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

 Abstract

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.

Index Terms

1.      DC ultra-fast charging

2.      Electric vehicles (EVs)

3.      Cascaded H-Bridge (CHB) converter

4.      Split battery energy storage

5.      Isolated DC/DC converter

6.      Dual half-bridge (DHB)

7.      Power Electronic Transformer (PET)

8.      State of charge (SoC) balancing

9.      Multiport converter

Block Diagram:

Fig. 1. Block diagrams for (a) branch and (b) submodule balancing of battery

State of Charges.

Expected Simulation Results:

Fig. 2. Simulation results of the M2PET-based ultra-fast EV charging station

for a hypothetical power profile, exploiting all operation modes.

Fig. 3. Detailed versions of the three-phase grid voltage and current quantities

for: (a) Mode I, (b) Mode II, (c) Mode III and (d) Mode IV corresponding to Fig. 5.

 

Fig. 4. Simulation results for the active power distribution within the four charging ports.

Fig. 5. Gain scheduling control behavior of the vertical SoC balancing controller.

Conclusion

This paper has proposed a multiport power electronic transformer-based concept for the realization of multifunctional medium voltage ultra-fast EV charging stations. All system components have been tailored for the specific application and chosen accordingly. A global system control structure has been described, which is capable of handling all different power flow directions as well as capacitor voltage and battery SoC unbalances. The operating modes of the system have been presented and simulated, verifying the versatility of the introduced converter structure. An additional advantageous SoC self-balancing operating mode of the delta-connected CHBBESS unit over the star-connected one has been proposed and discussed. Preliminary results from a down-scaled developed prototype have finally supported the theoretical investigations.

References

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