The objective of WP 4 was the development and standardisation of the technologies for central fast-charging and battery-swapping stations, including control and communication methods for optimal utilization of the battery capacity in the power system. This work package has been split into 6 subtasks focussing on the environment, design, hardware and software requirements.
The purpose of EDISON Work Package 4.1 is the evaluation of central (charging) stations design options for public charging of Electric Vehicle (EV). Future EV charging scenarios are assessed, with special emphasis on the options of fast charging and battery swapping stations.
The report identifies a number of possible central station sites, such as shopping malls, large car parking lots, and mixed gasoline stations. The central stations are planned in the Danish distribution grid, considering as a special case the Danish island of Bornholm, where high penetration of wind power is present. For details refer to Chapter 1.1.5 of the report “ WP 4.1 Central Station Design Options”.
The concepts for fast-charging stations and battery swapping stations developed as part of the EDISON project have been technically tested utilising the SYSLAB experimental research facility at Risø DTU – in close collaboration with the WP 6a of the project.
Three sets of EV battery packs and bi-directional inverters have been specified, purchased, installed (in individual containers) and integrated into the SYSLAB testing platform. The three battery packs are of two different Lithium types with DC voltage levels in the range 3-400 V and with energy capacities of 15 kWh and 25 kWh. The inverters, with capacities of 30 kW and 90 kW, control the power flow to / from the battery packages. The SYSLAB power system includes also wind power, solar power and real consumption from a building, and the SYSLAB power system is connected to the national grid.
This set-up can be used to proof-of-concept test of fast-charging stations with a common AC bus – specifically the impact of different fast-charging strategies and algorithms on the charging time, the charged energy, the degradation of the batteries, the requirements to the inverters, the requirements to the power system and the power quality in the power system.
The testing setup described above is also used to proof-of-concept test of the battery swapping station concept. One of the batteries is used to emulate a fast-charging post, while the other two batteries are used to emulate batteries to be swapped. The two swapping batteries can either be ‘on stock’ in the charging station or ‘out for driving’. When ‘on stock’ the batteries should be charged to be ready for swapping. But they can also be used as a temporary local storage capability to control the power drag from the grid.
A main aspect of the Network Analysis is the analysis of the geographical distribution of the fast charging stations. The analysis described in the report “WP 4.6 Case studies of grid impacts of fast charging” is based on an algorithm developed by DTU transport. Introductory a model is used to estimate the geographical position where 100 fast-charge stations will be optimally placed in Denmark, so that each EV trip has the lowest detour distance, when needing to fast-charge. On the basis of this placement, two locations are selected for case studies on which grid reinforcements are needed when connecting a fast-charge station.
Finally, the impact of harmonic distortion of chargers on exemplary distribution networks with different network structure is investigated. The main findings are:
▪ If the number of chargers with low charging power connected to the same interconnection point is increased, the impact on harmonic distortion is mitigated, especially when chargers of different manufacturers are used.
▪ Short-circuit level at the point of interconnection is very important for the value of harmonic distortion. A stronger MV network respectively higher short-circuit level usually reduces the overall harmonic distortion.
▪ When connecting chargers with high power rating (e.g. fast-charging stations) to the network, adequate voltage level and sufficiently high short-circuit power should be available at the interconnection point.
▪ High frequency (HF) current injections are not introduced into the MV network by converters connected to the LV level due to HF filters and the typical HF characteristics of the LV network (i.e. transformers and cables).
In the subtask 5 of WP4 we have looked into the complexity of controlling and operating a fast-charging station. Focus has been on the communication and control functions between the grid, the central charging stations and the vehicles (in particular the batteries).
The investment costs for two different fast charging station concepts are compared: The concept with a common AC bus – the AC bus concept – and the concept with a common DC bus – the DC bus concept.
DC Bus Bar Concept AC Bus Bar Concept
The result of the research during this project has shown that the recommended hardware solution is the AC busbar concept. If a battery bank is located together with the fast-charging station, the flexibility in the control schedules are much higher.
At the COP 15, UN Climate Conference in Copenhagen, we showed the first AC charger which could intelligently control charging of electrical cars. We implemented the concept in close cooperation between Eurisco working in WP 5 and Siemens working in WP4. The task for WP5 is to look into the communication between the charging spot and the electric vehicle.
The objective was to present a working demonstration model of a central charging station including EV, the user of the vehicle and the grid operator. For more detailes please refer to the Report, Concept Study on Fast Charging Station Design.
Different fast charging profiles, including constant current (CC), constant power (CP), forced power (FP) and pulsed power (PP), and different charging rates, corresponding to 2C, 3C and 4C, have been applied for the two different types of EV batteries.
With a limited cooling capacity of the battery packs in the test bench, the main limitation for fast charging of the battery packs turned out to be the temperature limits of the batteries. Both batteries could be charge with 50% of its capacity within 15 minutes and 25% within 5 minutes – more or less independent of the selected charging profile.