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Maximum Power Point Tracking Method for PV Array under Partially Shaded Condition Young-Hyok Ji Sungkyunkwan Univ. 300, Chunchun, Jangan, Suwon, Gyunggi, Korea [email protected]

Doo-Yong Jung

Chung-Yuen Won

Byoung-Kuk Lee

Jin-Wook Kim

Sungkyunkwan Univ. 300, Chunchun, Jangan, Suwon, Gyunggi, Korea [email protected]

Senior Member, IEEE Sungkyunkwan Univ. 300, Chunchun, Jangan, Suwon, Gyunggi, Korea [email protected]

Senior Member, IEEE Sungkyunkwan Univ. 300, Chunchun, Jangan, Suwon, Gyunggi, Korea [email protected]

SAMSUNG ElectroMechnics CO. LTD 314, Maetan3, Yeoungtong, Suwon, Gyunggi, Korea

Abstract -- Conventional popular MPPT methods are effective under uniform solar irradiance. However, under partially shaded conditions, these MPPTs can fail to track the real MPP because of the multiple local maxima which can be existed on PV characteristic curve under partially shaded condition. In spite of some researchers have worked on real MPP tracking under partial shading conditions, the methods have some drawbacks with complexity and requirement of additional circuit, etc. In this paper, a novel MPPT method that is capable of tracking the real MPP under partially shaded conditions is proposed. The performance of proposed MPPT is analyzed according to the position of real MPP and is verified by simulation and experimental results. Index Terms--Photovoltaic power systems, MPPT(Maximum power point tracking), Shaded condition.

I.

INTRODUCTION

The maximum power point tracking (MPPT) is usually an essential part of a photovoltaic power generation system, because of nonlinear characteristics of photovoltaic array. As such, many MPPT methods have been developed and implemented. When the entire array does not receive uniform solar irradiance (i.e. partial shading condition), the multiple local maxima appear on P-V characteristic curve of PV array. In spite of conventional popular MPPT methods (i.e. P&O, IncCond, RCC, Two-mode etc.) are effective under uniform solar irradiance, the presence of multiple local maxima reduces the effectiveness of the conventional MPPT methods. Actually, it is found that the power loss of commercial power conditioning system(PCS) can be as high as 70% under partial shading condition [1]-[4]. Because of the partial shading conditions (PSC) occur quite common due to clouds, trees, or buildings, it is necessary to develop special MPPT schemes that can track the real MPP under PSC. In spite of some researchers have worked on real MPP tracking under partial shading conditions [5]-[7], the methods have some drawbacks with complexity of method, tracking failure according to the real MPP position, and difficulties on the application to the installed power conditioning system, etc. In this paper, a new MPPT method for PV array under PSC

978-1-4244-2893-9/09/$25.00 ©2009 IEEE

[email protected]

is proposed. The operating principle and structure of proposed method is represented and the operation characteristics of the proposed MPPT are analyzed according to the various positions of real MPP as well as the change in solar irradiance. The simulation and experimental results are presented to verify the performance of proposed method. II.

EFFECTS OF THE PV ARRAY UNDER PSC

A.

Nonlinearity of PV Array Characteristics under PSC A PV array is composed of several PV modules connected in series-parallel to get the desired voltage and current. To protect modules from hot-spot problem, the bypass diodes are connected in parallel with each PV module. And the blocking diode is connected in series with each string, which is a group of series connected PV module, to protect the modules from the effect of potential difference between series connected strings. When the solar irradiance on PV array is identical, only one MPP is existed on the P-V characteristic curve of PV array. However, because of the bypass diodes and the blocking diodes, numerous local maximum power points (multiple local maxima) can be existed under partially shaded condition. Fig. 1 shows a PV array composed of 3×2 modules and its characteristic curves under PSC. There are two local MPPs on the P-V curve, however, only one of them is the real MPP.

307

IA

Ipv IscA

Blocking Diode

IscM PV Module

PV Module

Bypass Diode

Vpv V ocM

PV Module

VA

PV Module

P pv Real MPP

PV Module

V ocA

Local MPP

PV Module Shaded Module

V pv V ocM

V ocA

Fig. 1. Output characteristic curve of 3x2 PV array under PSC

To analyze the output characteristics of PV array under

PSC, an approximate model is presented in [8] as shown in Fig. 2. In this approximate model, it is assumed that the PV array is composed of NsM×NpM modules. A string is indicated that the group of series connected module, and the subscript x stands for the ‘string number’ (i.e. x = 1, 2, ⋯, NpM). Assume that the voltage drop across the bypass diode of shaded module equal to zero, the number of shaded module in a x-th string is expressed as NDx .

step (△V), the operating point is oscillated on vicinity of “point B”. At the same time, the difference in power capacity between PC and PB is lost due to this MPPT failure. To prevent this power loss, MPPT methods have to move the operating point to “point C”.

NpM IA IAx

String

PV Module

PV Module

PV Module

PV Module

PV Module

PV Module

VM

Fig. 3. MPPT failure in conventionl method under PSC

NsM

VA=VAx

PV Module

PV Module

PV Module

NDx

Shaded Module

X=1

X=2

X=NpM

Fig. 2. Approximate model of PV array for analysis the effect of PSC

The output current of PV array under PSC, IA, can be represented as in (1). N pM

I A = ∑ I Ax = x =1

N pM

∑I

x =1

scAx

  q (VA + RsAx I Ax − VocAx )   1 − exp     k A k BTN s ( N sM − N Dx )   

(1)

where the subscript A stands for ‘array’, and subscript M stands for ‘module’. VocAx is open-circuit voltage, and IscAx is short-circuit current of x-th string, respectively. T is temperature, and constants kA and kB mean the ideality factor and Boltzmann`s gas constant, respectively [8]. B.

MPPT Failures in Conventional Methods The efficiencies of conventional popular MPPT methods (i.e. P&O, IncCond, and Two-mode etc.) have been known as over 99% under uniform solar irradiance condition. However, the effectiveness of conventional MPPT methods could be reduced under PSC because of the multiple local maxima. Fig. 3 shows the reason that the tracking failure of conventional MPPTs under PSC. In Fig. 3, the operating point of PV array is on the “point A” before PSC is occurred. After PSC is occurred, the operating point is moved to “point B”. In this case, the real MPP is located on “point C”. Nevertheless, because of the conventional methods changes the operating point due to predetermined voltage reference

Actually, some researchers have worked on real MPP tracking under PSC. As mentioned above, however, these methods have some drawbacks with complexity of method, tracking failure according to the real MPP position, and difficulties on the application to the installed power conditioning system. Besides, some methods include shortcircuit or open-circuit condition that makes output power of PV array to be zero. To improve the drawbacks of conventional methods, the requirements that have to be considered in MPPT method under PSC are deduced. The requirements are as follows ; 1) It has to be applied to the installed power conditioning systems without additional circuits. 2) Under partial shading condition, it has to avoid shortcircuit or open-circuit condition. III. ALGORITHM FOR MPPT METHOD A.

MPPT Method under PSC Proposed MPPT method uses the simple linear function for tracking under PSC without any additional circuit. The block diagram of proposed MPPT is represented in Fig. 4. As shown in Fig. 4, proposed MPPT is based on the conventional incremental conductance (IncCond) MPPT with step-size variation. The step size is increased when the slope of P-V characteristic curve exceed predetermined value, e (in Fig. 4). And the step size is decreased again when the slope of P-V characteristic curve is less than predetermined value. The step-size variation technique is applied to minimize the energy loss due to the oscillation on vicinity of MPP, as well as, to make the dynamic performance better at rapid change in irradiance level. Proposed MPPT estimate that the PV array is under PSC, when the (2) and (3) are satisfied.

308

B.

Performance According to Real MPP Position The maximum number of local maxima can be affected by the number of parallel connected string as expressed in (1). As the number of strings is increased, the real MPP can be located in various positions on P-V characteristic curve. Therefore, the tracking performances related to the real MPP position have to be considered. In this paper, the performance of proposed MPPT is analyzed according to the real MPP positions, and the positions are divided into three-cases according to the voltage level. Fig. 6 shows the tracking performance in each case of the real MPP is on the low voltage region (case 1), Fig. 7 shows that is on the high voltage region (case 2), and Fig. 8 shows that is on the middle voltage region (case 3). From the following figures, it is confirmed that the proposed MPPT can track the real MPP at any voltage region.

Start

Sense Vpv[n], Ipv[n]

Calculation ΔVpv = Vpv[n] － Vpv[n-1] ΔI pv = I pv[n] － I pv[n-1]

G pv = | Gpv | < e Yes

Ipv ∆ Ipv + V pv ∆ V pv

No

α = Small

Yes Partial Shading ?

α = Large

Vpv_ref =

NS × VOC Ipv[n] NP × ISC

No Return α

ΔVpv = 0 No

Yes

Yes

Yes Gpv = 0

Δ I pv = 0

Gpv

Δ I pv > 0

No

No

Yes

Yes >

0

No

No Vpv_ref = Vpv_ref －ΔV×α

Vpv_ref = Vpv_ref －ΔV×α

Vpv_ref = Vpv_ref +ΔV×α

Vpv_ref = Vpv_ref ＋ΔV×α

Return

Fig. 4. Block diagram of proposed MPPT

Ipv ISC(array)

∆Vpv = V pv [ n ] − V pv [ n − 1] < ∆VSET

∆I pv

I pv [ n − 1]

=

I pv [ n] − I pv [ n − 1]

I pv [ n − 1]

< − ∆I SET = −

(2) I pv [ n]

C A

D

IA

B

(3)

N pM

IB

If (2) and (3) are satisfied, the voltage reference is changed by the linear function that is described in (4).

Ppv

VC

VD

VA

Vpv VOC(array)

A

 N ×V V pv* =  sM ocM  N pM × I scM 

  × I pv [ n ] 

(4) D

B

C

Equation (4) moves the operating point toward the lower voltage level. After the operating point is arrived at the reference point, MPP is tracked again by the IncCond MPPT. The operation principle of proposed MPPT is presented in Fig. 5. The ①~③ means that the operating sequence when the PSC is occurred.

Vpv VC

VD

VA

Fig. 6. Tracking performance in case 1

Fig. 7. Tracking performance in case 2

Fig. 5. Operation principle of proposed MPPT

309

The performance of proposed MPPT under slowly changed circumstance is almost same with that of conventional IncCond MPPT because the proposed method is based on IncCond method. Actually, the differences between proposed MPPT method and conventional method are appeared when the circumstance is changed rapidly.

Ipv ISC(array)

A

IA

C D B

IB

Ppv

VC

VD

VA

Vpv VOC(array)

A

D B

C

Vpv VC

VD

VA

(a) Conventional IncCond MPPT Fig. 8. Tracking performance in case 3

IV. SIMULATION RESULTS To confirm the performances, proposed MPPT method is compared with the conventional MPPT by using PSIM 6.0. The PV output characteristic curve is implemented by (1), and it is verified in early study as in [8]. Fig. 9 shows the change in operating point position when the surrounding conditions are changed slowly. Fig. 9(a) is under slowly changed uniform irradiance condition, and (b) is under slowly changed PSC.

Fig. 10. Performance comparison when the rapid PSC is occurred

(a) Due to slow change in overall irradiance

(a) Conventional IncCond MPPT

(b) Due to slow change in the number of shaded modules

(b) Proposed MPPT

Fig. 9. Change in operating point under slowly changed circumstance

Fig. 11. Performance comparison according to rapid change in irradiance

310

(b) Proposed MPPT

Fig. 10 shows the difference in performance between conventional and proposed MPPT method when the rapid PSC is occurred. As in Fig. 10(a), it is confirmed that the conventional method fails to track real MPP, and the operating point stay in about 1500[W] point. On the other hand, the proposed MPPT can track the real MPP. It is shown that the operating point is arrived in about 2300[W] point as in Fig. 10(b). Fig. 11 shows the difference in performance between conventional and proposed MPPT methods when the uniform irradiance on the entire PV array is rapidly changed. In this case, the performance of conventional IncCond method has better performance than the proposed method. It is because of the proposed MPPT moves the operating point when both (2) and (3) are satisfied. To overcome this drawback, the step size variation technique is essential. V.

EXPERIMENTAL RESULTS

To verify the performance of the proposed method, the proposed MPPT method is applied to two-stage power conditioning system rated in 3[kW]. Developed prototype is composed of a boost converter and a single-phase full-bridge inverter. The block diagrams of prototype test bed and its controller are shown in Fig. 12.

12 Modules

Fig. 13. Cofiguration of parallel connected PV array

Fig. 14 shows the prototype test bed. In this system, DSP TMS320F2812 of Texas Instruments is used as a main controller, and a Mitshubishi IPM PM75B5LB is used as the switching device.

Fig. 14. Prototype test bed used in experiment

Fig. 12. Block diagram of the test bed

PV array is composed of 12(6×2) modules rated 150[W], and total experimental capacity is 1.8[kW]. The PV array is shown in Fig. 13, and the specification of PV module is presented in table I. TABLE I SPECIFICATION OF PV MODULE Value Parameter Simbol Open-circuit Voltage Voc 43.4 Short-circuit Current Isc 4.8 Max. Power Voltage Vmp 34.1 Max. Power Current Imp 4.4 Max. Output Power Pmax 150

Unit V A V A Wp

Fig. 15 and 16 show the experimental results of MPPT. The PV voltage and current waveforms are represented in lower side of the figure, and its operating points are also represented on the I-V plane. Fig. 15 shows the performance of IncCond MPPT when the 3 modules of a string, which is composed of 6 modules, are shaded. It is confirmed that the output power under PSC is about 600[W], against the power before PSC is about 1100[W]. From this figure, it is clearly shown that the voltage of the operating point is rarely changed, and the current is reduced under PSC. Consequently, about 45% of the output power is lost due to the shading on 3 modules among the 12 modules. On the other hand, in Fig. 16, it is confirmed that the output power under PSC is about 550[W], against the power before PSC is about 850[W]. In Fig. 16, it is shown that the voltage of the operating point is reduced, and the current is increased under PSC. The power loss due to the same shading condition is presented about 35%.

311

VI. CONCLUSION Conventional popular MPPT methods, namely the P&O and the IncCond method, can not track the real MPP under partially shaded condition (PSC) because of the multiple local maxima which can be existed on P-V characteristic curve under PSC. To overcome this tracking failure, a new MPPT method for PV array under PSC is proposed in this paper. Proposed method is based on IncCond method with step-size variation. When PSC is detected, the proposed MPPT changes the voltage reference by linear function shown in (4). After the movement of operating point, proposed MPPT operates as conventional IncCond to find the real MPP. To verify the performance of proposed MPPT method, simulations and experiments are performed. From the experimental results, it is confirmed that the power loss of proposed method is lower than that of conventional IncCond MPPT about 10[%].

Fig. 15. Performance of conventional MPPT under PSC (100V/div, 2A/div, 200ms/div)

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[3]

[4]

[5]

Fig. 16. Performance of proposed MPPT under PSC (100V/div, 2A/div, 200ms/div)

[6]

Fig. 17 shows the output waveforms of the power conditioning system connected to the utility (220[V], 60[Hz]). It is shown that the system provides the active power to the utility.

[7]

[8] Grid voltage (100V/div)

Output current (5A/div)

Detected phase

DC-link voltage (100V/div)

Fig. 17. Output waveforms of power conditioning system (100V/div, 5A/div, 10ms/div)

312

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