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Three phase grid connected inverter using current
 

Three phase grid connected inverter using current

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    Three phase grid connected inverter using current Three phase grid connected inverter using current Document Transcript

    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME293THREE-PHASE GRID-CONNECTED INVERTER USING CURRENTREGULATORTran Quang Tho, Truong Viet AnhFaculty of Electrical & Electronic Engineering,HCM City University of Technical EducationABSTRACTThis paper presents an approach for a three-phase grid-connected inverter usingcurrent regulator. The switching frequency of hystereris in the current modulation is fixed bycomparing the current error with carrier wave with the constant frequency of the multiple of3. The LCL filter is installed at the inverter output to offer high harmonic attenuation. Inorder to determine simply the parameters of PI regulators, the methods of PSO, GA and theconventional Ziegler-Nichols are used to search the best values with high global stability. Thesimulation results in Simulink/Matlab show that the PI regulators designed by PSO methoddemonstrate better results than Ziegler-Nichols and even GA technique.Keywords: gen algorithm (GA), particle swarm optimization (PSO)I. INTRODUCTIONThe demand of renewable energy sources such as solar energy is becoming morepopular for sustainability and environment with enormous potentials [1]. In order to convertsolar DC source to three-phase AC power needs to have 3-phase inverters that have been wellresearched in recent years [2].The current modulation plays an important role in power electronic systems,especially in voltage source inverters [3]. The advantages of current regulator are verysimple, fast response, high robust and overload protection. In addition, it also keeps powerfactor unity and does not depend on voltage drop of switches [15]. However, the hysteresisPWM has unfixed switching frequency that increases loss of switches and current THD [16].The elimination of common mode voltage in VSIs aims to reduce THD by usingcompensation circuitry [4], harmonic filters [5], [6], [7] and carrier wave phase shift [8] isvery complicated. In order to meet grid-connected standard IEEE Std 929-2000 [9] withINTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING& TECHNOLOGY (IJEET)ISSN 0976 – 6545(Print)ISSN 0976 – 6553(Online)Volume 4, Issue 2, March – April (2013), pp. 293-304© IAEME: www.iaeme.com/ijeet.aspJournal Impact Factor (2013): 5.5028 (Calculated by GISI)www.jifactor.comIJEET© I A E M E
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME294harmonic attenuation [10], LCL filter is installed at inverter output. The simulation resultsdiscussed show that the PI regulators designed by PSO demonstrate better results thanZiegler-Nichols and even GA method.II. MODEL OF GRID-CONNECTED THREE PHASE INVERTER AND CONTROLSTRATEGYThe principle diagram of grid-connected three phase system is shown in Fig 1.Fig 1: Simplified model of the grid-connected inverter with L filterII.1. Current regulationThe three phase AC quantities Ia, Ib and Ic in the stationary frame are transformed intothe DC components Id and Iq in the synchronously rotating frame by the phase angle Θ ofPLL. With L filter in grid-connected VSI as Fig 1, voltage equation of phase A in thestationary frame is:)1(ViRVdtdiL gaagiaa−−=And phases B and C are similar. When neglecting resistor Rg, equation (1) became:)2(VVdtdiL gaiaa−=The equation (2) shows that phase current can be regulated by amplitude and phase angleof Vi at inverter output with constant Vg as Fig 2.Fig 2: Relationship between Vi and Ig in dq frame
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME295Active and reactive powers in dq frame are calculated as (3) and (4).( )( ) )4(IVIV23Q)3(IVIV23Pgqgdgdgqgqgqgdgd−=+=With reference P_ref and Q_ref, currents Idp and Iq can determine as:( ) )5(VVPQQPVV32IIgqgdref_ref_ref_ref_2gq2gdqdp−+=The current Idp depends on DC source power status of solar. So:)6(PP dcref =For optimization of generation, only active power is to be injected in the grid andreference Iq is zero. Pdc and Idp can be determined by MPPT technique. To obtain the closedloop response, Id and Iq are taken from the outputs of the inner loop PI regulator, as (7).Where Id and Iq are the reference currents. Kp and Ki are the proportional and integral gainconstants respectively. These gain constants are determined by tuning the regulators foroptimal response with methods of Ziegler-Nichols, GA and PSO.)7(IIIIsKK00sKKIIqgref_qdgref_diq_iiq_pid_iid_p*q*d−−++=The LCL filter of the inverter output is proposed as Fig 3.Fig 3: The proposed diagram of three phase inverter with LCL filter
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME296II.2. DC link voltage controlThe DC voltage is held at a constant value by using a PI regulator which provides thereal current reference as (8). Where V*dc is the DC voltage of MPPT.[ ] )8(VVsKKII dc*dcdc_idc_pdpref_d −+−=II.3. PWM modulationCurrent error: )9(III gref_gerror_ −=Then:( ) )10(VVdtIdL iref_ierror_−=Current errors are compared with carrier wave of fixed frequency and amplitude. Ifthe current error is positive and larger than the carrier wave, the switches are activated toapply +Vdc. On the other hand, if current error is positive and smaller than the carrier wave,the switches are activated to apply –Vdc as Fig 4.Fig 4: PWM modulationII.4. Tuning parameters of PI regulator:With LCL filter, parameters of PI regulators effect significantly on THD of inverteroutput current [11].The conventional tuning methods of PI regulator such as Ziegler-Nichols rules andGA have been applied to tune the controller recently. Randomly searching technique such asGA that has high efficient computational and global searching capabilities has been appliedsuccessfully to optimize the complex problems. But the premature convergence of GAdegrades its performance and reduces its searching capabilities. The PSO algorithm isproposed in this paper to tune PI regulator.The Ziegler-Nichols method:Fig 5: Single phase equivalent circuit of LCL filter
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME297In Fig 5, when assuming no harmonic at PCC, then Vg=0. For a balanced system thetransfer function of the LCL filter for every phase is given by (11).;RRC;RRCLLC;RLCRLCC;LLCC:where)11(CsCsCsC1sRCsLC)s(V)s(Iig4igfig3gifigf2igf1432231gf2gfii+=++=+==+++++=and Ri and Rg are resistors of inductances Li and Lg respectively.System parameters: Vdc=650V; Ldc=3mH; Rdc=1Ω; Cdc=500µF; grid voltage =380V;50Hz; short-circuit power=40KVA; Lg=1mH; Rg=0.1Ω; Cf=5µF; Li=2mH; Ri=0.2Ω; carrierwave frequency fc=9KHz.Kgh=30 and Tgh=3.888 are determined by Ziegler-Nichols method in (11).In the GA method with flowchart in Fig 6aFig 6a: GA flowchart Fig 6b: PSO flowchartIn the PSO method, velocity and position are updated by equations (12) and (13) in flowchartin Fig 6b.)13(V.PP)12()PP(R.)PP(R.)t(V.wVcurcurcurcurglobes2curlobes1curγ+=−β+−α+=Results of tuned parameters are shown in table 1Method Kp_Id Ki_Id Kp_Iq Ki_IqZiegler-Nichols13.5 4.182 13.5 4.182GA 5.6208 200.046 3.4441 1.0156PSO 4.3523 179.534 2.442 4.0112Table 1: parameters of PI regulators
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME298III. SIMULATION RESULTS AND DISCUSSIONFig 7: Simulation model in Simulink/Matlab.The current Id increases from 5A up to 10A at 3.803s.III.1. Results with Ziegler-Nichols:Fig 8a: Three-phase voltage (V)Terminator3PI_IdqPI_idqPI_VdcPI_Vdc0Iq=0Idp[gates]Goto[Vdc_ref]From6[Vdc]From5[Iabc]From4[Iabc]From3wtFrom2wtFrom1abcwtdq0abc_dq0EmbeddedMATLAB Function1dqwtabcdq0_abcEmbeddedMATLAB Function9KHzI*_abcCarrierI_abcgates6 xungwtwt650VsolABCThree-Phase SourceVabcABCabcThree-PhaseV-I MeasurementIabcABCabcThree-PhaseI MeasurementabcABCRi_LiabcABCRg_LgGatesVsoVDCABCInverter[Iabc]IVdcGoto3[gates]FromabcABCCVabc (pu)wt3-phase PLL3.75 3.76 3.77 3.78 3.79 3.8 3.81 3.82 3.83 3.84 3.85x 105-400-300-200-10001002003004003-phase voltage (V)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME299Fig 8b: Three-phase current (A)Fig 8c: Active power P (w) and reactive power Q (var)Fig 8d: THD spectrum of current3.75 3.76 3.77 3.78 3.79 3.8 3.81 3.82 3.83 3.84 3.85x 105-10-505103-phase current (A)0 1 2 3 4 5 6 7x 105-200002000400060003.63 3.64 3.65 3.66 3.67 3.68 3.69 3.7 3.71 3.72-4-2024FFT window: 5 of 288.7 cycles of selected signalTime (s)0 200 400 600 800 100001234Frequency (Hz)Fundamental (50Hz) = 4.521 , THD= 5.20%Mag(%ofFundamental)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME300III.2. RESULTS WITH GA:Fig 9a: Three-phase voltage (V)Fig 9b: Three-phase current (A)Fig 9c: Active power P (w) and reactive power Q (var)3.75 3.76 3.77 3.78 3.79 3.8 3.81 3.82 3.83 3.84 3.85x 105-400-300-200-10001002003004003-phase voltage (V)3.75 3.76 3.77 3.78 3.79 3.8 3.81 3.82 3.83 3.84 3.85x 105-10-505103-phase current (A)0 1 2 3 4 5 6 7x 105-20000200040006000P (w) & Q (var)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME301Fig 9d: THD spectrum of currentIII.3. RESULTS WITH PSO:Fig 10a: Three-phase voltage (V)Fig 10b: Three-phase current (A)3.61 3.62 3.63 3.64 3.65 3.66 3.67 3.68 3.69 3.7-505FFT window: 5 of 305.3 cycles of selected signalTime (s)0 200 400 600 800 100000.511.522.5Frequency (Hz)Fundamental (50Hz) = 5.007 , THD= 3.41%Mag(%ofFundamental)3.75 3.76 3.77 3.78 3.79 3.8 3.81 3.82 3.83 3.84 3.85x 105-400-300-200-10001002003004003-phase voltage (A)3.75 3.76 3.77 3.78 3.79 3.8 3.81 3.82 3.83 3.84 3.85x 105-10-505103-phase Current (A)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME302Fig 10c: Active power P (w) and reactive power Q (var)Fig 10d: THD spectrum of currentthod % THD of output currentZiegler-Nichols 5.20GA 3.41PSO 2.93Table 2: THD of output current at PCCIV. DISCUSSIONParameters of PI regulators in GA and PSO methods always give Kp_Id ≠ Kp_Iq andKi_Id ≠ Ki_Iq.Power responses in figures 8c, 9c and 10c demonstrate that GA and PSO methodsgive results better than Ziegler-Nichols method.The output currents harmonics in figures 8d, 9d and 10d also show that PSO methodin the table 2 gives the best result current THD is 2.93%.0 1 2 3 4 5 6 7x 105-20000200040006000P (w) & Q (var)3.63 3.64 3.65 3.66 3.67 3.68 3.69 3.7 3.71 3.72-505FFT window: 5 of 264.4 cycles of selected signalTime (s)0 200 400 600 800 100000.511.52Frequency (Hz)Fundamental (50Hz) = 5.009 , THD= 2.93%Mag(%ofFundamental)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME303V. CONCLUSIONThis paper presents an approach for a three-phase grid-connected inverter usingcurrent regulator with low current THD by using LCL filter at inverter output and goodresponse.The PSO algorithm is proposed in this paper to tune parameters of PI regulator givesglobal results better than Ziegler-Nichols and even GA method.Control strategies proposed is a good alternative to implement an inverter systemcontrol with reduced harmonic content injected into the grid and less computational load thanother methods.REFERENCES[1] Stéphan Astier, “Systèmes solaires photovoltaïques”, Seminar on 15/02/2011 at TechnicalUniversity HCM city.[2] Amirnaser Yazdani and Prajna Paramita Dash, “A Control Methodology andCharacterization of Dynamics for a Photovoltaic (PV) System Interfaced With a DistributionNetwork”, IEEE Transactions on Power Delivery, Vol. 24, No. 3, July 2009.[3] José Rodríguez, Jorge Pontt, “Predictive Current Control of a Voltage Source Inverter”,IEEE Transactions on Industrial Electronics, Vol. 54, No. 1, February 2007.[4] K. H. Edelmoser, “Common Mode Problematic of Solar Inverter Systems”, Proceedingsof the 11th WSEAS International Conference on Circuits, 2007 .[5] Eftichios Koutroulis, Frede Blaabjerg, “Methods for the Optimal Design of Grid-Connected PV Inverters”, International Journal of Renewable Energy Research, IJRER-vol.1,No.2,pp.54[6] Hyosung Kim, Kyoung-Hwan Kim, “Filter design for grid connected PV inverters”,ICSET 2008.[7] H. R. Karshenas, and H. Saghafi, “Performance Investigation of LCL Filters in GridConnected Converters”, IEEE PES Transmission and Distribution Conference andExposition Latin America, Venezuela, 2006.[8] M. Brenna, R. Chiumeo and C. Gandolfi, “Harmonic analysis: comparison betweendifferent modulation strategies for three phase inverter connecting Distributed Generation”,Politecnico di Milano-Department of Energy, Italy-2011 .[9] “IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) systems”, IEEEStandard 929-2000, Jan. 2000.[10] M.A.A. Younis, N. A. Rahim, and S. Mekhilef, “Harmonic Reduction In Three-PhaseParallel Connected Inverter”, World Academy of Science, Engineering and Technology 502009 - 64,2011[11] Erika Twining, Donald Grahame Holmes, “Modelling grid-connected voltage sourceinverter operation”, Power Electronics Group-Department of Electrical and ComputerSystems Engineering Monash University, Clayton[12] Byeong-Mun Song, Youngroc Kim, Hanju Cha, Hakju Lee, “Current HarmonicMinimization of a Grid-Connected Photovoltaic 500kW Three-Phase Inverter using PRControl”, IEEE 2011[13] Miguel Castilla, Jaume Miret, Antonio Camacho, José Matas, and Luis García deVicuña, “Reduction of Current Harmonic Distortion in Three-phase Grid-connectedPhotovoltaic Inverters via Resonant Current Control”, IEEE 2011
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME304[14] X.Q. Guo W.Y. Wu, “Improved current regulation of three-phase grid-connectedvoltage-source inverters for distributed generation systems”, IET Renewable PowerGeneration, September 2009[15] Leonardo Augusto Serpa, “Current Control Strategies for Multilevel Grid ConnectedInverters”, Swiss Federal Institute of Technology Zurich, 2007[16] George Alin Raducu, “Control of Grid Side Inverter in a B2B Configuration for WTApplications”, Aalborg University, 2008[17] Satyaranjan Jena, B.Chitti Babu, S.R.Samantaray and Mohamayee Mohapatra,“Comparative Study between Adaptive Hysteresis and SVPWM Current Control for Grid-connected Inverter System”,[18] R. Arivoli Dr. I. A. Chidambaram “Multi-Objective Particle Swarm OptimizationBased Load-Frequency Control Of A Two-Area Power System With Smes Inter ConnectedUsing Ac-Dc Tie-Lines” International Journal of Electrical Engineering & Technology(IJEET), Volume 3, Issue 1, 2012, pp. 1- 20, ISSN Print : 0976-6545, ISSN Online: 0976-6553.[19] Mr. Laith O. Maheemed, Prof. D.S. Bankar, “Harmonic Mitigation For Non-LinearLoads Using Three-Phase Four Wire Upqc Control Strategy” International Journal ofElectrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 247- 260, ISSNPrint : 0976-6545, ISSN Online: 0976-6553.[20] Pradeep B Jyoti, J.Amarnath and D.Subbarayudu “The Scheme of Three-Level InvertersBased On Svpwm Overmodulation Technique for Vector Controlled Induction Motor Drives”International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 2,2013, and pp. 245- 260, ISSN Print: 0976-6545, ISSN Online: 0976-6553.