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- 1. A Hybrid Wind-Solar Energy System: A New Rectifier Stage TopologyK. Hari Kishore. V.Chaitanya Prasad, (M-tech).08MP1A0229 Assoc.Prof. EEE Dept.Abstract—Environmentally friendly solutions INTRODUCTIONare becoming more prominent than ever as a With increasing concern of global warmingresult of concern regarding the state of our and the depletion of fossil fuel reserves, many aredeteriorating planet. This paper presents a new looking at sustainable energy solutions to preservesystem configuration of the front-end rectifier the earth for the future generations. Other thanstage for a hybrid wind/photovoltaic energy hydro power, wind and photovoltaic energy holdssystem. This configuration allows the two the most potential to meet our energy demands.sources to supply the load separately or Alone, wind energy is capable of supplying largesimultaneously depending on the availability of amounts of power but its presence is highlythe energy sources. The inherent nature of this unpredictable as it can be here one moment andCuk-SEPIC fused converter, additional input gone in another. Similarly, solar energy is presentfilters are not necessary to filter out high throughout the day but the solar irradiation levelsfrequency harmonics. Harmonic content is vary due to sun intensity and unpredictabledetrimental for the generator lifespan, heating shadows cast by clouds, birds, trees, etc. Theissues, and efficiency. The fused multi input common inherent drawback of wind andrectifier stage also allows Maximum Power Point photovoltaic systems are their intermittent naturesTracking (MPPT) to be used to extract that make them unreliable. However, bymaximum power from the wind and sun when it combining these two intermittent sources and byis available. An adaptive MPPT algorithm will be incorporating maximum power point trackingused for the wind system and a standard perturb (MPPT) algorithms, the system‟s power transferand observe method will be used for the PV efficiency and reliability can be improvedsystem. Operational analysis of the proposed significantly.system will be discussed in this paper. Simulation When a source is unavailable or insufficientresults are given to highlight the merits of the in meeting the load demands, the other energyproposed circuit. When a source is insufficient in source can compensate for the difference. Severalmeeting the load demands, the other energy hybrid wind/PV power systems with MPPTsource can compensate for the difference. control have been proposed and discussed inSeveral hybrid wind/PV power systems with works Most of the systems in literature use aMPPT control have been proposed, usage of separate DC/DC boost converter connected inrectifiers and inverters is also discussed. parallel in the rectifier stage as shown in Figure 1
- 2. to perform the MPPT control for each of therenewable energy power sources. A simpler multi- PROPOSED MULTI-INPUT RECTIFIERinput structure has been suggested that combine STAGEthe sources from the DC-end while still achieving A system diagram of the proposed rectifierMPPT for each renewable source. The structure stage of a hybrid energy system is shown inproposed by is a fusion of the buck and buck- Figure 2, where one of the inputs is connected toboost converter. The systems in literature require the output of the PV array and the other inputpassive input filters to remove the high frequency connected to the output of a generator. The fusioncurrent harmonics injected into wind turbine of the two converters is achieved by reconfiguringgenerators. The harmonic content in the generator the two existing diodes from each converter andcurrent decreases its lifespan and increases the the shared utilization of the Cuk output inductorpower loss due to heating. by the SEPIC converter. This configuration allows In this paper, an alternative multi-input each converter to operate normally individually inrectifier structure is proposed for hybrid the event that one source is unavailable. Figure 3wind/solar energy systems. The proposed design illustrates the case when only the wind source isis a fusion of the Cuk and SEPIC converters. available. In this case, D1 turns off and D2 turnsThe features of the proposed topology are: on; the proposed circuit becomes a SEPIC1) The inherent nature of these two converters converter and the input to output voltage eliminates the need for separate input filters for relationship is given by (1). On the other hand, if PFC [7]-[8], only the PV source is available, then D2 turns off2) It can support step up/down operations for each and D1 will always be on and the circuit becomes renewable source (can support wide ranges of a Cuk converter. The input to output voltage PV and wind input), relationship is given by (2). In both cases, both3) MPPT can be realized for each source, converters have step-up/down capability, which4) Individual and simultaneous operation provide more design flexibility in the system if duty ratio control is utilized to perform MPPT control. Vdc d2 = Vw 1 − d2 Vdc d1 = Vpv 1 − d1 Figure 3 illustrates the various switchingFigure 1: Hybrid System with multi-connected states of the proposed converter. If the turn on boost converter duration of M1 is longer than M2, then the
- 3. switching states will be state I, II, IV. Similarly, 𝑉𝑝𝑣 iL1 = IiPV + 𝑡 0 < t < d1Ts 𝐿1the switching states will be state I, III, IV if the 𝑉𝑐1+𝑉𝑐2switch conduction periods are vice versa. To iL2 = Idc + 𝑡 0 < t < d1Ts 𝐿2provide a better explanation, the inductor current 𝑉𝑤waveforms of each switching state are given as iL3 = Iiw + 𝑡 0 < t < d1Ts 𝐿3follows assuming that d2 > d1; hence only states I, STATE III (M1 OFF, M2 ON):III, IV are discussed in this example. In thefollowing, Ii,PV is the average input current fromthe PV source; Ii,W is the RMS input current afterthe rectifier (wind case); and Idc is the averagesystem output current. The key waveforms that illustrate theswitching states in this example are shown in Figure 4: M1 off, M2 onFigure 5. The mathematical expression that relates 𝑉𝑝𝑣 −𝑉𝑐1the total output voltage and the two input sources iL1 = IiPV + 𝑡 d1Ts < t < d2Ts 𝐿1will be illustrated in the next section. 𝑉𝑐2 iL2 = Idc + 𝑡 d1Ts < t < d2Ts 𝐿2 𝑉𝑤 iL3 = Iiw + 𝑡 d1Ts < t < d2Ts 𝐿3 STATE IV (M1 OFF, M2 OFF): Figure 2: Circuit diagram of Darlington configuration There are different states in the working of theproposed circuit of rectifier stage for a HybridWind/PV System and these states are as follows: Figure 5: M1 off, M2 offSTATE I (M1 ON, M2 ON): 𝑉𝑝𝑣 −𝑉𝑐1 iL1 = IiPV + 𝑡 d2Ts < t < Ts 𝐿1 𝑉𝑑𝑐 iL2 = Idc - 𝑡 d2Ts < t < Ts 𝐿2 𝑉𝑤 −𝑉𝑐2−𝑉𝑑𝑐 iL3 = Iiw + 𝑡 d2Ts < t < Ts 𝐿3 Figure 3: M1 on, M2 on
- 4. ANALYSIS OF PROPOSED CIRCUIT equivalent inductance of Cuk and SEPIC converter respectively. To find an expression for the output DC bus The PV output current, which is also equalvoltage, Vdc, the volt-balance of the output to the average input current of the Cuk converterinductor, L2, is examined d2 > d1. Since the net is given in (12). It can be observed that thechange in the voltage of L2 is zero, applying volt- average inductor current is a function of itsbalance to L2 results in (3). The expression that respective duty cycle (d1). Therefore by adjustingrelates the average output DC voltage (Vdc) to the the respective duty cycles for each energy source,capacitor voltages (vc1 and vc2) is then obtained maximum power point tracking can be achieved.as shown in (4), where vc1 and vc2 can then beobtained by applying volt-balance to L1 and L3[9]. The final expression that relates the averageoutput voltage and the two input sources (VW andVPV) is then given by (5). It is observed that Vdcis simply the sum of the two output voltages of theCuk and SEPIC converter. This further impliesthat Vdc can be controlled by d1 and d2individually or simultaneously. MPPT CONTROL OF PROPOSED CIRCUIT A common inherent drawback of wind and The switches voltage and current PV systems is the intermittent nature of theircharacteristics are also provided in this section. energy sources. Wind energy is capable ofThe voltage stress is given by (6) and (7) supplying large amounts of power but its presencerespectively. As for the current stress, it is is highly unpredictable as it can be here oneobserved from that the peak current always occurs moment and gone in another. Solar energy isat the end of the on-time of the MOSFET. Both present throughout the day, but the solarthe Cuk and SEPIC MOSFET current consists of irradiation levels vary due to sun intensity andboth the input current and the capacitors (C1 or unpredictable shadows cast by clouds, birds, trees,C2) current. The peak current stress of M1 and M2 etc. These drawbacks tend to make theseare given by (8) and (10) respectively. Leq1 and renewable systems inefficient. However, byLeq2, given by (9) and (11), represent the incorporating maximum power point tracking
- 5. (MPPT) algorithms, the systems‟ power transfer Figure 6 and 7 are illustrations of a powerefficiency can be improved significantly. To coefficient curve and power curve for a typicaldescribe a wind turbine‟s power characteristic, fixed pitch (β =0) horizontal axis wind turbine. Itequation (13) describes the mechanical power that can be seen from figure 7 and 8 that the poweris generated by the wind. curves for each wind speed has a shape similar to that of the power coefficient curve. Because thePm = 0.5𝜌ACp(λ, β)v3 w TSR is a ratio between the turbine rotational speed The power coefficient (Cp) is a nonlinear and the wind speed, it follows that each windfunction that represents the efficiency of the wind speed would have a different correspondingturbine to convert wind energy into mechanical optimal rotational speed that gives the optimalenergy. It is dependent on two variables, the tip TSR.speed ratio (TSR) and the pitch angle. The TSR,λ, refers to a ratio of the turbine angular speedover the wind speed. Figure 7: Power Curves for a typical wind turbine For each turbine there is an optimal TSRFigure 6: Power Coefficient Curve for a typical value that corresponds to a maximum value of the wind turbine power coefficient (Cp,max) and therefore the maximum power. Therefore by controlling The pitch angle, β, refers to the angle in rotational speed, (by means of adjusting thewhich the turbine blades are aligned with respect electrical loading of the turbine generator)to its longitudinal axis. maximum power can be obtained for different R ωb wind speeds. A solar cell is comprised of a P-N = Vw junction semiconductor that produces currents viaWhere, the photovoltaic effect.R = turbine radius,ωb = angular rotational speed Figure 8: PV cell equivalent circuit
- 6. PV arrays are constructed by placing The typical output power characteristics of anumerous solar cells connected in series and in PV array under various degrees of irradiation isparallel. A PV cell is a diode of a large-area illustrated by Figure 11. It can be observed inforward bias with a photo voltage and the Figure 11 that there is a particular optimal voltageequivalent circuit is shown by Figure 9. The for each irradiation level that corresponds tocurrent-voltage characteristic of a solar cell is maximum output power.derived in as follows:Where,Iph = photocurrent,ID = diode current,I0 = saturation current, Figure 9: Cell Current Vs Cell VoltageA = ideality factor, Therefore by adjusting the output current (orq = electronic charge 1.6x10-9, voltage) of the PV array, maximum power fromkB = Boltzmann‟s gas constant (1.38x10-23), the array can be drawn. Due to the similarities ofT = cell temperature, the shape of the wind and PV array power curves,Rs = series resistance, a similar maximum power point tracking schemeRsh = shunt resistance, known as the hill climb search (HCS) strategy isI = cell current, often applied to these energy sources to extractV = cell voltage maximum power. Typically, the shunt resistance (Rsh) is verylarge and the series resistance (Rs) is very small.Therefore, it is common to neglect theseresistances in order to simplify the solar cellmodel. The resultant ideal voltage-currentcharacteristic of a photovoltaic cell is given by(17) and illustrated by Figure 10 Figure 10: Output Power Vs Output Current
- 7. The HCS strategy perturbs the operating SIMULATION RESULTSpoint of the system and observes the output. If the In this section, simulation results fromdirection of the perturbation (e.g an increase or MATLAB-5 are given to verify that the proposeddecrease in the output voltage of a PV array) multi-input rectifier stage can support individualresults in a positive change in the output power, as well as simultaneous operation. Thethen the control algorithm will continue in the specifications for the design example are given indirection of the previous perturbation. Conversely, TABLE 1. Figure 11.1 shows output AC and DC.if a negative change in the output power is Figure11.2 Shows Input DC and AC. Figure 11.3observed, then the control algorithm will reverse Shows final Cuk-SEPIC mode.the direction of the pervious perturbation step. Inthe case that the change in power is close to zero(within a specified range) then the algorithm willinvoke no changes to the system operating pointsince it corresponds to the maximum power point(the peak of the power curves). Table 1: Design Specifications The MPPT scheme employed in this paperis a version of the HCS strategy. Figure 12 is theflow chart that illustrates the implemented MPPTscheme. Figure 11.1: Final Output AC & DCFigure 11: General MPPT Flow Chart for wind and PV Figure 11.2: Input DC & AC
- 8. REFERENCES [1] S.K. Kim, J.H Jeon, C.H. Cho, J.B. Ahn, and S.H. Kwon, “Dynamic Modeling and Control of a Grid-Connected Hybrid Generation System with Versatile Power Transfer,” IEEE Transactions on IndustrialElectronics, vol. 55, pp. 1677-1688, April 2008. [2] D. Das, R. Esmaili, L. Xu, D. Nichols, “An Optimal Design of a Grid Connected Hybrid Figure 11.3: Cuk-SEPIC Wind/Photovoltaic/Fuel Cell System for Distributed Energy Production,” in Proc. CONCLUSION IEEE Industrial Electronics Conference, pp. 2499-2504, Nov. 2005. The common inherent drawback of windand photovoltaic systems are their intermittent [3] N. A. Ahmed, M. Miyatake, and A. K. Al-natures that make them unreliable but by Othman, “Power fluctuations suppression ofcombining these two intermittent sources and by stand-alone hybrid generation combining solarincorporating maximum power point tracking photovoltaic/wind turbine and fuel cell(MPPT) algorithms, the system‟s power transfer systems,” in Proc. Of Energy Conversion andefficiency and reliability has been improved Management, Vol. 49, pp. 2711-2719, Octobersignificantly. 2008. When a source is unavailable or [4] S. Jain, and V. Agarwal, “An Integratedinsufficient in meeting the load demands, the Hybrid Power Supply for Distributedother energy compensates for the difference. In Generation Applications Fed bythis project a New Multi Input Cuk-SEPIC Nonconventional Energy Sources,” IEEERectifier Stage for Hybrid Wind/Solar Energy Transactions on Energy Conversion, vol. 23,systems has been implemented. The features of June 2008.this circuit are additional input filters are notnecessary to filter out high frequency harmonics, [5] Y.M. Chen, Y.C. Liu, S.C. Hung, and C.S.both renewable sources can be stepped up/down Cheng, “Multi-Input Inverter for Grid-(supports wide ranges of PV and wind input), Connected Hybrid PV/Wind Power System,”MPPT can be realized for each source, individual IEEE Transactions on Power Electronics, vol.and simultaneous operation is supported. 22, May 2007.Simulation results have been presented to verifythe features of the proposed topology.
- 9. [6] dos Reis, F.S., Tan, K. and Islam, S., “Using Intelligence and Industrial Application 2008 PFC for harmonic mitigation in wind turbine (PACIIA „08), Dec 2008, pp. 498-501 energy conversion systems” in Proc. of the IECON 2004 Conference, pp. 3100- 3105, Nov. 2004[7] R. W. Erickson, “Some Topologies of High Quality Rectifiers” in the Proc. of the First International Conference on Energy, Power, and Motion Control, May 1997.[8] D. S. L. Simonetti, J. Sebasti´an, and J. Uceda, “The Discontinuous Conduction Mode Sepic and ´ Cuk Power Factor Preregulators: Analysis and Design” IEEE Trans. On Industrial Electronics, vol. 44, no. 5, 1997[9] N. Mohan, T. Undeland, and W Robbins, “Power Electronics: Converters, Applications, and Design,” John Wiley & Sons, Inc., 2003.[10] J. Marques, H. Pinheiro, H. Grundling, J. Pinheiro, and H. Hey, “A Survey on Variable-Speed Wind Turbine System,” Proceedings of Brazilian Conference of Electronics of Power, vol. 1, pp. 732-738, 2003.[11] F. Lassier and T. G. Ang, “PhotovoltaicEngineering Handbook” 1990.[12] Global Wind Energy Council (GWEC),“Global wind 2008 report,” June 2009.[13] L. Pang, H. Wang, Y. Li, J. Wang, and Z. Wang, “Analysis of Photovoltaic Charging System Based on MPPT,” Proceedings of Pacific-Asia Workshop on Computational

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