IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 29, NO. 2, MARCH 2019 0602504

A New Isolated Multi-Port Converter WithMulti-Directional Power Flow Capabilities

for Smart Electric Vehicle Charging StationsShakil Ahamed Khan , Student Member, IEEE, Md. Rabiul Islam , Senior Member, IEEE,

Youguang Guo , Senior Member, IEEE, and Jianguo Zhu , Senior Member, IEEE

Abstract—If the batteries are charged by clean renewable en-ergy sources, electric vehicles (EVs) can have zero gas emission,contributing greatly toward the preservation of the green environ-ment. In a smart micro-grid, EVs together with other distributedenergy storage units can be used to supply electricity to the loadsduring the peak hours so as to minimize the effects of the loadshedding and improve the quality of electricity. To achieve thesegoals, an isolated hybrid multi-port converter is required to controlthe power flows and balance the energy among renewable energysources, EVs, and the grid. In this paper, a new isolated multi-portconverter is proposed, which can control the power flow in multi-ple directions. The converter is modeled in the MATLAB/Simulinksoftware environment and this validates the technology with a lab-oratory prototype test platform. The modeling, implementation,and results are discussed comprehensively.

Index Terms—Electric vehicles, battery charger, clean energy,micro-grid, multi-port converter, isolation.

I. INTRODUCTION

B ECAUSE the conventional power systems are short oflarge scale energy storage units, all electricity generated

must be consumed by either real or dummy loads. Insufficientgeneration may lead to load shedding, especially during the peakhours, which significantly increases the electricity price andaffects the power grid reliability, whereas excessive generationwill lead to waste of energy. Also, the fast consumption offossil fuels is causing serious problems of energy shortage indeveloping countries [1]. Recognized as an effective means toresolve these enormous challenges, the smart micro-grids basedon renewable energy sources, energy storage units, and electricvehicles (EVs) are becoming more and more popular around theworld [2], [3]. If the batteries are charged by clean renewable

Manuscript received October 31, 2018; accepted January 21, 2019. Dateof publication January 28, 2019; date of current version February 8, 2019.(Corresponding author: Shakil Ahamed Khan.)

S. A. Khan and Y. G. Guo are with the School of Electrical and Data Engineer-ing, University of Technology Sydney, Ultimo, NSW 2007, Australia (e-mail:,Shakil.A.Khan@student.uts.edu.au; Youguang.Guo-1@uts.edu.au).

M. R. Islam is with the School of Electrical, Computer and Telecommu-nications Engineering, University of Wollongong, Wollongong, NSW 2522,Australia (e-mail:,rabiulbd@hotmail.com; mrislam@uow.edu.au).

J. G. Zhu is with the School of Electrical and Information Engineering, TheUniversity of Sydney, Sydney, NSW 2006, Australia (e-mail:, Jianguo.zhu@sydney.edu.au).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TASC.2019.2895526

energy sources, EVs can have zero gas emission, contributinggreatly towards the preservation of green environment. In asmart micro-grid, EVs together with other distributed energystorage units can be used to supply electricity to the loads duringthe peak hours so as to minimize the effects of the load sheddingand improve the quality of electricity. However, an intelligenthybrid multiport converter is required to control the power flowsand balance the energy among renewable energy sources, EVs,and grids [4].

In [5], a motor integrated bidirectional power converter waspresented, where a relay was used to change the direction ofpower flow between EV and the ac or dc grids. Consideringthe efficiency, the solar photovoltaic (PV) system interfaced dcmicro-grid has found its application in interconnecting EVs inparking lot. However, the constant-power operation of dc–dcconverter-based EV chargers may affect the stability of the dis-tribution system [4]. Research in [6] focused to develop three-level dc–dc converter-based high-power fast charger for a park-ing lot to reduce the range anxiety of EV drivers. Althoughhigh-power charger may charge multiple EVs simultaneously,the sudden connection of multiple fast EV chargers may createfluctuations of distribution system voltage and power. High-temperature superconducting cables have already proven theirsuperiority in developing emerging power devices to overcomepower quality problems [7], [8]. In [9], a superconducting coilwas proposed for smoothing the system voltage and power.

A bidirectional buck-boost dc–dc converter with adaptablecontroller for grid connected EV charging lot was proposed tosupply electricity to the loads during the peak hours so as to min-imize the effects of the load shedding and improve the quality ofelectricity [10]. The proposed topology implies multiple powerconversion stages, which degrades the conversion efficiency.In [11], a modified Z-source converter-based EV charger hasbeen presented to reduce the power conversion stages. Althoughthe Z-source converter may improve the power conversion effi-ciency, its operation mostly depends on two capacitors and twoinductors. Recently, a common dc-link based three-port con-verter has been proposed to interface EV, solar PV and the gridwith reduced power conversion stages and component counts[12]. However, the galvanic isolation has not been considered inthe existing literatures. In this paper, a common magnetic-linkbased multi-port converter is proposed, where the magnetic-linkensures the galvanic isolation.

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0602504 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 29, NO. 2, MARCH 2019

Fig. 1. Circuit diagram of the proposed approach.

II. OPERATING PRINCIPLE OF THE PROPOSED CONVERTER

This section presents detail operation modes of the proposedapproach, which combines general multiport dual active bridge(DAB)/triple active bridge (TAB) converter, bidirectional dc–dc converter, and dc–dc unidirectional converter. The circuitdiagram of the proposed approach is illustrated in Fig. 1.

In the proposed approach, a bidirectional dc–dc converter isemployed to interface the EV battery to provide bidirectionalpower flow capability with the utility grid and the energy stor-ages. In addition, a conventional dc–dc boost converter is em-ployed to interface the photovoltaic module. In order to pro-vide bidirectional power flow capability with the utility grid, abidirectional ac–dc converter is used to generate sinusoidal accurrent in the grid side for high-quality power conversion. Inthis approach, a bidirectional dc-dc converter is used to inter-face the energy storage devices, which is used to control thecharging and discharging operations of the storages. Moreover,the proposed approach can be extended to N different dc busesthrough a multi-winding transformer (MWT) to handle a widevariety of dc voltage sources.

A. Bidirectional AC–DC Converter

During the EV charging or energy storage charging modes theconverter is controlled to generate the current (ic) in phase withthe grid voltage, and phase opposition during the active powerinjection into the grid. To realize controlled dc-link voltage,the active power reference is calculated dynamically to regulatethe dc-bus voltage. In the proposed system, the grid currentreference is calculated from the following expression:

ig ,ref =2Pref

vgdsin(θ) (1)

where Pref is the dynamically calculated active power referencevalue obtained from the proportional integral (PI) controller, θis the grid voltage phase angle obtained from the phase-lockedloop (PLL), and vgd is obtained from the αβ to dq conver-sion. The dynamic model of the converter in the dc-link can beexpressed as:

Pinst. =VDC.IDC =VDC (iC + iL ) = CVDCdVDC

dt+ VDC iL

(2)

where Pinst is the input instantaneous power. The controlleroutput can be defined as:

Pref =(

kpedc + ki

∫edcdt

)× VDC−link (3)

where edc is the dc-link voltage regulation error. The voltageregulation error can be expressed as:

edc = VDC−link − VDC−ref (4)

where VDC−link is the bus1 voltage and VDC−ref is the refer-ence dc-link voltage value. Similarly, in order to inject the activepower into the grid from the PV or energy storage devices, thereference grid current can be calculated from (1).

B. The DC–DC Converters

In the proposed approach, a bidirectional dc–dc converter isconsidered to transfer power between the energy storage de-vices and the utility grid. During the EV battery charging ordischarging, the converter is controlled to maintain a constantEV battery current. In the proposed approach, the EV batterycharging can be performed by power sharing from the utilitygrid and the energy storages. In this mode, the grid current ref-erence is calculated from (1) and rest of the required power istransferred from the energy storage devices. The bidirectionaldc–dc converter is controlled to provide constant charging anddischarging current. When the switching state of the switch Sb1is 1, the behavior of the dc–dc converter can be expressed as:

VDC−link = Ldibattery

dt+ Vbattery (5)

where VDC−link is the bus 2 voltage, L the value of the induc-tance, Vbattery the instantaneous value of the battery voltage, andibattery the instantaneous value of the battery current. Similarly,when the switching state of the switch Sb1 is 0, the dynamicalbehavior of the dc–dc converter can be expressed as:

Ldibattery

dt+ Vbattery = 0 (6)

Therefore, the mathematical model of the converter can berepresented as:

G.VDC−link = Ldibattery

dt+ Vbattery (7)

where G is the switching state. By using the forward Eulermethod with a sampling period TS , (7) can be expressed as:

ik+1battery = ikbattery +

TS

L(G.VDC−link − vbattery ) (8)

In order to provide constant charging or discharging opera-tion, the cost function can be defined as:

gcon. =(ik+1

b a t t e r y− iref

)2(9)

where iref is the desired charging or discharging current.

III. SIMULATION AND EXPERIMENTAL RESULTS

To verify the functionality of the proposed technology,the detailed simulation studies have been conducted in the

KHAN et al.: NEW ISOLATED MULTI-PORT CONVERTER WITH MULTI-DIRECTIONAL POWER FLOW CAPABILITIES 0602504

Fig. 2. Simulation results: (a) grid to EV, (b) grid to EV and energy storage,(c) PV to energy storage, (d) energy storage to EV, (e) energy storage to grid,and (f) PV to grid.

MATLAB/Simulink software environment. During the grid tovehicle (G2V) operation mode, the EV battery is charged fromthe grid and the energy storage battery is disconnected. Fig. 2(a)shows the EV battery state of charge (SOC) in this mode. Inthe proposed concept, the EV and energy storage batteries canbe charged from the grid at the same time. Fig. 2(b) shows thecharging currents of the batteries in this mode. The PV energycan be used to charge the energy storage with the proposed ar-chitecture. In this mode, the perturb and observe algorithm wasimplemented to control the unidirectional converter to extractthe maximum available power from the PV to charge the en-ergy storage battery. Fig. 2(c) shows the charging current in thismode.

Fig. 2(d) shows the EV battery and energy storage batterySOCs in this mode. As it can be seen that the energy storagebattery SOC is decreasing and EV battery SOC is increasing dueto the discharging and charging process. During energy storageto grid power transfer mode, the energy storage is discharged ata constant current rate as shown in Fig. 2(e), which depends onthe desired value of the active power, and the ac-dc converteris controlled to generate sinusoidal ac current in the grid-side.During PV to grid power transfer mode, the ac-dc converter inthe grid side is controlled to generate sinusoidal grid current asshown in Fig. 2(f).

Experimental studies have been conducted to confirm the va-lidity of the proposed concept for different modes of operation.A photograph of the experimental test platform is illustrated inFig. 3. In the proposed structure, the TAB converter is controlledto generate the leading or lagging voltages across the windingsto transfer power for the desired mode. Fig. 4(a) shows the exper-imental results for G2V operation mode. Clearly, the convertergenerated current (ic) is sinusoidal and in phase with the utility

Fig. 3. A photograph of the experimental test platform.

Fig. 4. Experimental results: (a) grid to EV and (b) energy storage to grid.

Fig. 5. Experimental results: (a) MWT winding voltages and excitation currentwaveforms and (b) EV to energy storage power transfer mode operation.

voltage (vg ), which demonstrates the unity power factor oper-ation. For the energy storage to grid power transfer mode, thegenerated current is out of phase with the grid voltage as shownin Fig. 4(b), which means that the utility grid receives powerfrom the energy storage. Fig. 5(a) shows the MWT winding volt-ages and excitation current waveforms for G2V operation mode.Moreover, EV to energy storage power transfer is possible withthe proposed structure. Fig. 5(b) shows the experimental resultswith the EV to energy storage power transfer mode operation.

IV. CONCLUSION

This paper presents a new architecture of an isolated multi-directional power conversion system, which is suitable for smartelectric vehicle charging station. This architecture is designedto interface EV battery, energy storage systems, and renewableenergy sources to interface with the grid through an isolatedsystem. This approach can be extended to N different dc busesthrough a MWT to handle a wide variety of dc voltage sourcesand provides the necessary isolation to meet the grid standards.

0602504 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 29, NO. 2, MARCH 2019

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