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Transcript
Comparative Analysis of Series and Parallel
Photovoltaic Arrays Under Partial Shading Conditions
Renan Diego de Oliveira Reiter, Leandro Michels, José
Renes Pinheiro
Rene Alfonso Reiter, Sérgio Vidal Garcia Oliveira,
Adriano Péres
Grupo de Eletrônica de Potência e Controle (GEPOC)
Universidade Federal de Santa Maria (UFSM)
Santa Maria, Brazil
[email protected], [email protected],
[email protected]
Departamento de Engenharia Elétrica e Telecomunicações
Universidade Regional de Blumenau (FURB)
Blumenau, Brazil
[email protected], [email protected], [email protected]
Abstract—This paper presents a comparative analysis among
different arrays of photovoltaic (PV) modules under partial
shading conditions. This scenario is typical of a residential area
where partial shading can occur due to trees or neighbor
buildings. The main objective is to evaluate the power extraction
under partial shading in parallel and series PV modules arrays,
and compare the results. Simulation and experimental results
are presented to validate the proposed analysis.
Keywords—Photovoltaic systems, parallel arrangement, DC-DC
converter.
I.
INTRODUCTION
Photovoltaic (PV) systems have been the fastest growing
energy technology over the last decade. This technology is
been disseminated in residential applications employing
underutilized spaces on rooftops [1].
Many different topologies of PV systems have been
developed for residential applications, such as central and
multi-string inverters [2]. Central inverter topology uses a
single converter to process energy from PV models while
multi-string uses more than one converter. Due to reduction of
cables and protective devices among PV modules and the
converter, most PV systems are based on central inverter
topologies.
Single-stage central inverter (CI) topologies have been
widely used due to its simplicity and high converter
efficiency, since it does not requires a step-up dc-dc converter.
Normally these PV systems use a large number of modules in
series for direct connection of the PV array to the inverter. The
main drawback of series arrays is the lower utilization factor
of the PV system under shading conditions [3]-[7].
This limitation is not a problem in most sites where
residential PV systems are widely used. In sub-tropical and
temperate climate sites such as in Germany, most residential
buildings are designed to receive the maximum quantity of
solar irradiation. Differently from many sub-tropical countries,
most residential areas in Brazil present leafy trees. These trees
are intentionally planted to reduce indoor heat due to the
tropical climate. Although the removal of these trees could
result in better performance of PV systems, most people are
not likely to do it. As a result, there is a demand for residential
PV systems optimized for partial shading conditions.
The reduction of energy production due to partial shading
can be mitigated in central inverters using parallel or seriesparallel arrangements of PV modules. However, the output
voltage of these PV arrays is lower than the series one. As a
result, series-parallel arrays of PV modules normally demand
the use of an additional dc-dc converter to step-up voltage
from PV array to inverter [3].
The objective of this paper is to evaluate the energy
production of series and series-parallel PV arrays under partial
shading conditions. Two different arrays with twelve PV
modules are analyzed. Simulation and experimental results are
provided to determine the advantages and disadvantages of
each PV array under this condition.
II.
CRITERIA OF COMPARISON
This section describes the criteria used to investigate the
impact of partial shading on energy production of central
inverter PV systems [1].
Irradiance, temperature, and partial shading pattern are the
factors with greater influence in the maximum peak of
available power from a PV array. It is worth mentioning that
the influence of partial shading pattern is directly related to the
arrangement of PV modules.
Shaded modules decrease the maximum available power
of the PV array and can produces multiple peaks in powervoltage curve. Moreover, it modifies the voltage of the PV
array at maximum power point (MPP) and can change voltage
range of the central inverter [8]-[12].
Assuming these factors, were considered the following
criteria for comparison of different PV arrangements under
partial shading conditions:




VDC
Array Utilization Factor (AUF): indicates the percentage
of generation in relation to the maximum potential
power. It is the ratio between the maximum output power
of PV array in relation to the maximum power
generation considering the array unshaded;
DC
V
DC
V
DC
V
DC
(a)
V
Fill-Factor (FF): indicates the percentage of generation
of available power under shading. It is the ratio between
the maximum output power of PV array in relation to the
sum of maximum output power of each individual PV
module [3];
DC
(b)
Voltage at Maximum Power Point (VMPP): indicate the
PV array voltage at global maxima power generation;
V
Number of Maximum Power Peaks (NMPP): indicate the
number of local maxima peaks of power versus voltage
curve.
III.
V
DC
(c)
MATERIALS AND METHODS
This paper presents a comparison based on simulation and
experimental results. Both analyses consider twelve Kyocera
KD135SX-UPU PV modules [13], which results in a total PV
installed power of 1620 Wp.
PV modules were arranged in two different series-parallel
configurations: i) four strings with three modules connected in
series (array 3x4); and, ii) two strings of six modules
connected in series (array 6x2). First configuration presents
characteristic of series array while second one of parallel
array.
Four cases were chosen to represent typical residential
roofs partial shaded by a building or a tree. Fig. 1 presents
four different shading patterns considered in this analysis: i)
Case I: no shading; ii) Case II: two modules are completely
shaded. iii) Case III: three modules are completely shaded;
and iv) Case IV: three modules are completely shaded and two
are partially shaded. In these figures, white PV modules are
not shaded, light gray modules are partially shaded, and dark
gray modules are completely shaded.
Both simulation and experimental curves were obtained
connecting an adjustable resistor in parallel with the PV array.
For each test condition this resistance was swept from zero
(short-circuit) to infinite (open-circuit). Measuring the current
and voltage for several points were plotted power versus
voltage curves.
V
(d)
Figure 1. PV array configuration: Left (Array 3x4), Right (Array 6x2).
(a) Case I. (b) Case II. (c) Case III. (d) Case IV.
in experimental tests. This difference is due to the
characteristic of the pyranometer that measured the global
horizontal irradiance, does not assuming that all PV modules
are tilted.
IV.
RESULTS
A. Simulation results
Four different simulations were performed to determine
the criteria of comparison defined in Sect. II, considering
shading patterns shown in Fig. 1. The irradiances used in the
simulation for the PV modules, according to Fig. 1, are 600
W/m² (white), 0 W/m² (dark gray), and 400 W/m² (light gray).
Fig. 3 presents simulation results of power versus voltage
curve for shading patterns. In this figure, the black plot is the
response of the 3x4 PV array while the gray plot is the 6x2 PV
array one. Fig. 3(a)-(d) show the results for shading patterns of
cases I to IV presented in Fig. 1, respectively. Table I presents
AUF, FF, VMPP, and NMPP obtained in these simulations.
Fig. 2 presents the setup used for experimental tests. Fig.
2(a) shows the PV array, Fig. 2(b) show the adjustable
resistors, Fig. 2(c) the data acquisition board for measurement
of current, voltage and global horizontal irradiance, and Fig.
2(d) the pyranometer Apogee SP-110 used to measure the
irradiance [14].
Simulations were carried out using PSIM® software [15].
The temperature of 48°C measured in experimental tests was
considered for all simulations. On the other hand, solar
irradiance used for simulations are lower than those obtained
DC
TABLE I.
SIMULATION RESULTS
Array 3x4
Case I
Case II
Case III
Case IV
AUF
(%)
FF
(%)
100
75
75
68
100
90
100
97
Array 6x2
AUF
VMPP
NMPP
(%)
(V)
43
43
43
44
1
1
1
1
100
75
75
68
FF
(%)
100
86
72
72
VMPP
NMPP
(V)
86
62
47
86
1
2
2
2
1200
Power (W)
1000
Array 3x4
881W Array 6x2
881W
800
600
400
200
0
0
20
40
60
80
Voltage (V)
(a)
(a)
661W
Power (W)
800
400
200
20
40
Power (W)
60
80
Voltage (V)
(b)
100
120
140
Array 3x4
Array 6x2
661W
600
475W
441W
400
200
0
20
40
800
Power (W)
140
441W
800
(c)
120
Array 3x4
Array 6x2
632W
600
0
(b)
100
60
80
Voltage (V)
(c)
100
425W
140
Array 3x4
Array 6x2
597W
600
120
441W
400
200
0
20
40
60
80
Voltage (V)
(d)
100
120
140
Figure 3. Simulation results: power versus voltage curve.
(a) Case I (b) Case II. (c) Case III. (d) Case IV.
modules were slightly changed due to small changes in the
environmental conditions.
(d)
Figure 2. Experimental setup. (a) Photovoltaic Array. (b) Adjustable resistor.
(c) Data acquisition system. (d) Pyranometer.
B. Experimental results
Experimental tests were performed considering similar
condition of the simulation results presented in Sect. IV A.
During these tests the temperature and irradiation levels of PV
Fig. 4 presents experimental results of power versus
voltage curve for shading patterns shown in Fig. 1. In these
tests, the horizontal global irradiances were approximately 675
W/m² (white), 445 W/m² (light gray), and 0 W/m² (dark gray).
The measured temperature of PV modules was 48°C. Fig.
4(a)-(d) show the results for shading patters of cases I to IV
presented in Fig. 1, respectively.
Table II presents AUF, VMPP, and NMPP obtained from
experimental results. The AUF was calculated considering
global maxima power obtained in this Case I. The FF can be
considered 100% in this case because there is no shading. It is
worth mentioning that FF was not measured experimentally
due to complexity of setup, since it requires an individual
converter for each PV module.
EXPERIMENTAL RESULTS
Array 3x4
Case I
Case II
Case III
Case IV
AUF
(%)
FF
(%)
100
76
74
67
-
AUF
VMPP
NMPP
(%)
(V)
39.4
39.4
39.4
37.8
1
1
1
1
100
66
51
53
Array 6x2
1200
FF
(%)
1000
-
VMPP
NMPP
(V)
88
56.5
83.6
83.6
1
2
2
2
Power (W)
TABLE II.
800
600
400
0
A comparative analysis between 3x4 array and 6x2 array is
presented in Fig. 5. This figure shows normalized results for
Cases II, III and IV in relation to Case I, where there is no PV
modules shaded.
Fig. 5(a) presents the normalized Fill-Factor obtained from
simulations results. This factor indicates the efficiency of the
PV array under partial shading conditions. One can observe
that arrays with few modules in series present high FF even
under partial shading conditions. On the other hand, the FF of
arrays with several modules in series decreases considerably if
some modules are shaded. Therefore, parallelism of PV
modules increases significantly the solar conversion
efficiency.
Fig. 5(b) shows the normalized array utilization factors
calculated from experimental results. Observe that the energy
production, in relation to unshaded case, decrease significantly
in 6x2 arrays even when few modules are shaded.
Fig. 5(c) presents the voltage changes of the maximum
power point. Due to multiple local maximum power peaks, the
voltage range of 6x2 array at MPP changes considerably. As a
result, the converter used to process the power from this PV
array must be designed to a wider input voltage range. On the
other hand, the voltage range at MPP for 3x4 arrays is narrow.
Finally, we conclude that PV arrays with few modules in
series and several strings in parallel present advantageous
characteristics, in relation to series arrays, if some modules are
20
Power (W)
40
60
80
Voltage (V)
(a)
782W
100
120
140
Array 3x4
Array 6x2
549W
722W
600
400
200
0
0
20
40
60
80
Voltage (V)
(b)
100
600
120
140
Array 3x4
Array 6x2
762W
800
Power (W)
The shape of power vs. voltage curve is similar in all four
cases for 3x4 array. On the other hand, under partial shading
condition, 6x2 array presents two local power peaks: one
around 90V and other around 50V. Depending on and
environmental conditions, the MPPT algorithm may not
converge to the global maxima, which reduce significantly the
efficiency of the array.
0
800
557W
500W
400
200
0
0
20
40
60
80
Voltage (V)
(c)
100
686W
800
576W
Power (W)
Simulation and experimental results for cases II, III and IV
present a good correspondence between them. One can
observe that global maxima of 3x4 array presents always
higher power peak and AUF in relation to 6x2 array.
Array 3x4
Array 6x2
200
C. Discussion
This section presents the analysis and discussion of
simulation and experimental results presented previously.
Case I illustrate a test condition where no PV modules are
shaded. In simulation, one can observe that global maxima
power of both arrays are the same because was neglected the
power losses in the circuit. On the other hand, experimentally,
there is a difference of maximum power between arrays 3x4
and 6x2. This difference is due the higher wiring losses in 3x4
array.
1097W
1023W
600
120
140
Array 3x4
Array 6x2
479W
400
200
0
0
20
40
60
80
Voltage (V)
(d)
100
120
140
Figure 4. Simulation results: power versus voltage curve.
(a) Case I (b) Case II. (c) Case III. (d) Case IV.
shaded. The complete system efficiency is normally increased
even when is required an additional dc-dc converter to step-up
the lower terminal voltage of parallel PV arrays.
V.
CONCLUSIONS
This paper presents a comparative analysis among
different arrays of photovoltaic modules under partial shading
conditions. This scenario is typical of a residential area where
partial shading can occur due to trees or neighbor buildings.
Simulation and experimental results have demonstrated
that PV systems with several modules connected in series can
reduce significantly the solar conversion efficiency when
some modules are partially shaded. On the other hand, solar
conversion efficiency can be increased using several PV
arrays in parallel with few modules in series.
1200
Power (W)
1000
800
90% 100%
600
86%
97%
Case II
Case III
Case IV
72% 72%
REFERENCES
[1]
[2]
400
200
[3]
0
Array 3x4
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[4]
Case II
Case III
Case IV
66%
51% 53%
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76% 74%
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[5]
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ACKNOWLEDGMENT
[11]
[12]
The authors would like to thanks to FAPESC, FAPERGS
and CAPES/PROCAD for financial support.
[13]
[14]
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