V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and                  Applications (IJERA)...
V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and                   Applications (IJERA...
V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and                    Applications (IJER...
V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and                    Applications (IJER...
V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and                    Applications (IJER...
V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and                  Applications (IJERA)...
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  1. 1. V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.778-783 Improving Power Quality by UsingMC-UPQC V. Surya Prakash (M.Tech) K. Kalyan Kumar M.Tech EEE – Department, Asst. Prof. EEE – Department, KSRM College of Engineering. Kadapa. KSRM College of Engineering. Kadapa.ABSTRACT This paper presents a new Unified Power- lines. The concept of GUPFC can be extended forQuality Conditioning System (MC- more lines if necessary. The device may be installedUPQC),capable of simultaneous compensation for in some central substations to manage power flows ofvoltage and current in multi-bus/multi-feeder multilines or a group of lines and provide voltagesystems. By using one shunt Voltage-Source support as well. By using GUPFC devices, theConverter(VSC) and two or more series VSCs the transfer capability of transmission lines can beconfiguration is made. The system can be applied increased significantly.to adjacent feeders to compensate for supply- Furthermore, by using the multiline-voltage and load current imperfections on the management capability of the GUPFC, active powermain feeder and full compensation of supply flow on lines cannot only be increased, but also bevoltage imperfections on the other feeders. The decreased with respect to operating and marketconfiguration will be designed as all converters transaction requirements. In general, the GUPFC canare connected back to back on the dc side and be used to increase the transfer capability and relieveshare a common dc-link capacitor. Therefore, congestions in a flexible way. This concept can bepower can be transferred from one feeder to extended to design multiconverter configurations foradjacent feeders to compensate for sag/swell and PQ improvement in adjacent feeders. For example,interruption. The proposed topology can be used the interline unified power-quality conditionerfor simultaneous compensation of voltage and (IUPQC), which is the extension of the IPFC conceptcurrent imperfections in both feeders by sharing at the distribution level, has been proposed in thepower compensation capabilities between two IUPQC consists of one series and one shuntadjacent feeders which are not connected. The converter. It is connected between two feeders tosystem is also capable of compensating for regulate the bus voltage of one of the feeders, whileinterruptions without the need for a battery regulating the voltage across a sensitive load in thestorage system and consequently without storage other feeder. In this configuration, the voltagecapacity limitations. By the simulation the regulation in one of the feeders is performed by theperformance of MC-UPQC as well as the adopted shunt-VSC. However, since the source impedance iscontrol algorithm will be illustrated. very low, a high amount of current would be needed to boost the bus voltage in case of a voltage sag/swellIndex – terms: Voltage-Source Converter(VSC), which is not feasible. It also has low dynamicInterline Power Flow Controller (IPFC),Unified performance because the dc-link capacitor voltage isPower-Quality Conditioning System (UPQC), not regulated. In this paper, a new configuration of aI. INTRODUCTION UPQC called the multiconverter unified power- When the power flows of two lines starting quality conditioner (MC-UPQC) is presented. Thein one substation need to be controlled, an Interline system is extended by adding a series-VSC in anPower Flow Controller (IPFC) can be used. An IPFC adjacent feeder. The proposed topology can be usedconsists of two series VSCs whose dc capacitors are for simultaneous compensation of voltage and currentcoupled. This allows active power to circulate imperfections in both feeders by sharing powerbetween the VSCs. With this configuration, two lines compensation capabilities between two adjacentcan be controlled simultaneously to optimize the feeders which are not connected. The system is alsonetwork utilization. The GUPFC combines three or capable of compensating for interruptions without themore shunt and series converters. It extends the need for a battery storage system and consequentlyconcept of voltage and power-flow control beyond without storage capacity limitations.what is achievable with the known two-converterUPFC. The simplest GUPFC consists of three II. PROPOSED MC-UPQC SYSTEMconverters—one connected in shunt and the other two A. Circuit Configurationin series with two transmission lines in a substation. The single-line diagram of a distribution systemThe basic GUPFC can control total five power with an MC-UPQC is shown in Fig.1.system quantities, such as a bus voltage andindependent active and reactive power flows of two 778 | P a g e
  2. 2. V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.778-783 end of Feeder1. VSC3 is connected in series with BUS2 at the Feeder2 end. Each of the three VSCs in Fig.3 is realized by a three-phase converter with a commutation reactor and high-pass output filter as shown in Fig.3.Fig.1 Single-line diagram of a distribution systemwith an MC-UPQC. As shown in this fig.1., two feedersconnected to two different substations supply theloads L1 and L2. The MC-UPQC is connected to twobuses BUS1 and BUS2 with voltages of ut1 and ut2, Fig 3 Schematic structure of a VSCrespectively. The shunt part of the MC-UPQC is alsoconnected to load L1 with a current of il1. Supply The commutation reactor (Lf )and high- passvoltages are denoted by u s1 and us2 while load output filter (Rf , Cf) connected to prevent the flow ofvoltages are ul1 and ul2 Finally, feeder currents are switching harmonics into the power supply. Asdenoted by i s1 and is2 load currents are il1 and il2 Bus shown in Fig, all converters are supplied from avoltages ut1 and ut2 are distorted and may be common dc-link capacitor and connected to thesubjected to sag/swell. The load L1 is a distribution system through a transformer. Secondarynonlinear/sensitive load which needs a pure (distribution) sides of the series-connectedsinusoidal voltage for proper operation while its transformers are directly connected in series withcurrent is non-sinusoidal and contains harmonics. BUS1 and BUS2, and the secondary (distribution) sideThe load L2 is a sensitive/critical load which needs a of the shunt-connected transformer is connected inpurely sinusoidal voltage and must be fully protected parallel with load L1.against distortion, sag/swell, and interruption. Thesetypes of loads primarily include production industries The aims of the MC-UPQC shown in Fig are:and critical service providers, such as medical 1) To regulate the load voltage (ul1) againstcenters, airports, or broadcasting centers where sag/swell and disturbances in the system tovoltage interruption can result in severe economical protect the nonlinear/sensitive load L1;losses or human damages. 2) To regulate the load voltage ul2 against sag/swell, interruption, and disturbances in theB. MC–UPQC Structure system to protect the sensitive/ critical load L2; 3) To compensate for the reactive and harmonic components of nonlinear load current (il1). In order to achieve these goals, series VSCs (i.e., VSC1 and VSC3) operate as voltage controllers while the shunt VSC (i.e., VSC2) operates as a current controller. C. Control Strategy As shown in Fig.4, the MC-UPQC consists of two series VSCs and one shunt VSC which are controlled independently. The switching control strategy for series VSCs and the shunt VSC are selected to be sinusoidal pulse width-modulationFig.2 Typical MC-UPQC used in a distribution (SPWM) voltage control and hysteresis currentsystem control, respectively. Details of the control algorithm, which are based on the d–q method, will be discussedThe internal structure of the MC–UPQC is shown in later. Shunt-VSC: Functions of the shunt-VSC are:Fig.2. 1) To compensate for the reactive component of It consists of three VSCs (VSC1, VSC2, and load L1 current;VSC3) which are connected back to back through a 2) To compensate for the harmonic components ofcommon dc-link capacitor. In the proposed load L1 current;configuration, VSC1 is connected in series with BUS1 3) To regulate the voltage of the common dc-linkand VSC2 is connected in parallel with load L1 at the capacitor. 779 | P a g e
  3. 3. V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.778-783 load, can also affect the dc link. In order to regulateFig. shows the control block diagram for the shunt the dc-link capacitor voltage, a proportional–integralVSC. The measured load current ( ) is (PI) controller is used as shown in Fig. The input of the PI controller is the error between the actualtransformed into the synchronous dq0 referenceframe by using capacitor voltage and its reference value . The output of the PI controller is added to … (1) the d component of the shunt-VSC reference current to form a new reference current as follows: … (9).... (2) Where the transformation matrix is shown ineq (2), at the bottom of the page. By this transform,the fundamental positive-sequence component, whichis transformed into dc quantities in the d and q axes,can be easily extracted by low-pass filters (LPFs).Also, all harmonic components are transformed intoac quantities with a fundamental frequency shift. As shown in Fig.4, the reference current in … eq(9) is then transformed back into the abc reference (3) frame. By using PWM hysteresis current control, the output-compensating currents in each phase are … (4) obtained.Where, , are d-q components of load current, , are dc components, and , are the accomponents of , .If is the feeder current and is the shunt VSCcurrent and knowing , then d–qcomponents of the shunt VSC reference current are Fig 5 Control block diagram of series VSCdefined as follows: … (5) … (10) … (6) Series-VSC: Functions of the series VSCs in eachConsequently, the d–q components of the feeder feeder are:current are 1) To mitigate voltage sag and swell; … (7) 2) To compensate for voltage distortions, such as harmonics; … (8) 3) To compensate for interruptions (in Feeder2 only). This means that there are no harmonic andreactive components in the feeder current. Switching The control block diagram of each serieslosses cause the dc-link capacitor voltage to decrease.Other disturbances, such as the sudden variation of VSC is shown in Fig. The bus voltage is 780 | P a g e
  4. 4. V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.778-783detected and then transformed into the synchronous power is consumed by the series VSC at steady state.dq0 reference frame using This is a significant advantage when UPQC mitigates sag conditions. The series VSC also shares the volt … (11) ampere reactive (VAR) of the load along with the shunt-VSC, reducing the power rating of the shunt-Where, VSC. Fig. shows the phasor diagram of this scheme under a typical load power factor condition with and without a voltage sag.… (12) When the bus voltage is at the desired value , the series-injected voltage According to control objectives of the MC- is zero Fig.(a).UPQC, the load voltage should be kept sinusoidalwith constant amplitude even if the bus voltage isdisturbed. Therefore, the expected load voltage in thesynchronous dq0 reference frame only hasone value.… (13)Where the load voltage in the abc reference frame is Fig 6 Phasor diagram of quadrature compensation … (14) a) Without voltage sag (b) With voltage sag. The shunt VSC injects the reactive component ofThe compensating reference voltage in the load current , resulting in unity input-power factor.synchronous dq0 reference frame is defined Furthermore, the shunt VSC compensates for not only the reactive component, but also the harmonicas components of the load current .The phasor … (15) diagram of Fig.6. explains the operation of this scheme in case of voltage sag. The compensating reference voltage is thentransformed back into the abc reference frame. Byusing an improved SPWM voltage control techniquethe output compensation voltage of the series VSCcan be obtained.III POWER-RATING ANALYSIS OFTHE MC-UPQC The power rating of the MC-UPQC is an Fig.7. Phasor diagram of inphase compensationimportant factor in terms of cost. Before calculation (supply voltage sag).of the power rating of each VSC in the MC UPQCstructure, two models of a UPQC are analyzed and A comparison between in phase (UPQC-P)the best model which requires the minimum power and quadrature (UPQC-Q) models is made forrating is considered. All voltage and current phasors different sag conditions and load power factors. It isused in this section are phase quantities at the shown that the power rating of the shunt-VSC in thefundamental frequency. There are two models for a UPQC-Q model is lower than that of the UPQC-P,UPQC - quadrature compensation (UPQC-Q) and and the power rating of the series-VSC in the UPQC-inphase compensation (UPQC-P). In the quadrature P model is lower than that of the UPQC-Q for acompensation scheme, the injected voltage by the power factor of less than or equal to 0.9. Also, it isseries- VSC maintains a quadrature advance shown that the total power rating of UPQC-Q isrelationship with the supply current so that no real 781 | P a g e
  5. 5. V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.778-783lower than that of UPQC-P where the VAR demandof the load is high. ...(20)As discussed in Section II, the power needed for Where is calculated. This part of the shunt VSCinterruption compensation in Feeder 2 must be current only exchanges reactive power (Q) with thesupplied through the shunt VSC in Feeder1 and the system.series VSC in Feeder 2. This implies that powerratings of these VSCs are greater than that of the 2) The second part provides the real power (P), whichseries one in Feeder1. If quadrature compensation in is needed for a sag or interruption compensation inFeeder1 and in phase compensation in Feeder 2 is Feeder2. Therefore, the power rating of the shuntselected, then the power rating of the shunt VSC and VSC can be calculated asthe series VSC (in Feeder 2) will be reduced. This isan important criterion for practical applications. As shown in Figs.6 and 7, load voltages in .both feeders are kept constant at regardless of bus =voltages variation, and the load currents in bothfeeders are assumed to be constant at their ratedvalues (i.e., and respectively) = 3 … (16) ...(21) Where is calculated. Finally, the power … (17) rating of the series-VSC in Feeder2 can be calculated. For the worst-case scenario (i.e., interruption The load power factors in Feeder 1 and compensation), one must considerFeeder2 are assumed to be and and . Therefore,the per-unit sags, which must be compensated inFeeder1 and Feeder2, are supposed to be x1 and x2, ... (22)respectively. IV. MATLAB DESIGN OF MC-UPQC il _ dqo  T abc iL _ abc dqo STUDY AND RESULTS If the MC-UPQC is lossless, the activepower demand supplied by Feeder 1 consists of twoparts:1) The active power demand of load in Feeder 1.2) The active power demand for sag and interruption compensation in Feeder 2. Thus, Feeder1 current can be found asFrom Fig., the voltage injected by the series VSC inFeeder1 and thus the power rating of this converter BUS1 voltage, series compensating voltage, and load( ) can be calculated as voltage in Feeder1....(18)The shunt VSC current is divided into two parts:1) The first part (i.e., ) compensates for the reactive component (and harmonic components) of Feeder1 current and can be calculated from Fig. 4.6. as BUS2 voltage, series compensating voltage, and load...(19) voltage in Feeder2. 782 | P a g e
  6. 6. V. Surya Prakash, K. Kalyan Kumar/ International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.778-783 VI. CONCLUSION In this paper, a new configuration for simultaneous compensation of voltage and current in adjacent feeders has been proposed. The new configuration is named multi-converter unified power-quality conditioner (MC-UPQC). Compared to a conventional UPQC, the proposed topology is capable of fully protecting critical and sensitive loads against distortions, sags/swell, and interruption in two-feeder systems. The idea can be theoretically extended to multibus / multifeeder systems by addingNonlinear load current, compensating current, more series VSCs. The performance of the MC-Feeder1 current, and capacitor voltage. UPQC is evaluated under various disturbance conditions and it is shown that the proposed MC- UPQC offers the following advantages: 1) Power transfer between two adjacent feeders for sag/swell and interruption compensation; 2) Compensation for interruptions without the need for a battery storage system. 3) Sharing power compensation capabilities between two adjacent feeders which are not connected. REFERENCES [1] D. D. Sabin and A. Sundaram, ―QualitySimulation results for an upstream fault on Feeder 2: enhances reliability,‖ IEEE Spectr., vol. 33,BUS2 voltage, compensating voltage, and loads L1 no. 2, pp. 34–41, Feb. 1996.and L2 voltages. [2] M. Rastogi, R. Naik, and N. Mohan, ―A comparative evaluation of harmonic reduction techniques in three-phase utility interface of power electronic loads,‖ IEEE Trans. Ind. Appl., vol. 30, no. 5, pp. 1149– 1155, Sep./Oct. 1994. [3] F. Z. Peng, ―Application issues of active power filters,‖ IEEE Ind. Appl. Mag., vol. 4, no. 5, pp. 21–30, Sep../Oct. 1998. [4] H. Akagi, ―New trends in active filters for power conditioning,‖ IEEE Trans. Ind. Appl., vol. 32, no. 6, pp. 1312–1322, Nov./Dec. 1996. [5] L. Gyugyi, C. D. Schauder, S. L. Williams, T. R. Rietman, D. R. Torjerson, and A.Simulation results for load change: nonlinear load Edris, ―The unified power flow controller: Acurrent, Feeder1 current, load L1 voltage, load L2 new approach to power transmissionvoltage, and dc-link capacitor voltage. control,‖ IEEE Trans. Power Del., vol. 10, no. 2, pp. 1085–1097, Apr. 1995.BUS1 voltage, series compensating voltage, and loadvoltage in Feeder1 under unbalanced source voltage. 783 | P a g e

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