A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJER...
A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJER...
A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJER...
A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJER...
A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJER...
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International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.

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Dw33741745

  1. 1. A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.741-745741 | P a g eGrid Interconnection of Renewable Energy Sources with Power-Quality Improvement Features at the Distribution Level1A.Rajashekar reddy, 2S.Swathi, 3P. sadanandam , 4Md.Yasin1P.G. Scholar, 3Associate Professor, 2,,4,Assistant Professor, Vaagdevi College of Engineering,Warangal, Andhrapradesh, India,AbstractThere is growing interest in renewableenergy around the world. Since most renewablesources are intermittent in nature, it is achallenging task to integrate renewable energyresources into the power grid infrastructure. Inthis grid integration, communication systems arecrucial technologies, which enable theaccommodation of distributed renewable energygeneration and play an extremely important rolein monitoring, operating, and protecting bothrenewable energy generators and powersystems. This paper presents a novel controlstrategy for achieving maximum benefits fromthese grid-interfacing inverters when installed in3-phase 4-wire distribution systems. Theinverter is controlled to perform as a multi-function device by incorporating active powerfilter functionality. The inverter can thus beutilized as: A) power converter to inject powergenerated from RES to the grid, and B) shuntAPF to compensate current unbalance, loadcurrent harmonics, load reactive power demandand load neutral current. All of these functionsmay be accomplished either individually orsimultaneously. With such a control, thecombination of grid-interfacing inverter and the3-phase 4-wire linear/non-linear unbalancedload at point of common coupling appears asbalanced linear load to the grid. This newcontrol concept is demonstrated with extensiveMATLAB/Simulink simulation studies andvalidated through digital signal processor-basedlaboratory experimental results.Index Terms— power quality (PQ), distributionsystem, grid interconnection, Active power filter(APF), distributed generation (DG), renewableenergy.I. INTRODUCTIONAccess to quality, reliable and affordableenergy is critical for promoting economic andsocial development in rural areas. Due to increasedstandard of living, growing population, rapidindustrialization etc., the energy demand hasincreased and hence the gap between generationand demand are increasing considerably.Distributed Generation (DG) is the powergeneration from locally available sources; generallyrenewable energy sources.DG is on the rise since distributed energy systemswith renewable sources have great potential inproviding reliable power to the rural areas wheregrid extension is difficult and uneconomical. Theincreasing demand for electrical energy and thefocus on environmental protection motivated theefforts to concentrate on developing renewableenergy sources. United Nations is planning 50% oftotal energy from renewable sources by 2050,Europe 20% by 2020 and India 10% by 2012 [1].Renewable energy source (RES)integrated at distribution level is termed asdistributed generation (DG). The utility isconcerned due to the high penetration level ofintermittent RES in distribution systems as it maypose a threat to network in terms of stability,voltage regulation and power-quality (PQ) issues.Therefore, the DG systems are required to complywith strict technical and regulatory frameworks toensure safe, reliable and efficient operation ofoverall network. With the advancement in powerelectronics and digital control technology, the DGsystems can now be actively controlled to enhancethe system operation with improved PQ at PCC.However, the extensive use of power electronicsbased equipment and non-linear loads at PCCgenerate harmonic currents, which may deterioratethe quality of power [1], [2].The non-linear load current harmonicsmay result in voltage harmonics and can create aserious PQ problem in the power system network.Active power filters (APF) are extensively used tocompensate the load current harmonics and loadunbalance at distribution level. This results in anadditional hardware cost. However, in this paperauthors have incorporated the features of APF inthe, conventional inverter interfacing renewablewith the grid, without any additional hardware cost.Here, the main idea is the maximum utilization ofinverter rating which is most of the timeunderutilized due to intermittent nature of RES. Itis shown in this paper that the grid-interfacinginverter can effectively be utilized to performfollowing important functions: a) transfer of activepower harvested from the renewable resources(wind, solar, etc.); b) load reactive power demandsupport; c) current harmonics compensation at
  2. 2. A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.741-745742 | P a g ePCC; and d) current unbalance and neutral currentcompensation in case of 3-phase 4-wire system.Moreover, with adequate control of grid-interfacinginverter, all the four objectives can beaccomplished either individually or simultaneously.The PQ constraints at the PCC can therefore bestrictly maintained within the utility standardswithout additional hardware cost.The paper is arranged as follows: Section IIdescribes the system under consideration and thecontroller for grid-interfacing inverter. A digitalsimulation study is presented in Section III.Extensive experimental results are discussed inSection IV and, finally, Section V concludes thepaper.II. SYSTEM DESCRIPTIONThe proposed system consists of RESconnected to the dc-link of a grid-interfacinginverter as shown in Fig. 1. The voltage sourceinverter is a key element of a DG system as itinterfaces the renewable energy source to the gridand delivers the generated power. The RES may bea DC source or an AC source with rectifier coupledto dc-link. Usually, the fuel cell and photovoltaicenergy sources generate power at variable low dcvoltage, while the variable speed wind turbinesgenerate power at variable ac voltage. Thus, thepower generated from these renewable sourcesneeds power conditioning (i.e., dc/dc or ac/dc)before connecting on dc-link [6]–[8]. The dc-capacitor decouples the RES from grid and alsoallows independent control of converters on eitherside of dc-link.A. DC-Link Voltage and Power ControlOperation:Due to the intermittent nature of RES, thegenerated power is of variable nature. The dc-linkplays an important role in transferring this variablepower from renewable energy source to the grid.RES are represented as current sources connectedto the dc-link of a grid-interfacing inverter. Fig. 2shows the systematic representation of powertransfer from the renewable energy resources to thegrid via the dc-link. The current injected byrenewable into dc-link at voltage level Vdc can begiven asWhere PRES is the power generated from RESFig. 2. DC-Link equivalent diagramThe current flow on the other side of dc-link can berepresented as,where Pinv, PG and PLoss are total poweravailable at grid-interfacing inverter side, activepower supplied to the grid and inverter losses,respectively. If inverter losses are negligible thenPRES = PGThe control diagramof grid- interfacinginverter for a 3-phase 4-wire system is shown inFig. 3. The fourth leg of inverter is used tocompensate the neutral current of load. The mainaim of proposed approach is to regulate the powerat PCC during:1) PRES = 0; 2) PRES < total load power PL ; and 3)PRES > PLWhile performing the power managementoperation, the inverter is actively controlled in sucha way that it always draws/ supplies fundamentalactive power from/ to the grid. If the loadconnected to the PCC is non-linear or unbalancedor the combination of both, the given control
  3. 3. A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.741-745743 | P a g eapproach also compensates the harmonics,unbalance, and neutral current.The duty ratio of inverter switches arevaried in a power cycle such that the combinationof load and inverter injected power appears asbalanced resistive load to the grid. The regulationof dc-link voltage carries the information regardingthe exchange of active power in between renewablesource and grid. Thus the output of dc-link voltageregulator results in an active current(Im).Fig.3 Block diagram representation of grid-interfacing inverter control.The multiplication of active currentcomponent (Im) with unity grid voltage vectortemplates (Ua, Ub and Uc) generates the referencegrid currents (Ia*, Ib* and Ic*). The reference gridneutral current (In*) is set to zero, being theinstantaneous sum of balanced grid currents. Thegrid synchronizing angle (θ) obtained from phaselocked loop (PLL) is used to generate unity vectortemplate as [9] – [11].The actual dc-link voltage (Vdc) is sensedand passed through a first order low pass filter(LPF) to eliminate the presence of switching rippleson the dc-link voltage and in the generatedreference current signals. The difference of thisfiltered dc-link voltage and reference dc-linkvoltage (Vdc*) is given to a discrete-PI regulator tomaintain a constant dc-link voltage under varyinggeneration and load conditions. The dc-link voltageerror Vdcerr(n) at nthsampling instant is given as:The output of discrete-PI regulator at nthsamplinginstant is expressed asWhere KPVdc = 10 and KIVdc = 0.05 are proportionaland integral gains of dc-voltage regulator. Theinstantaneous values of reference three phase gridcurrents are computed asThe neutral current, present if any, due to the loadsconnected to the neutral conductor should becompensated by forth leg of grid-interfacinginverter and thus should not be drawn from thegrid. In other words, the reference current for thegrid neutral current is considered as zero and can beexpressed asThe reference grid currents (Ia*, Ib*, Ic* and In*)are compared with actual grid currents (Ia, Ib, Icand In) to compute the current errors asThese current errors are given to hysteresis currentcontroller. The hysteresis controller then generatesthe switching pulses (P1 to P8 ) for the gate drivesof grid-interfacing inverter.The average model of 4-leg inverter can beobtained by the following state space equationsWhere VInva, VInvb, VInvc and VInvn are the three-phase ac switching voltages generated on the outputterminal of inverter. These inverter output voltagescan be modeled in terms of instantaneous dc busvoltage and switching pulses of the inverter asSimilarly the charging currents IInva, IInvb, IInvc andIInvn on dc bus due to the each leg of inverter can beexpressed as
  4. 4. A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.741-745744 | P a g eThe switching pattern of each IGBT inside invertercan be formulated on the basis of error betweenactual and reference current of inverter, which canbe explained as:If IInva < (I*Inva - hb) then upper switch S1 will beOFF (P1 = 0) and lower switch S2 will be ON (P4 =1) n the phase “a” leg of inverter.If IInva > (I*Inva - hb) then upper switch S1 will be ON(P1 = 1) and lower switch S2 will be OFF (P4 = 0) nthe phase “a” leg of inverter.where hb is the width of hysteresis band. On thesame principle, the switching pulses for the otherremaining three legs can be derived.III.SIMULATION RESULTSIn order to verify the proposed control approach toachieve multi-objectives for grid interfaced DGsystems connected to a 3-phase 4-wire network, anextensive simulation study is carried out usingMATLAB/Simulink.Fig. 4. Simulation results: (a) Grid voltages, (b)Grid Currents (c) Unbalanced load currents, (d)Inverter Currents.A 4-leg current controlled voltage source inverter isactively controlled to achieve balanced sinusoidalgrid currents at unity power factor (UPF) despite ofhighly unbalanced nonlinear load at PCC undervarying renewable generating conditions. A RESwith variable output power is connected on the dc-link of grid-interfacing inverter. An unbalanced 3-phase 4-wire nonlinear load, whose unbalance,harmonics, and reactive power need to becompensated, is connected on PCC. The waveformsof grid voltage (Va, Vb and Vc) grid currents (Ia, Ib,Ic, In), unbalanced load currents (Ila, Ilb, Ilc, Iln), andinverter currents (IInva, IInvb, IInvc, IInvn), are shown inFig.4. The corresponding active-reactive powers ofgrid (Pgrid, Qgrid) load (Pload, Qload) and inverter (Pinv,Qinv) are shown in Fig. 5. Positive values of gridactive-reactive powers and inverter active-reactivepowers imply that these powers flow from grid sidetowards PCC and from inverter towards PCC,respectively.The active and reactive powers absorbedby the load are denoted by positive signs.Initially, the grid-interfacing inverter is notconnected to the network (i.e., the load powerdemand is totally supplied by the grid alone).Therefore, before time t = 0.72 s, he grid currentprofile in Fig. 4(b) is identical to the load currentprofile of Fig. 4(c). At t = 0.72 s, the grid-interfacing inverter is connected to the network. Atthis instant the inverter starts injecting the currentin such a way that the profile of grid current startschanging from unbalanced non linear to balancedsinusoidal current as shown in Fig. 4(b). As theinverter also supplies the load neutral currentdemand, the grid neutral current (In) becomes zeroafter t = 0.72 s.Fig. 5. Simulation results: (a) PQ-Grid, (b) PQ-Load, (c) PQ-Inverter, (d) dc-link voltage.At s, the inverter starts injecting active powergenerated from RES(PRES ≈ Pinv) . Since thegenerated power is more than the load powerdemand the additional power is fedback to the grid.The negative sign of Pgrid, after time 0.72 s suggeststhat the grid is now receiving power from RES.Moreover, the grid-interfacing inverter alsosupplies the load reactive power demand locally.Thus, once the inverter is in operation the grid onlysupplies/receives fundamental active power.At t = 0.82 s, the active power from RES isincreased to evaluate the performance of systemunder variable power generation from RES. Thisresults in increased magnitude of inverter current.As the load power demand is considered asconstant, this additional power generated from RES
  5. 5. A.Rajashekar reddy, S.Swathi, S.Sadanandam, Md.Yasin / International Journal ofEngineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.741-745745 | P a g eflows towards grid, which can be noticed from theincreased magnitude of grid current as indicated byits profile. At t = 0.92 s the power available fromRES is reduced. The corresponding change in theinverter and grid currents can be seen from Fig. 4.The active and reactive power flows between theinverter, load and grid during increase and decreaseof energy generation from RES can be noticed fromFig. 5. The dc-link voltage across the grid-interfacing inverter (Fig. 5(d)) during differentoperating condition is maintained at constant levelin order to facilitate the active and reactive powerflow. Thus from the simulation results, it is evidentthat the grid-interfacing inverter can be effectivelyused to compensate the load reactive power, currentunbalance and current harmonics in addition toactive power injection from RES. This enables thegrid to supply/ receive sinusoidal and balancedpower at UPF.IV.CONCLUSIONThis paper has presented a novel control ofan existing grid-interfacing inverter to improve thequality of power at PCC for a 3-phase 4-wireDGsystem. It has been shown that the grid-inter-facing inverter can be effectively utilized forpower conditioning without affecting its normaloperation of real power transfer. The grid-interfacing inverter with the proposed approach canbe utilized to:i) inject real power generated from RES to the grid,and/or,ii) operate as a shunt Active Power Filter (APF).This approach thus eliminates the need foradditional power conditioning equipment toimprove the quality of power at PCC. ExtensiveMATLAB/Simulink simulation. When the powergenerated from RES is more than the total loadpower demand, the grid-interfacing inverter withthe proposed control approach not only fulfills thetotal load active and reactive power demand (withharmonic compensation) but also delivers theexcess generated sinusoidal active power to the gridat unity power factor.REFERENCES[1] J. M. Guerrero, L. G. de Vicuna, J. Matas,M. Castilla, and J. Miret, “A wirelesscontroller to enhance dynamicperformance of parallel inverters indistributed generation systems,” IEEETrans. Power Electron., vol. 19, no. 5, pp.1205–1213, Sep. 2004.[2] J. H. R. Enslin and P. J. M. Heskes,“Harmonic interaction between a largenumber of distributed power inverters andthe distribution network,” IEEE Trans.Power Electron., vol. 19, no. 6, pp. 1586–1593, Nov. 2004.[3] U. Borup, F. Blaabjerg, and P. N. Enjeti,“Sharing of nonlinear load in parallel-connected three-phase converters,” IEEETrans. Ind. Appl., vol. 37, no. 6, pp. 1817–1823, Nov./Dec. 2001.[4] P. Jintakosonwit, H. Fujita, H. Akagi, andS. Ogasawara, “Implementation andperformance of cooperative control ofshunt active filters for harmonic dampingthroughout a power distribution system,”IEEE Trans. Ind. Appl., vol. 39, no. 2, pp.556–564, Mar./Apr. 2003.[5] J. P. Pinto, R. Pregitzer, L. F. C. Monteiro,and J. L. Afonso, “3-phase 4-wire shuntactive power filter with renewable energyinterface,” presented at the Conf. IEEERnewable Energy & Power Quality,Seville, Spain, 2007.[6] F. Blaabjerg, R. Teodorescu,M. Liserre,and A. V. Timbus, “Overview of controland grid synchronization for distributedpower generation systems,” IEEE Trans.Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.[7] J. M. Carrasco, L. G. Franquelo, J. T.Bialasiewicz, E. Galván, R. C. P. Guisado,M. Á. M. Prats, J. I. León, and N. M.Alfonso, “Powerelectronic systems for thegrid integration of renewable energysources: A survey,” IEEE Trans. Ind.Electron., vol. 53, no. 4, pp. 1002–1016,Aug. 2006.[8] B. Renders, K. De Gusseme, W. R.Ryckaert, K. Stockman, L. Vandevelde,and M. H. J. Bollen, “Distributedgeneration for mitigating voltage dips inlow-voltage distribution grids,” IEEETrans. Power. Del., vol. 23, no. 3, pp.1581–1588, Jul. 2008.[9] V. Khadkikar, A. Chandra, A. O. Barry,and T. D. Nguyen, “Application of UPQCto protect a sensitive load on a polluteddistribution network,” in Proc. Annu.Conf. IEEE Power Eng. Soc. Gen.Meeting, 2006, pp. 867–872.[10] M. Singh and A. Chandra, “Powermaximization and voltage sag/swell ride-through capability of PMSG basedvariable speed wind energy conversionsystem,” in Proc. IEEE 34th Annu. Conf.Indus. Electron. Soc., 2008, pp. 2206–2211.[11] P. Rodríguez, J. Pou, J. Bergas, J. I.Candela, R. P. Burgos, and D. Boroyevich,“Decoupled double synchronous referenceframe PLL for power converters control,”IEEE Trans. Power Electron, vol. 22, no.2, pp. 584–592, Mar. 2007.

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