70

Improved Photovoltaic Conversion Chain with Interleaved Method.

C. Cabal*, A. Cid-Pastor**, L. Seguier*, B. Estibals*, C. Alonso*.* University of Toulouse/LAAS-CNRS, Toulouse, FRANCE. ** University of Rovira I Virgili/D.E.E.A, Tarragona, SPAIN.

Abstract - This paper presents a parallel connection of three-cell interleaved boost converters used as an adaptation stage dedicated to photovoltaic applications. Indeed, this structure associated to a MPPT control improves the efficiency and reliability of a photovoltaic power conversion chain. An additional current sharing control has been incorporated to assure a uniform distribution of power between each dc-dc converter. Every voltage and current ripples present on the load and photovoltaic (PV) source implying the switch stresses are reduced with this technique compared to a classical structure. Finally; experimental results show that the proposed technique can be used for impedance matching of a PV array.

I. INTRODUCTION

Generally, a photovoltaic conversion chain is constituted by a PV array, a dc-dc switching converter and a dc load. The dc-dc converter tracks permanently the maximum power point (MPP) of the PV array. The MPP depends on the irradiance, temperature, and shadowing conditions. Most of the photovoltaic energy systems are combined with power switching converters in accordance with the application. In order to obtain the MPP of this intermittent source, these power stages are associated to a function called maximum power point tracking (MPPT), which controls the PV dc source parameters (voltage or/and current) to set the PV operating point at its MPP (Fig. 1). Many MPPT techniques for photovoltaic systems have been established in the literature. Nevertheless, the Perturb and Observe (P&O), Incremental Conductance (IncCond) and Hill Climbing methods are widely used because of their easy implementation, despite the oscillations around the MPP and their drawbacks with respect to rapidly changing atmospheric conditions. Nowadays, these problems are partially solved by the digital circuits, such as microcontroller, Digital Signal Processor (DSP) and Field Programmable Gate Array (FPGA), which adapt the value of the step size to improve the accuracy and the fastness [1-5]. The MPPT efficiency obtained by this type of command is often up than 99%, which corresponds almost to the photovoltaic maximum power point.

On the other side, in order to transfer the maximum power to the load, the converter efficiency must be also improved. The reduction of the power losses in the adaptation stage can be obtained by means of parallel connection of several dc-dc converters. This connection mode, often used in several high power applications, can allow a uniform distribution of the global power among the dc-dc converters. This approach offers some advantages compared with a single high power converter.

Indeed, paralleling mode increases the power processing capability and improves the reliability because stresses are better distributed and fault tolerance is guaranteed [6-10]. Moreover, a uniform stresses is achieved when the input current is shared appropriately among the converters [11-14].

For the photovoltaic applications, this parallel solution requires a perfect compatibility between the MPPT control and the current-sharing control. Some concepts associating both control schemes are illustrated in the literature. For example, Kasemsan Siri proposes in [15] a control scheme for parallel connected converter system admitting the maximum available power transfer from a photovoltaic generator. A central limit current distribution is added to the system to improve the efficiency and reliability by uniformly distributing the total peak power among the buck converters connected in parallel. A coupled inductor boost converters are used in [16] to share the current and to distribute the PV maximum power trough each converter cell. One more, the interleaved mode is inserted in the system to decrease the input and output ripples of the electrical signals.

In this paper, an adaptation stage realized by means of three-cell interleaved boost converters is proposed. The motivations are based on the properties of interleaved mode and parallel connection. The interleaved approach allows a reduction of the voltage and current ripples and the size of the input and output filters. The parallel association increases the converter efficiency and presents modularity in converter design. This property can be adapted to high photovoltaic power applications with numerous dc-dc switching cells. The power distribution is obtained by means of a democratic current sharing without perturbing the MPPT behaviour.

PV

BatterySWITCHED

MODE POWER

CONVERTER

MPPT CONTROL

IPV

VPV

D

Figure 1 : Elementary photovoltaic chain conversion.

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

71

D1

D2

D3

31

Filter

Filte

r

Filter

+

++

ILref

+

IL1

IL3

IL2

-

+

+

Vc

1 Vc1

-

+

+

Vc

Vc2

+

-

+

+

Vc

Vc3

+

0

120

240

2

3

PWMGenerator

V PV

IPV

BOOST 3 Battery24 V

PIC

18F

1220 TRACK

D1

D2

D3

IPV

V PV

MPPT Control

DC LoadPV Generator BOOST 2

BOOST 1

C INC OUT

Integrator

IL1

IL2

IL3

VC

Figure 2 : Photovoltaic conversion chain realized with three interleaved Boost converters.

II.

A.

DESCRIPTION OF THE SYSTEM

The block diagram of the proposed system is illustrated in Fig. 2. The system consists of a non-ideal power source, three dc-to-dc boost converters, a load and associated control circuits. The converter topology is chosen depending on the load value and the open circuit voltage of the PV array. In this study, a battery having its nominal voltage above the typical voltage of the PV source is considered. Three-cell interleaved boost converters are associated to a MPPT control and a current distribution control for an optimum MPP tracking of the PV generator. The principle of each circuit is explained in next subsections.

Maximum Power Point Tracking The MPPT designed in our laboratory is based on the

Extremum Seeking principle. It has been recently demonstrated [17] that an Extremum seeking algorithm is stable since Lyapunov criterion and then, it can be used to track MPP of a PV generator. In practical case, it operates by varying the duty cycle of a PWM switched converter. This method checks permanently the sign of PV power derivative to obtain at each time the direction of MPP research.

Thus, a positive power derivative moves the operating point towards the MPP; and if not, the system goes away from its objective. The corresponding algorithm (Fig. 3) is implemented in a PIC microcontroller 18F1220 which guarantees a compromise between performances and cost [18]. The direction of MPP research is represented in the flowchart by the TRACK signal which has two typical values (0 or 1). For example, when the derivative is positive the TRACK signal keeps its value. If the derivative is now negative, the value of the TRACKsignal must theoretically change. In practical, a delay function is needed to avoid parasite change linked to electronic noises and dc-dc converters transients. Then, this function delay guarantees that the sign detected of the PV power derivative corresponds to the desired sign. It can be notice that some of other MPP controls lose the MPP when the PV array is subject to brutal

variations of irradiation. Then, the change of the TRACK signal is authorised after a time delay improving the dynamic behaviour of the research during a brutal solar variation ensuring a high robustness.

In our application, for a single switching converter, the TRACK signal is modified by a circuit integrator (VC) and then compared to a PWM generator providing a duty cycle value necessary to control the power stage. In summary, the Extremum seeking algorithm forces the PV system to reach the MPP at each time by increasing or decreasing the PV voltage VPV with a constant delay chosen higher than a time constant of the power stage. This mode of research induces little oscillations around the MPP inducing 1% of PV power losses.

Start

Measure of PVI et PVV

Calcul of PVP

PVPVPV IVP

Calcul of

PV

PVPV dV

dPD

Test0PVD

YES

Track = 1 Track = 0

NO

H = 0 H = 1

Start delay

H = 1

Legend : Track : logical input signal integrator H = 1 : Delay time achieved H = 0 : Delay time not achieved

H = 0

Track = 1 Track = 1 Track = 0Track = 0 Track = 0 Track = 1

Track = 0 Track = 1

Start delay

Figure 3 : Extremum seeking algorithm.

72

B. Current Sharing-Distribution.The stress distribution among the different boost

converters connected in parallel can be performed by means of different current distribution strategies. Fig. 2 shows a block diagram describing a current distribution scheme for three dc-dc converters connected in parallel. The current ILref establishes the reference current for each converter. This current is obtained by weighting the input values of each converter as follows:

332211 LLLLref IIII (1)

where IL1, IL2, and IL3 are respectively input currents of each boost converter.

The current distribution can be realized by means of a democratic current sharing [14] or a master slave distribution [11]. Fig. 2 illustrates three boost converters parallel connection with democratic current-sharing. This current distribution control implies μ1 = μ2 = μ3 = 1/3 which is the optimum value from component stress. Thus the equation (1) can be rewritten as:

)( 32131

LLLLref IIII (2)

where ILref represents the mean value of the photovoltaic current IPV. To generalize for N cell converters, the μ variable of the democratic current sharing can be determinate as :

N1

(3)

Globally, the MPPT behaviour is not modified. Thus, from its inputs VPV and IPV, it generates the output VCnear the instantaneous MPP. Also, different current error signals ( 1, 2, and 3), corresponding to the respective difference between the reference current ILrefand each converter input current, are established. Then, they are added respectively to VC in order to make a minor adjustment of each converter and then generate the different control signals VC1, VC2 and VC3. This type of control allows both a perfect track of instantaneous MPP and an optimum share of the photovoltaic current between each switching cell.

C.

III.

IV.

Interleaved ConverterThe interleaved mode requires that the N parallel

connected converters operate at constant switching frequency and each phase is shifted with respect to one another by 2 /N radians. As a result, the global input and output waveforms exhibit low ripple amplitudes and the switching and conduction losses are significantly reduced. In consequence, this operating mode can reduce sizes and/or losses of the input and output filtering stage. In our example case (Fig. 2), the interleaved mode conception is validated through three saw tooth signals working at the same frequency but shifted to one another by 120°. The comparison

between the different converter control signals VCi and the three saw tooth supplies each Di duty cycle value necessary for each converter to share correctly the PV current and to track the MPP.

EXPERIMENTAL SYSTEM DESCRIPTION

A 100W experimental prototype was developed to validate the MPPT control associated to the democratic current sharing. Each adaptation stage has been designed to process a nominal power of 33 W. Each commutation cell is made by a power MOSFET (IRF9410) with its driver circuit and a schottky diode 30CWQ04FN. The input and output passive elements are respectively L1 = L2 = L3 = 33μH, CIN = 3 μF and COUT = 4 μF. The PWM generator is realized with three voltage comparators LM311. The switching frequency imposed by the saw tooth is chosen equal to 180 kHz. The MPPT algorithm is implemented inside the PIC 18F1220. To make a comparison and show the improvement supplied by the new structure, a classical boost converter of 100W has been also designed. The MOSFET and the diode used are respectively referenced IRF7832 and 6CWQ04FN, and the passive elements are L = 33μH, CIN = 3 μF and COUT = 4 μF. These values are chosen to make a real comparison of performances.

EXPERIMENTAL RESULTS

A set of outdoor measurements has been made in order to investigate the influences of the proposed structure on the behaviour of the MPPT. The electrical features of the PV array are reported in the table I, and a lead storage battery with a nominal voltage of 24V is used as a DC load.

Steady state waveforms of the photovoltaic chain conversion for a constant radiation level are illustrated in Fig. 4. These waveforms show the achievement of the MPP and the uniformly photovoltaic current distribution among the different converters. It is shown in this figure the compatibility between the MPPT control and current-sharing control.

The information displayed allows to determinate the MPPT efficiency. This one corresponds to the ratio between the PV extracted average power and the maximum power supplied by the PV module. The average power in the Fig. 4, is 55 W for a maximum power of 55.53 W, given a MPPT efficiency of 99%.

TABLE I. ELECTRICAL FEATURES OF PV BP585.

Maximum power (Pmax) 85 W

Voltage at Pmax (Vmp) 18 V

Current at Pmax (Imp) 4.72 A

Short circuit current (Isc) 5.0 A

Open circuit voltage (Voc) 22.1V

73

Figure 4 : Steady state behaviour of photovoltaic chain conversion.

Figure 5 : Inductor (IL1, IL2 and IL3) currents and photovoltaic current.

Fig. 5, illustrates the photovoltaic current sharing between each converter and the photovoltaic current ripple elimination obtained by means of the interleaved mode.

Fig. 6, illustrates the control signals distribution effect on the MPPT signal (VC). This command adapts

PPV

VC1IPV

VCVPV VC2

IL1 VC3Ch1 = VCCh2 = VC1Ch3 = VC2Ch4 = VC3

IL2

Figure 6 : Converter control signals (VC1, VC2 and VC3) and the MPPT research signal (VC).

the converter control signals(VC1, VC2 and VC3) adding the current errors ( 1, 2 and 3) in order to obtain the MPP maintaining a good current sharing. IPV

The dynamic response of the system for step change of the solar radiation is exposed in Fig. 7. The democratic current-sharing is able to maintain equal distribution current without perturbing the MPP tracking when the system is submitted to this abrupt variation. The MPPT setting time necessary to achieve the new maximum power point is estimated at 15 ms.

IL1 IL2 IL3

Fig. 8.a and 8.b shows respectively the steady state behaviour of the PV electrical variables for a three boost converter operating in interleaving mode and for a single boost converter. Similarly, Fig. 9.a and 9.b shows the current and battery voltage.

The interleaved mode increases the switching frequency of the electrical signals by a factor N in the both input and output converter ports. As a result the filtering stages action is increased and the electrical signals ripple is almost eliminated compared to a classical structure. This result decreases the noise effect on the PV sampling signals implying an improvement of the MPPT performances.

a)

VPVVPVPPV

IPVIPV

IL1 IL2IL1 IL2

PPV

b) Figure 7 : Dynamic response

a) during a solar radiation increase, b) during a solar radiation decrease.

74

a)

VPV VPVIPV IPV

IL

IL1 IL2

b) Figure 8 : IPV and VPV waveforms

a) three cell interleaved boost converters, b) single boost converter.

a)

VBAT VBAT

IBAT IBAT

IL

IL1 IL2

b) Figure 9 : IBAT and VBAT waveforms

a) three cell interleaved boost converters, b) single boost converter.

V. CONCLUSION

It has been shown in this paper that three cell interleaved boost converters can be used to research the maximum power point of a PV array. A current sharing control is inserted to distribute the total peak power among the different DC-DC converters improving photovoltaic conversion chain efficiency. In addition, we reduce losses and stress in the power switches increasing reliability and lifetime of each converter. The interleaved mode decreases the noise level of the sensed signals implying better MPPT performances. The compatibility between the MPPT control and current-sharing control has been validated experimentally. Experimental results illustrate the benefits introduced by the interleaved current-sharing approach compared to a classical structure.

REFERENCES

[1] N. Jhaehintung, T. Wiangtong, S. Phaophak, “FPGA Implementation of MPPT Using Variable Step Size P&O

Algorithm for PV applications,” Communications and Information Technologies, 2006. ISCIT’06 International Symposium on. pp. 212–215, October 2006.

[2] D. Sera, T. Kerekes, R. Teodorescu, F. Blaabjerg, “Improved MPPT method for rapidly changing environmental conditions,” Industrial Electronics, 2006 IEEE International Symposium on. vol. 2, pp.1420–1425, July 2006.

[3] X. Weidong, W.G. Dunford, “A modified adaptive hill climbing MPPT method for photovoltaic power systems,” Power Electronics Specialists Conference, 2004. PESC 04. 2004 IEEE 35th Annual. vol. 3, pp.1957–1963, June 2004.

[4] J. H. Lee, H. S. Bae, B. H. Cho “Advanced Conductance MPPT Algorithm with a Variable Step Size,” Power Electronics and Motion Control Conference, 2006. EPE-PEMC 2006. 12th

International. pp.603–607, Aug. 2006. [5] H. Koizumi, K. Kurokawa, “Plane Division Maximum Power

Point Tracking Method for PV Module Integrated Converter,” Industrial Electronics, 2006 IEEE International Symposium on. .vol 2, pp.1265–1270, July 2006.

[6] L. Luo, Z. Ye, R.-L. Lin and F. C. Lee, “A classification and evaluation of paralleling methods for power supply modules,” in Proc. IEEE Power Electronics. pp.901–908, 1999.

[7] R. Giral, L. Martinez-Salamero, and S. Singer, “Interleaved converters operation based on CMC,” IEEE Trans., pp.643–652, July 1999.

75

[8] R. Giral, L. Martinez-Salamero, R. Leyva, J. Maixé, “Sliding-Mode control of interleaved boosts converters,” IEEE Trans. vol 47, pp.1330–1339, Sept. 2000.

[9] D. J Perreault, J. G. Kassakian, “Distributed interleaving of paralleled power converters,” IEEE Transaction on. vol 44, pp.728–734, Aug. 1997.

[10] S. C. Babu, M. Veerachary, “Predictive controller for interleaved boost converter,” IEEE International Symposium on. vol 2, pp.577–581, June 2005.

[11] Y. Panov, J. Rajagopalan, and F. C. Lee, “Analysis and design of N paralleled Dc-DC converters with master-slave current sharing control,” APEC 97 Conference Proceeding, vol 1, pp.436–442, Feb. 1997.

[12] K. Siri, C. Q. Lee, T. F. Wu, “Current distribution control for parallel connected converters: part I,” IEEE Trans. Aerospace and Electronic Systems, vol. 28, pp.829–840, July 1992.

[13] K. Siri, C. Q. Lee, T. F. Wu, “Current distribution control for parallel connected converters: part II,” IEEE Trans. Aerospace and Electronic Systems, vol. 28, pp.841–851, July 1992.

[14] M. Ponjavic and R. Djuric, “Current sharing for synchronized dc-dc converters operating in discontinuous conduction mode,” Proc. Inst. Elect. Eng., vol. 152, pp.119–127, Janv. 2005.

[15] K. Siri, V. Caliskan, “Maximum power point tracking in parallel connected converters,” IEEE Transactions on. Aerospace and Electronic Systems, vol. 29, pp.935–945, July 1993.

[16] M. Veerachary, T. Senjyu, and K. Uezato, “Maximum power point tracking of coupled inductor interleaved boost converter supplied PV system,” IEE proceeding Electric Power Applications, vol. 150, pp.71–80, Jan. 2003.

[17] R. Leyva, C. Alonso, and A. Cid-Pastor, “MPPT of photovoltaic applications using Extremum Seeking Control,” IEEE Aerospace and Electronic Systems, vol. 42, pp.249–258, Jan. 2006.

[18] C. Cabal, C. Alonso, A. Cid-Pastor, B. Estibals, L. Seguier, R. Leyva, G. Schweitz, J. Alzieu, “Adaptive digital MPPT control for photovoltaic applications,” Industrial Electronics, 2007, ISIE 2007, International Symposium on. pp.2414–2419, June 2007.