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  • 1. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-059 Application For Major Power Quality And Extend Pac For Upqc-S PETTA SABITHA (M.Tech), M.SRIDHAR Ph.d *(Department of Electrical Engineering, GIET, JNTUK, Rajahmundry, A .P, INDIAABSTRACT This paper introduces a new concept of of two voltage source inverters connected back tooptimal utilization of a unified power quality back using a common dc bus capacitor. This paperconditioner (UPQC). The series inverter of deals with a novel concept of optimal utilization of aUPQC is controlled to perform simultaneous 1) UPQC.voltage sag/swell compensation and 2) loadreactive power sharing with the shunt inverter.The active power control approach is used tocompensate voltage sag/swell and is integratedwith theory of power angle control (PAC) ofUPQC to coordinate the load reactive powerbetween the two inverters. Since the seriesinverter simultaneously delivers active andreactive powers, this concept is named as UPQC-S (S for complex power). A detailed mathematicalanalysis, to extend the PAC approach for UPQC-S, is presented in this paper. MATLAB/SIMULINK -based simulation results arediscussed to support the developed concept.Finally, the proposed concept is validated with adigital signal processor-based experimentalstudy.Keywords - Active power filter (APF), power Fig. 1. Unified power quality conditioner (UPQC)angle control (PAC), power quality, reactive system configuration.power compensation, unified power qualityconditioner (UPQC), voltage sag and swell The voltage sag/swell on the system is onecompensation. of the most important power quality problems [1], [2]. The voltage sag/swell can be effectivelyI. INTRODUCTION compensated using a dynamic voltage restorer, The modern power distribution system is series active filter, UPQC, etc. [7]–[28]. Among thebecoming highly vulnerable to the different power available power quality enhancement devices, thequality problems [1], [2]. The extensive use of UPQC has better sag/swell compensation capability.nonlinear loads is further contributing to increased Three significant control approaches for UPQC cancurrent and voltage harmonics issues. Furthermore, be found to control the sag on the system: 1) activethe penetration level of small/large-scale renewable power control approach in which an in-phase voltageenergy systems based on wind energy, solar energy, is injected through series inverter [16]–[22],fuel cell, etc., installed at distribution as well as popularly known as UPQC-P; 2) reactive powertransmission levels is increasing significantly. This control approach in which a quadrature voltage isintegration of renewable energy sources in a power injected [23], [24], known as UPQC-Q; and 3) asystem is further imposing new challenges to the minimum VA loading approach in which a serieselectrical power industry to accommodate these voltagenewly emerging distributed generation systems [3]. is injected at a certain angle, [25]–[28], in this paperTo maintain the controlled power quality called as UPQC-VA min. Among theregulations, some kind of compensation at all the aforementioned three approaches, the quadraturepower levels is becoming a common practice [5]– voltage injection requires a maximum series[9]. At the distribution level, UPQC is a most injection voltage, whereas the in-phase voltageattractive solution to compensate several major injection requires the minimum voltage injectionpower quality problems [7]–[9], [14]–[28]. The magnitude. In a minimum VA loading approach, thegeneral block diagram representation of a UPQC- series inverter voltage is injected at an optimal anglebased system is shown in Fig. 1. It basically consists with respect to the source current. Besides the series inverter injection, the current drawn by the shunt 52 | P a g e
  • 2. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-059inverter, to maintain the dc link voltage and the Since the series inverter of UPQC in this caseoverall power balance in the network, plays an delivers both active and reactive powers, it is givenimportant role in determining the overall UPQC VA the name UPQCS (S for complex power).loading. The reported paper on UPQC-VA min isconcentrated on the optimal VA load of the seriesinverter of UPQC especially during voltage sagcondition [25]–[28]. Since an out of phasecomponent is required to be injected for voltageswell compensation, the suggested VA loading inUPQC-VA min determined on the basis of voltagesag, may not be at optimal value. A detailedinvestigation on VA loading in UPQC-VA minconsidering both voltage sag and swell scenarios isessential. In the paper [15], the authors haveproposed a concept of power angle control (PAC) ofUPQC. The PAC concept suggests that with propercontrol of series inverter voltage the series invertersuccessfully supports part of the load reactive powerdemand, and thus reduces the required VA rating ofthe shunt inverter. Most importantly, thiscoordinated reactive power sharing feature isachieved during normal steady-state conditionwithout affecting the resultant load voltage Fig. 2. Concept of PAC of UPQC.magnitude. The optimal angle of series voltageinjection in UPQC-VAmin is computed using The key contributions of this paper are outlined aslookup table [26], [27] or particle swarm follows.optimization technique [28]. These iterative methods 1) The series inverter of UPQC-S is utilized formostly rely on the online load power factor angle simultaneous voltage sag/swell compensation andestimation, and thus may result into tedious and load reactive power compensation in coordinationslower estimation of optimal angle. On the other with shunt inverter.hand, the PAC of UPQC concept determines the 2) In UPQC-S, the available VA loading is utilizedseries injection angle by estimating the power angle to its maximum capacity during all the workingδ. The angle δ is computed in adaptive way by conditions contrary to UPQC-VA min where primecomputing the instantaneous load active/reactive focus is to minimize the VA loading of UPQCpower and thus, ensures fast and accurate estimation. during voltage sag condition. Similar to PAC of UPQC, the reactive 3) The concept of UPQC-S covers voltage sag aspower flow control utilizing shunt and series well as swell scenario.inverters is also done in a unified power flow In this paper, a detailed mathematical formulation ofcontroller (UPFC) [4], [5]. A UPFC is utilized in a PAC for UPQC-S is carried out. The feasibility andpower transmission system whereas a UPQC is effectiveness of the proposed UPQC-S approach areemployed in a power distribution system to perform validated by simulation as well as experimentalthe shunt and series compensation simultaneously. results.The power transmission systems are generallyoperated in balanced and distortion-free II. ACTIVE POWER FILTERSenvironment, contrary to power distribution systems The proliferation of microelectronicsthat may contain dc component, distortion, and processors in a wide range of equipments, fromunbalance. The primary objective of a UPFC is to home VCRs and digital clocks to automatedcontrol the flow of power at fundamental frequency. industrial assembly lines and hospital diagnosticsAlso, while performing this power flow control in systems has increased the vulnerability of suchUPFC the transmission network voltage may not be equipment to power quality problems. Thesemaintained at the rated value. However, in PAC of problems include a variety of electrical disturbances,UPQC the load side voltage is strictly regulated at which may originate in several ways and haverated value while performing load reactive power different effects on various kinds of sensitive loads.sharing by shunt and series inverters. In this paper, What were once considered minor variations inthe concept of PAC of UPQC is further expanded for power, usually unnoticed in the operation ofvoltage sag and swells conditions. This modified conventional equipment, may now bring wholeapproach is utilized to compensate voltage sag/swell factories to standstill. As a result of thiswhile sharing the load reactive power between two vulnerability, increasing numbers of industrial andinverters. commercial facilities are trying to protect themselves by investing in more sophisticate equipment to 53 | P a g e
  • 3. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-059improve power quality. Moreover, the proliferation quality related process stoppages and energyof nonlinear loads with large rated power has suppliers trying to maximize operating profits whileincreased the contamination level in voltages and keeping customers satisfied with supply quality,currents waveforms, forcing to improve the innovative technology provides the key to cost-compensation characteristics required to satisfy effective power quality enhancements solutions.more stringent harmonics standard. Between the However, with the various power quality solutionsdifferent technical options available to improve available, the obvious question for a consumer orpower quality, active power filters have proved to be utility facing a particular power quality problem isan important alternative to compensate for current which equipment provides the better solution.and voltage disturbances in power distributionsystems. Different active power filters topologies IV. SOLUTIONS TO POWER QUALITYhave been presented in the technical literature many PROBLEMSof them are already available in the market. This There are two approaches to the mitigationpaper will focus in the analysis of which to use with of power quality problems. The first approach istheir compensation characteristics. Shunt active called load conditioning, which ensures that thepower filters, series active topologies, will be equipment is less sensitive to power disturbances,presented allowing the operation even under significant voltage distortion. The other solution is to install lineIII. POWER QUALITY IN POWER conditioning systems that suppress or counteracts theDISTRIBUTION SYSTEMS power system disturbances. A flexible and versatile Most of the more important international solution to voltage quality problems is offered bystandards define power quality as the physical active power filters. Currently they are based oncharacteristics of the electrical supply provided PWM converters and connect to low and mediumunder normal operating conditions that do not voltage distribution system in shunt or in series.disrupt or disturb the customer’s processes. Series active power filters must operate inTherefore, a power quality problem exists if any conjunction with shunt passive filters in order tovoltage, current or frequency deviation results in a compensate load current harmonics. Shunt activefailure or in a bad operation of customer’s power filters operate as a controllable current sourceequipment. However, it is important to notice that and series active power filters operates as athe quality of power supply implies basically voltage controllable voltage source. Both schemes arequality and supply reliability. A voltage quality implemented preferable with voltage source PWMproblem relates to any failure of equipment due to inverters, with a dc bus having a reactive elementdeviations of the line voltage from its nominal such as a capacitor. Active power filters can performcharacteristics, and the supply reliability is one or more of the functions required to compensatecharacterized by its adequacy (ability to supply the power systems and improving power quality. As itload), security (ability to withstand sudden will be illustrated in this paper, their performancedisturbances such as system faults) and availability depends on the power rating and the speed of(focusing especially on long interruptions). Power response.quality problems are common in most of The selection of the type of active power filter tocommercial, industrial and utility networks. Natural improve power quality depends on the source of thephenomena, such as lightning are the most frequent problem as can be seen in Table 1.cause of power quality problems. Switchingphenomena resulting in oscillatory transients in theelectrical supply, for example when capacitors areswitched, also contribute substantially to powerquality disturbances. Also, the connection of highpower non-linear loads contributes to the generationof current and voltage harmonic components.Between the different voltage disturbances that canbe produced, the most significant and critical powerquality problems are voltage sags due to the higheconomical losses that can be generated. Short-termvoltage drops (sags) can trip electrical drives ormore sensitive equipment, leading to costlyinterruptions of production. For all these reasons,from the consumer point of view, power qualityissues will become an increasingly important factorto consider in order satisfying good productivity. Toaddress the needs of energy consumers trying toimprove productivity through the reduction of power 54 | P a g e
  • 4. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-059V. SHUNT ACTIVE POWER FILTERS Shunt active power filter compensatecurrent harmonics by injecting equal-but-oppositeharmonic Compensating current. In this case theshunt active power filter operates as a current sourceinjecting the harmonic components generated by theload but phase shifted by180 deg. This principle isapplicable to any type of load considered a harmonicsource. Moreover, with an appropriate controlscheme, the active power filter can also compensatethe load power factor. In this way, the powerdistribution system sees the non linear load and theactive power filter as an ideal resistor. The currentcompensation characteristic of the shunt activepower filter is shown in fig Fig.4. SERIES ACTIVE POWER FILTERS Unified Power Quality Conditioner The provision of both DSTATCOM and DVR can control the power quality of the source current and the load bus voltage. In addition, if the DVR and STATCOM are connected on the DC side, the DC bus voltage can be regulated by the shunt connected DSTATCOM while the DVR supplies the required energy to the load in case of the transient disturbances in source voltage. The configuration ofFig.3. SHUNT ACTIVE POWER FILTERS such a device (termed as Unified Power Quality Conditioner (UPQC)) is shown in Fig. 14.15. This isSERIES ACTIVE POWER FILTERS a versatile device similar to a UPFC. However, the It is well known that series active power control objectives of a UPQC are quite differentfilters compensate current system distortion caused from that of a UPFC.by non-linear loads by imposing a high impedancepath to the current harmonics which forces the highfrequency currents to flow through the LC passivefilter connected in parallel to the load. The highimpedance imposed by the series active power filteris created by generating a voltage of the samefrequency that the current harmonic component thatneeds to be eliminated. Voltage unbalance iscorrected by compensating the fundamentalfrequency negative and zero sequence voltagecomponents of the system. Fig.5. Unified Power Quality Conditioner. VI. CONTROL OBJECTIVES OF UPQC The shunt connected converter has the following control objectives 1. To balance the source currents by injecting negative and zero sequence components required by the load 2. The compensate for the harmonics in the load current by injecting the required harmonic currents 55 | P a g e
  • 5. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-0593. To control the power factor by injecting therequired reactive current (at fundamental frequency) (5)4. To regulate the DC bus voltage. where V ¤ L and I¤S are the reference quantities forOperation of UPQC the load bus voltage and the source current respectively. Ál is the power factor angle at the load bus while Ás is the power factor angle at the source bus (input port of UPQC). Note that V ¤ L(t) and I¤S (t) are sinusoidal and balanced. If the reference current (I¤C ) of the shunt converter and the reference voltage (V ¤ C) of the series converter are chosen as (6) with the constraintFig.6.Operation of UPQC (7)The operation of a UPQC can be explained from the we have,analysis of the idealized equivalent circuit shown inFig. 14.16. Here, the series converter is represented (8)by a voltage source VC and the shunt converter is Note that the constraint (14.30) implies that V 1p Crepresented by a current source IC. Note that all the is the reactive voltage in quadrature with the desiredcurrents and voltages are 3 dimensional vectors with source current, I¤S. It is easy to derive thatphase coordinates. Unlike in the case of a UPFC(discussed in chapter 8), the voltages and currentsmay contain negative and zero sequence components The above equation shows that for the operatingin addition to harmonics. Neglecting losses in the conditions assumed, a UPQC can be viewed as aconverters, we get the relation inaction of a DVR and a STATCOM with no active power °ow through the DC link. However, if the magnitude of V ¤ L is to be (1) controlled, it may not be feasible to achieve this byWhere X, Y denote the inner product of two vectors, injecting only reactive voltage. The situation getsdefined by complicated if V 1p S is not constant, but changes due to system disturbances or fault. To ensure the regulation of the load bus voltage it may be necessary to inject variable active voltage (in (2) Phase with the source current). If we expressLet the load current IL and the source voltage VS bedecomposed into two. Components given by (9) (3)Where I1p L contains only positive sequence,fundamental frequency components. Similar (10)comments apply to V 1pS . IrL and V rS contain rest In deriving the above, we assume thatof the load current and the source voltage includingharmonics. I1pL is not unique and depends on thepower factor at the load bus. However, the following (11)relation applies for I1p L . This implies that both ¢VC and ¢IC are perturbations involving positive sequence, fundamental frequency quantities (say, resulting (4) from symmetric voltage sags). the power balance onThis implies that hIrL ; VLi = 0. Thus, the the DC side of the shunt and series converter. Thefundamental frequency, positive sequence perturbation in VC is initiated to ensure thatcomponent in IrL does not contribute to the activepower in the load. To meet the control objectives,the desired load voltages and source currents must Thus, the objective of the voltage regulation at thecontain only positive sequence, fundamental load bus may require exchange of power betweenfrequency components and the shunt and series converters. 56 | P a g e
  • 6. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-059 condition. (j) Enlarged power angle δ during voltageRemarks swell condition.1. The unbalance and harmonics in the sourcevoltage can arise due to uncompensated nonlinearand unbalanced loads in the upstream of the UPQC.2. The injection of capacitive reactive voltage by theseries converter has the advantage of raising thesource voltage magnitude.UPQC-S CONTROLLER A detailed controller for UPQC based onPAC approach is described in [15]. In this paper, thegeneration of reference signals for series inverter isdiscussed. Note that, as the series inverter maintainsthe load voltage at desired level, the reactive powerdemanded by the load remains unchanged (assumingload on the system is constant) irrespective ofchanges in the source voltage magnitude.Furthermore, the power angle δ is maintained atconstant value under different operating conditions.Therefore, the reactive power shared by the seriesinverter and hence by the shunt inverter changes asgiven by (47) and (54). The reactive power sharedby the series and shunt inverters can be fixed atconstant values by allowing the power angle δ tovary under voltage sag/swell condition.The control block diagram for series inverteroperation is shown in Fig. 10. The instantaneouspower angle δ is determined using the proceduregive in [15]. Based on the system ratedspecifications, the value of the desired load voltageis set as reference load voltage k. The instantaneousvalue of factors kf and nO is computed by measuring VII. SIMULATION RESULTSthe peak value of the supply voltage in real time. The performance of the proposed conceptThe magnitudes of series injected voltage VSr and its of simultaneous load reactive power and voltagephase angle ϕSr are then determined using (15) and sag/swell compensation has been evaluated by(17). A phase locked loop is used to synchronize and simulation. To analyze the performance of UPQC-S,to generate the source is assumed to be pure sinusoidal.Instantaneous time variable reference signals v∗ Furthermore, for better visualization of results theSr,a , v∗ Sr,b , v∗ Sr,c . load is considered as highly inductive. The supplyThe reference signals thus generated give the voltage which is available at UPQC terminal isnecessary series injection voltages that will share the considered as three phase, 60 Hz, 600 V (line to line)load reactive power and compensate for voltage with the maximum load power demand of 15 kW + jsag/swell as formulated using the proposed 15 kVAR (load power factor angle of 0.707approach. The error signal of actual and reference lagging). The simulation results for the proposedseries voltage is utilized to perform the switching UPQC-S approach under voltage sag and swelloperation of series inverter of UPQC-S. The control conditions. The distinct features of the proposeddiagram for the shunt inverter is as given in [15]. UPQC-S approach are outlined as follows. 1) From Figure the load voltage profile is maintainedFig.7. Simulation results: Performances of the at a desired level irrespective of voltage sagproposed UPQC-S approach under voltage sags and (decrease) or swell (increase) in the source voltageswell conditions. (a) Supply voltage. (b) Load magnitudes. During the sag/swell compensation, tovoltage. (c) Series inverter injected voltage. (d) Self- maintain the appropriate active power balance in thesupporting dc bus voltage. (e) Enlarged power angle network, the source current increase during theδ relation between supply and load voltages during voltage sag and reduces during swell condition.steady-state condition. (f) Supply current. (g) Load 2) As illustrated by enlarged results, the power anglecurrent. (h) Shunt inverter injected current. (i) δ between the source and load voltages during theEnlarged power angle δ during voltage sag steady state, voltage sag and voltage swell is maintained at 21◦. 57 | P a g e
  • 7. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-0593) The UPQC-S controller maintains a self- Although the reactive power shared by the series andsupporting dc link voltage between two inverters shunt inverters is varied, the sum of their reactive powers always equals the reactive power demanded by the load. Table II gives the power losses associated with UPQC with and without PAC approach under different scenarios. The power loss is computed as the ratio of losses associated with UPQC to the total load power. This is an interesting outcome of the PAC approach even when the series inverter deals with both active and reactive powers due to δ shift between source and load voltages. One may expect to increase the power loss with the UPQC-S system. The reduction in the power loss is mainly due to the reduction in the shunt inverter rms current from 20.20 A (without PAC approach) to 13.18 A (with PAC approach). Second, the current through the series inverter (which is almost equal to the source current) remains unchanged. Similarly from the Table I, the power losses utilizing the PAC approach, during voltage sag and swell conditions, are observed lower than those without PACFig.8. Simulation results: active and reactive power approach. This study thus suggests that the PACflow through source, load, shunt, and series inverter approach may also help to reduce the power lossutilizing proposed UPQC-S approach under voltage associated with UPQC system in addition to thesag and swell conditions. (a) Source P and Q. (b) previously discussed advantages. The significantLoad P and Q. (c) Series inverter P and Q. (d) Shunt advantage of UPQC-S over general UPQCinverter P and Q. applications is that the shunt inverter rating can be reduced due to reactive power sharing of both theTABLE II inverters.LOSSES ASSOCIATED WITH UPQC UNDERDIFFERENT SCENARIOS VIII. CONCLUSION In this paper, a new concept of controlling complex power (simultaneous active and reactive powers) through series inverter of UPQC is introduced and named as UPQC-S. The proposed concept of the UPQC-S approach is mathematically formulated and analyzed for voltage sag and swell conditions. The developed comprehensive equations for UPQC-S can be utilized to estimate the required series injection voltage and the shunt compensating current profiles (magnitude and phase angle), and the overall VA loading both under voltage sag and swell conditions. The simulation and experimental studies demonstrate the effectiveness of the proposed concept of simultaneous voltage sag/swell and load reactive power sharing feature of series part of UPQC-S. The significant advantages of UPQC-S over general UPQC applications are: 1) the The reactive power supplied by the series multifunction ability of series inverter to compensateinverter during the voltage sag condition increases voltage variation (sag, swell, etc.) while laggingdue to the increased source current. As load reactive power factor; DC bus: dc bus capacitor = 1100power demand is constant, the reactive power μF/220 V, reference dc bus voltage = 150 V; UPQC:supplied by the shunt inverter reduces accordingly. shunt inverter coupling inductance = 5 mH, shuntOn the other hand, during the voltage swell inverter switching type = analog hysteresis currentcondition, the reactive power shared by the series controller with average switching frequency betweeninverter reduces and the shunt inverter increases. 5 and 7 kHz, series inverter coupling inductance = 2The reduction and increment in the shunt mH, series inverter ripple filter capacitance = 40 μF,compensating current magnitude, as seen from series inverter switching type = analog triangularFigure, also confirm the aforementioned fact. carrier pulse width modulation with a fixed 58 | P a g e
  • 8. Petta Sabitha, M.Sridhar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January-February 2013, pp.052-059frequency of 5 kHz, series voltage injection [15] V. Khadkikar and A. Chandra, ―A newtransformer turn ratio = 1:3, DSP sampling time = 0 control philosophy for a unified power qualityμs. conditioner (UPQC) to coordinate load- reactive power demand between shunt andREFERENCES series inverters,‖ IEEE Trans. Power Del., [1] R. C. Dugan, M. F. McGranaghan, and H. W. vol. 23, no. 4, pp. 2522–2534, Oct. 2008. Beaty, Electrical Power Systems Quality.. [16] M. Vilathgamuwa, Z. H. Zhang, and S. S. New York: McGraw-Hill, 1996, p. 265. Choi, ―Modeling, analysis and control of [2] C. Sankaran, Power Quality. Boca Raton, FL: unified power quality conditioner,‖ in Proc. CRC Press, 2002, p. 202. IEEE Harmon. Quality Power, Oct. 14–18, [3] R. A. Walling, R. Saint, R. C. Dugan, J. 1998, pp. 1035–1040. Burke, and L. A. Kojovic, ―Summary of [17] M. Gon, H. Liu, H. Gu, and D. Xu, ―Active distributed resources impact on power voltage regulator based on novel delivery systems,‖ IEEE Trans. Power Del., synchronization method for unbalance and vol. 23, no. 3, pp. 1636–1644, Jul. 2008. fluctuation compensation,‖ in Proc. IEEE Ind. [4] L. Gyugyi, ―Unified power-flow control Electron. Soc (IECON), Nov. 5–8,, 2002, pp. concept for flexible AC transmission 1374–1379. systems,‖ IEE – C Gene. Trans. Distr., vol. [18] M. S. Khoor and M. Machmoum, ―Simplified 139, no. 4, pp. 323–331, Jul. 1992. analogical control of a unified power quality [5] N. G. Hingorani and L. Gyugyi, conditioner,‖ in Proc. IEEE Power Electron. Understanding FACTS: Concepts and Spec. Conf. (PESC), Jun., 2005, pp. 2565– Technology of Flexible AC Transmission 2570. Systems. New York: IEEE Press, 2000, p. [19] V. Khadkikar, A. Chandra, A. O. Barry, and 432. T. D. Nguyen, ―Analysis of power flow in [6] V. K. Sood, HVDC and FACTS Controllers – UPQC during voltage sag and swell Applications of Static Converters in Power conditions for selection of device ratings,‖ in Systems. Boston, MA: Kluwer, 2004, p. 295. Proc. IEEE Electr. Computer Eng. (CCECE), [7] A. Ghosh and G. Ledwich, Power Quality May 2006, pp. 867–872. Enhancement Using Custom Power Devices. [20] B. Han, B. Bae, H. Kim, and S. Baek, Boston, MA: Kluwer, 2002, p. 460. ―Combined operation of unified power- [8] B. Singh, K. Al-Haddad, and A. Chandra, ―A quality conditioner with distributed review of active power filters for power generation,‖ IEEE Trans.Power Del., vol. 21, quality improvement,‖ IEEE Trans. Ind. no. 1, pp. 330–338, Jan. 2006. Electron., vol. 45, no. 5, pp. 960–971, Oct. 1999. [9] M. El-Habrouk, M. K. Darwish, and P. Mehta, ―Active power filters: A review,‖ IEE Electr. Power Appl., vol. 147, no. 5, pp. 403– 413, Sep. 2000. [10] Doncker, C. Meyer, R. W. De, W. L. Yun, and F. Blaabjerg, ―Optimized control strategy for a medium-voltage DVR—Theoretical investigations and experimental results,‖ IEEE Trans. Power Electron., vol. 23, no. 6, pp. 2746–2754, Nov. 2008. [11] C. N. Ho and H. S. Chung, ―Implementation and performance evaluation of a fast dynamic control scheme for capacitor-supported interline DVR,‖ IEEE Trans. Power Electron., vol. 25, no. 8, pp. 1975–1988, Aug. 2010. [12] Y. Chen, C. Lin, J. Chen, and P. Cheng, ―An inrush mitigation technique of load transformers for the series voltage sag compensator,‖ IEEE Trans. Power Electron., vol. 25, no. 8, pp. 2211–2221, Aug. 2010. [13] S. Subramanian and M. K. Mishra, ―Interphase AC–AC topology for voltage sag supporter,‖ IEEE Trans. Power Electron., vol. 25, no. 2,pp. 514–518, Feb. 2010. [14] H. Fujita and H. Akagi IEEE Trans. Power Electron., vol. 13, no. 2,pp. 315–322, Mar. 1998 59 | P a g e