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Published in IET Generation, Transmission & DistributionReceived on 5th September 2013Revised on 12th March 2014Accepted on 21st May 2014doi: 10.1049/iet-gtd.2013.0636

T Gener. Transm. Distrib., pp. 1–10oi: 10.1049/iet-gtd.2013.0636

ISSN 1751-8687

Improvement of unified power quality conditionerperformancewith enhanced resonant control strategyQuoc-Nam Trinh, Hong-Hee Lee

School of Electrical Engineering, University of Ulsan, Ulsan, South Korea

E-mail: [email protected]

Abstract: This study proposes an enhanced resonant control strategy for the unified power quality conditioner (UPQC) tosimultaneously tackle voltage sags, unbalance and distortions on the supply side as well as current harmonics on the loadside. The proposed control strategy is developed in both the series and shunt active power filters (APFs) of the UPQC. In theseries APF, a proportional–resonant controller and a resonant controller are employed to mitigate voltage sags, unbalance anddistortions, whereas a proportional–integral (PI) controller and three vector PI controllers are used in the shunt APF tocompensate harmonic currents. The performance of the proposed UPQC control scheme is significantly improved comparedwith the conventional control strategy owing to the superiority of the resonant controllers. In addition, voltage sag/harmonicand current harmonic detectors are not required in the proposed control scheme, which helps to simplify the control strategyand to improve the control accuracy. The proposed control strategy is theoretically analysed and its feasibility is validatedthrough experiments.

1 Introduction

Voltage sags are generally considered as common and costlypower quality problems affecting sensitive loads. Theyregularly result in undesired interruptions of electronic-based equipment and are responsible for large productivelosses in industrial processes [1, 2]. The series active powerfilter (APF), commonly known as the dynamic voltagerestorer (DVR), is an effective solution for protectingsensitive loads from voltage sags [3–10]. The DVR isinstalled in series between the supply and the load side inorder to inject a compensating voltage to maintain the loadvoltage balanced and sinusoidal at the desired value whenvoltage sags take place on the supply side.The performance of a DVR in compensating the voltage

sags is determined by the voltage sag detectors and voltagecontrollers. Voltage sags should be detected rapidly andaccurately without any delay time to ensure that the DVRcan quickly recognise and respond to the voltage sags.Monitoring the magnitude of the supply voltage in thefundamental (d–q) reference frame is a common method todetect voltage sag [9]. After accurately detecting voltagesag, a voltage reference for the DVR is generated and thevoltage controller then executes to ensure the load voltageis undisturbed. Hence, the voltage controller also plays avital role in a DVR control scheme to achieve a fasttransient response as well as good steady-state performanceof the load voltage during voltage sags.The conventional proportional–integral (PI) controller

combined with the supply voltage feed-forward in the d–qreference frame has been developed to tackle voltage sags[3–5]. However, this method has only been evaluated under

balanced sags, whereas the majority of faults on powersystems are asymmetrical, which results in unbalancedvoltage sags on the supply side. Under such conditions, theconventional PI controller fails to properly track thereference signal because of the presence of the negativesequence components [5]. To mitigate unbalanced voltagesags, several control strategies have been developed: two PIcontrollers in positive and negative d–q reference frames[6], a proportional–resonant (PR) controller [7] or a H∞controller in the stationary frame [8]. However, the gridvoltage may not only be unbalanced because of voltagesags, but also be distorted because of non-linear loads ondistribution systems. Therefore, those control strategies areinadequate for tackling voltage sags and distortionssimultaneously. In addition, from the view point of thepower system, harmonic currents produced by non-linearloads also cause serious problems, thereby degrading thepower quality [11]. Hence, they must be taken into accountand fully compensated.The unified power quality conditioner (UPQC) has been

introduced as an advanced solution to fulfil those goals[11–13]. Owing to the combination of a series APF and ashunt APF, the UPQC is able to simultaneously deal withvoltage and current-related problems to protect sensitiveloads and to enhance the power quality. In the UPQC, therole of the shunt APF is not only to compensate theharmonic currents, but also to support the series APF tocompensate long duration voltage sags by maintaining thecommon DC-link voltage of the series and the shunt APFsat a constant level.Various UPQC control strategies have been developed

to deal with voltage distortions and current harmonics

1& The Institution of Engineering and Technology 2014

Fig. 2 Block diagram of the proposed UPQC control scheme

Fig. 1 Block diagram of the conventional UPQC control scheme

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[11–23]. In [11–15], in order to regulate the load voltage andsupply current to be sinusoidal, harmonic components in thesupply voltage and the load current are detected throughharmonic detectors and regulated by hysteresis controllers.In [16–20], harmonic detectors are eliminated to simplifythe control scheme, but hysteresis controllers are still usedto regulate the load voltage and supply current to besinusoidal. Despite the simple structure and fast responsecharacteristics of the hysteresis controller, this controltechnique has two shortcomings: first, the variation ofswitching frequency may cause many switching noises onthe load voltage and the supply current; and second, thecontrol performance is limited because of the trade-offbetween the hysteresis band and the switching frequency.These drawbacks degrade the performance of the hysteresiscontroller. Meanwhile, in [21, 22], artificial neural networkand particle swarm optimisation-based controllers weredeveloped for the UPQC to deal with voltage and currentharmonics. However, these control algorithms are obviouslycomplicated since a large training time is required foroptimising the controller gains. Moreover, despite thecomplexity of the control algorithm, the performancepresented in [21] is not so good compared with otherconventional control methods. A novel control method forthe UPQC to tackle voltage and current harmonicsproblems was proposed in [23]. The control strategy in [23]requires fewer sensors compared with other methods and italso provides a good harmonic compensation performance.However, in that study, only the voltage and currentharmonic problems are considered. Other power qualityissues such as voltage sags and voltage unbalance are nottaken into account, which restrict the compensationcapability of the UPQC.In this paper, an enhanced resonant control strategy is
proposed for the UPQC to simultaneously tackle variouspower quality issues such as voltage sags, unbalance anddistortions as well as current harmonics. To mitigate thevoltage sags and distortions on the supply side, a PRcontroller tuned at the fundamental frequency (ωs) and aresonant controller tuned at six multiple of the fundamentalfrequency (6ωs) are employed in the series APF side.Meanwhile, to compensate the current harmonics ofnon-linear loads, a PI controller and three vector PI (VPI)controllers tuned at 6nωs (n = 1, 2, 3) are used in the shuntAPF side. The performance of the proposed UPQC controlscheme is significantly improved compared with theconventional control strategy owing to the effectiveness ofthe resonant controllers. In addition, in order to simplify thecontrol strategy and improve the control accuracy, thevoltage sag/distortion and current harmonic detectors areeliminated in the proposed control scheme. The feasibilityof the proposed control scheme is validated throughexperiments.

2 Proposed UPQC control scheme

2.1 Configuration of an UPQC

Fig. 1 shows the block diagram of a three-phase UPQC,which consists of two voltage source inverters connectedback-to-back through a common DC-link capacitor, whereone inverter is the shunt APF and another inverter is theseries APF. The series APF is connected in series betweenthe supply and the load through a series transformer.Meanwhile, the shunt APF is connected in parallel with theloads through an inductor Lsh. An LC (Lf, Cf) filter is


connected at the AC output voltage of the series APF toeliminate high-frequency switching ripples. The loads onthe system are non-linear, linear and sensitive.As illustrated in Fig. 1, to tackle the voltage sags and

distortions as well as current harmonics, the conventionalUPQC control strategies require a voltage sag/harmonicdetector, a current harmonic detector and hysteresiscontrollers where those parts must be designed properly toachieve good performance. It makes the control schemebecome too complex. Moreover, because of the use ofhysteresis controllers, the control performance of the UPQCis generally limited.

2.2 Proposed UPQC control scheme

To improve the control performance of the UPQC, anenhanced control strategy shown in Fig. 2 is proposed inthis paper with the aid of resonant controllers. In theproposed control scheme, the voltage sag/harmonic andcurrent harmonic detectors are not required. The loadvoltage and the supply current are directly sensed andregulated to follow their corresponding references in thed–q reference frame (v∗L, dq and i∗S, dq) through the proposed

IET Gener. Transm. Distrib., pp. 1–10doi: 10.1049/iet-gtd.2013.0636

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PR-R voltage controller and PI-VPI current controller,respectively. The PWM signals for the series APF and theshunt APF are generated from the voltage referencesv∗F , abc and v∗sh, abc, respectively, by using a sinusoidal PWMmethod. Since the proposed control strategy is executedwithout the voltage sag/harmonic and current harmonicdetectors, the control accuracy and dynamic response of theUPQC can be significantly improved. In addition, a PIcontroller is employed in the outer control loop of the shuntAPF for regulating the DC-link voltage of the UPQC.Moreover, since the proposed control algorithm is designedin the d–q reference frame, a phase-locked loop (PLL) isused to detect the phase angle of the supply voltage forcoordinate transformations. A second-order generalisedintegrator-PLL introduced in [24] is adopted in Fig. 2 toguarantee the precise operation of the PLL under theabnormal supply voltage condition.
3 Control strategy in the series APF

3.1 Voltage sags compensation

The main purpose of the series APF in an UPQC is to inject acompensating voltage to maintain the voltage at the loadterminal balanced and sinusoidal at the desired levelirrespective of the supply voltage conditions. As illustratedin Fig. 2, the load voltage is determined by the supplyvoltage and the injected voltage of the series APF as follows

vL(vt) = vS(vt)+ vf (vt) (1)

where vL(ωt), vS(ωt) and vf(ωt) are the load voltage, the supplyvoltage and the injected voltage of the series APF,respectively.Fig. 3 shows the locus of the load voltage phasor diagram

under the voltage sag situations. In Fig. 3, it is assumed thatthe supply voltage under the normal condition is sinusoidaland balanced at the nominal magnitude with the phaseangle θs and the load voltage vector is same as the supplyvoltage vector [VL = VS(pre− sag)] without any help of theseries APF. When the voltage sag occurs, the magnitude of

Fig. 3 Locus of the voltage phasor diagram under the voltage sagsituations


the supply voltage is reduced to V’S or V’’S according tothe voltage sag type: without or with a phase jump,respectively [2]. In case of the voltage sag without a phasejump, the supply voltage reduces to V’S and the series APFinjects a compensating voltage V’f so that the load voltagehas maintained its nominal magnitude. Since the voltagesag occurs without a phase jump, the compensating voltageV’f is in-phase with the supply voltage V’S and the loadvoltage VL. In case of the supply voltage V’’S, the voltagesag occurs with a phase jump δ; the series APF generatesthe compensating voltage V’’f with the phase angle (θs + δ)to keep the load voltage vector unchanged without anyphase jump. In the conventional control methods, tomaintain the load voltage undisturbed from the voltage sag,it is necessary to detect the voltage sag and to find thephase angle of the compensating voltage V’’f in the controlscheme. However, in the proposed control algorithm, theload voltage can maintain the desired voltage without thesource voltage sag detection in spite of the voltage sag withor without a phase jump since it is directly regulated.To maintain the constant load voltage, despite voltage sag,

the voltage controller of the series APF plays an importantrole. The PI controller in the d–q reference frame has beenintroduced and widely used for this purpose [3]. However,this method has a limitation in mitigating unbalancedvoltage sags because of the presence of the negativesequence component [5]. Therefore, a PR controller tunedat the fundamental frequency in the stationary frame is usedin this paper for dealing with unbalanced voltage sags. Theadvantage of PR controller is that it is capable of effectivelytracking an AC signal at the selected frequency; hence, itcan simultaneously regulate positive and negative sequencecomponents without any sequence decomposition process[25]. The open-loop transfer function of the PR controller isgiven as follows

GPR(s) = Kp +Kr1vcs

s2 + 2vcs+ (vs)2 (2)

where Kp is the proportional gain, Kr1 denotes the resonantgain of the resonant controller, ωs is the selected resonantfrequency of PR controller, which means the fundamentalfrequency of the grid voltage in this case and ωc is thecut-off frequency which determines the control bandwidthof the PR controller.

3.2 Voltage distortions mitigation

In practice, because of the intensive use of non-linear loads inpower systems, the voltage available at the supply bus isusually not pure sinusoidal but distorted. This distortedvoltage condition will harmfully affect sensitive loadsconnected at the load terminal if it is not taken into accountand fully compensated. The UPQC is also capable ofisolating voltage harmonics on the supply side to improvethe load voltage performance. The voltage controller, whichis used to mitigate voltage harmonics, is introduced in thissection.Assuming that the voltage available at the supply bus (vS)

is distorted and includes the fundamental (vS1) and harmoniccomponents (vSh) as identified in (3)

vS(vt) = vS1(vt)+∑

h=1

vSh(vt) (3)


Fig. 4 Block diagram of the proposed PR-R voltage controller

Fig. 5 Bode diagram of closed-loop PR-R controller with respectto different values of Lf and Cf

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To make the load voltage sinusoidal, the harmoniccomponents presented in (3) must be completelycompensated by the series APF. In three-phase systems, theharmonic voltages have odd orders: 6n ± 1 (n = 1, 2, 3, …)of the fundamental frequency of the network (ωS). Amongthem, the fifth and seventh harmonics are dominantcomponents that need to be eliminated. A resonantcontroller is an effective solution to compensate thoseharmonic voltages [26]. Since one resonant controller canonly track a specific AC component, two resonantcontrollers tuned at 5ωS and 7ωS are required to compensateboth the fifth and seventh harmonics. Fortunately, sinceboth the fifth and seventh harmonics become the sixthharmonic in the d–q reference frame, one resonantcontroller with a selected resonant frequency of 6ωS in thisframe is also capable of simultaneously compensating boththe fifth and seventh harmonic voltages. Accordingly, thecontrol scheme is simplified since only one controller isneeded to regulate two harmonics. The open-loop transferfunction of the resonant controller is given as

GR(s) =Kr6vcs

s2 + 2vcs+ (6vs)2 (4)

The block diagram of the proposed voltage controller for theseries APF is illustrated in Fig. 4.

3.3 Analysis and design of proposed voltagecontroller

Designing the gains of resonant controller is not a simple task,especially when compensating high-order harmonics [26, 27].In this paper, the Naslin polynomial technique is used todetermine the resonant controller gains [26]. In addition, Kp

and ωc values of the PR controller should be designed to besufficiently large to make the PR controller have a widebandwidth, and hence provide a fast response againstvoltage sags. According to the design procedure presentedin [26], the controller gains of PR and resonant controllersin Fig. 4 are selected based on the system parameters givenin Appendix as follows: Kp = 2, Kr1 = 2000, ωc = 10rad/sand Kr6 = 500.Since these controller gains depend on the LC filter

parameters, the effect of the filter parameter variations onthe system stability is considered. From Fig. 4, theclosed-loop transfer function of the PR-R controller is givenas follows

GC = GPR−R(s)GLC(s)

1+ GPR−R(s)GLC(s)(5)

where GLC = 1/(LfCfs2 + RfCfs + 1) is the transfer function of

the LC filter.Fig. 5 shows the Bode diagram of (5) with respect to

different values of Lf and Cf. In Fig. 5, the proposed PR-R


controller has two selected resonant frequencies: 60 and360 Hz. At 60 Hz, the frequency response of the PR-Rcontroller is almost the same regardless of the filterparameter variations. In contrast, at the vicinity of 360 Hz,the frequency response of the PR-R controller shows asmall difference with respect to different values of Lf andCf. However, the PR-R controller always provides unitygain and zero phase at selected resonant frequency 360 Hz,despite the variations of the filter parameters (Lf or Cf).Therefore, we can say that the variations of filter parametershave no impact on the system stability and performance ofthe proposed voltage controller.To implement these controllers by digital signal processors

(DSPs), the transfer function of the PR controller in (2) andthe resonant controller in (4) are discretised by consideringaccurate resonant pole placement [28]

GPR(z) = Kp + Kr1vcTs1− cos (vsTs)z

−1

1− (2 cos (vsTs)− 2vcTs)z−1 + z−2

(6)

GR(z) = Kr6vcTs1− cos (6vsTs)z

−1

1− (2 cos (6vsTs)− 2vcTs)z−1 + z−2

(7)

where z is the shift operator and TS = 100 μs denotes thesampling period.For digital implementation, (6) and (7) with the same gains

in continuous time domain are used because the samplingperiod TS is very short.

4 Control strategy in the shunt APF

4.1 Current harmonics compensation

The main purpose of the shunt APF is to mitigate harmoniccurrents produced by the non-linear load. The non-linearload under consideration is a three-phase diode rectifiersupplying a DC load. This type of load draws harmoniccurrents into the networks that have odd orders: 6n ± 1 (n =1, 2, 3, …) of ωS. As a result, the load current consists ofthe fundamental (iL1) and harmonic components (iLh) as

iL(vt) = iL1(vt)+∑

h=6n+1

iLh(vt) (8)


Fig. 6 Block diagram of the proposed PI-VPI current controller

Fig. 7 Bode diagram of the closed-loop transfer function of theproposed current controller

Table 1 Controller gains of PI-VPI controllers

Kp = 2 Ki = 50Kp6 = 0.8 Kr6 = 20Kp12 = 0.6 Kr12 = 15Kp18 = 0.3 Kr18 = 7.5

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To make the supply current sinusoidal, the harmonic currentspresented in (8) must be fully compensated by the shunt APF.In other words, the shunt APF must operate as a controlledcurrent source to generate harmonic currents to reject thoseof the load current. Since the current controller must dealwith harmonic currents which are high-frequency signals,the PI controller is not a sufficient solution because of itslimitation on the control bandwidth. To overcome thisissue, resonant controller could be considered as aneffective solution for regulating harmonic currents. The(6n ± 1) harmonic currents also become 6n harmonics in thed–q reference frame. Hence, one resonant controller with aresonant frequency of 6nωS is capable of regulating a pairof (6n ± 1) harmonic currents. The number of resonantcontrollers is thereby reduced by half. In fact, the effect ofvery high-order harmonics is usually negligible. In thispaper, only harmonic currents up to 19th order areconsidered for compensation. Accordingly, three resonantcontrollers are needed to regulate those harmonic currents.In fact, the VPI controller which is an alternative version of

the resonant controller is used in this paper because it hassome advantages over the standard resonant controllerswhen applied for the shunt APFs: it is robust in operationand capable of compensating very high-order (up to 37th)harmonic currents without requiring a time delaycompensator [29]. The open-loop transfer function of theVPI controller is described as

GVPI =∑

h=6, 12, 18

Kphs2 + Krhs

s2 + (hvs)2 (9)

where Kph and Krh are the proportional and the resonant gainsat the hth-order harmonic.In addition, the shunt APF is also responsible for regulating

the DC-link voltage of the UPQC. Hence, a PI controller isalso needed to regulate the fundamental current to chargethe DC-link capacitor. As a result, the current controller ofthe shunt APF consists of a PI controller and three VPIcontrollers as shown in Fig. 6.

4.2 Analysis of proposed current controller

According to Fig. 6, the closed-loop transfer function of theproposed current controller is determined as

GC = GPI−VPI(s)GL(s)

1+ GPI−VPI(s)GL(s)(10)

where GL = 1/(Lshs + Rsh) is the transfer function of the Lshinductor.By selecting the resonant gains as Krh =KphRsh/Lsh, the VPI

controller has only one gain to be tuned, that is, Kph. Fig. 7illustrates the Bode diagram of (10) with respect to differentvalues of Kph. In Fig. 7, it is obvious that the proposed


controller provides a unity gain and zero-phase shift atselected resonant frequencies, that is, 360, 720 and 1080Hz, irrespective of the value of Kph. It means that the VPIcontroller is able to regulate the selected harmonic currentswith zero steady-state error. Even though any value of Kph

can provide unity gain and zero-phase shift, Kph shouldbe carefully selected to achieve a good steady-stateperformance and a fairly fast response. To fulfil thatdemand, it is recommended to select Kph smaller than 1[29]. Finally, the controller gains of each VPI controller aretuned individually by using the system parameters shown inAppendix and listed in Table 1.The controller gains of VPI controllers are tuned based on

the relationship of Lsh and Rsh. When these parameterschange, the coefficient, Krh = KphRsh/Lsh, is no longer theassigned value. To investigate the robustness of the VPIcontroller against the parameter variations, Bode diagram of(10) is plotted in Fig. 8 with respect to the different valuesof Lsh. Judging from Bode diagram in Fig. 8, the inductorparameter variation has no effect on the system stability ofthe PI-VPI current controller because a unity gain andzero-phase shift at the selected resonant frequencies such as360, 720 and 1080 Hz are maintained regardless of thevariation of Lsh.Similar to the PR and resonant controllers, the transfer

function of the PI and the VPI controllers are discretised fordigital implementation

GPI(z)=Kp+KiTs1

1−z−1

GVPI(z)=∑

h=6,12,18

Kph+(KrhTs−2Kph)z−1−(KrhTs−Kph)z

−2

1−2cos(hvsTs)z−1+z−2

(11)

In the discrete time domain, the same controller gainsdesigned in Table 1 can be used for the PI-VPI controller in(11) because the sampling period TS is very short.


Fig. 8 Closed-loop Bode diagrams of the VPI controllers withdifferent values of the inductor parameters

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5 Experimental results

To validate the feasibility of the proposed control algorithm,an experimental platform is built in the laboratory as shown inFig. 9. The experimental system consists of a three-phasepower supply, an UPQC, a linear load, a non-linear loadand the system parameters are listed in Table 2. The supplyvoltage is generated by a programmable AC power source(Chroma 61704), which is able to produce either voltagesags or distorted supply voltage in order to investigate theperformance of the proposed control strategy. The seriesAPF and the shunt APF of the UPQC are developed by sixinsulated gate bipolar transistor modules –FMG2G75US120. The non-linear load is composed of thethree-phase diode rectifier with a resistance load with6 kVA (power)/380 V (voltage) ratings, whereas thethree-phase resistance load is used as the linear load. Thecontrol algorithm is implemented using a 32 bitfloating-point DSP TMS320F28335 by Texas Instruments.The control sampling and switching frequencies are set tobe 10 and 5 kHz, respectively. Fig. 9a shows the hardwareconfiguration to implement the proposed UPQC controlalgorithm. The PWM control signals are generated by DSP

Fig. 9 Hardware configuration and experimental platform of the propo

a Hardware configurationb Experimental platform


board and transferred to the driver circuit by fibre opticcables. The harmonic components and the total harmonicdistortions (THDs) are measured by a power analyser(HIOKI 3193). When the DC-link voltage reaches thesteady-state value Vdc after the STS is closed, the controlalgorithm of the UPQC is activated. To assess theperformance of the proposed UPQC experimentally, weconsider three cases: (i) voltage sag compensation;(ii) voltage distortions and current harmonic compensation;and (iii) voltage sag, voltage distortions and currentharmonic compensation.

5.1 Voltage sag compensation

Since the voltage sag is compensated by the series APF, theshunt APF is not activated and a DC power source issupplied at the DC-link voltage of UPQC. Also, only thethree-phase resistive load is connected at the load side forthe sake of simplicity. In addition, unbalanced voltage sagsare considered to investigate the compensating performanceof the UPQC since the voltage sags are generally occurredwith unbalance and the performance of the UPQC underunbalanced sags is more crucial.Fig. 10 shows the experimental results of the UPQC

compensating an unbalanced voltage sag by using theconventional PI controller. In this case, a 30% unbalancedsag accompanied by a 20° phase shift ( jump) is generatedduring 20 cycles. In Fig. 10, by using the PI controller, it isobvious that the load voltage (vLabc) is unable to bemaintained balanced at its rated level during the voltagesag. In fact, the PI controller cannot accurately regulate thenegative sequence component in the unbalanced voltagebecause it behaves as second harmonic in the d–q referenceframe. Therefore, the conventional PI controller is unable todeal with unbalanced sag conditions.In contrast, by using the proposed control scheme, the load

voltage can be effectively compensated to be sinusoidal asshown in Fig. 11. According to Fig. 11, thanks to theeffectiveness of the PR controller in regulating both thepositive and negative sequence components of unbalancedvoltage, the load voltage is maintained balanced at its ratedvalue even though an unbalanced voltage sag with a phase

sed UPQC


Fig. 10 Experimental result of UPQC by using the PI controller

Fig. 11 Experimental result of UPQC using the proposed control scheme under

a Sinusoidal supply voltageb Distorted supply voltage

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Fig. 12 Experimental results of UPQC to compensate voltage distortions and current harmonics

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jump occurs in Fig. 11a. The proposed control strategyprovides not only good steady-state performance, but alsogood dynamic response with the unbalanced voltage sag:the load voltage is always maintained undisturbed even atthe transient time. Meanwhile, in Fig. 11b, we can see thatthe proposed UPQC effectively compensates bothunbalanced voltage sag and voltage harmonics in the supplyvoltage to maintain the load voltage balanced andsinusoidal at the desired value. The THD of the loadvoltage is reduced to 1.2%, whereas the THD of the supplyvoltage is 8.1%.The experimental results verify that the proposed control

strategy is able to effectively tackle both voltage sags andvoltage harmonics to improve the load voltage performance.

5.2 Voltage distortions and current harmoniccompensation

The performance of the UPQC to compensate the voltagedistortions and the current harmonics is investigated in thissection. In the experimental test, a three-phase diode rectifieris connected at the load side as the non-linear load and theTHD of the non-linear load current is 25.2%. Meanwhile, thesupply voltage is distorted with a THD is about 8.1%. In thiscase, both the series and the shunt APF are activated tocompensate harmonics in supply voltage and load current.Fig. 12 shows that the harmonics in the supply voltage, as

well as the harmonic currents produced by the non-linearload, are effectively compensated to maintain both the load


voltage and the supply current sinusoidal. The THD valuesof the load voltage and the supply current are 1.2 and1.95%, respectively, which completely comply with theIEEE 519-1992 standard [30]. These results reveal that theproposed control strategy is capable of simultaneouslydealing with the harmonic voltage and harmonic currentproblems: both the load voltage and the supply current arecompensated to be sinusoidal without the demand of thevoltage harmonic or current harmonic detectors.

5.3 Voltage sag, voltage distortions and currentharmonics compensation

To verify the effectiveness of the proposed control in casevoltage sag, voltage distortions and current harmonics existall together, the total performance is evaluated. It can beseen in Fig. 13 that before the voltage sag occurs, theUPQC compensates the harmonics presented in the supplyvoltage and the load current to make the load voltage andthe supply current sinusoidal. At times when the voltagesag takes place, the UPQC injects not only the harmonics,but also the fundamental voltage to maintain the voltageat the load terminal sinusoidal at its rated value. In fact,to compensate the voltage sag, the UPQC requires acertain amount of active power from the supply sidethrough the shunt APF. Hence, during the voltage sag,both the shunt APF current (isha) and the source current(iSa) are increased as compared with before the voltagesag. The THD values of the load voltage and the supply


Fig. 13 Experimental results of UPQC to compensate the voltage sag, voltage distortions and current harmonics

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current after compensation are about 1.22 and 1.87%,respectively, which also comply with the IEEE-519standard. In this test case, the THD value of the supplycurrent is slightly reduced compared with Case B becausethe magnitude of the supply current is increased duringthe voltage sag.Finally, we can say that the UPQC with the proposed

control strategy is able to simultaneously tackle variouspower quality issues such as voltage sags, voltageunbalance, voltage distortions and current harmonics: theload voltage and the supply current are always maintainedsinusoidal regardless of the supply voltage and the loadcurrent conditions.

6 Conclusion

An enhanced resonant control strategy for the UPQC to dealwith various power quality issues such as voltage sags,voltage unbalanced, voltage distortion and currentharmonics was proposed in this paper. The feasibility of theproposed control strategy was verified through variousexperimental tests in which the load voltage is maintainedbalanced and sinusoidal at the desired value regardless ofthe voltage sag, unbalanced and distorted supply voltageconditions. In addition, the supply current is maintainedsinusoidal irrespective of the non-linear load current. Theproposed control strategy exhibited a fast dynamic responseas well as good steady-state performance under voltage sag


and distortion conditions. Under the distorted supplyvoltage with a non-linear load in the system, the UPQCreduced the THD of the load voltage and the supply currentto <1.22 and 1.95%, respectively, which completelycomply with the IEEE 519-1992 standard.The proposed control method not only improves the

compensation performance, but also can reduce the systemcost of the UPQC since it requires fewer sensorscompared with the conventional methods. Furthermore, theaccuracy and reliability of the UPQC was improvedsignificantly because the proposed control strategy wasoperated without the voltage sag/harmonic and currentharmonic detectors. Thanks to these advantages, theproposed control method can be applied for the UPQC as acost effective solution.

7 Acknowledgment

This work was supported by the National Research Foundationof Korea Grant funded by the Korean Government.

8 References

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2 Bollen, M.: ‘Understanding power quality problems, voltage sags andinterruptions’ (IEEE Press, Piscataway, NJ, 1999)


Table 2 UPQC system parameters

Parameters Values

supply line-line voltage[root mean square (RMS)]

VS 380 Vrms

frequency fS 60 Hzload three-phase resistive load Rac 30 Ω

diode rectifier resistive load RL 40 ΩDC-link reference voltage V ∗

dc 700 Vcapacitance Cdc 2.2 mF

shunt APF Lsh inductance Lsh 2 mHLsh resistance Rsh 0.05 Ω

series APF LC filter inductance Lf 0.7 mHLC filter capacitance Cf 27 µFdamping resistance Rf 5 Ω

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30 IEEE Recommended Practices and Requirements for Harmonic Controlin Electrical Power Systems, IEEE Standard 519-1992, 1992

8 Appendix

See Table 2.


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