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Mr.M.samuel Babu, Dr.V.Nagabhaskar Reddy and Dr.Ch.Sai Babu 1

IRET Transaction on Power Electronics and Drives (ITPED) Vol. 1, Issue. 1, Oct. 2013

PV System with Z.V.T.Interleaved BoostConverter

Mr. M. Samuel Babu, Dr. V.Nagabhaskar Reddy and Dr.Ch. Sai Babu

Abstract: A photo voltaic power system with Zero VoltageTransition (ZVT) interleaved boost converter and full bridgeconverter with bi-directional power flow control strategy isproposed in this paper and verified with simulation results.ZVT interleaved boost converter is a combination of windingcoupled inductors and active-clamp circuits which will step-upthe very low voltage of Photo Voltaic (PV) Cell voltage intovery high dc-bus voltage and steady-state model also explained.A bi-directional control strategy, which makes the full bridgeconverter as an inverter to transfer the power from PV Cell toutility grid and as well as rectifier to stabilize the high voltagedc-bus by drawing power from the grid.

Keywords: Bidirectional power flow control, Direct currentcontrol, Maximum power point tracking (MPPT) method,Photovoltaic (PV) system, steady-state model of ZVT-Boostconverter.

I. INTRODUCTION

The photovoltaic (PV) Technology is a technology whichdirectly converts the sunlight energy into the electricityusing semiconductors. Energy reaching the earth isincredible. By one calculation, 30 days of sunshine strikingthe earth have the energy equivalent of the total of all theplanet’s fossil fuels, both used and unused. Because of theseso many advantages and increase in the energy demandmakes today photovoltaic (PV) power systems more andmore popular. There are several ways to deploy PhotoVoltaic Distributed Generation (PVDG) systems. Amongthem four different system configurations are widelydeveloped in grid connected PV power applications. Theyare centralized inverter system, string inverter system, multi-string inverter system and the module-integrated invertersystem [1]–[4]. Centralized inverter system was used in themiddle of 1980’s and

1Mr.M.Samuel Babu PG-Student, Department of Electrical and ElectronicsEngg. RGMCET, Nandyal, India,E-Mail: [email protected]. V. Nagabhaskar Reddy Professor, Department of Electrical andElectronics Engg. RGMCET, Nandyal, India,E-Mail: [email protected]. Sai Babu***Professor, Department of Electrical and ElectronicsEngg. JNT University, Kakinada, India,E-Mail: [email protected].

Noticed some disadvantages like miss matching losses, Riseof electric arc in dc wiring and no design flexibility.Module-Integrated inverter system was introduced instarting of 1990’s to overcome the disadvantages ofcentralized system but reduces the efficiency. String invertersystem was used in the middle of1990’s with the combinedfeatures of the centralized and the module integrated invertersystem . Multi-string inverter system introduced in 2000’sand developed to combine the advantages of higher energyyield of string inverter with the low cost. A PV inverter hasto fulfill three main functions in order to feed energy from aPV array into the utility grid. The first one is that the PVarray voltage has to be boost and second one is to invert theDC current into AC current and finally shape the currentinto sinusoidal wave form. Two different methods ofinverter topologies are there for grid-connected PV powersystems as with the frequency transformers and withoutfrequency transformers. Maximum efficiency is achievedwith the line-commutated and self-commutated transformerless inverters. In every PVDG System the most importantdesign constraint is to obtain a very high voltage gain, whybecause an open circuit voltage is about 21V for a typicalPV module and the maximum power point is about 16V, butthe utility grid nominal voltage is much more than this PVmodule voltage. So, the high voltage amplification isnecessary to realize the grid connected function. Theconventional system requires large numbers of PV modulesconnected in series and parallel which may had a problem oflarge power losses. There is another way to achieve this highvoltage amplification by using line frequency transformersbut line frequency step-up transformer has the disadvantagesof larger size and weight. In transformer less photovoltaicpower systems the power electronic converters plays veryimportant role in the power conversion. Generally gridconnected pulse width modulation (PWM) voltage sourceinverter are widely applied in PV System.

Fig.1 Block diagram of Proposed PV Power System.

In this paper a photovoltaic power system with ZVTInterleaved boost converter and full bridge converter withbi-directional power flow control strategy is proposed and



verified with the simulation results. ZVT Interleaved boostconverter is a combination of winding coupled inductors andactive-clamp circuits which will step-up the

Fig. 2 proposed grid-connected PV Power System

Very low voltage of PV Cell into very high dc-bus voltageand steady-state model also explained. A bi-directionalpower flow control strategy which makes the full bridgeconverter as an inverter to transfer the power from PV Cellto utility grid and as well as rectifier to stabilize the highvoltage dc-bus by drawing the power from the grid.fig 1Represents the block diagram of this two stage gridconnected PV power system. And Fig 2 represents theproposed circuit.

II. MODELING OF A P.V.CELL

Fig.3 Equalent Circuit of a PV cell.The use of equivalent electric circuits makes it possible tomodel characteristics of a PV cell. The method used here isimplemented in MATLAB programs for simulations. Thesame modeling technique is also applicable for modeling aPV module. There are two key parameters frequently used tocharacterize a PV cell. Shorting together the terminals of thecell, the photon generated current( ) will flow through thecell is defined as short circuit current ( ). Thus, =when there is no connection to the PV cell (open-circuit), thephoton generated current is shunted internally by theintrinsic p-n junction diode. This gives the open circuitvoltage (Voc). The PV module or cell manufacturers usuallyprovide the values of these parameters in their datasheets.

The simplest model of a PV cell equivalent circuitconsists of an ideal current source in parallel with an ideal

diode as shown in the fig 3. The current source representsthe current generated by photons ( ) and its output isconstant under constant temperature and constant incidentradiation of light.

The PV panel is usually represented by the singleexponential model or the double exponential model. Thesingle exponential model is shown in fig. 3. The current isexpressed in terms of voltage, current and temperature asshown in equation 1.= − { [ ( )] − 1} (1)

Equation (1) Shows that the output characteristics of solarcell is non linear and vitally effected by radiation,temperature and load conditions.Photon generated current is which is equal to Short circuit

current (Isc) directly proportional to the radiation.( ) = (2)

Where ‘Gas’ is irradiance at standard test conditions and‘ ’ is the absolute irradiance. Short circuit current of asolar cell directly depends on the temperature (T) andstandard test conditions temperature (Ts).( ) = [1 + ∆ ( − ) (3)

Where is the short circuit current at the standard testconditions. Then photon current will be( , ) = [1 + ∆ ( − )]. (4)The saturation current also depends on irradiance and cell

temperature ( , ) = ( , )( )( ) . (5)

III. STEADY-STATE MODEL OF HIGH STEP-UP

ZVT INTERLEAVED BOOST CONVERTER

Fig. 4 ZVT-interleaved boost converter Equivalent circuitHere each winding-coupled inductor are modeled as thecombination of a magnetizing inductor, an ideal trans-former with corresponding turn’s ratio and a leakageinductor in series with the magnetizing inductor. The opencircles and asterisks are indicated the coupling method of theeach coupled inductor. Where Lm 1 and Lm 2 are themagnetizing inductors, Llk1 and Llk2 are the leakageinductors including the reflected leakage inductors of thesecond and third windings of the coupled inductorsrespectively. Cs1 and Cs2 are the parallel capacitors,including the parasitic capacitors of the switches. Cc1 and



Cc2 are the clamp capacitors. N is the turns ratio n2 /n1.In paper [5] the operating principle and the steady-statewave-forms of the high step-up ZVT Interleaved boostconverter was discussed. A full bridge DC-DC converteralso can be replaced this ZVT Interleaved boost converterbut for high step-up gain applications the conduction losseswill be increase and converter efficiency will be decreases.And other than this DC-DC converter a resonant converterslike LLC, LCC are attractive but most of these resonantconverters are induces some inherent problems likeelectromagnetic interference (EMI). Comparing with allthese power electronic converters proposed ZVT Interleavedboost converter has the following advantages.

1. As the turn’s ratio increases, the voltage gainincreases without extreme duty ratio, so that thecurrent ripples will reduces.= = . (6)

2. As increasing the turn’s ratio voltage stress up onthe main switches is reduced, The normalizedvoltage stress of the main switches is given by= (7)

3. For both main switches and auxiliary switches ZVTsoft switching is achieved during the wholeswitching transition

To simplify the calculation, the following conditions areassumed.

1) The capacitance of the active-clamp circuits should belarge enough so that the voltage Ripple on the mainswitches can be ignored and voltage across the eachswitch is considered as constant when they turn off.

2) The magnetizing inductance is much larger than theleakage inductance, so the magnetizing current istaken as a constant in one switching period.

3) The dead times of the main switches and thecorresponding auxiliary switches are ignored.

Based on the previous assumptions, the partial keywaveforms of this converter are shown in Fig. 5, which havereached a steady State.Applying KVL for the ZVT Interleaved boost converterEqualent circuit, the output voltage will be given by= + + ∗ (8)Where and ∗ respectively represent the voltage of thesecond winding L1b and the voltage of the third winding L2c.

Fig. 5 Partial key waveforms of the converter

A. Stage A (Main Switches S1 OFF and S2 ON).Based on the voltage-second balance to the magnetizinginductor, the switching voltage of S1 is given by= (9)

From the waveform of iLk1 shown in Fig. 5, it can b befound that= = △△ = △( )⁄ (10)

As shown in Fig. 5, the voltages on the winding coupledinductors are decided by= = − (11)= (12)∗ = ( − ) (13)

Therefore, substituting (9), (12), and (13) into (8), theequation of the output voltage in Stage a is obtained= ( + 1) − 2= − 2 △( ) (14)

B. Stage b (Both Main Switches S1 and S2 are ON)Likewise, from the wave forms shown in Fig. 5, it can beFound that = 0 (15)



= = △△ = △△ (16)

Considering the polarity of the voltages on the windingcoupled inductors in stage b, the voltage expressions for thewinding-coupled inductors are also obtained by= = − (17)= (18)

∗ = ( + ). (19)

Therefore, substituting (15), (16), and (18) into (8), theequation of the output voltage in Mode 2 is obtained= 2 = 2 △△ (20)

In addition, from the waveform of iDo1 shown in Fig. 5charge through the two output diodes in one switchingperiod is decided by= 2 = (△ +△ ) △ (21)

The charge through the load in one switching period is= (22)

Therefore, the charge-conservation equation can be foundthat (△ +△ ) △ = (23)

Therefore, the equations (11), (17), and (20) can be solved toobtain the expression for the steady-state model of theconverter.=

= ( + 1) [( ) ] ( ) (24)

Where Llk is the equivalent leakage inductance of thewinding coupled inductors, and Llk = Llk1 = Llk2, and R isthe equivalent load of the converter.

Table 1 presents the values calculated by the two steadystate models based on equation (8) and based on theequation (24) are compared with the simulation results toverify the proposed model. It is clear that the proposedsteady-state model approaches the simulation results closely.

IV. CONTROL STRATEGY OF FULL-

BRIDGE CONVERTER WITH

BIDIRECTIONAL POWER FLOW

Fig. 1 represents the block diagram of the two-stage grid-connected PV system. Here bi-directional power flow

concept is implemented by the full-bridge converter byusing a direct-current-control strategy. The full bridgeconverter is a voltage source pulse width modulatingconverter (VS-PWM). The instantaneous load current isforcefully made to accurately follow the sinusoidal referencecurrent by this VS-PWM converter, which synchronizeswith the utility grid voltage. Furthermore, the bidirectionalflow of power facilitates to compensate the dc-bus for theac-side voltage variations, which helps to stabilize the dc-bus voltage in startup and cloudy situations and improvesthe stability of overall system.

A. Control of the Bidirectional Power Flow

Fig. 6 Control block of full-bridge inverter with bidirectionalpower flow

Fig.6 represents control block simulation diagram. Theerror signal will go on increasing If > and theVS-PWM converter works as an inverter which transfers thePV array power to the utility grid. The energy generation ofPV power system is positively correlated with the magnitudeof .The error signal will go on decreasing with the negativevalue ( < 0) when < then the VS-PWM converterworks as a PWM rectifier, which draws the energy from theutility grid to the capacitor of dc bus, to stabilize the dc-busvoltage. The current in the negative direction finallyapproaches a quite small value, which is only used tocompensate for the switch losses of VS-PWM converter.This bi-directional control strategy guarantees that the dc-bus voltage is stabilized to a required value by the back-endVS-PWM converter, whether the front-end dc–dc converterworks or not. Therefore, it is avoided that the ZVTInterleaved Boost converter to works in an open-circuitstate.

B. Direct Current Control with Compensation UnitsFrom the Fig. 6, it is clear that the detected load currents

and reference current are compared and generatesan error signal and processed by a PI regulator in the currentfeedback control loop. In direct current control strategy willhave very low harmonics in steady state compared with theother control methods. Generally the grid voltage is not anideal sinusoidal waveform in practice. Therefore, it is hardto achieve low total harmonic distortion (THD) of the outputcurrent by using the simple direct current control strategy inthe real grid condition so here two compensation units areadded to the current control loop as the control units.Where the first one is with Compensation coefficientwhich is directly processes the magnitude of reference



currents iref, and inversely proportional dc-bus voltage. Andis defined as = / (25)

The PI regulator in the grid-voltage-feed forward controlmultiplies the real, defective grid voltage with a proportiongain as a second compensation unit. Its output uc and the

output ua of PI regulator in current feedback loop aretogether fed to the PWM modulator to produce the drivesignals for the inverter switches. Therefore, the modulationwave uout includes the defective component of grid voltageto compensate grid voltage fluctuation and obtain highlysinusoidal current waveform. The feed forward effectdepends on the value of .In addition, the frequency ω and phase φ of reference

current iref are calculated to synchronize with utility gridvoltage by a phase-locked-loop (PLL) system.

V. UTILITY GIRDGenerally utility grid is defined as summing point of all thegenerating stations near by the load and the distributionsystem. In this paper grid act as A.C. load with requirednominal voltage of 480V and 60 Hz when the full bridgeconverter works in a inverting mode and considered as avoltage source with a 480V, 60 Hz when full bridgeconverter works in rectifying mode.

VI. SIMPLE MPPT SOLUTION BASED ONPOWER BALANCE

Maximum Power Point tracking, frequently referred to asMPPT, is an electronic system that operates the Photovoltaic(PV) modules in a manner that allows the modules toproduce all the power. MPPT is a electronic system thatvaries the electrical operating point of the modules so thatthe modules are able to deliver maximum available power.Many MPPT solutions are developed to ensure the optimalutilization of PV modules [11], [12].some of the popularMPPT techniques are available like

1. Fractional Open-Circuit Voltage2. B Fractional Short-Circuit Current3. Perturb and Observe4. Incremental Conductance

The implementations generally involve sensing the outputcurrent and voltage of PV modules, and the MPPTalgorithms use the information to maximize power drawnfrom the PV modules. Unfortunately, such realizations arecostly and complex [11], [12].In this paper a simple perturband observe MPPT solution is adopted in view of the powerbalance. Table 1 describes the algorithm for employing Ue.

TABLE 1P&O ALGORITHM EMPLOYINGUe

Fig.7 flow chart of P & o MPPT MethodFig.7 represents the flow chart for the perturbs and observeMPPT technique. And perturbing the duty ratio D of theZVT Interleaved boost converter perturbs the PV arraycurrent and voltage, and consequently, perturbs the PV arraypower. The operating point is then adjusted to maximize PVarray power. The other advantage is the simple MPPTsolution ensures the maximization of the power transferredto the utility grid, but not the output power from PV array. Ifwe ignore the whole system losses, the power thattransmitted from PV array is equal to the utility grid. And asdiscussed earlier, the magnitude of the load current ioutdirectly depends on the output Ue of the negative PIcontroller in the voltage loop. Therefore, the majority ofMPPT algorithms can be implemented by controlling thevalue Ue rather than the calculated power value bymultiplying the inputs from voltage and current sensors,which holds universality.

VII. SIMULATION RESULTS ANDDISCUSSION

The fig.8 represents the input and out voltage wave form ofZ.V.T. Interleaved boost converter. A P.V.cell voltage of50v is boosted up to around 366 which is 7 times to theinput voltage.Fig.9 represents the voltage wave form of grid connectedP.V.System when full bridge inverter is operated inrectification mode , here the Z.V.T.Interleaved boostconverter is failed to boost up the P.V.Cell voltage to busvoltage .then the inverter is act as rectifier and power drawsfrom the grid and stabilize the grid connected P.V.SystemFig.10 represents voltage wave forms of grid connectedP.V.System when the full bridge inverter is operated ininverting mode, here boosted D.C.voltage is converted intoA.C. and power is fed to the grid.



Fig.8 Z.V.T.Interleaved Boost Converter voltages

Fig.9 When Circuit Operated In Rectification Mode

Fig.10 When Circuit Operated In Inverting Mode

Table 2 presents that the values calculated by the two steadystate models are compared with simulation results to verify

the proposed model. It is clear that the proposed steady-statemodel approaches the simulation results closely.

TABLE IISTEADY-STATE MODEL VERIFICATION



Vin 48v fs 50khzCc1 ,Cc2 2.2µF Cs1,Cs2 2.2nFLm1,Lm2 2ooµH R 100ΩN Llk(µH) D Model

based onEquation

6

Modelbased onEquation

24

MATLABsimulation

Result

2 4 6.5 411.4 368.4 366.63 4 6.5 548.6 443.3 4422 5 6.5 411.4 360.0 358.33 5 6.5 548.6 426.7 423.32 6 6.5 411.4 352.3 352.23 6 6.5 548.6 411.1 4132 4 7 480 415.9 4153 4 7 640 490.0 4912 5 7 480 402.3 402.33 5 7 640 466.7 469.22 6 7 480 392.3 3933 6 7 640 450.0 449.6

VIII. CONCLUSION

The proposed PV system employs a ZVT-interleaved boostconverter which can boost the low voltage of the PV arrayup to a high dc-bus voltage. Accordingly an accurate steady-state model is obtained and verified by the simulation resultsand a full bridge inverter with bi-directional power flow isused which can inverting the dc current into a sinusoidalwaveform synchronized with the utility grid and stabilizedc bus voltage .

REFERENCES[1]. S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of

single-phase grid-connected inverters for photovoltaicmodules,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005.

[2]. Q. Li and P.Wolfs, “A review of the single phasephotovoltaic module integrated converter topologies withthree different DC link configurations,” IEEE Trans. PowerElectron., vol. 23, no. 3, pp. 1320–1333, May 2008.

[3]. M. Calais, J.Myrzik, T. Spooner, and V. G. Agelidis,“Inverters for single zphase grid connected photovoltaicsystems—An overview,” in Proc. IEEEPESC, 2002, vol. 4,pp. 1995–2000.

[4]. S. B. Kjær, J. K. Pedersen, and F. Blaabjerg, “Power invertertopologies for photovoltaic modules—A review,” in Proc.IEEE IAS Conf., 2002, pp. 782–788.

[5]. W. Li and X. He, “ZVT interleaved boost converters forhigh-efficiency, high step-up DC/DC conversion,” IETElectr. Power Appl., vol. 1, no. 2, pp. 284–290, Mar. 2007.

[6]. Q. Zhao and F. C. Lee, “High-efficiency, high step-up DC–DC converters,” IEEE Trans. Power Electron., vol. 18, no. 1,pp. 65–73, Jan. 2003.

[7]. 7. R. J.Wai and R. Y. Duan, “High step-up converter withcoupled-inductor,” IEEE Trans. Power Electron., vol. 20, no.5, pp. 1025–1035, Sep. 2005.

[8]. K. C. Tseng and T. J. Liang, “Novel high-efficiency step-upconverter,” IEE Electr. Power Appl., vol. 151, no. 2, pp.182–190, Mar. 2004.

[9]. A. Bellini, S. Bifaretti, and V. Iacovone, “Resonant DC–DCconverters for photovoltaic energy generation systems,” inProc. IEEE SPEEDAM Conf., 2008, pp. 815–820.

[10]. D. Fu, F. C. Lee, Y. Liu, and M. Xu, “Novel multi-elementresonant converters for front-end dc/dc converters,” in Proc.IEEE PESC Conf., 2008, pp. 250–256.

[11]. T. Esram and P. L. Chapman, “Comparison of photovoltaicarray maximum power point tracking techniques,” IEEE

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Mr.M.samuel Babu received the B.Tech(Electrical and Electronics Engineering)degree from the Neduramalli balakrishnareddy institute of science and technologyaffiliated to S.V.University in 2004 andpursuing M.Tech (Power Electronics) fromRGMCET (Autonomous). His field of

interest includes D.C – D.C Converters, pulse widthModulation, and Photovoltaic Power Systems (E-mail:babu4net @gmail.com).

Dr.V.Naga Bhaskar Reddy was born inKurnool, India. He received the B.Tech(Electrical and Electronic Engineering)degree from the Bangalore University,Bangalore in 2000, M.Tech (PowerElectronics and Drives) from the Bharath

Institute of Higher Education Research [BIHER], Chennaiin 2005. He attained his doctoral degree from JawaharlalNehru Technological University; Kakinada in 2012.He iscurrently a Professor of the Dept. of Electrical andElectronic Engineering, R.G.M College of Engineering andTechnology, Nandyal. His area of interest is PowerElectronics, Microcontrollers, Power Electronic converters.(Email: [email protected])

Dr.Ch. Sai Babu received the B.E fromAndhra University (Electrical &Electronics Engineering), M.Tech inElectrical Machines and Industrial Drivesfrom REC, Warangal and PhD inReliability Studies of HVDC Convertersfrom JNTU, Hyderabad. Currently he is

working as a Professor in Dept. of EEE in JNTUCEA,Anantapur. He has published several National andInternational Journals and Conferences. His area of interestis Power Electronics and Drives, Power System Reliability,HVDC Converter Reliability, Optimization of ElectricalSystems and Real Time Energy Management.E-mail: [email protected]

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