PSIM Simulation of partial shading effect on photovoltaic ...

Zebiri & al./ Appl. J. Envir. Eng. Sci. 5 N°2(2019) 193-201

193

PSIM Simulation of partial shading effect on photovoltaic modules

with bypass diode

M. Zebiri1,*

, M. Mediouni1

1 Laboratory of Engineering Sciences and Energy Management (LASIME), ENSA, University Ibn Zohr, PO Box 1136,

80000, Agadir, Morocco

*

* Corresponding author. E-mail: [email protected]

Received 9Apr2019, Revised 3Jun2019, Accepted 21Jun 2019

Abstract.

The production of renewable energy depends on weather conditions. In the case of a photovoltaic generator,

the partial shading considerably reduces its output power capacity and can even cause the destruction of part

or all of a panel. Once shaded, a cell is operating in reverse bias and it acts as a charge and dissipates energy,

then the cell can be irreversibly damaged, we usually called these hot spot phenomena. In order to avoid this

possible phenomenon, manufacturers have resorted to the use of bypass diodes and anti-return diodes. In this

paper, we present a simulation of the shading effect of a photovoltaic generator composed of panels mounted

in series and in parallel. We will compare the characteristics of power voltage (P-V) for different irradiances

in the presence and absence of bypass diode. For the simulation, we will use the single diode model and

PSIM software to perform the simulation.

Key word: Photovoltaic, partial shading, modelling and simulation, bypass diode, PSIM software

1. Introduction

The electrical power produced by a photovoltaic module is the sum of the powers delivered by all the cells

that compose it [1]. But in reality, it depends on the meteorological conditions: solar irradiation, temperature,

shadow, etc. the shading effect is one of the conditions that considerably affect this production; in this

article, we will present its effects. Some modules or cells of a photovoltaic module can be in the shade

because of clouds, buildings, trees, bird droppings ... Under these conditions, the characteristics (P-V)

become more complex and not linear presenting several peaks, and they vary with solar irradiance.

However, it is important to understand and plan for them to extract as much electrical power as possible. In

this objective, over the years researchers have studied the characteristics of PV modules and the

meteorological factors affecting them. Herin Patel [2] has compiled a list of the work done in the study of

these characteristics.

mailto:[email protected]


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We will simulate six panels in different configurations that will be subjected to different irradiances and

compare the characteristics obtained with and without bypass diode.

2. Materials and methods

2.1. Equivalent model

In the literature a photovoltaic cell can be represented with an equivalent circuit model, operating in

various conditions (single diode, double diode, bishop model ...). The single-diode model is the most widely

used to represent a PV cell, thanks to its simplicity and precision [2, 3]. In this paper, we will use the single

diode to represent a PV cell figure 1. It consists of a current generator (Iph), a diode, an internal resistance

(Rs) and a shunt resistor (Rh). The model with by-pass diode figure 2, [4, 5, 6] is used to study the effect of

shading on the photovoltaic panels with this diode in anti parallel. For the simulation, without loss of

generality [5], we will place one bypass diode in anti-parallel with the whole module.

2.1.1. One diode model

Figure 1: Electrical model of a PV cell.

I = Iph - Id - Ih (1)

( ( )) ⁄ (2)

( (

) ) (3)

Where Vd = (V+IRs)

( )

( ( ( ))

)

(4)

Ih = (V+IRs) / Rh (5)

( (

) ) ( ) ⁄ [7] (6)

With Vt,d = NsaKT/q the thermal voltage (Ns is the number of cells connected in series, q, a, K and T

respectively are the charge of the cell electron, diode ideality factor, Boltzmann constant and p-n junction

temperature), and Iph is the photo-current of a cell, G is the sun irradiation, Ki is the temperature coefficient

of Isc, Id is the current of the diode of a cell, Isat,dis its inverse saturation current, Kv is the temperature

coefficient of Voc, V is the voltage across the cell, I is the cell current, Rs and Rsh are the series and shunt

resistors of the cell.

2.1.2 One diode model with bypass diode

This is the same model as in Figure 1 with a diode Db in anti-parallel (bypass diode).


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Fig. 2: Electrical model of a PV module with bypass diode Db [2].

In normal operating mode of the photovoltaic module, the open circuit voltage is in order of 0.6 V for the

crystalline cells while in reverse bias operation, the voltages can reach more than -20V [4, 5, 6]

Applying the Kirchhoff’s law in figure2 we get the current generated by the PV cell:

I = Iph - Id - Ih + Idb (7)

Ih = (V+IsRs) / Rh = (V+ Rs (I-Idb)) / Rh (8)

( (

) ) (9)

I = Iph - ( (

) )- (V+ Rs (I-Idb)) / Rh + (

(

) ) (10)

With Idb, and Vt,dbare respectively current and thermal voltage of the bypass diode.

2.2. Partial shading and bypass diodes

Because of the shadows of trees or buildings, passing clouds and variations of the solar angle the

uniform irradiation is not always satisfied. And precisely the effect of shading causes the loss of power also

this may damage the modules even a panel due to the high current flowing through it in reverse. The impact

of this effect is reduced by uses of the bypass diode. In practice, bypass diodes are connected in anti-parallel

with each group of 18 cells in general to prevent damage due to reverse bias on shaded cells.

The bypass diode starts operating as soon as a shaded cell is reverse biased. It allows the current from

the non-shaded cells to drift around the shaded ones. The effect of shading on the output of a PV module is

non-linear because a small amount of shadow on part or the entire module can result in a significant

reduction in output power.

When the generated current (Iph) decreases to a level lower than the current generated by all the

modules in series, the bypass diode limits the reverse voltage to a value lower than the breakdown voltage of

the PV cells. In other words, the bypass diode begins to operate when equation (13) is satisfied[8]

Ipva>Iph (11)

2.3. Simulation

Simulation procedure

This section presents the procedure used for simulating the (I –V) and (P –V) characteristics of a

partially shaded PV panel at standard temperature condition of 25 °C. This procedure involves defining

module assemblies for use with the PSIM model as illustrated below.


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Illustration 1: first we will use a panel composed of three modules (MSX60) connected in series. One

of the panels will be subjected to different solar irradiances. The simulation is performed without a bypass

diode, figure 3a, and then a bypass diode is used for each of them in figure 3b.

Figure 3: Three PV modules connected in series without bypass diode (a), with bypass diode (b), and

Three PV modules connected in parallel without bypass diode (c), with bypass diode (d).

Then in illustration two the same scenario is used with a panel where the three modules are connected

in parallel figure 3c and figure 3d. The results of the different characteristics obtained will be compared.

The panels are illuminated first by irradiation of 1000 w/m² each in both cases, then they will be

illuminated respectively by following variants irradiances (1000, 600 and 200 w/m²) in both configurations.

After this, we use a PV array composed of two panels with different configurations where we use both

connections in series and parallel Figures 4a and 4b.

Figure 4a: Two PV panels connected in

parallelwith three modules connected in series

every one.

Figure 4b: Two PV panels connected in series

with three modules connected in parallel every

one.

In this simulation, two modules will be shaded and have the illumination of 600 and 200 w/m². First,

two modules in one panel will be shaded figure 5a and 5c, then one module of each panel will be shaded

figures 5b and 5d. Every module has a bypass diode in anti-parallel.

Figure 5a: Two PV panels connected in parallel

one of them has two modules shaded.

Figure 5b: Two PV panels connected in parallel

with one module shaded in every one.


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Figure 5c:Two PV panels connected in series one

of them has two modules shaded.

Figure 5d:Two PV panels connected in series

with one module shaded in every one

Table1: Typical electrical characteristics of Solarex MSX60

Maximum power (Pmax) 60W

Voltage @ Pmax (Vmp) 17.1V

Current @ Pmax (Imp) 3.5A

Short-circuit current (Isc) 3.8A

Open-circuit voltage (Voc) 21.1V

Temperature coefficient of open-circuit voltage –(80±10)mV/°C

Temperature coefficient of short-circuit current (0.065±0.015)%/°C

Temperature coefficient of power –(0.5±0.05)%/°C

While figure 6 shows the mathematical model realized under PSIMenvironment [9, 10, 11] using (2),

(3) and (4) which is used to realize the photovoltaic module, figure 7 shows the PV module circuit model

with bypass diode (a) and without bypass diode (b). This module is used to constitute the studied panels.

Figure 6: Mathematical model built under PSIM and Parameters and specifications used.


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Figure 7 PV module circuit model with bypass diode (a) and without bypass diode (b) and panel with

modules connected in series (c) and in parallel (d).

3. Results and discussion

By comparing the characteristics (PV) in figures 8a and 8b without bypass diode we find that in a series

connected modules the power losses are greater ( + 70%) than in a parallel connection (40%). Another

remark in series connected module the panel produces only the power of the most shaded module, it’s similar

effect as if the whole panel is under shading. We get the same maximum power point for subsequent

illuminations (1000, 1000 and 200 or 1000, 600 and 200) which is not the case for a parallel connected

modules. This is due to the fact that in connection in series the total voltage is the sum of the voltages of the

modules composing the panel.


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Figure8: P-V Characteristics of three modules in series (a) and in parallel (b) with different irradiance

without bypass diode

Figure 9: P-V Characteristics of three modules in series (a) and in parallel (b) with different irradiance with

bypass diode

0 10 20 30 40 50 60

50

100

150

200

a

3X1 Kw/m²2X1, 0.6 Kw/m²2X1, 0.2 Kw/m²1, 0.6, 0.2 Kw/m²

Voltage (V)

Po

we

r (W

)

0 10 20

50

100

150

200

b

3X1 Kw/m²2X1, 0.6 Kw/m²2X1, 0.2 Kw/m²1, 0.6, 0.2 Kw/m²

Voltage (V)

Po

we

r (W

)

0 10 20 30 40 50 60

50

100

150

200

a

3X1000 w/m²2X1000 and 600 w/m²2X1000 and 200 w/m²1000, 600 and 200 w/m²

Voltage (V)

Po

we

r (W

)

0 5 10 15 20

50

100

150

200

b

3X1000 w/m²2X1000 and 600 w/m²2X1000 and 200 w/m²1000, 600 and 200 w/m²

Voltage(V)

Po

we

r(W

)


200

In figures 9a and 9b we compare the power characteristics with bypass diode. For the parallel

connected modules there is no difference if there is a bypass diode or no. That means the bypass diodes

have no effect in the case where the modules are parallel connected. But for the series connected modules

the bypass diode has an important impact in the power produced,the maximum power reduction is only 55%

compared to 70% of reduction when there is no bypass diode, but it is less than power produced with

parallel connected modules (40%).

Figure10: P-V Characteristics of two panels in parallel with three modules in series every one.Two

modules of one panel are shaded (a) and one module from every panel is shaded (b).

Figure 10 shows the P-V characteristics of two panels in parallel that every one consists of three

modules connected in series with partially shading figure 5. The reduction in output power is more important

when the shadowed modules are from every panel figure 5b. For the case in figure 5a, the unshaded panel

produces full power so the losses come from the shaded one.

Figure 11 shows the P-V characteristics of two panels in series that every one consists of three modules

connected in parallel with partially shading figure 6. In this configuration, the reduction in output power is

more important when the shadowed modules are from one-panel figure 6a. The appearance of characteristics

is the same as in figure 9. The bypass diode has no influence in the parallel connection.

0 10 20 30 40 50 60

100

200

300

400

a

6X1 Kw/m²1, 2X0.6/ 3X1 Kw/m²1, 0.6, 0.2/ 3X1 Kw/m²1, 2X0.2/3X1 Kw/m²

Voltage (V)

Po

we

r (W

)

0 10 20 30 40 50 60

100

200

300

400

b

6X1 Kw/m²2X1, 0.6 /2X1, 0.6 Kw/m²2X1, 0.6/ 2X1, 0. 2 Kw/m²2X1, 0.2/ 2X1, 0.2 Kw/m²

Voltage (V)

Po

we

r (W

)


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Figure11: P-V Characteristics of two panels in series with three modules in parallel every one.Two

modules of one panel are shaded (a) and one module from every panel is shaded (b).

Conclusion

In this work, we studied the effect of different shading conditions and the difference between parallel and

series connected modules on the output power of PV modules. We used PSIM to perform our simulations

and used the Solarex MSX60 module specifications. The results obtained show clearly that the performance

of the panels varies and strongly depends on the shading effect and the type of connection use. We also

highlighted the role of the bypass diode in different configurations. The bypass diode is important for the

series connected module.

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0 10 20 30 40

100

200

300

400

a

6X1 Kw/m²1, 2X0.6/ 3X1 Kw/m²1, 2X0.2/ 3X1 Kw/m²1, 0.6, 0.2 / 3X1 Kw/m²

Voltage (V)

Po

we

r (W

)

0 10 20 30 40

100

200

300

400

b

6X1 Kw/m²2X1, 0.6/ 2X1, 0.6 Kw/m²2X1, 0.6/ 2X1, 0.2 Kw/m²2X1, 0.2/ 2X1, 0.2 Kw/m²

Voltage (V)

Po

we

r (W

)

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