A Voltage-supportive Controller for Ultra-fast Electric Vehicle Chargers in Islanded DC Microgrids

This paper presents a controller for fast and ultrafast electric vehicle(EV)charging stations.Without affecting the charging efficiency,the proposed controller enables the charger to provide support to the interconnection voltage to counter and damp its transients.Existing solutions are either hardware-based such as using supercapacitors and flywheels which increase the cost and bulkiness of the charging station,or software-based such as P/V droop methods which are still unable to provide a robust and strong voltage support.This paper proposes an emulated supercapacitor concept in the control system of the ultra-fast EV charger in an islanded DC microgrid.Thus,it converts the EV from a static load to a bus voltage supportive load,leading to reduced bus voltage oscillations during single and multiple ultra-fast EV charging operations,and rides through and provides supports during extreme external disturbances.Detailed analysis and design guidelines of the proposed controller are presented,and its effectiveness and improved performance compared with conventional techniques are shown for different case studies.

Distributed DC-Bus Signaling Control of Photovoltaic Systems in Islanded DC Microgrid

The stability of an islanded DC microgrid(DCMG)is highly dependent on the presence and performance of the backup energy storage system(BESS),due to the lack of main grid support.This condition makes the DCMG vulnerable to the critical situation of absence of the BESS,which could be caused by a fault or being fully charged or flat.This paper presents an enhanced distributed DC-bus signaling control strategy for converters of photovoltaic systems(PVs)to make the islanded DCMG less dependent on the BESS.Unlike a conventional control approach that utilizes PVs to operate in maximum power point tracking(MPPT)mode and the BESS solely regulating DC-bus voltage,the proposed control method maintains DC-bus voltage via intelligently managing output powers of the PVs.The proposed control method continuously monitors DC-bus voltage and regulates the output powers of all the PVs via switching between MPPT mode and voltage regulating mode.Accordingly,if the DC-bus voltage level is less than a predefined maximum level,the PVs work in MPPT mode;otherwise,the PVs work in voltage regulating mode to maintain DC-bus voltage at an acceptable range.Such switching between MPPT and voltage regulating control operations results in protecting the DCMG from unavoidable shutdowns conventionally necessary during the absence of the BESS unit.Moreover,the proposed control method reduces oscillations on the DC-bus voltage during existence of the BESS.The performance and effectiveness of the proposed control strategy are validated through different case studies in MATLAB/Simulink.