ambatam

1490 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 3, JUNE 2014

Optimal Sizing of UPQC Considering VALoading and Maximum Utilization of

Power-Electronic ConvertersBharath Babu Ambati and Vinod Khadkikar, Member, IEEE

AbstractThis paper introduces an optimum method to designa unified power-quality conditioner (UPQC) system with theminimum possible VA rating based on the compensation require-ments. A set of generalized VA loading equations for the UPQC isderived, which are valid for all of the UPQC control approaches(such as UPQC-P, UPQC-Q, and ). The vari-ation in series and shunt inverters VA loadings of UPQC forthe given compensation requirements is analyzed for all existingcontrol approaches. A novel design method and the correspondingalgorithm are proposed to size the major components in an UPQC,such as the series inverter, shunt inverter, and series transformercorresponding to the minimum possible overall VA rating. TheVA rating and the utilization of power-electronic converters usingthe proposed design method are compared with those of UPQC-P,and approaches to show the effectiveness ofthe proposed design method.

Index TermsActive power filters (APF), minimum VA loading,optimum rating, reactive power sharing, unified power-qualityconditioner (UPQC), voltage sag, voltage swell.

NOMENCLATURE

Rated source voltage and current.

Rated load voltage and current.

Rated load power factor angle.

Voltage injected by the series part of UPQC.

Current injected by shunt inverter with .

Current injected by the shunt inverter with.

Displacement angle between source and loadvoltages.

Displacement angle between the source andseries-injected voltages.

Ratio between actual source and rated sourcevoltages.

Manuscript received April 07, 2013; revised August 29, 2013; accepted De-cember 07, 2013. Date of publication January 09, 2014; date of current versionMay 20, 2014. Paper no. TPWRD-00397-2013.The authors are with the Institute Center for Energy, Masdar Institute

of Science and Technology, Abu Dhabi, United Arab Emirates (e-mail:[email protected]; [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPWRD.2013.2295857

Minimum possible value of during the voltagesag.

Maximum possible value of during voltageswell.

Active power handled by the shunt inverter.

Reactive power handled by the shunt inverter.

VA loading of the shunt inverter.

Active power handled by the series inverter.

Reactive power handled by the series inverter.

VA loading of the series inverter.

Rated source/load power.

Total VA loading of the UPQC.

VA rating of the series transformer.

I. INTRODUCTION

I N MODERN power systems, distribution utilities man-date the connected loads compliance with the strictpower-quality standards. This is to improve the reliability ofthe distribution system to cater the needs of critical loads andsensitive automation systems.The major challenges to maintain good quality power are:

1) fundamental reactive power requirements of the connectedloads; 2) voltage sags and swells at the point of common cou-pling (PCC) due to connection and disconnection of large in-dustrial loads and reactive power compensating capacitors; and3) voltage and/or current harmonic distortion due to the pres-ence of nonlinear loads. Active power filters (APFs) are themostpromising and widely used solutions for improving the powerquality (PQ) at the distribution level [1], [2]. These APFs canbe classified as shunt APF, series APF, and hybrid APF. Thecombination of both series and shunt APFs, to mitigate almostall of the voltage- and current-related PQ problems, is a uni-fied power-quality conditioner (UPQC). Superior performanceand the ability to mitigate almost all major PQ problems makeUPQC the most attractive solution for PQ improvement despiteits high cost, complex structure, and control [1][3]. The systemconfiguration of a UPQC is shown Fig. 1.Current trends in the area of UPQC are directed toward op-

erating the UPQC with minimum volt-ampere (VA) loading toreduce the overall system losses [3][12]. However, all of the

0885-8977 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

AMBATI AND KHADKIKAR: OPTIMAL SIZING OF UPQC CONSIDERING VA LOADING AND MAXIMUM UTILIZATION 1491

Fig. 1. Schematic diagram of the unified power-quality conditioner (UPQC).

reported work centered on minimizing the VA loading duringvoltage sag conditions [3], [4], [7][12]. The sizing aspect ofthe UPQC system (including the shunt inverter, series inverter,and series transformer) considering individual shunt and seriesinverter VA loadings under different operating conditions (suchas steady state, voltage sag, voltage swell, and voltage and cur-rent harmonics compensation scenarios) is themain focus of thispaper and has not been addressed and studied so far.Based on the control strategy being employed for voltage sag

or swell compensation, the UPQC systems can be classified asUPQC-P, UPQC-Q, and UPQC-S [2]. The UPQC-P is consid-ered to be a conventional UPQC, where voltage sag and swellcompensation are performed by injecting/absorbing the activepower (in-phase or out-of-phase voltage) through the series partof the UPQC whereas the shunt inverter supports the load re-active power, active power required by the series inverter, andthe losses in the system. For the same percentage of voltage sagand swell compensation, the VA loadings of series and shunt in-verters will be maximum during the UPQC-P, compensating forthe maximum voltage sag. Hence, UPQC-P should be designedbased on the maximum voltage sag compensation.While in caseof UPQC-Q the voltage injected through a series transformer isin quadrature with the source current. Thus, series inverter doesnot require any active power for compensating the voltage sagexcept for the switching and filtering losses. The UPQC-Q ap-proach is limited to voltage sag compensation since it cannotcompensate for the voltage swell [2], [4]. For the same amountof sag compensation, UPQC-Q requires larger series injectionvoltage magnitude compared to UPQC-P [2][4], [7], [8]. Thisincreases the VA rating of the series transformer significantly.Generally, the voltage sags and swells are short duration PQ

problems. Thus, in UPQC-P and UPQC-Q, series inverter VAloading will only be utilized for short durations. On the otherhand, the shunt inverter VA loading is fully utilized throughoutthe operation, due to continuous load reactive power supportand current harmonic compensation. To enhance the utilizationof series part of UPQC during steady state, part of load reactivepower is supported by the series inverter in UPQC-S [5], [6].This role of series inverter not only improves its utilization, butalso reduces the shunt inverter VA loading. Due to the load re-active power sharing feature of the series part, the rating of theshunt inverter in UPQC-S may be less than that in the UPQC-P.But this is at the expense of a slightly increased series trans-former rating and reduction in the percentage of swell compen-sation capability.

In [3][12], several methods have been proposed for theoptimization of total instantaneous VA loading of UPQC (i.e.,algebraic sum of VA loadings of shunt and series inverters).In [9] and [10], the authors have used an offline optimizationmethod to compute the optimum angle (displacement anglebetween source and load voltages) that ensure the minimuminstantaneous VA loading of UPQC for the given percentage ofvoltage sag and load power factor. An 80 80 matrix-based2-D lookup table is computed and used for the control of theseries inverter during the voltage disturbance (sag/swell) con-ditions. This algorithm minimizes the total UPQC VA loadingat any given operating condition; however, it does not considerthe variation in individual VA loadings of series and shuntinverters under different operating conditions.In [11] and [12], along with the load power factor, load cur-

rent, and percentage of voltage sag, the authors have includedthe allowable total harmonic distortions (THDs) of load voltageand source current as variables in the optimization problem. Aparticle swarm optimization (PSO)-based technique is used tocompute the instantaneous optimum angle. However, its impacton VA ratings of series and shunt inverters, and series trans-formers is not considered.After examining different techniques for minimizing the VA

loading of UPQC [3][12], it is clear that all the techniquesdirectly or indirectly control the displacement angle. The sat-isfactory results supporting minimum VA loading claims, at aparticular operating point, are found in the literature. It shouldbe noted that obtaining minimum UPQCVA loading at a certaincondition (such as voltage sag) does not guarantee minimumVAratings of the shunt inverter, series inverter, series transformer,and, thus, the overall UPQC system.Although general voltage sags and swells exist only for short

durations, the UPQC sizing should be carried out consideringthe voltage sag/swell as steady-state operation for an uninter-rupted operation of critical loads in the events of long durationsags/swells (up to a few hours).This paper deals with the sizing of the UPQC system with

minimum possible ratings of shunt and series inverters withoutcompromising any of its compensation capabilities under dif-ferent operating conditions. An algorithm is proposed to mini-mize the overall VA rating of UPQC which determines the cor-responding displacement angle , fundamental VA ratings of se-ries inverter, shunt inverter, and series transformer. Similar tothe UPQC-S, the series inverter in the designed UPQC sharespart of the load reactive power. Moreover, the proposed algo-rithm indirectly identifies the amount of load reactive powerthat needs to be shared by the series inverter. The VA load-ings and utilization of power-electronic converters of the de-signed UPQC under different operating conditions are com-pared with those of UPQC-P and UPQC with minimum VAloading obtained by using [7][12].

II. VA LOADING AND TRANSFORMER RATING IN UPQC-PThe vector diagrams representing operation of UPQC-P

where load voltage is always in phase with the instanta-neous source voltage during: 1) steady-state ; 2)voltage sag ; and 3) voltage swell conditions isshown in Fig. 2. The magnitudes of and are constant at


Fig. 2. Operation of the UPQC-P during (a) steady state, (b) voltage sag, and (c) voltage swell (a) 1, (b) 1, and (c) 1.

rated values during all three operating conditions irrespectiveof the source voltage magnitude . To keep the magnitudeof constant when , the series voltage to be in-jected by UPQC is always in line with the instantaneous sourcevoltage. To maintain the unity power factor operation at sourceterminals, shunt inverter supplies entire load reactive current

requirement during , and load reactive current plusactive current required for series inverter, during .Due to the UPQC-P operation, the source current is alwaysin phase with the source voltage (unity power factor) and theload do not see any change in voltage magnitude. This meansthat the source ( and ) and load ( and ) powersremain constant even under the voltage sag/swell conditions.Approximating the total power loss in the UPQC systemremains constant during all operating conditions, and the UPQCpower balance equation in steady state can be written as

(1)

This implies

(2)

Based on the aforementioned discussion and (1) and (2), thesource power prior and after the disturbance can be treated asconstant, i.e.,

(3)

Assuming and are the instantaneous source voltageand current at any operating condition (including sag/swell) inFig. 2(a)(c) such that . Therefore, from (3), thesource current during any operating condition is

(4)

From (4), it is clear that the current drawn from the sourcevaries with as shown in Fig. 2.

A. VA Loading of the Series Part of UPQCFrom Fig. 2(a)(c), the active power handled by the series

inverter of UPQC at any operating point is the productof series-injected voltage and source current , andcan be written as

(5)

Since the injected voltage is always in-phase with thesource current , reactive power handled by series inverteris always zero . Therefore, the VA loading of theseries inverter is

(6)

In case of the series transformer, voltage rating is directlyproportional to the maximum amount of sag/swell that needs tobe compensated and the current rating is equal to the maximumcurrent that flows in the winding during voltage sag. Thus, VArating of the series transformer is given as

(7)

B. VA Loading of the Shunt Part of UPQCThe role of the shunt inverter is to maintain a unity power

factor at PCC by compensating the load reactive power demand,to maintain the dc-link voltage and to support the active powerrequirements of the series inverter. If we neglect the losses inthe system, the active power handled by the shunt inverter isequal and opposite to that of the series inverter active power

(8)

whereas the reactive power handled by the shunt inverter isequal to the maximum load reactive power demand

(9)

VA loading of the shunt inverter at any operating point is

(10)

C. Total VA Loading of the UPQC-PThe total VA loading of the UPQC-P system is the sum of

individual VA loadings of the shunt and series inverters. Byadding (6) and (10)

(11)

III. GENERALIZED VA LOADING EQUATIONS OF UPQCThe operation of UPQC with any arbitrary displacement

angle between the source voltage and load voltage


Fig. 3. Operation of UPQC with the any arbitrary displacement angle during (a) steady state (b) voltage sag, and (c) voltage swell: (a) 1, (b) 1, and(c) 1.

under: 1) steady-state ; 2) voltage sag ; and3) voltage swell conditions are shown in Fig. 3(a)(c).To keep the magnitudes of load voltage and the displace-ment angle constant, the voltage injected by the seriesinverter and its angle with source voltage areto be controlled according to the operating condition as shownin Fig. 3(a)(c). The behavior of load current, shunt invertercurrent, and source current under different operating condi-tions with an arbitrary displacement angle can be seenin Figs. 3(a)(c). For better correlation, steady-state currentsduring the operation of conventional UPQC-P (or UPQC with

) are shown with dotted lines. It can be seen that themagnitude of load current and load power factor angle

are constant despite angle shift in the load voltage on thelocus. For any fixed displacement angle , a fixed operating

point on the locus exists irrespective of any source voltagedisturbance. The locus represents the maximum load currentat rated voltage with the least possible power factor.In Fig. 3, the angles and resemble the opera-

tion of UPQC-P and UPQC-Q, respectively, whereas any anglein the range resembles the operation of UPQC-S.A generalized vector diagram shown in Fig. 4 can be drawn

to determine the magnitude and phase angle of voltage to beinjected by the series inverter and current to be injected by theshunt inverter. This vector diagram is applicable for all controlstrategies of UPQC, including voltage sag/swell conditions.

A. VA Loading of the Series Part of UPQC

From Fig. 4, the magnitude of injected series voltage can bewritten as

(12)

The angle between the source voltage and injected voltagecan be calculated as

(13)

Fig. 4. Determination of , , , and .

The active and reactive powers handled by the series inverteras a function of and are

(14)

(15)

The VA loading of the series inverter at any operating condi-tion is

(16)

The series transformer should be capable of handling themaximum series voltage and maximum possible winding cur-rent. Hence, per-phase VA rating of the series transformer canbe computed as

(17)


B. VA Loading of the Shunt Part of UPQC

The current injected by the shunt inverter can be calculatedfrom Fig. 4 as

(18)

In (18), we can write as the maximum rms loadcurrent magnitude remains the same while the UPQC is in op-eration. The angle computed using the vector calculations is

(19)

Using (18) and (19), the active and reactive powers handledby the shunt inverter are represented as

(20)(21)

The VA loading of the shunt inverter at any operating condi-tion is

(22)

C. Total VA Loading of the UPQC

Adding (16) and (22), the total VA rating of UPQC as a func-tion of and can be determined as

(23)

Equation (23) represents the VA loading of any UPQC systemfor any given load during different operating conditions such assteady state , voltage sag , and voltage swell

.

IV. DESIGN OF THE UPQC WITH MAXIMUM UTILIZATION

The design procedure is illustrated by considering the case ofload supplied by a three-phase 400-V, 50-Hz

utility system. The sizing of the UPQC is evaluated to achieveunity power factor at the source side with 40% voltage sag and40% voltage swell compensation capability. A balanced supplyand a balanced harmonic-free load are considered in this designexample. By substituting the worst case operating conditions,in the generalized VA loading equations, the maximum possibleVA loading for which the UPQC should be sized is determined.The total VA loadings of the UPQC (23) with variation fromto , under: 1) steady state without sag/swell (i.e., );

2) 40% voltage sag (i.e., ); and 3) 40% voltage swell(i.e., ) are plotted in Fig. 5.Any successful attempt to optimize the total VA loading of

the UPQC will converge to the valley points marked in Fig. 5.During 40% voltage sag, the minimum possible total UPQCVA loading/phase is 4625 VA and that occurs atwhereas the minimum possible VA loading/phase during 40%

Fig. 5. VA loading of the UPQC under different operating conditions.

swell compensation is 4106 VA at . During steady-stateoperation, the minimum possible VA loading/phase is 3334 VAat . The use of any optimization algorithm in [7][12]guarantees the minimum possible VA loading at any particularoperating condition. But this does not guarantee the minimumoverall VA rating (i.e., size) of the UPQC system since thisapproach does not deal with the individual VA loadings ofshunt and series inverters. To demonstrate this aspect, theindividual loadings of shunt and series inverters, together withthe total UPQC VA loading, are provided in Fig. 6(a) for thesteady state, (b) 40% sag, and (c) 40% swell conditions. TheseVA loading curves are drawn by using generalized equationsderived in Section III by varying the from to .The parameters, for the case of UPQC-P (when )

without any optimization, and computed using (6)(11) aregiven in Table I. From Table I or in Fig. 6, the total VArating of UPQC-P for the specifications considered in this de-sign example is 6230 VA or 18 690VA for the three-phase system. Note that the VA loading ofthe series transformer is identical to that of the series inverter.The maximum voltage per phase that needs to be injectedto compensate 40% voltage sag and swell is 93 V. From theTable I, it can be observed that the sizing of the series inverter,shunt inverter, and series transformer in UPQC-P should bedesigned based on voltage sag compensation VA loadings (ifthe design is for the same amount of voltage sag and swellcompensation).A detailed analysis on the [7][12] ap-

proach employing the optimization techniques is also carriedout. To design the rating of the , the VA load-ings of series and shunt parts of UPQC, and total VA loadingof UPQC should be obtained for different operating condi-tions. The use of any optimization technique for minimizingthe operational VA loading in converges to

for , for and foras indicated in Fig. 5. These points can be identified

in Fig. 6(a)(c) to obtain the VA loadings of the shunt andseries inverters, total VA loading of the UPQC, and the voltageinjected by the series transformer. The results obtained using


Fig. 6. VA loading during (a) steady-state operation , (b) 40% voltagesag , and (c) 40% voltage swell .

the minimum VA loading approach of are aslisted in Table II.As noticed from Table II, despite minimum UPQC VA

loading design (4625 VA in compared to6230 VA in UPQC-P), the required maximum VA loading ofthe series inverter is 3577 VA during 40% sag (higher thanUPQC-P). Thus, in order to achieve this minimum total UPQCVA loading with the aforementioned approach, the series in-verter should be rated/sized at 3577 VA and the shunt inverter

TABLE IVA LOADING IN UPQC-P WITHOUT OPTIMIZATION

TABLE IIVA LOADING IN

should be rated/sized at 3334 VA, which are the maximumvalues of shunt and series inverter VA loadings out of steadystate, sag, and swell conditions. Thus, the overall VA rating ofthe system is 20 733 VA .The maximum voltage/phase that needs to be injected by theseries transformer is 149 V. These ratings are higher thanthose of the UPQC-P. Hence, the advantage of minimum VAloading for a particular operating condition is at the expense ofincreased overall UPQC system size.After examining the individual VA loading variation of shunt

and series inverters with different angles under different oper-ating conditions, a comprehensive design procedure is proposedto minimize the overall VA rating of the system. Furthermore, itidentifies the optimal at which the total VA rating of the UPQCis minimum.In the proposed design method, for every small step change

in , the individual VA loadings of the series inverter,shunt inverter, and series transformer are computed underthe full-load condition with: 1) ; 2) ; and3) separately using (12)(23). Then, it selects themaximum VA loading of the series inverter among the set ofthree values computed separately. This occurs simultaneouslyfor shunt inverter VA loading, series-injected voltage, andthe series transformer VA rating. These individual VA load-ings/ratings are stored in an array against the corresponding(i.e., , , , , C versus as shown in Fig. 7).This process continues until the delta reach a value of .Any angle beyond cannot be the optimal since the activeand reactive powers need to be handled by the series inverterand transformer rise significantly afterwards. The sum of theshunt inverter rating and series inverter rating ,which represents the total VA rating of the UPQC, is


Fig. 7. Flowchart to achieve the optimal sizing of UPQC by solving the gen-eralized equations.

TABLE IIIOUTPUTS OF THE PROPOSED DESIGN ALGORITHM

then plotted against the angle stored in the same array. Thevalley point on the curve is the minimum possible VArating of the UPQC , and the corresponding is theoptimum displacement angle between the source voltage andload voltage that guarantees minimum VA rating of the system.Once the optimum angle is obtained, corresponding valuesare given as outputs from the data stored in internal memory asshown in Table III.The flowchart for solving (12)(23) and finding the optimum

size using the proposed design method is shown in Fig. 7. The

Fig. 8. Pplot of , , and versus .

proposed approach is applicable for any load power factor angle.The intermediate results (i.e., , , and versus )obtained with the proposed design method are shown in Fig. 8.In Fig. 8 of the present design example, sag compensation ismore demanding in terms of VA loadings of the series inverterand shunt inverter up to . Therefore, the sizing ofthe total UPQC is solely decided by sag compensation up to

. Afterwards, swell compensation decides the shuntinverter rating and sag compensation continues to decide theseries inverter rating.From Fig. 8, the valley point on the total VA rating curve of

UPQC is ( , 5397 VA). The ratings of the series and shunt in-verters corresponding to the minimum VA rating, using the pro-posed design method are: 2490 VA and 2907 VA, respectively.Hence, the total VA rating of the designed UPQC is 16 191 VA

.This shift between the source voltage and the load

voltage should be maintained by the control algorithm to keepthe VA loadings of both inverters below these maximum ratingsat any operating condition. As may be observed from Figs. 3 and4, the shift between the source voltage and load voltage is tobe maintained by the series inverter by injecting series voltage

at an angle with source voltage. These and canbe calculated using (12) and (13) which are independent of loadcondition, that is, load current. Replacing the online optimiza-tion approaches to determine and in [5][12] with thestraightforward approach using (12) and (13) is the simplest wayto realize the control implementation.It is important to note that sizing is carried out for full load

and worst case sag/swell operations with a fixed optimum .In case of reduced load and/or less severe voltage sag/swells,online optimization methods can be employed in control algo-rithms to minimize the operational losses of UPQC by tem-porarily changing the displacement angle keeping shunt andseries inverters ratings, and series transformer rating, maximumseries voltage injection as constraints for optimization.Furthermore, due to generalization of VA loading equations,

the VA ratings of different types of UPQC for the same func-tional specifications can be obtained directly from Fig. 8 when

. VA ratings of the series inverter, shunt inverter, and total


TABLE IVVA LOADING IN UPQC WITH MAXIMUM UTILIZATION BY

USING THE PROPOSED SIZING APPROACH

TABLE VSIZING COMPARISON OF THE UPQC SYSTEM WITH DIFFERENT DESIGN

APPROACHES FOR THE SAME COMPENSATION REQUIREMENTS

VA rating of the UPQC, marked on the -axis of Fig. 8, repre-sent the VA ratings of UPQC-P. These results can be verifiedwith the ratings obtained in Table I.With the computed optimumdisplacement angle , the maximum VA loadings of theseries and shunt inverters, and the total UPQC under differentoperating conditions computed using (12)(23) are shown inTable IV.During the steady state (i.e., ), from Table IV, as long

as the load reactive power is more than 2610 VA 3870], thecontrol strategy of the series inverter should maintain .If the load reactive power demand goes below 2610 VA, thecontrol strategymay be switched to UPQC-P operation (i.e.,) to avoid the overutilization to reduce the losses in UPQC.The utilization of the series inverter, shunt inverter,

and the total UPQC under different operating condi-tions in three different designs such as: 1) UPQC-P; 2)

[7][12]; and 3) UPQC with the proposedmaximum utilization are illustrated in Fig. 9. The utilization ofthe series and shunt inverters in UPQC-P andis lower compared with those of the proposed approach. Thebetter utilization of the inverters in all operating modes, in-cluding steady state, gives the least sizing of the UPQC in theproposed design method. The comparison of VA ratings of thetotal UPQC system and series transformer between UPQC-P

and UPQC with maximum utilization is madein Table V.Considering UPQC-P as the base design for comparison anal-

ysis, the total VA rating of UPQC with minimum VA loadingdesign will increase by 2043 VA and the VA rating of the se-ries transformer will rise by 4041 VA. The increased sizing of

Fig. 9. Utilization of the overall rating of UPQC, series and shunt parts ofUPQC in: (a) UPQC-P, (b) UPQC with minimum VA loading, and (c) UPQCwith maximum utilization using the proposed design method.

power-electronic converters and series transformer will add tothe cost of whereas the VA rating of the seriestransformer in the proposed design is 1732 VA higher than inUPQC-P, but the rating of the UPQC inverters is brought downby 2499 VA. This reduction in the VA rating of power-electronicconverters comparatively brings down the manufacturing costof the entire UPQC system.


V. DESIGN OF UPQC FOR HARMONIC ANDUNBALANCE ENVIRONMENTS

The rating optimization algorithm developed in the previoussections deals with the variables which are positive-sequencecomponents at the fundamental frequency only. This is due tothe fact that source voltage harmonics/unbalance can be mit-igated by the series part of the UPQC only and current har-monics/unbalance can be tackled by the shunt part of the UPQConly. Unlike the sharing of fundamental load reactive powerin maintaining unity power factor operation, compensation ofthese kinds of disturbances cannot be shared between the seriesand shunt inverters.The sizing procedure can further be extended to include har-

monic (both voltage and current) compensation simply by con-sidering appropriate distortion levels. For example, with a per-unit total source voltage harmonic distortion and a per-unit total load current harmonic distortion , the actual rat-ings of the series inverter and shunt inverterwith the proposed approach would be as follows:

(24)

(25)

Similarly, the actual VA rating of the series transformercan be given by

(26)

where , , and are outputs of the developedalgorithm shown in Fig. 7. This increase in VA ratings to incor-porate harmonic compensation will be very minimal comparedto the fundamental VA ratings. Similar to harmonic compensa-tion capability, for source voltage and/or load current unbalancecompensation, the required voltage and current unbalance fac-tors should be included in (24)(26).

VI. CONCLUSION

In this paper, the phenomena of variation in the rating ofthe UPQC system with the variation in displacement angle be-tween source and load voltages (i.e., fundamental load reactivepower sharing) has been studied by formulating the generalizedVA loading equations of UPQC systems. Based on the concep-tual study made, an algorithm has been developed to identifythe minimum possible VA rating of the UPQC system and thatresults in the corresponding optimal displacement angle , se-ries inverter, shunt inverter, and series transformer ratings. Thispaper thus provides the guidelines for the futuristic researchstudies to develop the control strategies for UPQC that are basedon online/offline optimization of instantaneous VA loading forreduced power losses and the VA burden on the UPQC system.

REFERENCES[1] B. Singh, K. Al-Haddad, and A. Chandra, A review of active power

filters for power quality improvement, IEEE Trans. Ind. Electron., vol.45, no. 5, pp. 960971, Oct. 1999.

[2] V. Khadkikar, Enhancing electrical power quality using UPQC: Acomprehensive overview, IEEE Trans. Power Electron., vol. 27, no.5, pp. 22842297, May 2012.

[3] W. C. Lee, D.M. Lee, and T. K. Lee, New control scheme for a unifiedpower-quality compensator-Q with minimum active power injection,IEEE Trans. Power Del., vol. 25, no. 2, pp. 10681076, Apr. 2010.

[4] M. Yun, W. Lee, I. Suh, and D. Hyun, A new control scheme of uni-fied power quality compensator-Q with minimum power injection, inProc. IEEE 30th Annu. Ind. Electron. Soc. Conf., Nov. 26, 2004, pp.5156.

[5] V. Khadkikar and A. Chandra, UPQC-S: a novel concept of simul-taneous voltage sag/swell and load reactive power compensations uti-lizing series inverter of UPQC, IEEE Trans. Power Electron., vol. 26,no. 9, pp. 24142425, Sep. 2011.

[6] V. Khadkikar and A. Chandra, A new control philosophy for a unifiedpower quality conditioner (UPQC) to coordinate load-reactive powerdemand between shunt and series inverters, IEEE Trans. Power Del.,vol. 23, no. 4, pp. 25222534, Oct. 2008.

[7] D. Kisck, V. Navrapescu, and M. Kisck, Single-phase unified powerquality conditioner with optimum voltage angle injection for minimumVA requirement, in Proc. IEEE Int. Symp. Ind. Electron., Jun. 1721,2007, pp. 24432448.

[8] H. Ryoo, G. Rim, T. Kim, and D. Kisck, Digital-controlled single-phase unified power quality conditioner for non-linear and voltage sen-sitive load, in Proc. IEEE 30th Annu. Ind. Electron. Soc. Conf. , Nov.26, 2004, pp. 2429.

[9] Y. Y. Kolhatkar, R. R. Errabelli, and S. Das, A sliding mode controllerbased optimum UPQC with minimum VA loading, in Proc. PowerEng. Soc. Gen. Meeting, Jun. 1216, 2005, pp. 871875.

[10] Y. Y. Kolhatkar and S. Das, Experimental investigation of a single-phase UPQC with minimum VA loading, IEEE Trans. Power Del.,vol. 22, no. 1, pp. 371380, Jan. 2007.

[11] G. S. Kumar, P. H. Vardhana, B. K. Kumar, and M. K. Mishra,Minimization of VA loading of unified power quality conditioner(UPQC), in Proc. Power Eng., Energy Elect..Drives, Mar. 1820,2009, pp. 552557.

[12] G. S. Kumar, B. K. Kumar, and M. M. Kumar, Optimal VA loading ofUPQC during mitigation of unbalanced voltage sags with phase jumpsin three-phase four-wire distribution system, in Proc. Int. Conf. PowerSyst. Technol., Oct. 2428, 2010, pp. 18.

Bharath Babu Ambati received the B.E. degree inelectrical and electronics engineering from Sir C. R.Reddy College of Engineering (affiliated to AndhraUniversity), Eluru, India, in 2009, the M.Tech. de-gree in power electronics, electrical machines, anddrives (PEEMD) from the Indian Institute of Tech-nology (IIT) Delhi, New Delhi, India, in 2011, and iscurrently pursuing the Ph.D. degree in interdiscipli-nary engineering at Masdar Institute of Science andTechnology, Abu Dhabi, United Arab Emirates.From 2011 to 2012, he was with Schneider Electric

India Private Ltd., as a Product Expert of Motion & Drives. His current researchinterests include power electronics, electrical machines, renewable energy gen-eration, and power quality.

Vinod Khadkikar (S06M09) received the B.E.degree in electrical engineering from the Gov-ernment College of Engineering, Dr. BabasahebAmbedkar Marathwada University, Aurangabad,India, in 2000, the M.Tech. degree in electricalengineering from the Indian Institute of Technology(IITD), New Delhi, India, in 2002, and the Ph.D.degree in electrical engineering from the cole deTechnologie Suprieure (E.T.S.), Montral, QC,Canada, in 2008.From 2008 to 2010, he was a Postdoctoral Fellow

at the University of Western Ontario, London, ON, Canada. Since 2010, he hasbeen an Assistant Professor with Masdar Institute of Science and Technology,AbuDhabi, UnitedArab Emirates. In 2010, he was a visiting faculty at theMass-achusetts Institute of Technology, Cambridge, MA, USA. His research interestsinclude applications of power electronics in distribution systems and renewableenergy resources, grid interconnection issues, power-quality enhancement, aswell as active power filters and electric vehicles.

ambatam

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