## 5 Conclusions  
This paper has presented the regulated transitions among the different operating modes of a hybrid microgrid devoted to the ultrafast charging of electric vehicles. The operating modes represent the interaction between the primary energy input source (ac grid), the hybrid energy storage system (HESS), and the main dc bus of the microgrid.  
The theoretical predictions of the transitions have been validated by means of PSIM simulations that use parametric information and appropriate time scales derived from the simulation of the steady-state operation of the charging station.  
The transitions are regulated by a two-level hierarchical control method whose superior level supervises the operating mode that is underway and establishes the passage to  
Figure 15: Operating modes of the HESS in interval B: (**a**) current supply of ChPs. (**b**) operation modes of the HESS.  
the next mode according to a state machine design. The control of the ongoing mode and the regulation of the transitions are performed by the low layer of the hierarchical control using a two-loop control approach.  
The inner loop of the low-level two-loop control uses an SMC with a switching surface based on the inductor current of a bidirectional converter equally for the battery and the supercapacitor, which facilitates the power decoupling and the robustness of the regulated system in the face of disturbances.  
This paper is a natural continuation of the work reported in reference [14], in which the charging station was simulated in steady-state operation. It can be concluded now that all possible transitions among the different operating modes disclosed in that paper can take place automatically and can be easily regulated.  
Further research will contemplate the discretization of the control system with the aim of developing a digital implementation in a reduced scale demonstrator of the hybrid microgrid. This will require the discretization of the continuous-time description of the two-loop control scheme, which will probably bring about new interesting subjects for discussion.  
Conceptualization, A.B.-F., D.Z.-P., O.L.-S., L.V.-S. and L.M.-S.; Methodology, A.B.-F., D.Z.-P. and L.M.-S.; Software, A.B.-F. and D.Z.-P.; Validation, A.B.-F. and D.Z.-P.; Formal analysis, A.B.-F., D.Z.-P., O.L.-S. and L.M.-S.; Investigation, L.M.-S. and A.E.A.; Supervision, D.Z.-P. and L.M.-S. All authors hareread and agreed to the published version of the manuscript.  
This work has been sponsored by the Spanish Ministerio de Ciencia e Innovacion under grants PID 2019-111443 RB-I00 and PID2020-120151 RB-100 and partially supported by grant MSCA IF EF-ST 2020/PCI2021-122066-2B, financed by MCIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR.  
Data are contained within this article.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
UFCSUltrafast charging station  
HESSHybrid energy storage system  
EVsElectric vehicles  
CC-CVConstant current-constant voltage  
acAlternating current  
dcDirect current  
SMCSliding mode control  
MVMedium voltage  
SSTSolid-state transformer  
ESSEnergy storage system  
ESDEnergy storage device  
BESSBattery energy storage system  
SESSupercapacitor energy storage system  
ChPsCharging points  
SOCState of charge