design and implementation of photovoltaic

7/30/2019 Design and Implementation of Photovoltaic

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Journal of Electrical Engineering & Technology Vol. 5, No. 4, pp. 606~613, 2010DOI:10.5370/JEET.2010.5.4.606

606

Design and Implementation of Photovoltaic Power Conditioning

System using a Current-based Maximum Power Point Tracking

Sanghoey Lee*, Jae-Eon Kim** and Hanju Cha

Abstract This paper proposes a novel current-based maximum power point tracking (CMPPT)

method for a single-phase photovoltaic power conditioning system (PV PCS) by using a modified in-cremental conductance method. The CMPPT method simplifies the entire control structure of the

power conditioning system and uses an inherent current source characteristic of solar cell arrays.Therefore, it exhibits robust and fast response under a rapidly changing environmental condition. Digi-

tal phase locked loop technique using an all-pass filter is also introduced to detect the phase of gridvoltage, as well as the peak voltage. Controllers of dc/dc boost converter, dc-link voltage, and dc/acinverter are designed for coordinated operation. Furthermore, a current control using a pseudo syn-chronous d-q transformation is employed for grid current control with unity power factor. A 3 kW pro-totype PV PCS is built, and its experimental results are given to verify the effectiveness of the pro-

posed control schemes.

Keywords: Single-phase photovoltaic power conditioning system, Digital phase locked loop, dc/dcboost converter, dc/ac inverter, CMPPT, dP/dI

1. Introduction

Photovoltaic (PV) energy is currently considered as one

of the most useful renewable natural energy sources in theworld because it is clean, free, abundant, pollution-free,

and inexhaustible. Due to the rapid growth in solar cellsand power electronics technology, PV energy has receivedincreasing interest in electrical power applications.

However, present energy-conversion efficiency of PV ar-ray is still low. It requires maximum power point tracking

(MPPT) control techniques to extract the maximum possi-ble power from PV arrays in order to achieve maximumoperating efficiency [1].

A PV array currently exhibits an extremely nonlinearvoltage that generally varies with array temperature andsolar isolation, making the maximum power point (MPP)difficult to locate. To overcome this problem, variousmethods, such as the perturbation and observation method

[2], [3] and incremental conductance methods [4]-[6], havebeen proposed for the MPPT algorithm of PV arrays.

In the perturbation and observation method, the operat-ing voltage of PV array changes the duty ratio in order tolocate variations in directions for maximizing PV arraycurrent. If power increases, the operating voltage is furtherperturbed in the same direction; if it decreases, the direc-tion of the perturbation is reversed. This method does not

require solar panel characteristics, but it remains unsuitablefor applications under rapidly changing atmospheric condi-tions.

The disadvantage of the perturbation and observation

method can be minimized by comparing the incremental

and instantaneous conductance of PV arrays. This methodis more accurate and can provide good performance underrapidly changing conditions.

In this paper, a current-based incremental conductancemethod that produces smooth transition and fast responseto the MPP is proposed. The proposed current-basedmaximum power point tracking (CMPPT) method adjusts a

reference current proportional to the power slope with re-spect to PV array current, and provides advantages ofsmooth and rapid transition to the MPP. In addition, digitalphase locked loop (DPLL), which detects the phase of gridvoltage as well as its peak voltage, is addressed. A PI con-troller using a pseudo synchronous d-q transformation is

employed for grid current control in the single-phase dc/acinverter. Results from analysis, simulation, and hardware

implementation of the power conditioning system are de-tailed. Experimental results are also obtained by using a 3kW prototype PV PCS, which then verifies the feasibilityof the proposed control schemes.

2. Photovoltaic Power Conditioning System

Fig. 1 shows the circuit configuration of a 3 kW trans-former-less PCS with grid connection. The transformer-lessPCS is composed of a PV array, dc/dc boost converter, dc-link,and dc/ac inverter. L-C filter PV voltage Vpv is set in widerange variation (150-450 V). The dc/dc boost converter

Corresponding Author: Department of Electrical EngineeringChungnam National University Daejeon, Korea. ([email protected])

* Department of Electrical Engineering Chungnam National Univer-sity Daejeon, Korea. ([email protected])

** Department of Electrical Engineering Chungbuk National Univer-sity Cheongju, Korea. ([email protected])

Received: January 28, 2010; Accepted: July 21, 2010


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Sanghoey Lee, Jae-Eon Kim and Hanju Cha 607

Fig. 1. Circuit configuration of PV PCS.

controls the PV current Ipv. These two actions perform theproposed CMPPT function. The dc/ac inverter controls thegrid current with unity power factor.

2.1 Proposed CMPPT

The PV array is nonlinear with existing operating points,

and the PV array produces maximum power, as illustratedin Fig. 2. The perturbation and observation method meas-ures voltage and current, allowing for the evaluation of themomentary operating region. In accordance with the region,the reference voltage is either increased or decreased, suchthat the system operates close to MPP [2], [3].

Implementing the perturbation and observation methodis simple because it only increases or decreases reference

voltage. However, this method cannot readily track anyimmediate and rapid change in the environment. One alter-native is the incremental conductance method; it can track

MPP accurately by comparing the incremental conductanceand instantaneous conductance of a PV array. [4], [5]. Inthis paper, the proposed CMPPT is improved from the per-spective of the incremental conductance method.

The P-I and V-I curves for CMPPT is presented in Fig. 2.

The flowchart shown in Fig.3 presents the systematic pro-gress of CMPPT, where Vpv(n) andIpv(n) are the present volt-

age and current of the PV array, and Vpv(n-1)and Ipv(n-1) aretheir previous values, respectively. When Vpv(n)/Ipv(n)+ dVpv/ dIpv< 0 ordIpv = 0 and dVpv < 0, decreasing reference cur-rent Ipv

* forces Vpv(n)/Ipv(n) + dVpv/ dIpv to approach zero.

When Vpv(n)/Ipv(n)+ dVpv/ dIpv< 0 ordIpv = 0 and dVpv > 0,increasing reference current Ipv

*forces Vpv(n)/Ipv(n)+ dVpv/

dIpv to approach zero. When Vpv(n)/Ipv(n)+ dVpv/ dIpv = 0 or

Fig. 2. P-I curve and V-I curve for CMPPT.

Fig. 3. Flowchart of CMPPT.

dIpv = 0 and dVpv = 0, as the PV array is at the MPP, refer-ence currentIpv

*is kept at a constant value, and thus, oscil-

lation is reduced. The low-frequency ac ripple mitigationmethod [12] is used to reduce PV array current ripple.

2.2 Current and Voltage MPPs in PV Array Charac-

teristics

Fig. 4 shows the MPP changes of voltage and current due

to the irradiation characteristics from 0 to 1 at the fixedtemperature of 25 C. To show the relative values between

the voltage and current MPP, the unit is shown in perunit (p.u.). At the midpoint of irradiation change duration,

voltage MPP changes slowly while current MPP exhibits abig slope, as shown in Fig. 4.

The current curve is increased linearly by PV array irra-diation characteristics, indicating that the current MPPvalue always changes at any irradiation. The voltage curvehas a small variation in VMPPT; therefore, the controller isnot always needed to control MPP. However, VMPPT can-

not reach an accurate MPP at any irradiation and mightencounter MPP confusion at low irradiation because of thePV array characteristics, as shown in Fig 4. CMPPT, on theother hand, can always control the current; it can reachMPP easily and accurately because of the short circuit cur-rent characteristics in the PV array. Therefore, CMPPT isdesired if the aim is to increase system efficiency.

Fig. 5 shows the MPP changes of both voltage and cur-rent due to temperature characteristics (0-45) at a fixed

irradiation of 1. To show the relative values between volt-age and current MPP, the unit is shown in per unit (p.u.).

At 250V, voltage MPP changes slowly while currentMPP exhibits a big slope. Fig. 5 shows that the currentcurve decreases linearly with PV array temperature, indi-cating that CMPPT always changes current MPP values at


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Design and Implementation of Photovoltaic Power Conditioning System using a Current-based Maximum~608

Fig. 4. CMPP and VMPP under irradiation-changing condi-tions [p.u. / 0.5 Irradiation MPP].

Fig. 5. CMPP and VMPP under temperature-changing con-ditions [p.u. / 25 Temperature MPP].

any temperature. Therefore, CMPPT controller can alwaysoperate to track MPP. The reverse is true for VMPPT be-cause of the small change in voltage MPP value.

Figs. 4 and 5 show the comparison of the inherent cur-rent source characteristics of solar cell array and voltage.Comparative results show that CMPPT has a robust andfast response under rapidly changing environmental condi-

tions compared with VMPPT.Fig. 6 shows a VMPPT P-V curve and slope calculated

by dP/dV. As shown in this figure, the slope is 0 when P-Vcurve is in MPP. The blue color refers to the value of theoperation region slope.

Fig 7 shows a CMPPT P-I curve and the slope calculated

by dP/dI. Results using different methods show that bothcurve powers are the same, but the slopes differ. In VMPPT,slope operation region value changes from 0 to 30, while itchanges from 0 to 350 in CMPPT. This result indicates thatthe CMPPT method has more comparative points com-pared with VMPPT. Furthermore, by using the same sam-pling period, results reveal that VMPPT has smaller steps,hence causing longer comparative time compared withCMPPT. In effect, CMPPT can track MPP more quicklyand accurately, Hence, CMPPT can track quickly and accu-

rately MPP by using slope as modified MPPT method.Implementing the CMPPT method is more convenient

due to the independent of slope value on array temperature

and irradiation., as opposed to the VMPPT approach [10], [11].

Fig. 6. P-V curve and dP/dV slope.

Fig. 7. P-I curve and dP/dI slope.

2.3 Dc/dc Boost Converter

Fig. 8 shows the block diagram of the dc/dc boost con-verter controller. This controller is composed of a PI com-

pensator, voltage limiter, and PWM generator, whereI*pv iscalculated from CMPPT method and Va is a dc-compensating variable for voltage across switch Sb.

Fig. 8. Block diagram of dc-dc boost converter.

The transfer function of the dc/dc boost converter can bederived as

ipvpv

ipvpv

pv

L

KsKsL

KsK

I

I

2

1

*

1

(1)

2.4 Dc-link Voltage Control

Fig. 9 shows the block diagram of the dc-link voltagecontrol, where Vdc

*is the reference dc-link voltage; Pin is

the input power to the dc-link capacitorCd from solar cellarray, such as (2); Vpeak is the peak value of grid voltage;

and Pout is the output power from the dc-link capacitorCdto the grid, as shown (3).


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pvpvin IVP (2)

22

L

peak

out IV

P (3)

Band stop filter is used to mitigate the effect of the 2ndharmonic voltage resulting in performance degradation.IL2

*

is a command value of the grid current.To balance Pin and Pout, the dc-link voltage controller

(Fig. 9) is employed. Its transfer function can be derived as(4) [13]:

idcdc

peak

dcdc

idcdc

dc

dc

KsKsV

VC

KsK

V

V

2

** 2

(4)

Fig. 9. Block diagram of dc-link voltage control.

2.5 Digital Phase Locked Loop

In the utility system, PLL control is needed to synchro-nize inverter output voltage to the interconnected utility.Generally, a PLL control system in single-phase is con-structed with zero crossing detection. However, DPLL isused in this paper because the phase detection time of itsmethod is faster compared with the conventional zerocrossing detection method.

The single-phase DPLL is implemented in virtual phase;that is, it is delayed by 90from the measured grid voltage

and is generated by passing through an all-pass filter, such as

s

ssH

)(

(5)

The all-pass filter in s domain is transferred into z do-

main by using a bilinear transformation, such as

1

1

)2(2

)2(2)(

zTT

zTTzH

cc

cc

(6)

Therefore, output y (k) (i.e., in virtual phase) from themeasure grid voltage Vgridcan be obtained as follows:

)1()()1()( kvkcvkcyky gridgrid (7)

where

2

2

c

c

T

Tc (8)

Fig. 10 shows the block diagram of digital PLL, where

Vgrid is the grid voltage, after which it transforms to Vds; Vqsis a virtual voltage through the aforementioned all-pass

filter; and Vds and Vqsare converted to Vde and Vqe in thesynchronous frame, where Vde denotes a difference be-tween grid voltage phase and estimated phase [7] [8] . Inaddition to phase estimation, digital PLL can calculate theinstantaneous peak value of the grid voltage. Vpeak is thesame as Vqe., and Vpeak is used to detect voltage sag and

swell in the utility.

Fig. 10. Block diagram of digital PLL.

2.6 Ac/dc Inverter for Unity Power Factor

Fig. 11 shows a block diagram of the current controller

in the dc/ac inverter, where current control using a pseudod-q transformation is employed, and which suggests stablecontrol performance. The proposed current control scheme

is verified through simulation, as shown in Fig. 12, whereIde is measured grid currentIgridandIqe is the virtual current

through all-pass filter. All voltages are in a synchronousframe. In this figure, Iqe and Ide are the dc values, whileVgridis in phase withIgrid.

Fig. 11. Block diagram of dc-ac inverter current control.


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3. Experimental Results

The overall system of the 3 kW PV PCS with line con-nection, as shown in Fig. 13, is implemented fully in a

software that adopts digital signal processing, TMS320F2812.The switching times of each converter are implementedfully in the software, and PWM pulses are generatedthrough an internal pulse generator of the DSP. Voltage andcurrent signals are measured by using an internal 12-bit-resolution analog-to-digital converter in the DSP. Further-more, a 4-channel 12-bit digital-to-analog converter is usedto display all of the waveforms. The switching frequencyof the inverter is 10 kHz while dead time is 3 sec. Hence,output command PWM is compensated by the dead time

effect.To verify the proposed CMPPT, the PV PCS is examined

by a test bed composed of a simulated utility source

(3P4S/1P2S; 12 kVA; model: NIS31411001), simulateddistribution line (12 kVA; model: NIS31477), and dcpower supply for PV simulation (Vmax = 600 V,Imax = 30 A,15 kW) from NF (Japan). This test bed is used to certifythe Korean license-to-sell protocol for the PV PCS. Theadopted PV array model is based on sol_dow_181U1F_ssPV module with the following electrical characteristicsduring standard testing: maximum delivered power, PM =186.792 W; short-circuit current,ISC = 7.9A; and open cir-cuit voltage, VOC = 32.4 V. Experimental results are pre-sented in Figs. 14-22. All experiments are conducted on thetest bed.

In the experimental test of PV PCS using CMPPT, cur-

rent reference Ipv* is either increased or decreased by adP/dI slope value every 1 ms. Therefore, CMPPT can reach

MPP more quickly and accurately because the dP/dI slopeis bigger than the dP/dV slope. Moreover, oscillation is

reduced by maintaining reference current Ipv*

at constantvalue. The experimental results are given to confirm thegood performance of CMPPT.

Fig. 12. Proposed PV PCS simulation by using PSIM.

Fig. 13 Prototype of the 3 kW PV PCS.

Fig. 14 shows the operation of DPLL, while traces (a)

and (b) represent the measured dq-transformation voltageVds and Vqs, where Vqs is generated from the all-pass filter.As shown in Fig. 15(a), programmable power supply

ES2000S (NF in Japan) generates voltage sag of 30% withrespect to utility grid (220 V, 60 Hz). Fig. 15(b) shows

negative peak voltage Vpeck , which is same as Vqe,. Thispeak voltage can be used to detect immediately the voltagesag. Figs. 16 and 17 show the experimental results by usingCMPPT when irradiation is at 0.5. Fig. 16(a) shows theutility current that is increased to MPP and then maintainedat MPP with perturbation. The PV array voltage, which isdecreased because of PV array characteristics, is shown inFig. 16(b). The PV array current increased to MPP current

by CMPPT is shown in Fig. 16(c).Fig. 17 shows MPP in V-I and P-V curves when irradia-

tion is at 0.5 and MPPT efficiency is around 99%.Figs. 18 and 19 show the experimental results of irradia-

tion change from 0.5 to 0.7 by using CMPPT. It exhibitssmooth transition and fast response for MPP.

Fig. 18(a) shows the utility current with changes in irra-diation ranging from 0.5 to 0.7. The utility current is in-

creased rapidly by the CMPPT controller, thereby generat-ing maximum power. Fig. 18(b) shows the decreased PV

Fig. 14. Experimental results of DPLL: (a) utility voltage

Vgrid;(b) all-pass filterVqs;(c) synchronized trans-

formation Vde;and (d) utility phase [4 ms/div].


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Fig. 15. Experimental results of peak voltage: (a) utilityvoltage Vgrid; (b) negative peak voltage Vpeck,Vqe;

(c) DA conversion Vgrid; and (d) utility phase

[10 ms/div].

Fig. 16. Experimental results of CMPPT (irradiation: 0.5):(a) utility current Igrid [2.5 A/div]; (b) PV arrayvoltage Vpv[30 V/div]; and (c) PV array current Ipv[1 A/div][2.5 s/div].

Fig. 17. Experimental results of CMPPT (0.5 irradiation).

array voltage used for tracking until MPP voltage respondsto irradiation change. Fig. 18(c) shows the PV array currentthat is increased rapidly until MPP current change occursas a response to CMPPT. Fig. 19 shows the MPP in P-Vcurve used to verify CMPPT operation maintained ataround 99% MPPT efficiency. Fig. 20 shows the CMPPToperation when irradiation is influenced by a sudden

change (i.e., from 0.7 to 0.2). With abrupt change, irradia-tion PV voltage is increased and PV current is decreased by

Fig. 18. Experimental results of CMPPT (irradiation change:0.5 - 0.7): (a) utility current Igrid[2.5 A/div]; (b) PVarray voltage Vpv[30 V/div]; and (c) PV array cur-rent Ipv[1 A/div][2.5 s/div].

Fig. 19. Experimental results of CMPPT (irradiation change:

0.5 - 0.7).

Fig. 20. Experimental results of CMPPT (irradiation change:0.7 - 0.2 ): (a) utility voltage Vgrid[100 V/div]; (b)PV array voltage Vpv[60 V/div]; (c) PV array cur-rent Ipv[2.5 A/div]; and (d) utility current Igrid[8A/div] [10 ms/div].

the CMPPT controller. At any condition, the CMPPT algo-

rithm operates continuously to locate MPP. Fig. 21 showsthe manual soft start from standstill to 3 kW power genera-tion. The utility voltage is shown in Fig. 21(a), while Fig.21 (b) shows the utility current that is quickly increased.The PV array voltage is reduced as characteristics in Fig.

21(c). Fig. 21(d) shows the PV array current that is in-creased in order to track MPP current by CMPPT


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The output grid current and voltage in 3 kW power gen-eration are shown in Fig. 22. It is noted that the grid cur-rent is exactly in phase with the grid voltage and is sinu-soidal current with low distortion.. The power efficiency of

PV PCS is around 96%.

Fig. 21. Experimental results of manual soft start fromstandstill to 3 kW power generation: (a) utility

voltage Vgrid [50 V/div]; (b) utility current Igrid[2.5A/div]; (c) PV array voltage [20 V/div]; and

(d) PV array current [1.6 A/div][0.25 s/div].

Fig. 22. Experimental results of utility: (a) utility voltageVgrid [50 V/div] and (b) utility current Igrid [2.5A/div][5 ms/div].

4. Conclusion

In this paper, the proposed CMPPT method has gener-ated a solar cell array current command directly by using a

modified incremental conductance method. In addition,controllers of dc/dc boost converter, dc-link voltage, anddc-ac inverter are designed and realized for the coordinatedoperation of these elements. The DPLL using an all-passfilter, which can detect phase and peak voltage, is pre-sented. PI control using a pseudo synchronous d-q trans-

formation is proposed for grid current control. A 3k W pro-totype PV PCS is built and tested to verify the proposedschemes. Experimental results of the PV PCS have re-vealed the advantages of CMPPT with respect to VMPPT.

Acknowledgements

This work is the outcome of the Manpower Develop-ment Program for Energy and Resources supported by the

Ministry of Knowledge and Economy (MKE)

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Sanghoey Lee received his B.S. degreein Instrumentation Control Engineeringfrom Konyang University, Korea in2002, and M.S. degree in Electrical

Engineering from Chungnam NationalUniversity, Korea in 2005. From 2005to 2007, he worked at the Institute forAdvanced Engineering in Yong-in,

Korea. He is currently pursuing his PhD degree in Electri-cal Engineering in Chungnam National University, Dae-

jeon, Korea. His research interests are power quality, ad-vanced converter and control for renewable energy systems,

and micro-grids.

Jae-Eon Kim received his B.S. andM.S. degrees from the University of

Hanyang in 1982 and 1984, respec-tively. He was affiliated with KERI asa researcher from 1984 to 1989; a sen-ior researcher form 1989 to 1996; and ateam leader of advanced distributionsystems and custom power lab from

1997 to 1998. He received his PhD from Kyoto University,Japan in 1996. He has been an assistant professor from1998 to 2004; an associate professor from 2004 to 2009;

and currently, a professor at Chungbuk National University.His current interests are analysis of power quality; opera-

tion and design of power distribution systems with distrib-uted generation; and advanced distribution systems, such

as micro-grids or smart grids.

Hanju Cha received his B.S. degree inElectrical Engineering from Seoul Na-tional University, Korea, and M.S de-gree in the same field from Pohang

Institute of Science and Technology,Korea in 1988 and 1990, respectively.He obtained his PhD in Electrical En-

gineering from Texas A&M University,College Station, Texas in 2004. From 1990 to 2001, he waswith LG Industrial Systems in Anyang, Korea where hewas engaged in the development of power electronics andadjustable speed drives. In 2005, he joined the Departmentof Electrical Engineering, Chungnam National University,Daejeon, Korea. He worked as a visiting professor inUnited Technology Research Center, Hartford CT, USA in2009. His research interests are high power dc-dc con-verter; ac/dc, dc/ac, and ac/ac converter topologies; power

quality and utility interface issues for distributed energysystem; and micro-grids.

design and implementation of photovoltaic

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