Central Station Design Options

Abstract

The purpose of EDISON Work Package 4.1 is the evaluation of possible Central (charging) Stations design options for making possible the public charging of Electric Vehicles (EVs). A number of scenarios for EVs are assessed, with special emphasis on the options of Fast Charging and Battery Swapping. The work identifies the architecture, sizing and siting of prospective Central Stations in Denmark, which can be located at shopping centers, large car parking lots or gas stations. Central Stations are planned to be integrated in the Danish distribution grid. The Danish island of Bornholm, where a high penetration of wind power is present, is considered as special case. The distribution grid in Denmark is built using larger secondary distribution transformers (e.g. 630 kVA) which in general allows higher flexibility for the installation of Central Stations, compared to Bornholm’s distribution grid. With the 3-phase AC quick charging options of 11, 22 kW and 43 kW, (IEC 62196, EN60038, EN 61851) both the Danish and Bornholm environments offer a good chance for EV integration [1]. Dealing with Fast Charging, the study determined that 300kW Fast Charging, which corresponds to 10 minutes charging for a 50 kWh battery-EV is not feasible in Bornholm at the 0.4 kV level, due to predominantly small size secondary distribution transformers, in the range of 100 - 200 kVA. This is possible at the 10kV level (MV level), if the Fast Charging station is equipped with its own dedicated transformer. With DC charging, rated at 50 kW, fast charging would be easier in both Denmark and Bornholm scenarios. For each scenario and charging power level, the possible number of EVs is estimated and finally architectural design options are proposed. Technical assessment is performed for evaluating the benefits of different charging concepts as Fast Charging and Battery Swapping as well as different loading options as DC loading versus AC loading, single phase (230V) versus three phase (400V) loading. A study on possible siting for Central Stations is performed for the Danish main land, where the average distance covered by EVs is used as input data. A finding of the study is that a reasonable number of fast charging and swapping stations are about 15, assuming that all EVs have a range of 100-120 Km. Furthermore the Bornholm case is not very relevant for fast charging or battery swapping stations, due to the relatively small dimensions of the island. Several architectures of Central Station are assessed based on different hardware components onfiguration: in particular we could have a Central Station with dedicated transformer sized ad-hoc for the DC charging equipment. On the other hand, it is possible to have a Central Station connected to an existing distribution transformer; in this case the transformer load management becomes crucial. Furthermore we could have the options of AC versus DC power distribution in a Central Station. In case of DC distribution, we could have multiple fast chargers, implemented as DC/DC converters, operating in parallel and serving different vehicles. Local active harmonics compensation is ecommended as a valid option for mitigating the effect on the grid due to fast charging equipment, in compliance to IEC/TS 61000-3-12. Among power quality issues in distribution grids with EVs, voltage drops and transformer overloading are identified in this work as most critical situations [1]. The mitigation of voltage drops within a Central Station requires the local analysis of the site and eventually the installation of energy storage systems for mitigating the drop during transients or parallel charging, in compliance to EN 50160. Transformer overloading can be prevented, providing real-time access to the local power flow data. The assumption of 50% transformer average loading is used as a single reference point in the current report. For future investigations on transformer overloading, more reference points might be necessary to represent various transformer loading levels. The subject of safety in Central Station is also addressed. A number of safety rules based on European standards apply to AC charging equipment up to 44 kW. The connection interlock and the automatic de-energization are identified as fundamental requirements for safety in such a charging station. The connection interlock is a solution which ensures that no power is applied to the DC cable when the EV connector is not connected. The automatic de-energization device ensures that whenever a strain on the cable is detected, e.g. due to vandalism, the charge supply circuit is disconnected. More electrical vehicles on the market are capable today of quick charging up to 50 kW power level. The feasibility of Central Stations with fast charging/swapping option, their capacity, design, costs and grid impact, as well as battery lifetime issues [1]-[2], are actual concerns for their development. At the same time fast charging and battery swapping stations can offer the opportunity of a new concept of electrical mobility, which is somehow similar to the common refueling practice. From a grid perspective, fast charging and battery swapping could be an opportunity for a clean utilization of renewable energies, if this technology is integrated with the excess of power generation from green sources.