Modular Converter Architecture for Medium Voltage Ultra Fast EV Charging Stations: Global System Considerations

Electric Vehicles (EVs) are expected to contribute significantly to the reduction of CO2 emissions and other types of pollution within the next years. However, the concept of their ultra fast charging still poses demanding requirements, both in terms of the EV battery and the impact on the power grid. Towards this direction, emerging power electronics interfaces as well as energy storage technologies will play a significant role into making EV charging stations competitive with the existing gas station infrastructures. This paper focuses on the proposal of a power converter architecture, which aims for the interface between the three-phase medium voltage AC grid and the Electric Vehicle (EV) batteries. The transformerless AC/DC conversion stage motivates the choice of a suitable multilevel topology, namely the Cascaded H-Bridge Converter (CHB), where the medium voltage is split into several dedicated low voltage DC buses. At each of these levels, integrated stationary Battery Energy Storage Systems (BESS) play the role of power buffers, reducing thus the influence of the charging station on the distribution grid. The EV battery is then charged through parallel-connected isolated DC/DC converters, in order to achieve high currents and meet the standards for galvanic isolation.

A flexible and robust power management mechanism based on single-phase Modular Multilevel Converter with integrated energy storage elements

Alfred Rufer, Rosa Martel Danoary Tsirinomeny, Minghao Xu

This paper deals with a power management mechanism using Modular Multilevel Converter (MMC) with integrated energy storage elements, the typical use of which could be a grid-interface for a photovoltaic park or a charging park for electric vehicles. In comparison with the currently commonly used topologies of MMC, which use submodule DC/DC converters to control the submodule voltages at the half-bridge side and see the energy storage elements as the distributed voltage sources of MMC, the proposed topology in this paper put additionally a buffer capacitor between the submodule halfbridge and the DC/DC converter, taking this capacitor as the direct voltage source of MMC. Consequently, the energy balancing system will be targeted on the energy stored in these buffer capacitors, regarding the power flow gone from these capacitors to the final energy storage elements as disturbances for the controllers. One of the main advantages of this proposal is the healthy operation of MMC under fault-condition of its submodule energy storage elements, which serves to guarantee a stable and robust connection to power grids. Moreover, the power flow of the energy storage elements is free from the energy balancing task and thus can be controlled independently and individually, ranging from zero to the maximum output power of the energy storage elements. This is a beneficial characteristic for a PV park with large scale and various input power of its battery packages or for an EV-charging park to charge different battery packages inside different vehicles with their own best charging profile at the same time within a single MMC.

2015

Modular Converter Architecture for Medium Voltage Ultra Fast EV Charging Stations: Dual Half-Bridge-based Isolation Stage

Behrooz Bahrani, Niklaus Burger, Alfred Rufer, Michail Vasiladiotis

The concept of ultra fast charging of Electric Vehicles (EVs) is expected to contribute significantly to their widespread utilization in long-distance travels. Towards this direction, emerging power electronic topologies will be the key components of the developed charging infrastructures. Lately, a novel converter architecture has been proposed for such a purpose. The latter is based on a Cascaded H-Bridge (CHB) converter with integrated battery energy storage systems, which play the role of a buffer in such a power demanding application. This paper is focusing on the isolated DC/DC conversion stage, which is needed for the achievement of the high charging currents. The dual half-bridge (DHB) converter is chosen, which fulfills several requirements for the studied application. The soft switching operation of the converter under wide input-output voltage variations is discussed. Moreover, a current controller is designed, capable of dealing with the topology-specific challenges. Finally, the development of a down-scaled laboratory prototype is described and experimental results are provided.

2014

A Modular Multiport Power Electronic Transformer With Integrated Split Battery Energy Storage for Versatile Ultra-Fast EV Charging Stations

Alfred Rufer, Michail Vasiladiotis

This paper proposes a power converter architecture for the implementation of an ultra-fast charging station for electric vehicles (EVs). The versatile converter topology is based on the concept of the Power Electronic Transformer (PET). For the direct transformerless coupling to the medium voltage grid, a Cascaded H-Bridge (CHB) converter is utilized. On the level of each submodule, integrated split battery energy storage elements play the role of power buffers, reducing thus the influence of the charging station on the distribution grid. The power interface between the stationary split storage stage and the EV batteries is performed through the use of parallel-connected dual half-bridge (DHB) DC/DC converters, shifting the isolation requirements to the medium frequency range. By choosing several different submodule configurations for the parallel-connection, a multiport output concept is achieved, implying the ability to charge several EVs simultaneously without the use of additional high-power chargers. All possible charging station operating modes among with the designed necessary control functions are analyzed. The state of charge (SoC) self-balancing mode of the delta-connected CHB converter is also introduced. Finally, the development of a down-scaled laboratory prototype is described and preliminary experimental results are provided.

Institute of Electrical and Electronics Engineers2015