from the grid via a DC station. Bi-directional power converters in the charging stations support power flows in both directions. This may be a future option to adapt to emerging concepts of load management, respectively flexible current prices on demand and supply.
Authors: Andreas Lamert, Project Sales Engineer Power Quality and Energy Storage | Holger Kretzschmar, Head of Sales Low Voltage Solutions PQM | Maschinenfabrik Reinhausen
Image: Maschinenfabrik Reinhause at the ees Award 2019
Maschinenfabrik Reinhausen was one of the winners of the ees Award 2019. Learn more: www.thesmartere-award.com
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