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Energy 109 (2016) 525e536

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Energy

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Transformer-less 3P3W SAPF (three-phase three-wire shunt activepower filter) with line-interactive UPS (uninterruptible power supply)and battery energy storage stage

Wajahat Ullah Tareen*, Saad Mekhilef*

Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, Faculty of Engineering, University of Malaya,Kuala Lumpur, 50603, Malaysia

a r t i c l e i n f o

Article history:Received 8 December 2015Received in revised form19 April 2016Accepted 1 May 2016Available online 27 May 2016

Keywords:3P3W SAPFTransformer-lessDiode rectifiers as non-lineal loadDistributed power energy networkSRF (Synchronous reference frame)Battery storage equipment

* Corresponding author.E-mail addresses: wajahattareen@gmail.com (W.U

(S. Mekhilef).

http://dx.doi.org/10.1016/j.energy.2016.05.0050360-5442/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this article, a transformer-less 3P3W SAPF (three-phase three-wire shunt active power filter) inte-grated into the distributed energy network which embeds line-interactive UPS and bi-directionalchopper-fed energy storage equipment. The proposed system operates in two operating modes ofline-interactive UPS (uninterruptible power supply) with bi-chopper-fed battery energy storage stage. Innormal mode of operation, the main power supply current charges the standby bi-chopper fed batteryenergy storage equipment, in addition to APF feature. When the main power supply is severed, itoperates as a backup power source to the load. The feasibility of the distributed energy network isimproved, by eliminating the transformer and reduced power component, to provide accurate perfor-mance, less cost and reduced size as compare to previous topologies. The PI (proportional integral)controller ensures the regulated sinusoidal voltage with unity power factor, phase amplitude, and lowTHD (total harmonic distortion) into the distributed energy grid. For proper batteries energy storagestage, active power filtering and UPS operation, the SRF (synchronous-reference-frame) method with PLL(phase locked loop) scheme is implemented. The test result of 5 kW prototype system tested in simu-lation and laboratory validates the claimed performance in the distribution power energy network.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

In a distribution energy system, the increasing demands fornonlinear loads, such as diodes, thyristor, and rectifiers is currentlyincreasing, especially in the context of harmonic propagation.These rectifiers result in the deterioration of waveforms and thegeneration of current harmonics, which affects its performance.The distortion will ultimately result in a low dc voltage output atthe UPS (uninterruptible power supply) system, which causes po-wer loss, high current flow, and faults in the system. The hybridpower filters, combining passive and active filters to compensatefor these harmonics [1]. The proposed UPS configuration improvesthe system's poor quality problem, with reactive and harmonicspower losses, by compensating for the voltage and current distur-bances [2]. The integrated shunt active power filter reduces the

. Tareen), saad@um.edu.my

THD (total harmonics distortion) to mark the harmonic mitigationstandard, such as IEEE 519 and IEC EN 61000-3 [3]. It helps thesystem provide unity input power factor with stable sinusoidalinput voltage and input current.

UPS (Uninterruptible Power Supply) technology is a very com-mon solution for power source quality, power failure, and industrialand commercial units [4]. The main purpose is to provide stability,continuity, and quality to critical loads, including office equipment,computers, and communication systems, along with isolation,regulated sinusoidal voltage, low sinusoidal input current, unitypower factor, and low THD (total harmonic distortion). It alsoprovides fast response transition during the change in operatingmodes.

In Refs. [5e8], many three-phase line interactive UPS system hasbeen configured for harmonic and reactive power compensation,but none was integrated with the shunt active power filter ability.The proposed UPS configuration filters the AC power supply har-monic current, resulting in excellent performance in terms of lowTHD and output voltage regulation in the distribution energynetwork.

Nomenclature

vs (a, b, c) Source voltage of phase-a, phase-b and phase-crespectively

is (a, b, c) Instantaneous Source current of phase-a, phase-b andphase-c respectively

iLh (a, b, c) Harmonics load current of phase-a, phase-b andphase-c respectively

iF (a, b, c) Compensator current of phase-a, phase-b and phase-crespectively

vf (a, b, c) Inverter output voltage of phase-a, phase-b andphase-c respectively

id Instantaneous active currentiq Instantaneous reactive currentsidAC

Instantaneous active AC currentiqAC Instantaneous reactive AC currentsv�AF Voltage reference of each phase VAf ; VBf ; VCfu1 Fundamental frequencyq Phase angleVDC DC voltagezs Source impedanceZF Filter impedance

VAF Voltage sourceVbat Battery voltageVL Load voltagesVdVq Load voltages in the dq frameidc DC-link current

List of abbreviations3P3W SAPF three-phase three-wire shunt active power filterSAPF Shunt Active Power FilterPWM pulse width modulationSAPF shunt active power filterVSC Voltage Source ConverterTHD Total Harmonic DistortionPF Power FactorSRF synchronous-reference-framePLL phase locked loopPI proportional integral controllerUPS uninterruptible power supplyPCC point of common couplingHPF high pass filterKI Integral gainKp Proportional gain

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536526

This paper proposed a 3.7 kVA transformer-less 3P3WSAPFwithline-interactive UPS and battery energy storage equipment stageembedded into a distributed energy network. The main drawbackof the previous stated configuration is the high number of powerdevices [9,10]. The arrangement of the proposed topology elimi-nates the need of line-interactive rectifier [11e13], thereforereducing the system size and cost. The system design reliability wasproven by the experimental prototype setup, and successfullyreduced the THD of the utility supply current to approximately4.2%. To avoid distortion, across passive components such as LCfilters [14], a simple coupling inductor is implemented for properfiltering and voltage regulation. The system enhances both thereactive and harmonic power filtering compensations, resulting inlow THD, cost reductionwith effective power-factor correction, andthe suppression of harmonic currents. Moreover, it provides fastdynamic response during step load transient and steady stateperformance results to verify the effectiveness of the proposedsystem.

Fig. 1 shows the proposed configuration of the APF with line-interactive UPS and battery storage equipment. The filter inverteroffers the best harmonic filtering performance, followed by thebattery energy storage equipment stage bi-directional DCeDCconverter and the battery bank [15]. The fast static switch transfersthe load to the UPS system during the event of power cut, overload,blackout, circuit fault, or component failure. The transfer time isless than ¼ line cycle, not affecting the system load [16]. A main-tenance parallel static by-pass switch is added in the circuit designto disable the UPS system for repair without affecting the contin-uous power to the load. This parallel static switch providestransient-free transfer operation and clears the fault by blowing afuse while protecting the UPS system from the output circuit fault.

2. System configuration

The proposed circuit consists of a 3.7-kVA transformer-lessshunt active power filter operates at 10 kHz switching frequency[17] into a 200 V, 5 kW rated three-phase diode rectifier load. Theexperimental circuit consists of a DC/AC inverter, DC/DC bi-

directional converter, and a DC battery bank as depicted in Fig. 2.The active power filter is installed in series to a coupling inductor LFconnected in parallel at the PCC (point of common coupling). Thecoupling inductors operate in a manner similar to a switchingripple filter, where it filters the switching ripples generated fromthe PWM (pulse width modulation) inverter operation.

The active power filter consists of a six IGBTs modules rated at600 V for three-phase PWM inverter and DC capacitor Cd, rated atVdc ¼ 300 V and Cdc ¼ 4700 mF. The proper rating of the DCcapacitor is selected, such that it reduces the voltage ripple to 1%[18]. Tomaintain the harmonic current time derivatives, the voltagereference level is kept high using a proportional integral (PI)controller. The PI controller marks the DC voltage level, and noexternal power source is needed. The dc bus voltage Vdc is kepthigh at 300 Vmore than the peak value of the AC supply voltage forthe proper active filtering.

3. Control of active power filter

The control for line-interactive UPS energy storage equipmentsystem is implemented in d-SPACE based intelligent under Matlab-Simulink environment. It operates the complete system in threemain sections: 1) To eliminate the harmonics, by controlling theSAPF operation. 2) To control the DC Voltage across the PWMinverter capacitor 3) and to perform the UPS operation duringnormal mode (battery storage stage) and backup operating mode.The control circuit of the parallel active power filter [19] is classifiedinto three sub-sections, as explain in Fig. 3, such as: 1) PLL (Phaselock loop), 2) APF system reference generator, and 3) DC voltagecontrol.

3.1. PLL (Phase lock loop)

The PLL (phase lock loop) provides a transient-free locking tothe output voltage in phase with the input supply waveform. ThePLL circuit maintains constant amplitude of output line voltage andlock in-phase with the input line voltage between ð0 to 2pÞ. Itkeeps the operation on the fundamental input component and

Fig. 1. Complete structure of the proposed configuration.

Fig. 2. A 200 V, 5 kW prototype system.

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536 527

Fig. 3. A 5 kW experimental different control system (a) APF control (b) Back-up mode control (c) Buck-boost converter control.

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536528

controls the input current in accordance with the input voltageharmonics.

3.2. APF system reference generator

The three phase instantaneous supply currents ð iSA; iSB; iSC Þare detected and inputs to the control system. By using the refer-ence frames, it converts three-phase supply current into a two-phase instantaneous active id and instantaneous reactive iq cur-rents at a fundamental frequency of u1. This synchronous rotatingframe is in phase with the positive sequence of three-phase supplyvoltages ð vA; vB; vC Þ by using the phase lock loop PLL. The zerosequence is neglected because of the three-phase system. Thevoltage source inverter provides the AC voltage to damp the systemharmonics [19].

24 idiqi0

35 ¼

ffiffiffi23

r2666666664

sinðustÞ sin�ust � 2p

3

�sin

�ust þ 2p

3

�

cosðustÞ cos�ust � 2p

3

�sin

�ust þ 2p

3

�

1ffiffiffi2

p 1ffiffiffi2

p 1ffiffiffi2

p

3777777775

24 iSAiSBiSC

35

(1)

It transforms both the components of the fundamentalu1(u1 ¼ 50 Hz) and harmonics frequencies. Both the active andreactive quantities are decomposed into DC and AC values. A phaselock loop circuit is implemented for an accurate phase angle q totrack the utility voltage.

�idiq

�¼

�idDC

iqDC

�þ�idAC

iqAC

�(2)

It decomposed the supply currents into the instantaneous activecurrent component id and instantaneous reactive currentcomponent iq. The fundamental component can be the DC signalquantities, while the harmonics quantities can be the AC signalcomponents, respectively.

�idAC

iqAC

�¼

�idiq

���idDC

iqDC

�(3)

The ac current harmonics are extracted using 2 s-order HPF(high pass filter) idAC

and iqAC at cutoff frequency of 50 Hz [20]. Thesampling time delay 50usec of the digital filter is short enough to beneglected for calculation, but the two-filter delay effects the dy-namic voltage damping performance of the shunt active filter [21].At the end of the operation, the supply harmonics current com-ponents are generated by the inverse d-q transformation operation.Each current component is amplified by the gain K

24 iCAiCBiCC

35 ¼

ffiffiffi23

r26666664

sinðustÞ cosðustÞ

sin�ust � 2p

3

�cos

�ust � 2p

3

�

sin�ust þ 2p

3

�cos

�ust þ 2p

3

�

37777775"i*di*q

#(4)

All the APF filtering parameter and characteristics are depen-dent upon this gain (K) value. As a voltage reference, the v�AF foreach phase

�VAf ; VBf ; VCf

�is compared to the proper switching

gate signals for PWM inverter switching operations.

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536 529

v�AF ¼ K � iFabc (5)

3.3. DC voltage control

A PI (proportional integral) control scheme is used to build andmaintain the required voltage level at the DC capacitor of the PWMinverter for harmonic component generator. To maintain the con-stant voltage level, it leads the current to the DC capacitor elimi-nating the need of external power source. During active power filteroperation, the excessive absorption of the active power will affectthe active filter operation by increasing the DC side voltage levels[21]. To control this problem, controlling the DC value of thereactive current iqDC of the quadrature axis, and setting the directaxis to zero is adopted. The detected DC voltage is compared to thereference value, a low pass filter LPF can be used to eliminate theripples in the detected DC voltage VDC , which is to be set high300 V, limiting the high rating voltage of IGBTs devices. To maintainthe constant DC-link voltage level, the controller parameters areestimated, as the transfer function for PI compensator is defined in(6) as:

GvðsÞ ¼ Kp þ KI

S(6)

The proportional and integral gains are derived usingKp ¼ (2.x.un. Cdc), it determines the dynamic response of the DC-link voltage controller and the integral gain is derived usingKI ¼ Cdc: u2

nv, determines it's settling time [22,23]. The proportionalðKpÞ and integral ðKI Þ gain values are selected as 0.1 U�1 and24 U�1

4. APF filtering characteristics

Thewhole idea of APF control operation is represented as virtualimpedance. This virtual impedance operation is frequency-dependent and must be kept at high frequencies to stop the flowof load current harmonic from flowing back into the utility. It isconnected in series with the source impedance zs. At a funda-mental frequency, the APF operates as zero impedance. It pushesthe harmonics to follow the inductor path, and behave as adamping resistor to the harmonics frequencies. In practical terms,the gain K value is increased to amaximum limit due to the stabilityproblems. The two controls operate as an independent controllablesources vs ; iLh and one as voltage controllable source vAF [24].

Fig. 4. (a) Equivalent circuit of single phase voltage mode

Fig. 4a illustrates the equivalent circuit of the voltage mode controlstructure of the proposed configuration. The load is considered asan ideal current source IL, operating as pure sinusoidal waveform,with filter impedance ZF . In practical terms, it is difficult to operateas an ideal harmonic current source, due to the system's impedance[25].

The APF operates as a controlled voltage source VAF to controlthe output voltage compensation, as depicted in Fig. 4b. It operatesas voltage source to the harmonic line current ðVAF ¼ K � ishÞwhere K is the gain of filter, the is is the source current, iF is APFcompensation current and iLh is the current-source harmonics loadcurrent, respectively. When the filtering control operates, the ratioish=iLh of harmonic components of the source current is present in(7).

ish ¼�

ZFK þ ZF þ Zs

�� iLh (7)

The impedance value is kept high in order to push the flow ofharmonics towards the APF leg. At low frequencies the APF stop thefundamental current and shift the source current towards the loadside. It behaves as an inductor at harmonic frequency and preventsthe current harmonics from flowing towards the utility grid. Thevalue of the gain K is kept high to control the filtering operationduring the frequency control schemes. Fig. 5 illustrates the struc-ture for harmonic compensator in dq-control scheme.

5. Operation of line-interactive UPS and battery energy stage

Generally, there are two operating modes in the experimentalline-interactive prototype. The operation of the input static switchdetermines these two modes of operations. During the normalcharging mode, when the input static switch is closed or the powersupply is available, the energy is supplied from the main AC sourceto the critical load. It charges the standby connected battery energystorage system in addition with filter operation and currentcompensation. Normally, the active power filters are installed nearthe non-linear load at the PCC to compensate for the effect of thecircuit non-linearity [26]. For a proper shunt active filter operation,the output DC bus voltage of voltage source inverter is kept high[27,28]. This voltage level cannot be used directly for charging thebattery bank, so it should be reduced to a suitable battery charginglevel.

As depicted in Fig. 3c, a bi-directional buck-boost DCeDC con-verter control is designed to eliminate the high rated step-up or

control (b) Single phase harmonic equivalent circuit.

Fig. 5. Scheme for harmonic compensating in dq control structure.

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536530

step-down transformer. In addition, it reduces the battery bank sizeat a rated voltage. The main objective is to design a simple, robust,and low cost control for reducing the current rating of theswitching components and battery bank, which effectively reducesswitching losses and increased operating time. To protect the bat-tery bank from overcurrent damages, the current limiter is set tothe normalized value of the battery bank. The bi-directional buck-boost DCeDC converter operates as a buck regulator during thenormal operating mode. When the main supply is cutoff, the boostregulator steps up the lower battery voltage to higher output DCvoltage level across APF inverter [29]. It charges the capacitor

Table 1Advantages and disadvantages of the PI controller and VLP (voltage loop controller).

Advantages

PI controller - Simple design and structure.- Effective control in the zero steady-state error in the d-q fram- High gain and- Fast transient response.

VLC (Voltageloop controller)

- The reference currents is taken from the load currents irrespecthe source voltage. The reference signal is not effective by the vsource distortion, helps in better harmmonic compensation anvoltage controller.

- show robust performance for periodic disturbances.- Ensures a zero steady-state error at all the harmonic frequenc- Less control effort and fast startup response, due to the feedfovoltages.

- Lowest current THD at small switching frequencies and at AC vdisturbances.

- Lowest coupling between active and reactive powers control,the decoupling network.

- Advantage of instantaneous reactive power compensation.- Do not need an accurate mathematical model, easy to implemnonlinearity inputs.

- Simplify the mathematical model, using frames of referencethree quantities to the two quantities.

- Show superior transient response, robust design, and dynamicresponse in all the three phases independently

voltage to the main supply voltage required level equal to thebattery voltage Vbat ¼ 100 V [30,31].

When the switch is opened or the main AC power grid is cutoff(blackout), the power is supplied from the standby battery energystorage equipment. In standbymode, the UPS system can operate asa backup power source and supply power to be loaded through thePWM inverter. During this mode, because of zero utility current, thedesired reference load voltage [32,33] is compared to the triangularwave for PWM switching processing as shown in Fig. 3b. The outputis a control voltage; it compels the output voltage to follow thesinusoidal voltage for generating the PWM gating switching signal

Disadvantages

e.- The circuit conditions changes or degrades the controller operationperformance.

- Difficult to tune controller gains.- Requires a precise linear mathematical model for perfect gains tuning.- Under unbalanced disorders, parameters variations and non-linearityconditions, display poor performance.

- Poor performance to compensate the higher orders harmonics.- In grid instability and disturbances, show low order harmonics poorcompensation capability.

- Complexity of the model increases as in term of transfer function.tive ofoltaged

ies.rward

oltage

due to

ent in

s from

speed

- Complex structure including a decoupling network, and referenceframe transformations.

- Not independent between coupling control variables (Id, Iq), needs adecoupling network.

- In some cases, requires the values of filter inductance in thedecoupling network.

- Unknown load disturbances effects the system stability performances.- Cannot achieved a fast response for fluctuating load.- Cause a slow dynamic response and poor THD results in case of linevoltage distortion.

- Low dynamic response and- Poor efficiency under the non-ideal grid voltages

Table 2200 V, 5 kW system parameters in experiment.

Diode rectifier rating 5 kWline to line RMS voltage (Vs) 200 VLine frequency 50 HzSupply inductor (Ls) 0.21 mHAC load inductor (LAC ) 3 mHRectifier DC capacitor (Cd) 1500 mFDC inductor (LDC ) 675 mHDC Capacitor (Ccf ) 1.54 mFActive filter rating 3.7 kVAFilter AC inductor (LF ) 1.5 mHDC capacitor of Active filter (Cdc) 4700 mFDC voltage of active filter (VDC ) 300 VBattery voltage (Vbatt ) 100 VHPF cutoff frequency (FHPF ) 50 HzGain (K) 35 U (p.u)

Fig. 7. Simulation waveforms of utility voltage, load current, utility current, filtercurrent and capacitor DC voltage (APF operation).

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536 531

with a low THD. Upon transitioning to the charging mode from thebackup mode when the input power is restored. The PLL (phaselock loop) provides a transient-free locking, once theworking rangeis achieved, and the static bypass switches are closed to restore thecharging operating mode. To avoid the zero utility current, a sinu-soidal PWM switching scheme is adopted to lower the harmoniccontents and ripples in the output voltage waveform. Table 1,summarizes the comparison between the advantages and disad-vantages of the PI controller and voltage oriented controller. Thecomparison list of each controller is evaluated in term of structurerobustness compared to model parameters, stability analysis, har-monic compensation, unbalanced conditions and nonlinearity inthe system [34].

6. Simulation results

To verify the simulation results, the proposed system is simu-lated in Matlab-Simulink software under the non-linear load con-dition. The specification of the system is shown in Table 2, which isused in the experimental and simulation circuit setup. Fig. 6 showsthe simulation results of proposed APF operation under normalcondition, around 0.12 s, the APF start the current harmonic in-jection operation. A typically three-phase rectifier load with filter

Fig. 6. Simulation waveforms of utility voltage, load current, utility current, filtercurrent and capacitor DC voltage (point of injection).

capacitor and resistor is used for distorting the utility waveform.The THD of supply current is reduces from 40.1% to 4%, with powerfactor of 0.99. As notice, the DC voltage of 300 V is sufficient forproper and effective APF operation. As see in Fig. 7, the sourcecurrent is nearly sinusoidal after the APF operation but the loadcurrent is seriously distorted because of three-phase rectifier load.By comparing the Figs. 7, 11 and 12 results it concludes that thesimulation and experimental results are nearly same.

Fig. 8 illustrates the harmonic spectrum of the source currentbefore and after compensation. It confirm that the proposed systemcompensate the THD of the source current. The THD of the sourcecurrent is 40.1% and after compensation, it reduces to 4% less than5%.

7. Experimental results

7.1. System specification

To understand the operation of APF, a laboratory prototypesystem of a three-phase line interactive system is combined with a

Fig. 8. Harmonic spectra of uncompensated source current (series 1) and compensatedsource current (series 2) waveforms.

Fig. 9. Experimental result a) input utility voltage V_A (X-axis: 10 ms/div; Y-axis: 250 V/div) b) input load current ISA (X-axis: 10 ms/div; Y-axis: 5 A/div).

Fig. 10. Experimental result of SAPF under normal condition a) utility voltage (X-axis: 10 ms/div; Y-axis: 150 V/div) b) load current (X-axis: 10 ms/div; Y-axis: 10 A/div) c) utilitycurrent (X-axis: 10 ms/div; Y-axis: 10 A/div).

Fig. 11. Experimental result under filtering condition a) Utility voltage (X-axis: 10 ms/div; Y-axis: 100 V/div) b) Load current (X-axis: 10 ms/div; Y-axis: 10 A/div) c) Utility current(X-axis: 10 ms/div; Y-axis: 10 A/div).

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536532

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536 533

3.7 kVA three-phase voltage source IGBT inverter. The prototype isbuilt and tested on a 200-V 5-kW nonlinear diode rectifier loadhaving total harmonic distortion of 39.48%. All the experimentalparameters are listed in Table 2.

7.2. Unity power factor correction

Fig. 9 illustrates the supply voltage VA and supply current ISA ofphase A. Both the waveforms are in phase for unity power factoroperation, having a measured power factor of 0.98. During opera-tion, the DC voltage VDC is set to 300 V, while the modulation indexis around M ¼ 0.9.

8. Filtering characteristics

As depicted in Fig. 10, the utility voltage, load current and utilitycurrent waveforms under the nonlinear load conditions prior to the

Fig. 12. Experimental results under filtering co

current harmonic compensation. Both the currents are distortedand non-sinusoidal.

8.1. Steady state filtering performance

Fig. 11 illustrations that prior to current injection the THD of theload current is 39.48%, but after the APF harmonic compensation,the THD of the utility current is reduced to 4.2%. The APF providesan output voltage THD of 3% and constant RMS output sinusoidalvoltage waveform. After APF compensation, all utility currents arenearly sinusoidal, even without the use of passive filters. Fig. 11shows the steady-state filtering waveform of the proposedconfiguration. The control gain is set to K ¼ 34 U for good filteringcharacteristics. Due to the pulse-width modulation PWM, the VINVhave a fundamental voltage and large amounts of switching ripplein the waveform. These ripples can be eliminated by setting anadditional small switching passive filter, and could also be reducedusing two first-order high-pass filters HPF. Fig. 12 shows the

ndition a) APF current b) DC-bus voltage.

Fig. 13. Experimental result of SAPF under transient conditions a) utility voltage (X-axis: 10 ms/div; Y-axis: 100 V/div) b) load current (X-axis: 10 ms/div; Y-axis: 10 A/div) c) utilitycurrent (X-axis: 10 ms/div; Y-axis: 10 A/div).

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536534

inverter filtering current and DC-link voltage across the DCcapacitor.

8.2. Transient state filtering performance

Fig. 13 demonstrations the transient response of the proposedAPF system, during the transient operation, the load was changedusing step on and step off operations. As can see, the response timeof the APF system is fast, showing excellent operation.

The transient response is observedbychanging the step load from5 kW to 2.5 kW. During the step load change, the IS waveform isdistorted forhalf cycle, but reaches a steady state, causingnonegativeeffect(s) on the filtering operation. As we can see, is high during thetransient state, causing a voltage increase of nearly 4%, above 300 V.

Fig. 14. Experimental results at utility interruption conditions a) utility current (X-axis: 10voltage (X-axis: 10 ms/div; Y-axis: 100 V/div).

8.3. Outage test

The proposed system is tested under utility interruption. Whenthe utility AC fails, the power is continuously supply to the criticalload from the battery bank. As seen in Fig. 14, during the backupmode, input current, output voltage and load current waveformschanges from utility to battery power immediately without anywithout interruption.

When the utility line is restored, the output voltage is controlledwithout transient. Fig. 15 shows that the proposed system caneasily transfer the input current, output voltage and load currentwaveforms to utility power. As depicted in Figs. 14 and 15, bothvalidate the experimental results during the normal and backupoperating modes.

ms/div; Y-axis: 10 A/div) b) load current (X-axis: 10 ms/div; Y-axis: 10 A/div) c) load

Fig. 15. Experimental results at utility recovery conditions a) utility current (X-axis:10 ms/div; Y-axis: 10 A/div) b) load current (X-axis: 10 ms/div; Y-axis: 10 A/div) c) load voltage(X-axis:10 ms/div; Y-axis: 100 V/div).

W.U. Tareen, S. Mekhilef / Energy 109 (2016) 525e536 535

9. Conclusion

In this paper, the main advantage of the proposed line-interactive UPS with battery energy storage stage configurationwas that it could operate as an APF during a normal operatingmodewithout the need for matching transformer in the distributed en-ergy system. The main focus of this work was to reduce the utilityTHD, system size, manufacturing costs, and eliminate harmonics.By using the shunt active power filter operation, it corrected thedistributed energy system power factor under non-linear distortedload. At full input power rating, it uses smaller power rating com-ponents, with effective optimal design and high UPS efficiency.Both the PI controller improves the active filtering operation andstabilizes the DC voltage across the DC capacitor, including controlparameters such as HPF cutoff frequency for harmonics extraction,stable DC capacitor voltage, and feedback gain K of filter. To increasethe filtering performance of the SAPF, an additional feedforwardcontrol loop can be integrated with the feedback control loop. Theproposed circuit demonstrated excellent performance in both thetransient and steady state operations.

Acknowledgment

The authors would like to acknowledge the financial supportfrom the High Impact Research-Ministry of Higher Education(HIRMOHE Project No. UM.C/HIR/MOHE/ENG/24) and FundamentalResearch Grant Scheme (FRGS), FP014-2014A.

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