Research ArticleExperimental Verification of a Battery Energy Storage Systemfor Integration with Photovoltaic Generators
Rajkiran Singh1 Seyedfoad Taghizadeh1 Nadia M L Tan1 and Saad Mekhilef2
1Department of Electrical Power Engineering Universiti Tenaga Nasional Selangor Malaysia2Power Electronics and Renewable Energy Research Laboratory (PEARL) Department of Electrical EngineeringUniversity of Malaya Kuala Lumpur Malaysia
Correspondence should be addressed to Nadia M L Tan nadiaunitenedumy
Received 26 June 2016 Revised 20 September 2016 Accepted 22 December 2016 Published 24 January 2017
Academic Editor Antonio J Marques Cardoso
Copyright copy 2017 Rajkiran Singh et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
This paper presents the experimental verification of a 2 kW battery energy storage system (BESS)The BESS comprises a full-bridgebidirectional isolated dc-dc converter and a PWM converter that is intended for integration with a photovoltaic (PV) generatorresulting in leveling of the intermittent output power from the PV generator at the utility side A phase-shift controller is alsoemployed tomanage the charging and discharging operations of the BESS based on PV output power and battery voltageMoreovera current controller that uses the 119889-119902 synchronous reference frame is proposed to regulate the dc voltage at the high-voltage side(HVS) to ensure that the voltage ratio of the HVS with low-voltage side (LVS) is equivalent to the transformer turns ratio Theproposed controllers allow fast response to changes in real power requirements and results in unity power factor current injection atthe utility side In addition the efficient active power injection is achieved as the switching losses areminimizedThe peak efficiencyof the bidirectional isolated dc-dc converter is measured up to 954 during battery charging and 951 for battery discharging
1 Introduction
Current grid codes for low-voltage grid-connected PV sys-tems consider a low PV penetration and in many countriesstipulate the regulation for anti-islanding and total harmonicdistortion of injected current to be less than 5 A highpenetration of PV systems will require strict adherence to thegrid codes because a large amount of varying active powerwill result in frequency variations and eventually instabilityof the power gridTherefore the integration of battery energystorage system a type of high energy density storage deviceis needed to level the output power from PV generatorsMoreover future grid codes are expected to include the faultride-through (FRT) capability and reactive power injectionof the PV systems [1] Battery energy storage systems couldbe employed to absorb active power from PV during the FRTconditions
A bidirectional isolated dc-dc converter with high-frequency galvanic isolation is one of the technologies thatenables the integration of energy storage devices such as
batteries and electric double-layer capacitors to the utilitygrid [2ndash6]The bidirectional operation of the converter easilycharges and discharges energy storage devices Moreoverthe high-frequency galvanic isolation increases the powerdensity and the reliability of the energy storage system Theefficiency of the bidirectional isolated dc-dc converter hasimproved since the converter was introduced in 1991 [7] Atthat time the first generation of IGBT suffered from highswitching losses The performance of the latest-generationIGBT improved in device switching losses FurthermoresuperjunctionMOSFET has a low on-state resistance119877DS[ON]that further reduces conduction loss of the switching device[2]
A 6 kW a 10 kW and a 100 kWbidirectional isolated dc-dcconverter achieved maximum efficiency of 969 [3] 974[4] and 987 [5] respectively The authors of [5] showedthat with the usage of silicon carbide MOSFETs the perfor-mance of the bidirectional isolated dc-dc converter is greatlyimproved as compared to the dc-dc converter using silicon-based switching devices The dc-dc converter operation is
HindawiAdvances in Power ElectronicsVolume 2017 Article ID 8158964 10 pageshttpsdoiorg10115520178158964
2 Advances in Power Electronics
Bridge 1Bridge 2
Bidirectional isolated dc-dc converterThree-phaseAC power
supply
PWM converter50ndash60V IB
VB CD2
CSL
CSLCSL
CSL
2
PD
LAHLAL
LACi116
transformer4 kHz CSH
CSHCSH
CSH
1VD1
CD1
CF
is
150V50Hz
300ndash360V
Figure 1 The experimental configuration of the BESS in charging mode 119871AC = 5mH (785) and 119862F = 1 120583F on a three-phase 150V 2 kWand 50Hz base
Bridge 1Bridge 2
Bidirectional isolated dc-dc converter
DC powersupply
Resistiveload
50ndash60V IB
VBCD2
CSLCSL
CSL CSL
2
PD
LAHLAL
LACi116
transformer4 kHz CSH
CSHCSH
CSH
1VD1 CD1
CF
is
PWM converter300ndash360V
Figure 2 The experimental configuration of the BESS in discharging mode 119871AC = 5mH (785) and 119862F = 1 120583F on a three-phase 150V2 kW and 50Hz base
optimum only when the voltage ratio of the HVS and LVSis equal to the turn ratio of the transformer Otherwise thecirculating current will be high leading to an increase inswitching loss due to high peak switching current and highturn-off overvoltage across the semiconductor switches Thecirculating current should beminimized so that the efficiencyof the converter is maintained high in applications with abroad range of operating voltageTherefore a reliable controlsystem is required to adjust the voltage of HVS with respectto the voltage of LVS of the converter so that the voltageratio of the HVS with LVS is close to the transformer turnsratio
This paper describes a battery energy storage system(BESS) that consists of a battery unit a bidirectional isolateddc-dc converter and a PWM converter that can be appliedto regulate the output power of a PV system This paperverifies the feasibility of operating a 2 kWBESS that respondsthe changes of a varying PV output power In order toachieve this purpose a current controller that uses 119889-119902synchronous reference and also a phase-shift controller areemployed The main function of the current controller is toregulate the voltage at HVS to maintain the voltage ratioof the HVS with LVS equal to the transformer turns ratioThe phase-shift controller is also employed to control thecharging and discharging modes of the battery based onPV output power and battery voltage Consequently theoperation of the both control systems maintains the grid-injected power at a constant value Reference [8] has shownonly the simulation results of the proposed systemThis paperaims to validate the proposed system by the constructionof a laboratory prototype and to show that the controllershave fast response and are feasible to be integrated with a PVsystem
2 Experimental System
Figures 1 and 2 present the experimental circuit of thecharging and discharging modes of the proposed BESSrespectively The system consists of a three-phase PWMconverter a 2 kW bidirectional isolated dc-dc converter anda three-phase resistive load bank The PWM converter isconnected at ac sidewith a 150V 50Hz power supply throughac-link inductors 119871AC and filter capacitors 119862F The proposedoperating voltage of the battery 119881B varies between 50Vand 60V and at the HVS 119881D1 is regulated between 300Vand 360V to adjust the ratio HVS and LVS close to thetransformer turns ratio In Figure 1 a single-phase resistiveload bank is used to represent the battery bank that isbeing charged In Figure 2 a three-phase resistive load isconnected in delta configuration at the ac side to verify theBESS in the dischargingmode Table 1 summarizes the systemparameters
The BESS is sized for active power injection of up to 2 kWThe power transfer 119875D of the BESS can easily be controlledby adjusting the phase-shift angle 120575 between two ac voltagesV1 and V2 and it is expressed as
119875D = 119881D1119881B120596119871 120575(1 minus |120575|120587 ) (1)
where119881D1 is the voltage at HVS119881B is the battery voltage 120596 isthe angular switching frequency and 119871 is the total inductanceincluding the leakage inductance of the transformer andauxiliary inductances referring to the high-voltage side Thedc-dc converter is operating in charging mode when voltageV1 leads voltage V2 During the charging mode 120575 is denotedas positive When operating in the discharging mode voltageV2 leads voltage V1 and 120575 is denoted as negative
Advances in Power Electronics 3
PWMconverter
converterLC filter
dc-dc
LVS
4 kHztransformer HF
inductor HVS
DSP
Resistiveload
dSPACE
Figure 3 Photo of the experimental setup at the laboratory
Table 1 Circuit parameters of the bidirectional isolated dc-dcconverter
Rated power 119875D 2 kWTransformer turn ratio 119873 6 1Auxiliary inductor (HVS) 119871AH 40 120583H (155)Snubber capacitor (HVS) 119862SH 10 nFDC capacitor (HVS) 119862D1 2mFAuxiliary inductor (LVS) 119871AL 40120583H (5583)Snubber capacitor (LVS) 119862SL 150 nFDC capacitor (LVS) 119862D2 96mFSwitching frequency 119891 4 kHzHVS is based on 360V 4 kHz and 2 kWLVS is based on 60V 4 kHz and 2 kW
Bridge 1 consists of four 600V 40A superjunctionMOS-FETs (TK40J60U) A film capacitor 119862SH is connected inparallel with each of the superjunction MOSFETs to achievezero-voltage turn-on and to minimize turn-off overvoltageacross the switching devicesThe on-state resistance119877DS(ON)is as low as 65mΩ
Bridge 2 consists of four 100V 200A superjunctionMOSFETs (IXFN200N10P) 119877DS(ON) is as low as 75mΩ Afilm capacitor 119862SL is connected in parallel with each of thefour MOSFETs to reduce its switching loss and to damp outany overvoltage
The PWM converter consists of six 600V 20A Field StopIGBTs (FGH20N60UFD) The saturation voltage is as low as18 V The IGBTs have internal parallel diodes
During the experiment a three-phase portable powersupply [KOSIJAYA model KA19530] was used for supply-ing the three-phase voltage A real-time interface systemdSPACE with DS1104 control card which consists of TexasInstruments TMS320F240 subprocessor and the Power PC603 e250MHz main processor is used for implementingthe proposed current controller This dSPACE control deskworks together with MathWorks MATLABSimulink R2011breal-time workshop and real-time interface (RTI) controlcards to implement the proposed PWM controller Anotherreal-time system Texas Instrument TMS F28335 controllerboard is used as the controller of the bidirectional isolateddc-dc converter Figure 3 presents the photo of the actualexperiment setup for battery charging Further details on theimplementation of the controllers are presented in Section 3
3 Control System
Figure 4 presents the overall control system of the proposedBESS Even though the control system has been presented in[8] it is explained in this paper for the sake of completenessand easy referencing The controller is designed to regulatethe power at the low-frequency ac side or the so-called ldquopointof common coupling (PCC)rdquo of the dc-dc converter at 2 kWThe phase-shift controller monitors the output power of PV
4 Advances in Power Electronics
dcdc converter PWM converterBattery
PWMSolar panel
Phase shiftcontroller
Gatedriver
Currentcontrol
CD1CD2
Ppv
Lsyss
VB
VB
iabcabc
VD1
Figure 4 Configuration of the proposed control system
Startingsequence
Check PVoutput power
BESS charging
BESS on standby
BESS discharging
NoNo
YesYes
PV gt 2kWPV = 2kWPV lt 2kW
50V lt VB le 60V VB le 60V
Figure 5 Determination of the three BESS operating modes
119875PV to determine the desired output power from the BESSsuch that
119875BESS = 119875PCC minus 119875PV (2)
The controller also considers the battery voltage 119881B and theHVS dc-link voltage 119881D1 Then it determines the requiredphase-shift angle 120575 needed to level the PV output powerby charging or discharging the battery Note that in theexperiment the PV output power is arbitrary and the mod-eling of PV generation system is outside the scope of thispaper
Figure 5 shows the typical operating modes of a BESSwhich are the charging discharging and standby modesWhen the PV output power is less than 2 kW the controllerchecks the battery voltage If the battery voltage is between50V and 60V (50V lt 119881B le 60V) the BESS is operated inthe discharging mode When the PV output power is morethan 2 kW the controller checks the battery voltage If thebattery voltage is equal to or less than 60V (119881B le 60V) theBESS is operated in the charging mode to absorb the surpluspower When the output power of PV is 2 kW the BESS goes
to standbymode by changing the phase-shift angle to zero Asshown in Figure 4 there are two other conditions where theBESS goes on standby mode to protect the battery bank frombeing overcharged and overdischarged
(i) When the battery voltage is less than 50V and 119875PV isless than 2 kW
(ii) When the battery voltage is more than 60V and 119875PVis more than 2 kW
Figure 6 presents the control system of the bidirectionalisolated dc-dc converterThe battery is charged or dischargedby the bidirectional isolated dc-dc converter connected tothe PWM converter Accordingly the control system deter-mines the required power in order to regulate the PCCpower at 2 kW The principal of this operation is based on(1)
The controller monitors 119875PV 119881D1 and 119881B By using (1) 120575will be obtained for both charging and discharging modes asfollows
Advances in Power Electronics 5
Equations (3) Gate driver
dcdc converter8 signals
PPCC= 2kW
PD
Ppv
VD1
VB
120575+minus
Figure 6 Phase-shift control of the bidirectional dc-dc converter
Bridge 1Gate driverEZDSPF28335
Gate driver Bridge 2
0 to 15V
minus7V to 15V
Figure 7 The overall controller connection of the dc-dc converter
4 + 120596120587119871119875D119881D1119881B119873Battery discharging 997888rarr 120575 = 1205872 minus radic120587
2
4 minus 120596120587119871119875D119881D1119881B119873(3)
Equations (3) are used to calculate the desired phase-shiftangle to achieve the required active power during chargingand discharging modes of the battery [3]
Figure 7 illustrates the overall controller connection ofthe dc-dc converter The specified phase-shift modulationfrequency is 4 kHz and the system clock frequency of theDSPF28335 is 150MHz The phase-shift angle is calculated in theDSPThe gate driver forHVS provides four signals of 0 to 15Vwhile the gate driver for LVS provides four signals of minus7Vto 15V to avoid any accidental turn-on at the LVS becauseany parasitic inductance effect can be amplified with a highercurrent flow
In order to minimize a high circulating current in theconverter a large mismatch between the transformer turnsratio and the voltage ratio of theHVS and LVS dc sides shouldbe prevented Therefore a current controller is designed toregulate the voltage across the dc-link capacitor 119862D1 usingthe three-phase 119889-119902 synchronous reference frame controltechnique Accordingly the controller monitors119881B and keeps119881D1 at a level where the voltage ratio between 119881D1 and 119881B isclose to the transformer turns ratio
Figure 8 shows the control system of the PWM converterThe employed control system regulates the three-phase cur-rents 119894a 119894b and 119894c to achieve unity power factor and toregulate the dc-link voltage119881D1 at a reference voltage equal tothe transformer turns ratio multiplied by the battery voltage
Table 2 The gains of the three PI controllers
PI1 PI2 PI3Proportional gain 119870119901 01 09 09Integral gain 119870119894 5 001 004
119881B The detailed derivation of the controller model has beenpresented in [8] Table 2 presents the parameters of each PIcontrollerThe parameters119870119901 and119870119868 are selected by trial anderror method
Figure 9 presents the implementation of the PWM con-verter control system that consists of three main blocks thatare the sensors the dSPACEDS1104 controller and the PWMconverter Two-phase ac currents and ac voltages are sent tothe ADCport and the third current and voltage are calculatedin MATLABSimulink environment The measured HVS dc-link voltage is also sent to the ADC port The carrier signalhas a frequency of 10 kHz and the modulation index is 09In addition the dead time of the PWM converter is 1 120583s TheSPWM signals are fed to the gate drivers to trigger the IGBTs
4 Experimental Results
The experimental prototype and the control systems are builtbased on Figures 1 and 2 The parameters and gains aresummarized in Tables 1 and 2 respectively The BESS systemis designed to supply a constant power of 2 kW into the gridat the PCC
Figure 10 illustrates the dc operating waveforms of theBESS during changes of the PVoutput powerThe step changeis assumed to be from 25 kW to 28 kWThe battery voltage isconstant at 50V and subsequently voltage 119881D1 is regulated at300V by operation of the control systemTheBESS is chargedwith 500W when 119875PV = 25 kW Accordingly the BESS ischarged with 800W when 119875PV = 28 kW to level the PVoutput power at the PCC The transient response caused bythe step change in power demand lasts for 70ms This showsthat the controller reacts fast and is feasible for PV outputleveling
Figure 11 shows the ac voltage and current waveformsat the HVS and LVS of the transformer when the BESSpower transfer is changed from minus500W to minus800WThere arechanges to the phase-shift angle from 120575 = minus1382∘ (minus0241 rad)
6 Advances in Power Electronics
+ +
+
++
+
abc
dq
0
PWM
6
Gatesignal
+
+
VB
VD1ref
VD1
PI1Idref PI2
Id
Iq
Iqref PI3minus
minus minusminus
Vpwmd
Vpwmq
120596Lsys
120596Lsys
Varef
Vbref
Vcref
Vsq
Vsd
lowast
Figure 8 Current control of the PWM converter
Voltage andcurrent
measurement
ADC
Voltageregulator
Slave DSP
PWM block
Gatedriver
PWMconverter Filter
Loadacpowersupply
dSPACE
Three-phase ac voltage and current
6
Figure 9 Block diagram of the dSPACE controlled PWMconverter
to 120575 =minus215∘ (minus0375 rad) during the change in power transferrequirement The maximum voltages of transformer at theLVS and HVS are close to the transformer turns ratio whichare 50V and 300V respectively The LVS have a so-calledldquoflat toprdquo current that reduces the rate of change of thecurrent 1198942 over the time interval of conduction Thus thiswill minimize the peak switching current which leads toa lower switch turn-off overvoltage and improve converterefficiency
Figure 12 presents the operation of the BESS when PVoutput power varies from 15 kW to 1 kWThe battery voltageis constant at 60V and voltage VD1 is regulated at 360Vby operation of the control system Initially when 119875PV =15 kW 119875D = 500W to regulate the power at the 119875PCC to 2 kWWhen the PV output changes to 1 kW the BESS transfers1 kW to the PCC to level the PV output The transient
response caused by the step change in power demand lastsfor 100ms and the peak of the transient current at the LVS is22A
Figure 13 shows that the magnitude of the phase-shiftangle 120575 is increased from 863∘ (0151 rad) to 219∘ (0382 rad)during the discharging mode Voltage V2 leads V1 in thedischarging mode Low-side transformer voltage is 60V andhigh-side transformer voltage is 360V This shows that thecontroller is able to regulate the voltage of HVS accordingto the transformer turns ratio At the HVS current 1198941is low and hence would not cause significant amount ofswitching loss The LVS current appears as having the so-called ldquoflat toprdquo It reduces the rate of change of the current1198942 over the time interval of conduction Thus this willminimize the peak switching current which can cause ahigher switch turn-off overvoltage thus improving converterefficiency
Figure 14(a) presents the three-phase ac current duringthe charging of BESS The transient of the ac current lastedfor 200ms Figure 14(b) presents the ac voltage The THD ofvoltage during charging of the 119862D1 is around 2 and duringthe discharging operation of the dc-dc converter the THD is1 The THD of grid current is measured with FLUKE 1735Power Logger The accuracy of this power logger is plusmn02 ofits full scale
Figure 15 shows the enlarged ac voltage and currentduring the charging operation of the BESS at the power of1 kW and battery voltage of 60V The synchronized voltageand current waveforms show that the power factor is close tounity
Figure 16 shows the converter efficiency when the BESSis operating in the charging and discharging modes Theefficiency of the dc-dc converter in the discharging modeis measured by choosing the LVS as the input and HVS asthe output On the other hand the efficiency of the dc-dcconverter in the charging mode is measured by choosingthe HVS as the input and LVS as the output At batterycharging the measured converter efficiency peaks at 954at 119875D = minus400W At battery discharging measured converter
Advances in Power Electronics 7
0
600
400
200VD1
(V)
300V
200ms(a)
0
100
50
VB
(V)
50V
200ms(b)
0
minus20
minus10
minus30
I B(A
)
minus16A
200ms(c)
minus15
0
minus05
minus1
PD
(kW
)
minus08 kW
200ms(d)
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
Bidirectional isolated dc-dc converterThree-phaseAC power
supply
PWM converter50ndash60V IB
VB CD2
CSL
CSLCSL
CSL
2
PD
LAHLAL
LACi116
transformer4 kHz CSH
CSHCSH
CSH
1VD1
CD1
CF
is
150V50Hz
300ndash360V
Figure 1 The experimental configuration of the BESS in charging mode 119871AC = 5mH (785) and 119862F = 1 120583F on a three-phase 150V 2 kWand 50Hz base
Bridge 1Bridge 2
Bidirectional isolated dc-dc converter
DC powersupply
Resistiveload
50ndash60V IB
VBCD2
CSLCSL
CSL CSL
2
PD
LAHLAL
LACi116
transformer4 kHz CSH
CSHCSH
CSH
1VD1 CD1
CF
is
PWM converter300ndash360V
Figure 2 The experimental configuration of the BESS in discharging mode 119871AC = 5mH (785) and 119862F = 1 120583F on a three-phase 150V2 kW and 50Hz base
optimum only when the voltage ratio of the HVS and LVSis equal to the turn ratio of the transformer Otherwise thecirculating current will be high leading to an increase inswitching loss due to high peak switching current and highturn-off overvoltage across the semiconductor switches Thecirculating current should beminimized so that the efficiencyof the converter is maintained high in applications with abroad range of operating voltageTherefore a reliable controlsystem is required to adjust the voltage of HVS with respectto the voltage of LVS of the converter so that the voltageratio of the HVS with LVS is close to the transformer turnsratio
This paper describes a battery energy storage system(BESS) that consists of a battery unit a bidirectional isolateddc-dc converter and a PWM converter that can be appliedto regulate the output power of a PV system This paperverifies the feasibility of operating a 2 kWBESS that respondsthe changes of a varying PV output power In order toachieve this purpose a current controller that uses 119889-119902synchronous reference and also a phase-shift controller areemployed The main function of the current controller is toregulate the voltage at HVS to maintain the voltage ratioof the HVS with LVS equal to the transformer turns ratioThe phase-shift controller is also employed to control thecharging and discharging modes of the battery based onPV output power and battery voltage Consequently theoperation of the both control systems maintains the grid-injected power at a constant value Reference [8] has shownonly the simulation results of the proposed systemThis paperaims to validate the proposed system by the constructionof a laboratory prototype and to show that the controllershave fast response and are feasible to be integrated with a PVsystem
2 Experimental System
Figures 1 and 2 present the experimental circuit of thecharging and discharging modes of the proposed BESSrespectively The system consists of a three-phase PWMconverter a 2 kW bidirectional isolated dc-dc converter anda three-phase resistive load bank The PWM converter isconnected at ac sidewith a 150V 50Hz power supply throughac-link inductors 119871AC and filter capacitors 119862F The proposedoperating voltage of the battery 119881B varies between 50Vand 60V and at the HVS 119881D1 is regulated between 300Vand 360V to adjust the ratio HVS and LVS close to thetransformer turns ratio In Figure 1 a single-phase resistiveload bank is used to represent the battery bank that isbeing charged In Figure 2 a three-phase resistive load isconnected in delta configuration at the ac side to verify theBESS in the dischargingmode Table 1 summarizes the systemparameters
The BESS is sized for active power injection of up to 2 kWThe power transfer 119875D of the BESS can easily be controlledby adjusting the phase-shift angle 120575 between two ac voltagesV1 and V2 and it is expressed as
119875D = 119881D1119881B120596119871 120575(1 minus |120575|120587 ) (1)
where119881D1 is the voltage at HVS119881B is the battery voltage 120596 isthe angular switching frequency and 119871 is the total inductanceincluding the leakage inductance of the transformer andauxiliary inductances referring to the high-voltage side Thedc-dc converter is operating in charging mode when voltageV1 leads voltage V2 During the charging mode 120575 is denotedas positive When operating in the discharging mode voltageV2 leads voltage V1 and 120575 is denoted as negative
Advances in Power Electronics 3
PWMconverter
converterLC filter
dc-dc
LVS
4 kHztransformer HF
inductor HVS
DSP
Resistiveload
dSPACE
Figure 3 Photo of the experimental setup at the laboratory
Table 1 Circuit parameters of the bidirectional isolated dc-dcconverter
Rated power 119875D 2 kWTransformer turn ratio 119873 6 1Auxiliary inductor (HVS) 119871AH 40 120583H (155)Snubber capacitor (HVS) 119862SH 10 nFDC capacitor (HVS) 119862D1 2mFAuxiliary inductor (LVS) 119871AL 40120583H (5583)Snubber capacitor (LVS) 119862SL 150 nFDC capacitor (LVS) 119862D2 96mFSwitching frequency 119891 4 kHzHVS is based on 360V 4 kHz and 2 kWLVS is based on 60V 4 kHz and 2 kW
Bridge 1 consists of four 600V 40A superjunctionMOS-FETs (TK40J60U) A film capacitor 119862SH is connected inparallel with each of the superjunction MOSFETs to achievezero-voltage turn-on and to minimize turn-off overvoltageacross the switching devicesThe on-state resistance119877DS(ON)is as low as 65mΩ
Bridge 2 consists of four 100V 200A superjunctionMOSFETs (IXFN200N10P) 119877DS(ON) is as low as 75mΩ Afilm capacitor 119862SL is connected in parallel with each of thefour MOSFETs to reduce its switching loss and to damp outany overvoltage
The PWM converter consists of six 600V 20A Field StopIGBTs (FGH20N60UFD) The saturation voltage is as low as18 V The IGBTs have internal parallel diodes
During the experiment a three-phase portable powersupply [KOSIJAYA model KA19530] was used for supply-ing the three-phase voltage A real-time interface systemdSPACE with DS1104 control card which consists of TexasInstruments TMS320F240 subprocessor and the Power PC603 e250MHz main processor is used for implementingthe proposed current controller This dSPACE control deskworks together with MathWorks MATLABSimulink R2011breal-time workshop and real-time interface (RTI) controlcards to implement the proposed PWM controller Anotherreal-time system Texas Instrument TMS F28335 controllerboard is used as the controller of the bidirectional isolateddc-dc converter Figure 3 presents the photo of the actualexperiment setup for battery charging Further details on theimplementation of the controllers are presented in Section 3
3 Control System
Figure 4 presents the overall control system of the proposedBESS Even though the control system has been presented in[8] it is explained in this paper for the sake of completenessand easy referencing The controller is designed to regulatethe power at the low-frequency ac side or the so-called ldquopointof common coupling (PCC)rdquo of the dc-dc converter at 2 kWThe phase-shift controller monitors the output power of PV
4 Advances in Power Electronics
dcdc converter PWM converterBattery
PWMSolar panel
Phase shiftcontroller
Gatedriver
Currentcontrol
CD1CD2
Ppv
Lsyss
VB
VB
iabcabc
VD1
Figure 4 Configuration of the proposed control system
Startingsequence
Check PVoutput power
BESS charging
BESS on standby
BESS discharging
NoNo
YesYes
PV gt 2kWPV = 2kWPV lt 2kW
50V lt VB le 60V VB le 60V
Figure 5 Determination of the three BESS operating modes
119875PV to determine the desired output power from the BESSsuch that
119875BESS = 119875PCC minus 119875PV (2)
The controller also considers the battery voltage 119881B and theHVS dc-link voltage 119881D1 Then it determines the requiredphase-shift angle 120575 needed to level the PV output powerby charging or discharging the battery Note that in theexperiment the PV output power is arbitrary and the mod-eling of PV generation system is outside the scope of thispaper
Figure 5 shows the typical operating modes of a BESSwhich are the charging discharging and standby modesWhen the PV output power is less than 2 kW the controllerchecks the battery voltage If the battery voltage is between50V and 60V (50V lt 119881B le 60V) the BESS is operated inthe discharging mode When the PV output power is morethan 2 kW the controller checks the battery voltage If thebattery voltage is equal to or less than 60V (119881B le 60V) theBESS is operated in the charging mode to absorb the surpluspower When the output power of PV is 2 kW the BESS goes
to standbymode by changing the phase-shift angle to zero Asshown in Figure 4 there are two other conditions where theBESS goes on standby mode to protect the battery bank frombeing overcharged and overdischarged
(i) When the battery voltage is less than 50V and 119875PV isless than 2 kW
(ii) When the battery voltage is more than 60V and 119875PVis more than 2 kW
Figure 6 presents the control system of the bidirectionalisolated dc-dc converterThe battery is charged or dischargedby the bidirectional isolated dc-dc converter connected tothe PWM converter Accordingly the control system deter-mines the required power in order to regulate the PCCpower at 2 kW The principal of this operation is based on(1)
The controller monitors 119875PV 119881D1 and 119881B By using (1) 120575will be obtained for both charging and discharging modes asfollows
Advances in Power Electronics 5
Equations (3) Gate driver
dcdc converter8 signals
PPCC= 2kW
PD
Ppv
VD1
VB
120575+minus
Figure 6 Phase-shift control of the bidirectional dc-dc converter
Bridge 1Gate driverEZDSPF28335
Gate driver Bridge 2
0 to 15V
minus7V to 15V
Figure 7 The overall controller connection of the dc-dc converter
4 + 120596120587119871119875D119881D1119881B119873Battery discharging 997888rarr 120575 = 1205872 minus radic120587
2
4 minus 120596120587119871119875D119881D1119881B119873(3)
Equations (3) are used to calculate the desired phase-shiftangle to achieve the required active power during chargingand discharging modes of the battery [3]
Figure 7 illustrates the overall controller connection ofthe dc-dc converter The specified phase-shift modulationfrequency is 4 kHz and the system clock frequency of theDSPF28335 is 150MHz The phase-shift angle is calculated in theDSPThe gate driver forHVS provides four signals of 0 to 15Vwhile the gate driver for LVS provides four signals of minus7Vto 15V to avoid any accidental turn-on at the LVS becauseany parasitic inductance effect can be amplified with a highercurrent flow
In order to minimize a high circulating current in theconverter a large mismatch between the transformer turnsratio and the voltage ratio of theHVS and LVS dc sides shouldbe prevented Therefore a current controller is designed toregulate the voltage across the dc-link capacitor 119862D1 usingthe three-phase 119889-119902 synchronous reference frame controltechnique Accordingly the controller monitors119881B and keeps119881D1 at a level where the voltage ratio between 119881D1 and 119881B isclose to the transformer turns ratio
Figure 8 shows the control system of the PWM converterThe employed control system regulates the three-phase cur-rents 119894a 119894b and 119894c to achieve unity power factor and toregulate the dc-link voltage119881D1 at a reference voltage equal tothe transformer turns ratio multiplied by the battery voltage
Table 2 The gains of the three PI controllers
PI1 PI2 PI3Proportional gain 119870119901 01 09 09Integral gain 119870119894 5 001 004
119881B The detailed derivation of the controller model has beenpresented in [8] Table 2 presents the parameters of each PIcontrollerThe parameters119870119901 and119870119868 are selected by trial anderror method
Figure 9 presents the implementation of the PWM con-verter control system that consists of three main blocks thatare the sensors the dSPACEDS1104 controller and the PWMconverter Two-phase ac currents and ac voltages are sent tothe ADCport and the third current and voltage are calculatedin MATLABSimulink environment The measured HVS dc-link voltage is also sent to the ADC port The carrier signalhas a frequency of 10 kHz and the modulation index is 09In addition the dead time of the PWM converter is 1 120583s TheSPWM signals are fed to the gate drivers to trigger the IGBTs
4 Experimental Results
The experimental prototype and the control systems are builtbased on Figures 1 and 2 The parameters and gains aresummarized in Tables 1 and 2 respectively The BESS systemis designed to supply a constant power of 2 kW into the gridat the PCC
Figure 10 illustrates the dc operating waveforms of theBESS during changes of the PVoutput powerThe step changeis assumed to be from 25 kW to 28 kWThe battery voltage isconstant at 50V and subsequently voltage 119881D1 is regulated at300V by operation of the control systemTheBESS is chargedwith 500W when 119875PV = 25 kW Accordingly the BESS ischarged with 800W when 119875PV = 28 kW to level the PVoutput power at the PCC The transient response caused bythe step change in power demand lasts for 70ms This showsthat the controller reacts fast and is feasible for PV outputleveling
Figure 11 shows the ac voltage and current waveformsat the HVS and LVS of the transformer when the BESSpower transfer is changed from minus500W to minus800WThere arechanges to the phase-shift angle from 120575 = minus1382∘ (minus0241 rad)
6 Advances in Power Electronics
+ +
+
++
+
abc
dq
0
PWM
6
Gatesignal
+
+
VB
VD1ref
VD1
PI1Idref PI2
Id
Iq
Iqref PI3minus
minus minusminus
Vpwmd
Vpwmq
120596Lsys
120596Lsys
Varef
Vbref
Vcref
Vsq
Vsd
lowast
Figure 8 Current control of the PWM converter
Voltage andcurrent
measurement
ADC
Voltageregulator
Slave DSP
PWM block
Gatedriver
PWMconverter Filter
Loadacpowersupply
dSPACE
Three-phase ac voltage and current
6
Figure 9 Block diagram of the dSPACE controlled PWMconverter
to 120575 =minus215∘ (minus0375 rad) during the change in power transferrequirement The maximum voltages of transformer at theLVS and HVS are close to the transformer turns ratio whichare 50V and 300V respectively The LVS have a so-calledldquoflat toprdquo current that reduces the rate of change of thecurrent 1198942 over the time interval of conduction Thus thiswill minimize the peak switching current which leads toa lower switch turn-off overvoltage and improve converterefficiency
Figure 12 presents the operation of the BESS when PVoutput power varies from 15 kW to 1 kWThe battery voltageis constant at 60V and voltage VD1 is regulated at 360Vby operation of the control system Initially when 119875PV =15 kW 119875D = 500W to regulate the power at the 119875PCC to 2 kWWhen the PV output changes to 1 kW the BESS transfers1 kW to the PCC to level the PV output The transient
response caused by the step change in power demand lastsfor 100ms and the peak of the transient current at the LVS is22A
Figure 13 shows that the magnitude of the phase-shiftangle 120575 is increased from 863∘ (0151 rad) to 219∘ (0382 rad)during the discharging mode Voltage V2 leads V1 in thedischarging mode Low-side transformer voltage is 60V andhigh-side transformer voltage is 360V This shows that thecontroller is able to regulate the voltage of HVS accordingto the transformer turns ratio At the HVS current 1198941is low and hence would not cause significant amount ofswitching loss The LVS current appears as having the so-called ldquoflat toprdquo It reduces the rate of change of the current1198942 over the time interval of conduction Thus this willminimize the peak switching current which can cause ahigher switch turn-off overvoltage thus improving converterefficiency
Figure 14(a) presents the three-phase ac current duringthe charging of BESS The transient of the ac current lastedfor 200ms Figure 14(b) presents the ac voltage The THD ofvoltage during charging of the 119862D1 is around 2 and duringthe discharging operation of the dc-dc converter the THD is1 The THD of grid current is measured with FLUKE 1735Power Logger The accuracy of this power logger is plusmn02 ofits full scale
Figure 15 shows the enlarged ac voltage and currentduring the charging operation of the BESS at the power of1 kW and battery voltage of 60V The synchronized voltageand current waveforms show that the power factor is close tounity
Figure 16 shows the converter efficiency when the BESSis operating in the charging and discharging modes Theefficiency of the dc-dc converter in the discharging modeis measured by choosing the LVS as the input and HVS asthe output On the other hand the efficiency of the dc-dcconverter in the charging mode is measured by choosingthe HVS as the input and LVS as the output At batterycharging the measured converter efficiency peaks at 954at 119875D = minus400W At battery discharging measured converter
Advances in Power Electronics 7
0
600
400
200VD1
(V)
300V
200ms(a)
0
100
50
VB
(V)
50V
200ms(b)
0
minus20
minus10
minus30
I B(A
)
minus16A
200ms(c)
minus15
0
minus05
minus1
PD
(kW
)
minus08 kW
200ms(d)
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
Figure 3 Photo of the experimental setup at the laboratory
Table 1 Circuit parameters of the bidirectional isolated dc-dcconverter
Rated power 119875D 2 kWTransformer turn ratio 119873 6 1Auxiliary inductor (HVS) 119871AH 40 120583H (155)Snubber capacitor (HVS) 119862SH 10 nFDC capacitor (HVS) 119862D1 2mFAuxiliary inductor (LVS) 119871AL 40120583H (5583)Snubber capacitor (LVS) 119862SL 150 nFDC capacitor (LVS) 119862D2 96mFSwitching frequency 119891 4 kHzHVS is based on 360V 4 kHz and 2 kWLVS is based on 60V 4 kHz and 2 kW
Bridge 1 consists of four 600V 40A superjunctionMOS-FETs (TK40J60U) A film capacitor 119862SH is connected inparallel with each of the superjunction MOSFETs to achievezero-voltage turn-on and to minimize turn-off overvoltageacross the switching devicesThe on-state resistance119877DS(ON)is as low as 65mΩ
Bridge 2 consists of four 100V 200A superjunctionMOSFETs (IXFN200N10P) 119877DS(ON) is as low as 75mΩ Afilm capacitor 119862SL is connected in parallel with each of thefour MOSFETs to reduce its switching loss and to damp outany overvoltage
The PWM converter consists of six 600V 20A Field StopIGBTs (FGH20N60UFD) The saturation voltage is as low as18 V The IGBTs have internal parallel diodes
During the experiment a three-phase portable powersupply [KOSIJAYA model KA19530] was used for supply-ing the three-phase voltage A real-time interface systemdSPACE with DS1104 control card which consists of TexasInstruments TMS320F240 subprocessor and the Power PC603 e250MHz main processor is used for implementingthe proposed current controller This dSPACE control deskworks together with MathWorks MATLABSimulink R2011breal-time workshop and real-time interface (RTI) controlcards to implement the proposed PWM controller Anotherreal-time system Texas Instrument TMS F28335 controllerboard is used as the controller of the bidirectional isolateddc-dc converter Figure 3 presents the photo of the actualexperiment setup for battery charging Further details on theimplementation of the controllers are presented in Section 3
3 Control System
Figure 4 presents the overall control system of the proposedBESS Even though the control system has been presented in[8] it is explained in this paper for the sake of completenessand easy referencing The controller is designed to regulatethe power at the low-frequency ac side or the so-called ldquopointof common coupling (PCC)rdquo of the dc-dc converter at 2 kWThe phase-shift controller monitors the output power of PV
4 Advances in Power Electronics
dcdc converter PWM converterBattery
PWMSolar panel
Phase shiftcontroller
Gatedriver
Currentcontrol
CD1CD2
Ppv
Lsyss
VB
VB
iabcabc
VD1
Figure 4 Configuration of the proposed control system
Startingsequence
Check PVoutput power
BESS charging
BESS on standby
BESS discharging
NoNo
YesYes
PV gt 2kWPV = 2kWPV lt 2kW
50V lt VB le 60V VB le 60V
Figure 5 Determination of the three BESS operating modes
119875PV to determine the desired output power from the BESSsuch that
119875BESS = 119875PCC minus 119875PV (2)
The controller also considers the battery voltage 119881B and theHVS dc-link voltage 119881D1 Then it determines the requiredphase-shift angle 120575 needed to level the PV output powerby charging or discharging the battery Note that in theexperiment the PV output power is arbitrary and the mod-eling of PV generation system is outside the scope of thispaper
Figure 5 shows the typical operating modes of a BESSwhich are the charging discharging and standby modesWhen the PV output power is less than 2 kW the controllerchecks the battery voltage If the battery voltage is between50V and 60V (50V lt 119881B le 60V) the BESS is operated inthe discharging mode When the PV output power is morethan 2 kW the controller checks the battery voltage If thebattery voltage is equal to or less than 60V (119881B le 60V) theBESS is operated in the charging mode to absorb the surpluspower When the output power of PV is 2 kW the BESS goes
to standbymode by changing the phase-shift angle to zero Asshown in Figure 4 there are two other conditions where theBESS goes on standby mode to protect the battery bank frombeing overcharged and overdischarged
(i) When the battery voltage is less than 50V and 119875PV isless than 2 kW
(ii) When the battery voltage is more than 60V and 119875PVis more than 2 kW
Figure 6 presents the control system of the bidirectionalisolated dc-dc converterThe battery is charged or dischargedby the bidirectional isolated dc-dc converter connected tothe PWM converter Accordingly the control system deter-mines the required power in order to regulate the PCCpower at 2 kW The principal of this operation is based on(1)
The controller monitors 119875PV 119881D1 and 119881B By using (1) 120575will be obtained for both charging and discharging modes asfollows
Advances in Power Electronics 5
Equations (3) Gate driver
dcdc converter8 signals
PPCC= 2kW
PD
Ppv
VD1
VB
120575+minus
Figure 6 Phase-shift control of the bidirectional dc-dc converter
Bridge 1Gate driverEZDSPF28335
Gate driver Bridge 2
0 to 15V
minus7V to 15V
Figure 7 The overall controller connection of the dc-dc converter
4 + 120596120587119871119875D119881D1119881B119873Battery discharging 997888rarr 120575 = 1205872 minus radic120587
2
4 minus 120596120587119871119875D119881D1119881B119873(3)
Equations (3) are used to calculate the desired phase-shiftangle to achieve the required active power during chargingand discharging modes of the battery [3]
Figure 7 illustrates the overall controller connection ofthe dc-dc converter The specified phase-shift modulationfrequency is 4 kHz and the system clock frequency of theDSPF28335 is 150MHz The phase-shift angle is calculated in theDSPThe gate driver forHVS provides four signals of 0 to 15Vwhile the gate driver for LVS provides four signals of minus7Vto 15V to avoid any accidental turn-on at the LVS becauseany parasitic inductance effect can be amplified with a highercurrent flow
In order to minimize a high circulating current in theconverter a large mismatch between the transformer turnsratio and the voltage ratio of theHVS and LVS dc sides shouldbe prevented Therefore a current controller is designed toregulate the voltage across the dc-link capacitor 119862D1 usingthe three-phase 119889-119902 synchronous reference frame controltechnique Accordingly the controller monitors119881B and keeps119881D1 at a level where the voltage ratio between 119881D1 and 119881B isclose to the transformer turns ratio
Figure 8 shows the control system of the PWM converterThe employed control system regulates the three-phase cur-rents 119894a 119894b and 119894c to achieve unity power factor and toregulate the dc-link voltage119881D1 at a reference voltage equal tothe transformer turns ratio multiplied by the battery voltage
Table 2 The gains of the three PI controllers
PI1 PI2 PI3Proportional gain 119870119901 01 09 09Integral gain 119870119894 5 001 004
119881B The detailed derivation of the controller model has beenpresented in [8] Table 2 presents the parameters of each PIcontrollerThe parameters119870119901 and119870119868 are selected by trial anderror method
Figure 9 presents the implementation of the PWM con-verter control system that consists of three main blocks thatare the sensors the dSPACEDS1104 controller and the PWMconverter Two-phase ac currents and ac voltages are sent tothe ADCport and the third current and voltage are calculatedin MATLABSimulink environment The measured HVS dc-link voltage is also sent to the ADC port The carrier signalhas a frequency of 10 kHz and the modulation index is 09In addition the dead time of the PWM converter is 1 120583s TheSPWM signals are fed to the gate drivers to trigger the IGBTs
4 Experimental Results
The experimental prototype and the control systems are builtbased on Figures 1 and 2 The parameters and gains aresummarized in Tables 1 and 2 respectively The BESS systemis designed to supply a constant power of 2 kW into the gridat the PCC
Figure 10 illustrates the dc operating waveforms of theBESS during changes of the PVoutput powerThe step changeis assumed to be from 25 kW to 28 kWThe battery voltage isconstant at 50V and subsequently voltage 119881D1 is regulated at300V by operation of the control systemTheBESS is chargedwith 500W when 119875PV = 25 kW Accordingly the BESS ischarged with 800W when 119875PV = 28 kW to level the PVoutput power at the PCC The transient response caused bythe step change in power demand lasts for 70ms This showsthat the controller reacts fast and is feasible for PV outputleveling
Figure 11 shows the ac voltage and current waveformsat the HVS and LVS of the transformer when the BESSpower transfer is changed from minus500W to minus800WThere arechanges to the phase-shift angle from 120575 = minus1382∘ (minus0241 rad)
6 Advances in Power Electronics
+ +
+
++
+
abc
dq
0
PWM
6
Gatesignal
+
+
VB
VD1ref
VD1
PI1Idref PI2
Id
Iq
Iqref PI3minus
minus minusminus
Vpwmd
Vpwmq
120596Lsys
120596Lsys
Varef
Vbref
Vcref
Vsq
Vsd
lowast
Figure 8 Current control of the PWM converter
Voltage andcurrent
measurement
ADC
Voltageregulator
Slave DSP
PWM block
Gatedriver
PWMconverter Filter
Loadacpowersupply
dSPACE
Three-phase ac voltage and current
6
Figure 9 Block diagram of the dSPACE controlled PWMconverter
to 120575 =minus215∘ (minus0375 rad) during the change in power transferrequirement The maximum voltages of transformer at theLVS and HVS are close to the transformer turns ratio whichare 50V and 300V respectively The LVS have a so-calledldquoflat toprdquo current that reduces the rate of change of thecurrent 1198942 over the time interval of conduction Thus thiswill minimize the peak switching current which leads toa lower switch turn-off overvoltage and improve converterefficiency
Figure 12 presents the operation of the BESS when PVoutput power varies from 15 kW to 1 kWThe battery voltageis constant at 60V and voltage VD1 is regulated at 360Vby operation of the control system Initially when 119875PV =15 kW 119875D = 500W to regulate the power at the 119875PCC to 2 kWWhen the PV output changes to 1 kW the BESS transfers1 kW to the PCC to level the PV output The transient
response caused by the step change in power demand lastsfor 100ms and the peak of the transient current at the LVS is22A
Figure 13 shows that the magnitude of the phase-shiftangle 120575 is increased from 863∘ (0151 rad) to 219∘ (0382 rad)during the discharging mode Voltage V2 leads V1 in thedischarging mode Low-side transformer voltage is 60V andhigh-side transformer voltage is 360V This shows that thecontroller is able to regulate the voltage of HVS accordingto the transformer turns ratio At the HVS current 1198941is low and hence would not cause significant amount ofswitching loss The LVS current appears as having the so-called ldquoflat toprdquo It reduces the rate of change of the current1198942 over the time interval of conduction Thus this willminimize the peak switching current which can cause ahigher switch turn-off overvoltage thus improving converterefficiency
Figure 14(a) presents the three-phase ac current duringthe charging of BESS The transient of the ac current lastedfor 200ms Figure 14(b) presents the ac voltage The THD ofvoltage during charging of the 119862D1 is around 2 and duringthe discharging operation of the dc-dc converter the THD is1 The THD of grid current is measured with FLUKE 1735Power Logger The accuracy of this power logger is plusmn02 ofits full scale
Figure 15 shows the enlarged ac voltage and currentduring the charging operation of the BESS at the power of1 kW and battery voltage of 60V The synchronized voltageand current waveforms show that the power factor is close tounity
Figure 16 shows the converter efficiency when the BESSis operating in the charging and discharging modes Theefficiency of the dc-dc converter in the discharging modeis measured by choosing the LVS as the input and HVS asthe output On the other hand the efficiency of the dc-dcconverter in the charging mode is measured by choosingthe HVS as the input and LVS as the output At batterycharging the measured converter efficiency peaks at 954at 119875D = minus400W At battery discharging measured converter
Advances in Power Electronics 7
0
600
400
200VD1
(V)
300V
200ms(a)
0
100
50
VB
(V)
50V
200ms(b)
0
minus20
minus10
minus30
I B(A
)
minus16A
200ms(c)
minus15
0
minus05
minus1
PD
(kW
)
minus08 kW
200ms(d)
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
Figure 4 Configuration of the proposed control system
Startingsequence
Check PVoutput power
BESS charging
BESS on standby
BESS discharging
NoNo
YesYes
PV gt 2kWPV = 2kWPV lt 2kW
50V lt VB le 60V VB le 60V
Figure 5 Determination of the three BESS operating modes
119875PV to determine the desired output power from the BESSsuch that
119875BESS = 119875PCC minus 119875PV (2)
The controller also considers the battery voltage 119881B and theHVS dc-link voltage 119881D1 Then it determines the requiredphase-shift angle 120575 needed to level the PV output powerby charging or discharging the battery Note that in theexperiment the PV output power is arbitrary and the mod-eling of PV generation system is outside the scope of thispaper
Figure 5 shows the typical operating modes of a BESSwhich are the charging discharging and standby modesWhen the PV output power is less than 2 kW the controllerchecks the battery voltage If the battery voltage is between50V and 60V (50V lt 119881B le 60V) the BESS is operated inthe discharging mode When the PV output power is morethan 2 kW the controller checks the battery voltage If thebattery voltage is equal to or less than 60V (119881B le 60V) theBESS is operated in the charging mode to absorb the surpluspower When the output power of PV is 2 kW the BESS goes
to standbymode by changing the phase-shift angle to zero Asshown in Figure 4 there are two other conditions where theBESS goes on standby mode to protect the battery bank frombeing overcharged and overdischarged
(i) When the battery voltage is less than 50V and 119875PV isless than 2 kW
(ii) When the battery voltage is more than 60V and 119875PVis more than 2 kW
Figure 6 presents the control system of the bidirectionalisolated dc-dc converterThe battery is charged or dischargedby the bidirectional isolated dc-dc converter connected tothe PWM converter Accordingly the control system deter-mines the required power in order to regulate the PCCpower at 2 kW The principal of this operation is based on(1)
The controller monitors 119875PV 119881D1 and 119881B By using (1) 120575will be obtained for both charging and discharging modes asfollows
Advances in Power Electronics 5
Equations (3) Gate driver
dcdc converter8 signals
PPCC= 2kW
PD
Ppv
VD1
VB
120575+minus
Figure 6 Phase-shift control of the bidirectional dc-dc converter
Bridge 1Gate driverEZDSPF28335
Gate driver Bridge 2
0 to 15V
minus7V to 15V
Figure 7 The overall controller connection of the dc-dc converter
4 + 120596120587119871119875D119881D1119881B119873Battery discharging 997888rarr 120575 = 1205872 minus radic120587
2
4 minus 120596120587119871119875D119881D1119881B119873(3)
Equations (3) are used to calculate the desired phase-shiftangle to achieve the required active power during chargingand discharging modes of the battery [3]
Figure 7 illustrates the overall controller connection ofthe dc-dc converter The specified phase-shift modulationfrequency is 4 kHz and the system clock frequency of theDSPF28335 is 150MHz The phase-shift angle is calculated in theDSPThe gate driver forHVS provides four signals of 0 to 15Vwhile the gate driver for LVS provides four signals of minus7Vto 15V to avoid any accidental turn-on at the LVS becauseany parasitic inductance effect can be amplified with a highercurrent flow
In order to minimize a high circulating current in theconverter a large mismatch between the transformer turnsratio and the voltage ratio of theHVS and LVS dc sides shouldbe prevented Therefore a current controller is designed toregulate the voltage across the dc-link capacitor 119862D1 usingthe three-phase 119889-119902 synchronous reference frame controltechnique Accordingly the controller monitors119881B and keeps119881D1 at a level where the voltage ratio between 119881D1 and 119881B isclose to the transformer turns ratio
Figure 8 shows the control system of the PWM converterThe employed control system regulates the three-phase cur-rents 119894a 119894b and 119894c to achieve unity power factor and toregulate the dc-link voltage119881D1 at a reference voltage equal tothe transformer turns ratio multiplied by the battery voltage
Table 2 The gains of the three PI controllers
PI1 PI2 PI3Proportional gain 119870119901 01 09 09Integral gain 119870119894 5 001 004
119881B The detailed derivation of the controller model has beenpresented in [8] Table 2 presents the parameters of each PIcontrollerThe parameters119870119901 and119870119868 are selected by trial anderror method
Figure 9 presents the implementation of the PWM con-verter control system that consists of three main blocks thatare the sensors the dSPACEDS1104 controller and the PWMconverter Two-phase ac currents and ac voltages are sent tothe ADCport and the third current and voltage are calculatedin MATLABSimulink environment The measured HVS dc-link voltage is also sent to the ADC port The carrier signalhas a frequency of 10 kHz and the modulation index is 09In addition the dead time of the PWM converter is 1 120583s TheSPWM signals are fed to the gate drivers to trigger the IGBTs
4 Experimental Results
The experimental prototype and the control systems are builtbased on Figures 1 and 2 The parameters and gains aresummarized in Tables 1 and 2 respectively The BESS systemis designed to supply a constant power of 2 kW into the gridat the PCC
Figure 10 illustrates the dc operating waveforms of theBESS during changes of the PVoutput powerThe step changeis assumed to be from 25 kW to 28 kWThe battery voltage isconstant at 50V and subsequently voltage 119881D1 is regulated at300V by operation of the control systemTheBESS is chargedwith 500W when 119875PV = 25 kW Accordingly the BESS ischarged with 800W when 119875PV = 28 kW to level the PVoutput power at the PCC The transient response caused bythe step change in power demand lasts for 70ms This showsthat the controller reacts fast and is feasible for PV outputleveling
Figure 11 shows the ac voltage and current waveformsat the HVS and LVS of the transformer when the BESSpower transfer is changed from minus500W to minus800WThere arechanges to the phase-shift angle from 120575 = minus1382∘ (minus0241 rad)
6 Advances in Power Electronics
+ +
+
++
+
abc
dq
0
PWM
6
Gatesignal
+
+
VB
VD1ref
VD1
PI1Idref PI2
Id
Iq
Iqref PI3minus
minus minusminus
Vpwmd
Vpwmq
120596Lsys
120596Lsys
Varef
Vbref
Vcref
Vsq
Vsd
lowast
Figure 8 Current control of the PWM converter
Voltage andcurrent
measurement
ADC
Voltageregulator
Slave DSP
PWM block
Gatedriver
PWMconverter Filter
Loadacpowersupply
dSPACE
Three-phase ac voltage and current
6
Figure 9 Block diagram of the dSPACE controlled PWMconverter
to 120575 =minus215∘ (minus0375 rad) during the change in power transferrequirement The maximum voltages of transformer at theLVS and HVS are close to the transformer turns ratio whichare 50V and 300V respectively The LVS have a so-calledldquoflat toprdquo current that reduces the rate of change of thecurrent 1198942 over the time interval of conduction Thus thiswill minimize the peak switching current which leads toa lower switch turn-off overvoltage and improve converterefficiency
Figure 12 presents the operation of the BESS when PVoutput power varies from 15 kW to 1 kWThe battery voltageis constant at 60V and voltage VD1 is regulated at 360Vby operation of the control system Initially when 119875PV =15 kW 119875D = 500W to regulate the power at the 119875PCC to 2 kWWhen the PV output changes to 1 kW the BESS transfers1 kW to the PCC to level the PV output The transient
response caused by the step change in power demand lastsfor 100ms and the peak of the transient current at the LVS is22A
Figure 13 shows that the magnitude of the phase-shiftangle 120575 is increased from 863∘ (0151 rad) to 219∘ (0382 rad)during the discharging mode Voltage V2 leads V1 in thedischarging mode Low-side transformer voltage is 60V andhigh-side transformer voltage is 360V This shows that thecontroller is able to regulate the voltage of HVS accordingto the transformer turns ratio At the HVS current 1198941is low and hence would not cause significant amount ofswitching loss The LVS current appears as having the so-called ldquoflat toprdquo It reduces the rate of change of the current1198942 over the time interval of conduction Thus this willminimize the peak switching current which can cause ahigher switch turn-off overvoltage thus improving converterefficiency
Figure 14(a) presents the three-phase ac current duringthe charging of BESS The transient of the ac current lastedfor 200ms Figure 14(b) presents the ac voltage The THD ofvoltage during charging of the 119862D1 is around 2 and duringthe discharging operation of the dc-dc converter the THD is1 The THD of grid current is measured with FLUKE 1735Power Logger The accuracy of this power logger is plusmn02 ofits full scale
Figure 15 shows the enlarged ac voltage and currentduring the charging operation of the BESS at the power of1 kW and battery voltage of 60V The synchronized voltageand current waveforms show that the power factor is close tounity
Figure 16 shows the converter efficiency when the BESSis operating in the charging and discharging modes Theefficiency of the dc-dc converter in the discharging modeis measured by choosing the LVS as the input and HVS asthe output On the other hand the efficiency of the dc-dcconverter in the charging mode is measured by choosingthe HVS as the input and LVS as the output At batterycharging the measured converter efficiency peaks at 954at 119875D = minus400W At battery discharging measured converter
Advances in Power Electronics 7
0
600
400
200VD1
(V)
300V
200ms(a)
0
100
50
VB
(V)
50V
200ms(b)
0
minus20
minus10
minus30
I B(A
)
minus16A
200ms(c)
minus15
0
minus05
minus1
PD
(kW
)
minus08 kW
200ms(d)
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
4 + 120596120587119871119875D119881D1119881B119873Battery discharging 997888rarr 120575 = 1205872 minus radic120587
2
4 minus 120596120587119871119875D119881D1119881B119873(3)
Equations (3) are used to calculate the desired phase-shiftangle to achieve the required active power during chargingand discharging modes of the battery [3]
Figure 7 illustrates the overall controller connection ofthe dc-dc converter The specified phase-shift modulationfrequency is 4 kHz and the system clock frequency of theDSPF28335 is 150MHz The phase-shift angle is calculated in theDSPThe gate driver forHVS provides four signals of 0 to 15Vwhile the gate driver for LVS provides four signals of minus7Vto 15V to avoid any accidental turn-on at the LVS becauseany parasitic inductance effect can be amplified with a highercurrent flow
In order to minimize a high circulating current in theconverter a large mismatch between the transformer turnsratio and the voltage ratio of theHVS and LVS dc sides shouldbe prevented Therefore a current controller is designed toregulate the voltage across the dc-link capacitor 119862D1 usingthe three-phase 119889-119902 synchronous reference frame controltechnique Accordingly the controller monitors119881B and keeps119881D1 at a level where the voltage ratio between 119881D1 and 119881B isclose to the transformer turns ratio
Figure 8 shows the control system of the PWM converterThe employed control system regulates the three-phase cur-rents 119894a 119894b and 119894c to achieve unity power factor and toregulate the dc-link voltage119881D1 at a reference voltage equal tothe transformer turns ratio multiplied by the battery voltage
Table 2 The gains of the three PI controllers
PI1 PI2 PI3Proportional gain 119870119901 01 09 09Integral gain 119870119894 5 001 004
119881B The detailed derivation of the controller model has beenpresented in [8] Table 2 presents the parameters of each PIcontrollerThe parameters119870119901 and119870119868 are selected by trial anderror method
Figure 9 presents the implementation of the PWM con-verter control system that consists of three main blocks thatare the sensors the dSPACEDS1104 controller and the PWMconverter Two-phase ac currents and ac voltages are sent tothe ADCport and the third current and voltage are calculatedin MATLABSimulink environment The measured HVS dc-link voltage is also sent to the ADC port The carrier signalhas a frequency of 10 kHz and the modulation index is 09In addition the dead time of the PWM converter is 1 120583s TheSPWM signals are fed to the gate drivers to trigger the IGBTs
4 Experimental Results
The experimental prototype and the control systems are builtbased on Figures 1 and 2 The parameters and gains aresummarized in Tables 1 and 2 respectively The BESS systemis designed to supply a constant power of 2 kW into the gridat the PCC
Figure 10 illustrates the dc operating waveforms of theBESS during changes of the PVoutput powerThe step changeis assumed to be from 25 kW to 28 kWThe battery voltage isconstant at 50V and subsequently voltage 119881D1 is regulated at300V by operation of the control systemTheBESS is chargedwith 500W when 119875PV = 25 kW Accordingly the BESS ischarged with 800W when 119875PV = 28 kW to level the PVoutput power at the PCC The transient response caused bythe step change in power demand lasts for 70ms This showsthat the controller reacts fast and is feasible for PV outputleveling
Figure 11 shows the ac voltage and current waveformsat the HVS and LVS of the transformer when the BESSpower transfer is changed from minus500W to minus800WThere arechanges to the phase-shift angle from 120575 = minus1382∘ (minus0241 rad)
6 Advances in Power Electronics
+ +
+
++
+
abc
dq
0
PWM
6
Gatesignal
+
+
VB
VD1ref
VD1
PI1Idref PI2
Id
Iq
Iqref PI3minus
minus minusminus
Vpwmd
Vpwmq
120596Lsys
120596Lsys
Varef
Vbref
Vcref
Vsq
Vsd
lowast
Figure 8 Current control of the PWM converter
Voltage andcurrent
measurement
ADC
Voltageregulator
Slave DSP
PWM block
Gatedriver
PWMconverter Filter
Loadacpowersupply
dSPACE
Three-phase ac voltage and current
6
Figure 9 Block diagram of the dSPACE controlled PWMconverter
to 120575 =minus215∘ (minus0375 rad) during the change in power transferrequirement The maximum voltages of transformer at theLVS and HVS are close to the transformer turns ratio whichare 50V and 300V respectively The LVS have a so-calledldquoflat toprdquo current that reduces the rate of change of thecurrent 1198942 over the time interval of conduction Thus thiswill minimize the peak switching current which leads toa lower switch turn-off overvoltage and improve converterefficiency
Figure 12 presents the operation of the BESS when PVoutput power varies from 15 kW to 1 kWThe battery voltageis constant at 60V and voltage VD1 is regulated at 360Vby operation of the control system Initially when 119875PV =15 kW 119875D = 500W to regulate the power at the 119875PCC to 2 kWWhen the PV output changes to 1 kW the BESS transfers1 kW to the PCC to level the PV output The transient
response caused by the step change in power demand lastsfor 100ms and the peak of the transient current at the LVS is22A
Figure 13 shows that the magnitude of the phase-shiftangle 120575 is increased from 863∘ (0151 rad) to 219∘ (0382 rad)during the discharging mode Voltage V2 leads V1 in thedischarging mode Low-side transformer voltage is 60V andhigh-side transformer voltage is 360V This shows that thecontroller is able to regulate the voltage of HVS accordingto the transformer turns ratio At the HVS current 1198941is low and hence would not cause significant amount ofswitching loss The LVS current appears as having the so-called ldquoflat toprdquo It reduces the rate of change of the current1198942 over the time interval of conduction Thus this willminimize the peak switching current which can cause ahigher switch turn-off overvoltage thus improving converterefficiency
Figure 14(a) presents the three-phase ac current duringthe charging of BESS The transient of the ac current lastedfor 200ms Figure 14(b) presents the ac voltage The THD ofvoltage during charging of the 119862D1 is around 2 and duringthe discharging operation of the dc-dc converter the THD is1 The THD of grid current is measured with FLUKE 1735Power Logger The accuracy of this power logger is plusmn02 ofits full scale
Figure 15 shows the enlarged ac voltage and currentduring the charging operation of the BESS at the power of1 kW and battery voltage of 60V The synchronized voltageand current waveforms show that the power factor is close tounity
Figure 16 shows the converter efficiency when the BESSis operating in the charging and discharging modes Theefficiency of the dc-dc converter in the discharging modeis measured by choosing the LVS as the input and HVS asthe output On the other hand the efficiency of the dc-dcconverter in the charging mode is measured by choosingthe HVS as the input and LVS as the output At batterycharging the measured converter efficiency peaks at 954at 119875D = minus400W At battery discharging measured converter
Advances in Power Electronics 7
0
600
400
200VD1
(V)
300V
200ms(a)
0
100
50
VB
(V)
50V
200ms(b)
0
minus20
minus10
minus30
I B(A
)
minus16A
200ms(c)
minus15
0
minus05
minus1
PD
(kW
)
minus08 kW
200ms(d)
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
Figure 9 Block diagram of the dSPACE controlled PWMconverter
to 120575 =minus215∘ (minus0375 rad) during the change in power transferrequirement The maximum voltages of transformer at theLVS and HVS are close to the transformer turns ratio whichare 50V and 300V respectively The LVS have a so-calledldquoflat toprdquo current that reduces the rate of change of thecurrent 1198942 over the time interval of conduction Thus thiswill minimize the peak switching current which leads toa lower switch turn-off overvoltage and improve converterefficiency
Figure 12 presents the operation of the BESS when PVoutput power varies from 15 kW to 1 kWThe battery voltageis constant at 60V and voltage VD1 is regulated at 360Vby operation of the control system Initially when 119875PV =15 kW 119875D = 500W to regulate the power at the 119875PCC to 2 kWWhen the PV output changes to 1 kW the BESS transfers1 kW to the PCC to level the PV output The transient
response caused by the step change in power demand lastsfor 100ms and the peak of the transient current at the LVS is22A
Figure 13 shows that the magnitude of the phase-shiftangle 120575 is increased from 863∘ (0151 rad) to 219∘ (0382 rad)during the discharging mode Voltage V2 leads V1 in thedischarging mode Low-side transformer voltage is 60V andhigh-side transformer voltage is 360V This shows that thecontroller is able to regulate the voltage of HVS accordingto the transformer turns ratio At the HVS current 1198941is low and hence would not cause significant amount ofswitching loss The LVS current appears as having the so-called ldquoflat toprdquo It reduces the rate of change of the current1198942 over the time interval of conduction Thus this willminimize the peak switching current which can cause ahigher switch turn-off overvoltage thus improving converterefficiency
Figure 14(a) presents the three-phase ac current duringthe charging of BESS The transient of the ac current lastedfor 200ms Figure 14(b) presents the ac voltage The THD ofvoltage during charging of the 119862D1 is around 2 and duringthe discharging operation of the dc-dc converter the THD is1 The THD of grid current is measured with FLUKE 1735Power Logger The accuracy of this power logger is plusmn02 ofits full scale
Figure 15 shows the enlarged ac voltage and currentduring the charging operation of the BESS at the power of1 kW and battery voltage of 60V The synchronized voltageand current waveforms show that the power factor is close tounity
Figure 16 shows the converter efficiency when the BESSis operating in the charging and discharging modes Theefficiency of the dc-dc converter in the discharging modeis measured by choosing the LVS as the input and HVS asthe output On the other hand the efficiency of the dc-dcconverter in the charging mode is measured by choosingthe HVS as the input and LVS as the output At batterycharging the measured converter efficiency peaks at 954at 119875D = minus400W At battery discharging measured converter
Advances in Power Electronics 7
0
600
400
200VD1
(V)
300V
200ms(a)
0
100
50
VB
(V)
50V
200ms(b)
0
minus20
minus10
minus30
I B(A
)
minus16A
200ms(c)
minus15
0
minus05
minus1
PD
(kW
)
minus08 kW
200ms(d)
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
Figure 10 Experiment waveforms of the step change of PV output power from 25 kW to 28 kW at119881B = 50V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus200
200
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 1382∘ 40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
40120583s120575 = 215∘
(b)
Figure 11 Experimental waveforms when the dc-dc converter is in charging mode (a) 119875D = minus05 kW (b) 119875D = minus08 kW
efficiency peaks at 949 at119875D = 600WThebattery voltage is60V for discharging mode and the battery voltage is 50V forcharging modes Efficiency is low during low power transferas the current is low
At low current levels the current could not reach theminimum required current for ZVS [4] Thus hard switch-ing occurs Furthermore the efficiency increases when thecurrent exceeds the minimum current for ZVS At the ratedpower the current increases increasing the conduction lossesand reducing the converter efficiency The efficiency of the
dc-dc converter is measured with YOKOGAWA WT1800Precision Power Analyzer The accuracy of this power loggeris plusmn02 of its full scale
5 Conclusion
This paper proposed a 2 kW BESS for integration with aPV generator that is based on a bidirectional isolated dc-dc converter and a PWM The feasibility of the proposedsystem is verified via experimental results The proposed
8 Advances in Power Electronics
0
600
400
200
VD1
(V)
360V
200ms(a)
0
100
20
6080
40VB
(V)
200ms
60V
(b)
0102030
I B(A
)
167A
200ms
22A
100ms84A
(c)
18
0
1206P
D(k
W)
1kW
200ms
05 kW
(d)
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
Figure 12 Experiment waveforms of the step change of PV output power from 15 kW to 1 kW at VB = 60V (a) High-voltage side capacitorvoltage (b) Battery voltage (c) Battery current (d) BESS power
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 863∘40120583s
(a)
40
0
minus40
10
minus10
0
0
minus600
600
minus100
100
0
1(V
) 2
(V)
i 1(A
)i 2
(A)
120575 = 219∘ 40120583s
(b)
Figure 13 Experimental waveforms when the dc-dc converter is in discharging mode (a) 119875D = 05 kW (b) 119875D = 1 kW
current controller regulates the voltage ratio of the HVSwith LVS of the dc-dc converter equal to the transformerturns ratio Accordingly the switching losses are minimizedto achieve an efficient power injection The phase-shift con-troller manages the charging and discharging modes of theBESS to provide the required amount of powerThe proposedBESS operates with unity power factor injected current
total harmonic distortion of less than 5 and fast dynamicresponse
Competing Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Advances in Power Electronics 9
5
minus5
0
i s(A
)
11A
200ms
(a)
200
minus200
0
s(V
)
200ms(b)
Figure 14 Variation of the charging mode from minus05 kW to minus08 kW (a) Three-phase ac current (b) Three-phase ac voltage
0
s(V
)i s
(A)divide
10
250
minus250
122V
42A
10ms
Figure 15 Ac voltage and current of phase-A during the charging operation of the BESS
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
The authors wish to thank the Ministry of Higher Education(MOHE) and Universiti Tenaga Nasional for the finan-cial support through FRGS12012TK07UNITEN0211 andInternal Grant no J510050610 respectively
References
[1] Y Yang P Enjeti F Blaabjerg and H Wang ldquoWide-scaleadoption of photovoltaic energy grid code modifications areexplored in the distribution gridrdquo IEEE Industry ApplicationsMagazine vol 21 no 5 pp 21ndash31 2015
[2] J-S Lai B-M Song R Zhou A Hefner Jr D W Berning andC-C Shen ldquoCharacteristics and utilization of a new class of low
on-resistance MOS-gated power devicerdquo IEEE Transactions onIndustry Applications vol 37 no 5 pp 1282ndash1289 2001
[3] NM L Tan T Abe andH Akagi ldquoDesign and performance ofa bidirectional isolated DCndashDC converter for a battery energystorage systemrdquo IEEE Transactions on Power Electronics vol 27no 3 pp 1237ndash1248 2012
[4] S Inoue and H Akagi ldquoA bidirectional isolated DC-DCconverter as a core circuit of the next-generation medium-voltage power conversion systemrdquo IEEE Transactions on PowerElectronics vol 22 no 2 pp 535ndash542 2007
[5] H Akagi T Yamagishi N M L Tan S-I Kinouchi YMiyazaki and M Koyama ldquoPower-loss breakdown of a 750-V 100-kW 20-kHz bidirectional isolated DC-DC converterusing SiC-MOSFETSBD dual modulesrdquo IEEE Transactions onIndustry Applications vol 51 no 1 pp 420ndash428 2015
10 Advances in Power Electronics
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
[6] N M L Tan S Inoue A Kobayashi and H Akagi ldquoVoltagebalancing of a 320-V 12-F electric double-layer capacitor bankcombined with a 10-kW bidirectional isolated DC-DC con-verterrdquo IEEE Transactions on Power Electronics vol 23 no 6pp 2755ndash2765 2008
[7] R W A A De Doncker D M Divan and M H KheraluwalaldquoA three-phase soft-switched high-power-density DCDC con-verter for high-power applicationsrdquo IEEE Transactions onIndustry Applications vol 27 no 1 pp 63ndash73 1991
[8] R Singh S Taghizadeh NM L Tan and J Pasupuleti ldquoBatteryenergy storage system for PV output power levelingrdquo Advancesin Power Electronics vol 2014 Article ID 796708 11 pages 2014
