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    Cmp0200 ieee Cmp0200 ieee Document Transcript

    • 2012 First International Conference on Renewable Energies and Vehicular Technology PHOTOVOLTAIC-GRID INTEGRATED SYSTEM Sameer Khader, Abdel-Karim Daud Palestine Polytechnic University emails: Sameer@ppu.edu, daud@ppu.edu ABSTRACT at realizing the required voltage level during different periods of day for certain application type such as This paper proposed solution for directly energizing pumps, motors in general and power supplies.of ac load throughout Photovoltaic Solar Array during During the design process of PV array poweredthe daytime by applying so called variable voltage systems; a simulation must be performed for systemtracking system (VVT). The main function of VVT is analysis and parameter settings. Therefore an efficientto maintain the average output chopped voltage at fixed user friendly simulation model of the PV array withvalue irrespective of solar radiation rate, in turn the various control strategies is always needed.chopped voltage is converted into ac voltage suitable The proposed model consists of several modules asfor grid-connected loads. This solution is realized by shown in Figure1 with the following functions:integrating both complementary buck-boost chopper - PV Photovoltaic Module (PV) that converts theand dc to ac converter. The ac-grid contributes to the solar irradiation into voltage Vpv and current Ipv.load in two cases, first when there is a power shortageduring the daytime due to weak irradiation rates, and - Complementary Buck-Boost DC Chopper Modulesecond during the night time. The power estimator unit that boosts up the PV voltage to the predeterminedis used to determine the grid contribution intervals. levels. Conversely in case of high Vpv the output This solution excludes the use of battery bank voltage is reduced.which is the main obstacle in massive use of solar Variableenergy due to their weight, short life time, maintenance Voltageand cost. Matlab/Simulink is used to simulate the Vg_Q Trackingproposed model, where the obtained simulation resultsconfirm and justify the proposed approach for further Vout Vrefstudy and looking for optimized solutions for cost PV Complement- Array ary Buck-Boostreduction and energy savings. GridIndex Terms-- Photovoltaic Systems, DC Choppers, Vg_sl Selector InverterSmart Grids, Soft Switching, Inverters, Buck-BoostChopper. Vac AC Grid Adapting Grid System 1. INTRODUCTION Load Grid-Drop Iout Photovoltaic energy resources presents alternative Compensationand friendly to the environment sources. It presentsunique solution for providing remote area with clean Pac Power Statusand sustainable energy during the daytime in heating, Ppv Estimatorlighting, refrigeration and water pumps systems [1-3]without the need of battery system, while during the Figure 1. PV-Grid system block diagramnight time the accumulated energy can be fully orpartially used to cover the energy domain. The output circuit connected to the photovoltaic - Variable Voltage Tracking Module that generatessystem is usually dc-dc converters mainly boost switching pulses according to the required outputchoppers in order to boost the voltage to the voltage level in order to maintain Vout at fixed value.predetermined levels. - Grid Adapting Module that converts the ac grid The DC/DC converters are widely used in regulated voltage into dc voltage in case of grid connection.switch mode power supplies, where the input voltage to - Grid Drop Compensation Module that compensatesthese converters varies in wide range especially in the the voltage drop according to the drawn load currentcase of photovoltaic (PV) supply source due to and generates reference voltage.unpredictable and sudden change in the solar - Power-Status Estimator that detects the availableirradiation level as well as the cell operating Ppv power, the consumed load power and the value oftemperature. Several connection topologies concerning power shortage that should be supplied from AC-grid.the switching systems have been proposed [4-8] aiming978-1-4673-1170-0/12/$31.00 ©2012 IEEE 60
    • The displyed in fig.1 parameters Ppv, Vac, Pac, Iout boosting up the output voltage to predetermined valueare PV power, AC-grid voltage and power, and load it is necessary to illustrate the obtained PV voltage andcurrent respectively; Vgsl, Vref and Vgo are grid selector current for boost chopper according to specificationssignal, reference voltage and complementary buck- given in table 1 at reference irradiation 1000W/m2.boost driving signals. The remainder of the paper is organized as follows: Table 1: Data specification for PV Array.Section (2) Modelling & simulation of PV array; q K Iph Id RS RP TCSection (3) The behaviours of PV-Grid integrated 1.602e- 1.38e-system; Section (4) Discusses the simulation results 4A 0.2mA 1mΩ 10kΩ 25°C 19 C 23J/°Kand conclusion. NS NP VO VOC ISC VMPP IMPP 38 4 0.6V 21.5 V 4A 17.5V 3.7A 2. MODELING OF PV ARRAY NSm NPm Vpv out Rload 6 1 130V 44Ω2.1 Characteristics of PV Array The PV Array voltage can be obtained byBasically, PV cell is a P-N semiconductor junction that multiplying the module voltage and current by Nsm anddirectly converts light energy into electricity. It has the Npm that represents number of series and parallelequivalent circuit shown in Figure 2 [8-10]. connected modules respectively. Continuous powergui G_T 1 11 .2903 T + - v T T_var V2 Figure 2. Equivalent circuit for PV cell G_var2 G Where Iph represents the cell photocurrent; Rp and i Lo Output 6 Ns +Vpv + -Rs are the intrinsic shunt and series resistance of the Nsm Icell respectively; Id is the diode saturation current; Vo 1 Npand Io are the cell output voltage and current Npm + v R-L -respectively. The following are the simplified equations GND V1 Rf-Cfdescribing the cell output voltage and current: PV Array A.K.Tc ⎛ Iph + Id − Io ⎞ Vo = ln ⎜ ⎟ − R s.Io (1) q ⎝ Io ⎠ ⎛ qAVo ./Tc . Ns ⎞ a) Proposed model for PV Array in simulink ⎜ e .K Io = N p ( Iph − Id ⎜ − 1⎟ (2) environment ⎟ ⎝ ⎠ 5 I-V performance 4.5 1200W/m2 3 q . Eg ⎛ 1 1 ⎞ ⎛ Tc ⎞ 4 ⎜ − ⎟ 1000W/m2 I d = I or ⎜ ⎟ .e B . K ⎝ Tr Tc ⎠ (3) 3.5 ⎝ Tr ⎠ 3 800W/m2 Ipv,A 2.5 I ph = N p.{I sc .φ n + I t ( T c − T r ) } (4) 2 600W/m2 1.5 400W/m2 Where, K- Boltzman constant; Np and Ns are the 1number of parallel and series connected cells 0.5respectively; Eg is the band gap of the semiconductor; 0 0 5 10 15 20 25Tc and Tr are the cell and the reference temperature Vpv, Vrespectively in Kelvin, A and B are the diode ideality b) I-V Performance of PV module.factors with values varies between 1 and 2; Φn is the Figure 3. PV model with I-V performances.normalized insulation; Isc is the short circuit current Figure 3 illustrates the proposed PV array built ingiven at standard condition; It and Ior are constants Matlab/ simulink [11] with R-L load, where thegiven at standard conditions. obtained results for different variation levels are presented. From these performances it is shown that the2.1.1. Photovoltaic I-V Performance total output PV voltage and current varies according to irradiation level with approximated 65W maximum In order to study the I-V performance of the PV power at G=1000W/m2.circuit and to look for appropriate dc chopper for 61
    • 2.2 Double-chopper PV Array Solar irradiation 2000 Regulating the output chopped voltage according to G, W/m2, V 1500reference or grid voltage can be realized by modifyingthe conventional boost chopper into double chopper 1000circuit with buck converter called "Complementary 500 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2buck-boost converter" as shown in Figure 4. Power Reference & actual chopped voltageswitched Q1 and Q2 operates in complementary mode 400 Voutboosting up the input PV voltage, while Q3 regulates 300 Vact, Vthis output voltage toward increase or decrease 200according to Vref. 100 Vref 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Laod current 8 6 Ich-out, A 4 2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Time, S Figure 6. Output chopped voltage and current at various Figure 4. Complementary-chopper circuit irradiation rates. The obtained output voltage according to these 3. PV-GRID INTEGRATED SYSTEMmodels [12] is illustrated in Figure 5 for differentirradiation levels, and can be presented as follows: According to Figure 1, the generated PV voltage is D = D Q1 = D Q 2 adjusted by complementary buck-boost converter and (5) being applied to the load via grid selector. The power– D VO = V pv status estimator generates switching pulses required to 1− D operate the grid selector. The ac-grid contribution can be described into two approaches: Where DQ1 and DQ2 are duty cycles of choppers Q1and Q2 respectively.The actual average voltage • Fully inverted circuit;Vact=Vout of both choppers operation can be determinedas follows: • Partially inverted circuit. 1 ⎧ t1 t2 ⎫ In case of fully inverted circuit, the ac-grid voltage V out = ⎨ ∫ (V pv + V Lb 2 )dt + ∫ (V pv + V Lb 1 )dt ⎬ is converted into dc throughout grid-adapting module, T ch ⎩ 0 t1 ⎭ and then added to the output chopped dc voltage as V Lb 1 = L b 1 . di Lb 1 ; V Lb 2 = L b 1 . di Lb 2 shown in Figure 7. dt dt t 1 = D .T ch ; t 2 = (1 − D ) . T ch In partially inverted circuit, the PV voltage is (6) converted into ac voltage, while the ac-grid voltage is directly connected to the load after being synchronized. Where Lb1 and Lb2 are boost inductances for both In present paper first approach will be describedbranches respectively, and equals each other; Tch=1/fch hereinafter.is the chopping period. Introducing variable voltage tracking system VVT The consumed by the load effective power and thecauses voltage regulation and adjustment of output power delivered by the PV and ac-grid are by assumingvoltage as shown in Figure 6 for various irradiation that the system operates at unity power factor:levels. Reference & actual average voltage P Rrms = V inv .Iload P Rrms = (P pvo + P gac ).η inv 350 300 G=1200W/m2 (7) where , 250 Vref=220V P pvo = V out .Io ; P gac = V acrms .Ig Vref, V act, V 200 150 G=400W/m2 100 50 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Time, S Figure 5. Output voltage of complementary chopper Figure 7. Principle PV-Grid connected fully inverted circuit. circuit. 62
    • Where Ppvo, Pgac are the effective power provided by grif power, and generates the requirred switching pulsesthe PV and the ac-grid respectively; Vinv, Io are output for grid integration with the PV source.inverted voltage and current of fundamental harmonic Table 2. Main parameters of PV-Grid circuitrespectively; ηinv is the inverter efficiency; Vout, Io arethe effective output voltage and current of the G, Lb1,2 Vpv, V Vout, V R,Ω Cf, nFcomplementary chopper which are proportional to PV W/m2 mHmaximum power respectively; and Vacrms, Ig are the 1200 145 1.42 16.1 110V 44...250effective grid voltage and current respectively. 400 77 3.25 fch, kHz Lb, mH Rloss Lf~ Co ηinv Start 10 3.25 0.2Ω 2mH 480uF 92% 4.2. Grid-drop compensation Read Vinv, Iload, Vout, Igmax Io, Vacrms Grid-drop compensation module is required to calculate the voltage drop of the grid circuit with purpose generates accurat reference voltage according Calculate: to eq.(8), and generates appropriate switching PWM Prms, Ppvo, Pgac pulses that drives buck chopper Q3. The simulink circuit for this module is illustrated in Figure 10. ΔP= Ppvo- Prms .. 1 dp PL -K- Scope4 Ppv Scope6 Add Gain1 2 Z-OH Sat 0 No Yes PG=0 >= - ΔP >0 1 DP<0 PGmax Vcar-Vref2 PG=1 Repeating 3 Sequence1 Scope3 Q4=ON≡1 Q4=OFF≡0 Vcar-Vref3 Product Variable Step Go if Stateflow Ptot Io1 1 AND Pulse_G |u| T Abs -C- F small step Vcar-Vref1 c DGmax Divide Chopper Q4/ Return <= 0 1 Vdg, Vtr Vdg Vref=1 Pulse_Gr Figure 8. Functional flowchart for power-status estimator. Display 2 Sum(P) Vdg1 The reference voltage according to the consumedload current can be determined as follows: Figure 9. Simulink model of power-status estimator Vref = Vinv + ΔV; (8) 4.3. Simulation results at various irradiations ΔV = (Rloss Iinv )2 + (Xf Iinv )2 . . Figure 11 illustrates the main results when solar Where Vinv, Rloss and Xf are inverter voltage, grid irradiation varies in wide range causing significantresistance and circuit reactance including the inductive variation in the PV output voltage, while the outputfilter Lf respectively. According to consumed power, chopped voltage is kept constant according to the reference voltage. The obtained results shows thethe power status estimator module estimates wheather inverted voltage and current approximately haveor not the ac-grid contribution. Functional flowchart constant amplitude with negligible fluctuations due toillustrating the operation of this module is shown in transitions from one irradiation to another.Figure 8. The generated pulses required to drive Q4 are 1 Vpvproportional to the rate of power difference, and gives Cuurent Sat inputs 0.2 -1the status of integrating the grid with the PV system. 2 Iload_rms/Ipv Ig Rloss Ig.sinwt -K- Vinv Eta 1 sqrt(2) Output sqrt(2) du/dt 4. SIMULATION RESULTS SW Derivative 2 3 Vgrid+DV Vac_grid_rms 1 -K- Irms_load 0.002 2 -K- Eta4 Lf Vac Eta2 The proposed simulation model is built in Product4matlab/simulink environment and consists of several 1 Pulse output PI <= VG_ch pisub-models. Taking into account main PV-grid data 4 Vout_boost 1 -given in table 2, the sub-models are as follows: 1 gain1 1 gain4.1. Power- Status Estimator The simulink model for power status estimator is Scope5shown in Figure 9, where the model process the PV and Figure 10. Simulink model of Grid-drop compensation 63
    • PV output voltage PV-Grid Contribution .... 300 2000 V o u t-in v , V V re f & V c h -a v g , V V p v-o u t, V G , W /m 2 200 1000 100 0 0.5 1 1.5 2 2.5 3 Ref.& out. average voltage 0 200 0 0.5 1 1.5 2 2.5 3 300 P p v , P lo a d , W 100 Ppv 200 0 Pload 0 0.5 1 1.5 2 2.5 3 100 Inverted voltage 200 0 0 0.5 1 1.5 2 2.5 3 0 -200 200 0 0.5 1 1.5 2 2.5 3 Out. inverted current 0 dp, W 5 Io u t-in v ,A -200 0 0 0.5 1 1.5 2 2.5 3 -5 1 0 0.5 1 1.5 2 2.5 3 P u l s e -Q 4 Time, S 0.5 Grid-on Grid-off Figure 11. Solar irradiation profile and corresponds PV voltage. 0 0 0.5 1 1.5 2 2.5 3 Time, S4.4. Simulation results at various reference voltages When the reference voltage varies according to load Figure 13. PV & Grid power contribution diagram forrequirements at constant irradiation the system regulates various solar irradiation intervals.the output chopped voltage to be equal to the referencevoltage as shown in Figure 12, where the actual outputchopper voltage tracks the reference value with high 5. COMPLETE SIMULINK MODELdegree of accuracy. Figure 14 shows the complete PV-Grid functional4.5. The power contribution profile model built in Matlab/ simulink environment, where several modules are connected and integrated together According to eq.(7) changing the solar irradiation resulting in complete simulation process of PV arrayrate affects the extracted from the PV array power, behaviors according to different load requirements.therefore, in case of power shortages the grid willcontribute with certain amount of watts as shown infig.13 for three levels of solar irradiations (G=400 6. CONCLUSIONW/m2 , G=1700W/m2 & G=1000W/m2). From this figure it is shown that, the region where In this work a simulation study for PV-Gridthe grid is connected to the circuit throughout transistor integrated model has been conducted, where theswitch Q4. following conclusions can be drawn: PV output voltage 600 - The proposed PV model consists of variable tracking Vpv-out, V 400 module and voltage drop compensating module that 200 can be used for either dc or ac loads with precise 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 voltage tracking procedure. The added power-status Ref.& out. average voltage estimator modules create new aspect to this model, Vref & Vch-avg, V 400 200 where the power shortages can be measured and delivered from alternative sources or main ac-grid. 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 - The proposed model can be used for simulating 500 Inverted voltage photovoltaic system individually or combined with battery charging unit. During the daytime there is no Vout-inv, V 0 need of battery unit, resulting in efficiency -500 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 enhancement, reliability of the system and long life 2 Out. inverted current time. Meanwhile, during the night time the load is directly energized from the grid, which in turn Iout-inv,A 0 enhances the system reliability and reduces the total -2 cost. - The use of battery bank as alternative power source 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time, S during the nigh time can be applied when the ac grid plays the role of standby energy source that could be Figure 12: Reference voltage profile and corresponds contribute only in case of energy shortages . inverted voltage and current. 64
    • - The proposed model can be scaled and used for large a high performance boost converter", Solar energy energy converted systems and energy saving with 80 (2006) pp.772-778. battery control unit. [7] Azab M.," Improved circuit model of photovoltaic array, PWASET, Vol.34, Oct.2008, pp.857-860. [8] Atlas H., Sharaf A.M.," A Photovoltaic array 7. REFERENCES simulation model for Matlab-simulink GUI environment, IEEE, Trans., 2007, pp.341-345.[1] Ho-sung Kim, Jong-Hyun Kim, Byung-Duk Min, “ A highly efficient PV system using a series [9] Chouder A., Silvester S., Malek A., " Simulation of connection of DC-DC converter output with a photovoltaic grid connected inverter in case of grid- photovoltaic panel", Renewable Energy 34(2009), failure", Revue des energes Renouveables Vol. 9, pp2432-2436. No4, 2006, pp.285-296. [10] Buresch M.," Photovoltaic energy systems design[2] Tseng S.Y., Li Y.L., Wu J.Y," Buck Converter and Installation", McGraw-Hill, New York, 1983. Associated with Active Clamp Flyback Converter for PV Power System", ICSET 2008, pp.916-921. VVT C ontinuous Vact_rms Vact powergui 1 220 Gate Vref Vref1 Vref_var Vout_rms Sv Vgrid_rms 1 20 T RMS discrete G_var T T_var Ipv G RMS Sg 821.1984 (discrete) Ns Iout_rms1 Io2 G_var1 [Vg_p] Vg PV i Output inverter + DCO + - Lb2 D2 v From DCi - 1 Np Grid Output chopper +Vpv Vout2 Ls OCP G g + E 6 GND v C - Q3 Ns DC + PV Array + v Lb1 D1 D4 v AC1 - 1 - D3 AC2 Vout3 R-L Vout Npm Lb4 Io G Vg_Q1 Inverter i Lf + - Vpv_rms Ppv_rms RMS VGT NOT discrete 1 2 + - . R v g g C C - Q1 Q2 Vout-ac 1:1 E E Vpv Iout_rms VG_ch [Vg_p] RMS Ipv Iload_rms/Ipv (discrete) Goto 110 Irms_load Vac_grid_rms Vgrid+DV RMS 110.1 Vac_grid_rms (discrete) Vout_boost Vrms+dv PV-Grid Compensation current P_status Q4 Voltage D5 Pulse_G g i Lb3 Max current E + - 20 C Io1 Igrid-max1 Ptot Grid_connector PV-Power Scope1 Power Status Estimater D7 A + Lb7 AC Grid Voltage B - UB Figure 14. Matlab/ simulink model for PV–Grid integrated system.[3] Khaligh A., " A Multiple-input dc-dc positive buck- [11] Matlab and Simulink, The Mathworks, Inc., boost converter topology", APEC2008, Twenty- version R2008a, http://www.mathworks.com Third Annual IEEE, 24-28 Feb., 2008, pp.1522- [12] Hart D.W, " Power Electronics", Valparaiso 1526. University, 2010, McGraw Hill, pp.196-230.[4] Ahmed N.A.," Modeling and simulation of ac-dc buck-boost converter fed dc motor with uniform PWM technique", Electric Power systems Research 73 (2005), pp363-372.[5] Balkarishnan A.,Toliyat and Alexander W.C.," Soft switched ac link buck-boost converter", APEC 2008, Twenty-Third Annual IEEE, 24-28 Feb., 2008, pp.1334-1339.[6] Santos J.L, Antunes F, Chehab A., and Cruz C.," A maximum power point tracker for PV systems using 65