Asymmetrical Multilevel Inverters for Partially Shaded PV Systems

2013 International Conference on Circuits, Power and Computing Technologies [ICCPCT-2013]

Asymmetrical Multilevel Inverters for Partially Shaded PV Systems S.Malathy

B.L.Mathur

R.Ramaprabha

Assistant Professor Department of Electrical and

Assistant Professor Department o{Electrical and Electronics Engineering SSN College o{Engineering,

Electronics Engineering SSN College of Engineering,

Professor, Department o{Electrical and Electronics Engineering SSN College of Engineering,

Rajiv Gandhi Salai, Kalavakkam-603110, Rajiv Gandhi Salai, Kalavakkam-603110, Rajiv Gandhi Salai, Kalavakkam-603110, Chennai, Tamilnadu, India. Chennai, Tamilnadu, lndia. Chennai, Tamilnadu, lndia. [email protected] Abstraet- This paper proposes an economic way to address

PV panels are connected in series and/or parallel to meet

partial shading condition in photovoltaic (PV) array. When series

the required voltage and current. The panels are connected in

connected PV panels are not uniformly iIIuminated, local hot

series to form a string. Few such strings may be connected in

spots develop due to the increased stress on low iIIuminated PV

parallel if current requirement is high. The overall efficiency

panels. This may eventually cause permanent damage to the low iIIuminated

cells/

panels.

Anti-parallel

diodes

are

of the interconnected PV system is lesser than the expected

normally

value due to losses. The losses are mainly due to panel

connected to protect the panels from this kind of damage and this

mismatch, partial shading and operation at non-optimal point.

introduces multiple peaks in the power characteristics. This

By employing a maximum power point tracking (MPPT)

increases the burden on maximum power point algorithm and

technique, the PV panel can be operated at its optimal point

tracker as it has to track the global peak among the multiple peaks. Grouping of PV panels based on insolation is proposed in

and panels with similar ratings can be used to avoid panel

this work. Panels with same insolation are grouped together to

mismatch. Partial shading is a common phenomenon which

form a string and these strings are then interfaced with an

occurs when all the modules in the interconnected PV array do

asymmetrical multilevel inverter for further power conversion.

not

This paper further analyses the performance of the asymmetrical multilevel

inverter

controlled

by

different

pulse

width

uniform

insolation.

Shading

degrades

the

sometimes leads to permanent damage of the shaded PV

modulation (PWM) schemes under partial shaded conditions.

Keywords-

receive

performance of the PV array by developing local hot spots and panel. The shading effect is much pronounced when PV

pv, partial shading, asymmetrie multilevel inverter,

panels

PWM

are

installed

in

a

residential

building

where

the

available space is limited. The shading may be caused by a I.

nearby buildings, chimney, a near by tree, any permanent

INTRODUCTION

fixtures or even by the other panels. These types of shadings

The ever increasing demand for power and the limited

can not be avoided and they exist all through the year and the

availability of non-renewable sources led to active research on

shading pattern is known. This paper addresses this issue by

the alternate sources of energy. Of the various available

grouping PV modules with similar insolation and interfacing it

renewable

with asymmetrical multilevel inverter. The performance is

energy

sources

like

solar,

wind,

tidal

and

geothermal, solar energy is readily available. Besides being a

further analyzed by applying various PWM techniques to the

clean source of energy, it is available in abundance. But only

inverter.

0.026% of this energy is directly converted in to electrical

11.

energy world wide. During the process of conversion losses occur. Some of these losses are inevitable but few losses can

PARTIAL SHADING CONDITIONS

To study the effect of shading in a series connected array,

be reduced by properly configuring the PV modules and

Matlab model [2] of individual PV panel is developed as

operating them at their optimal power points by adapting

shown in Fig.l.

maximum power point tracking (MPPT) techniques. The efficiency of the PV cell depends not only on the material and technology

but

also

on

the

environmental

factors

like

temperature and insolation. The efficiency of the PV cell drops when the external circuit draws a current that is very much lesser or greater than the current it can produce optimally.

The

PV

current

is

directly

proportional

to

insolation and the open circuit voltage of the PV cell is inversely proportional to the working temperature [I]. Usually 36 PV cells are connected in series to form a PV module. The

Ramp

voltage current characteristics of the PV module are highly

Fig.l. Matlab/Simulink model of a PV panel

non-linear.

978-1-4673-4922-2/13/$31.00 ©2013 IEEE

579


based

The performance of the PV panel depends highly on proportional to the insolation and the open circuit voltage is

insolation

IV.

The V-I

inversely proportional to working temperature.

on

and

selection

of

appropriate

power

conditioning unit (PCU).

insolation. The photon current or the panel current is directly

GROUPING BASED ON INSOLATION

characteristics and the V-P characteristics of the individual

When PV panels are installed on roof top of residential

module and the variation in the characteristics with change in

buildings, the shading may be caused by the permanent fixtures

insolation are presented in Fig.2.

or by nearby buildings. The shade pattern is almost known in these cases and it is unavoidable due to space limitation. In

;;3r--�-�-�---r---"]

such circumstances, going for complex tracking algorithms to

! I-----�

track global maxima or incorporating a boost converter with

�2f_---"""�

__

i

1

each individual module to operate at optimal point will increase the cost and size. Instead, the modules can be grouped based on

f-----':!...--.:._

the insolation levels. It is a cost effective solution since the

E � Ol'o

shading pattern is known a priori. ---�-�-c:__-�

Py module Yoltage M

Let a system of nine panels be considered where four panels 2 receive an insolation of 1000 W/m , three panels receive an 2 insolation of 600 W/m and two panels receive an insolation of 2 400 W/m . The modules with same insolation are connected in

F'VmoduleVoll;ge(Vo�s)

Fig.2. V-I and V-P characteristics at different insolation levels

When two or more PV panels are connected in series, the output voltage is equal to the sum of the individual panel

series to form a string. Three such strings are formed based on

voltages. The operating current is equal to the individual panel current.

Under

partially

receive

non-uniform

shaded

insolation,

conditions, the

when

operating

their

panels

current

insolation

levels

and

the

respective

V-I

and

V-P

characteristics are shown in Fig. 3. Let Isl be the current of the

is

first string which has four fully illuminated PV panels, Is2 be

determined by the panel which receives low insolation [4].

the current of the second string with three PV panels at an 2 insolation level of 600 W/m and Is3 be the current of the third 2 string with two PV panels at an insolation of 400 W/m . In this

When two panels are connected in series and both the panels are uniformly illuminated, then the voltage and current are given by (1) and (2).

case, Is3

D..

of

with the next one), phase displacement ( all the earries are in

techniques (APOD, PD, CO, VF and ISC) are used to eontrol

D..

Ö Q) 5: o

arrangement

these carrier waves give rise to different control schemes like 0 alternate phase opposition ( eaeh carrier is 180 out of phase

the asymmetrical CMU [7]-[8]. The carrier arrangements of

50

the PWM sehemes for sine referenee are shown in Fig.8.

O L----�---�----L--_L� o 50 100 150 200

Voltage of PV string (Volts)

Fig.6. V-P characteristics with bypass diodes

3 ,------r--�---.--,

0.005 ,) co PWM

0.01 Tlmelseesl

0.015

0.02

b)ISCPWM Time (sees)

- - - - - - --,-�__ IL>Is3 -

-

-

-

-

-

-I - - - - - -

c

� 0.5

:::l ()

O

c)VFPWM

Timelsecsl

0.005 d)PDPWM

0.01 Time (sees)

0.015

L---�---��----�--L�

o

50 100 150 Voltage of PV string (Volts)

200

Fig.7. V-I characteristics with bypass diodes

o �� o 0.005 0.01 0.015 0.02 e)APOD PWM Time (sees)

For further power conversion from dc-ac, an inverter is to be interfaced. Instead of going for a single phase or three phase voltage source inverter, the paper suggests incorporating

Fig.8. Carrier arrangement for various Sine PWM schemes

an asymmetrical multilevel inverter which employs unequal

581

0.02


In

trapezoidal

PWM

schemes,

a

single

trapezoidal

�

�

modulating signal is compared with multiple triangular carrier

150 ,-----�--�--__,

�

signals. The intersection of the trapezoidal signal and carrier signals defines the switching instant of the PWM pulse. The trapezoidal signal is characterized by its slope angle, u.

The shape of the trapezoidal waveform varies with angle

If

u

u

=

=

ro

� -150 L---�--�----' 0.06 0.02 0.04 0.08 0.1 Time (sees)

0, the trapezoidal waveform resembles a square wave;

o

90°, will result in triangular wave. As a result, for different

values of

u,

-50

o

� -100

u.

Fig.ll. output voltage of AMLI for PDPWM

different harmonics profile is obtained.

The gating pulses are generated for various PWM schemes by comparing the carries with the sine /trapezoidal modulating signal.

; - I;� � 1Im (o

0.005

o

0.0115

001 Time(s)

0.01 Tlme(secs)

0015

0.015

002

o

0.02

0.005

o

0005

0.01 Time(s)

001 Time (8)

0.015

0015

The

%

THD

and

the

Vnns

of

the

fundamental

component for various PWM schemes are tabulated in Table I and Table 11. TABLE I COMPARlSON OF %THD FOR VARlOUS SINE PWM SCHEMES

0.02

002

Ma

PD

APOD

VF

CO

ISC

1 .95 .9 .8 .7

9. 56 12.6 IIA5 13.34 14.29

10.15 12.39 12.16 13.65 15A3

9A2 11.14 10.68 11.96 15. 79

12.65 13.23 12. 54 13. 58 16.73

9.83 12.72 12A3 13.35 13.83

The VF PWM scheme is found to produce waveform with better harmonie profile. The RMS value of the fundamental component is more when ISC PWM scheme is employed. TABLE 11 COMPARlSON OF VR.\lS FOR VARlOUS SINE PWM SCHEMES

Ma

PD

APOD

VF

CO

ISC

Fig.9. Carrier arrangement for various Trapezoidal PWM schemes

1 .95 .9

80A 74.72 70.6

79. 9 74. 95 71.12

80. 2 75A9 71.13

82.71 79.09 75. 26

83.74 79. 52 73. 75

The complete Matlab model [9] of AMLI with PV interface

.8 .7

63.94 55.95

64.09 56.01

63.96

67. 1 59.92

66.82 57.84

is given in Fig.lO.

In

trapezoidal

PWM

55.66

schemes,

trapezoidal

modulating

signal is used instead of sine modulating signal. The carries arranged in different ways are compared with this reference and

the

generated

pulses

control

the

switches

of

the

asymmetrical multilevel inverter. The % THD and the ViruS of the fundamental component are shown in Table III and Table IV. TABLE III COMPARISON OF %THD FOR VARIOUS TRAPEZOIDAL PWM SCHEMES

Ma

PD

APOD

VF

CO

ISC

1 .95 .9 .8 .7

18.23 21.42 22. 5 20.82 22.66

18.66 20.3 22.09 23. 55 25. 72

18.74 20.01 21.63 21.33 23.16

20.79 20.03 21.64 23.53 24.93

18.64 20.69 22.15 20.62 22A

TABLE IV COMPARlSON OF VR.\lS FOR VARlOUS TRAPEZOIDAL PWM SCHEMES

G.lling puls�s

Ma I .95 .9 .8 .7

String3

Fig.IO. Matlab model of PV fed AMLI

APOD

VF

CO

93. 7 87.82 82. 7 74.22 66. 58

93. 9 88 83.06 74. 2 66. 2

97.74 93. 56 89.01 78A9 71.44

ISC 97.35 93.47 88. 85 76.16 67. 5

Tables 111 and IV show that the harmonie profiles of all the

The output voltage waveform for PD PWM technique is shown in fig. 1l.

PD 93.72 88.6 83. 56 74.21 65. 81

PWM techniques are equally good but the Vnns is high if

For other techniques the parameters are

carrier overlapping technique is used. By employing proper

tabulated trom Table I to Table IV.

filter circuit at the output side the harmonie distortions can

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R. Ramaprabha,B. L.Mathur,"Characteristics of Solar PV Array under Partial Shaded Conditions", TENCON 2008 -2008 IEEE Region 10 Conference, 19-21- Nov.2008. [5] R.Ramaprabha and B.L. Mathur, Comparative Study of Series and Parallel configurations of Solar PV Array under Partial Shaded Conditions, International Review on Modeling and Simulation, vol. 3. no. 6, pp. 1363-1371, 2010. [6] Muhammad H . Rashid, Power Electronics circuits,devices and applications, Prentice H all, 2nd Ed. [7] Jose Rodriguez, Jih-Sheng Lai and Fang Zheng Peng, "Multilevel Inverters: A Survey of Topologies, Controls, and Applications", IEEE Transactions on Industrial Electronics, Vol. 49, No. 4, pp.724-738 August 2002. [8] V. G. Agelidis and M.Calais,"Application specific harmonie performance evaluation of multi carrier PWM techniques", proceedings of IEEE conference, PESC'98, Vol.l, pp.I72-l78,1998. [9] www.mathworks.com [10] J.A.Duffie and W.A. Beckman, Solar Engineering ofThermal Processes (John Wiley Sons, 3rd edition, 2006). [4]

further be reduced. Of all the schemes, CO PWM scheme along with the trapezoidal reference is found to give the maximum output voltage and VF PWM with sine reference is found to give better harmonie spectrum. VI.

CONCLUSION

The effect of partial shading condition is analyzed with the Matlab/Simulink model. Development of MPPT algorithm to track the global maxima under partial shaded condition becomes challenging when bypass diodes are used. A simple cost effective solution of grouping cells based on insolation is proposed in this paper for building integrated PV systems where shading is unavoidable and the shading pattern is known a priori. Asymmetrical multilevel inverter which employs unequal dc sources is found to be the best choice for the dc- ac power

conversion

under

shaded

conditions.

The

series

connection of differently iIIuminated PV panels is avoided. The performance of asymmetrie CMU fed by PV strings is further studied for various PWM control schemes. The tabulated data can be used to decide the appropriate PWM scheme based on the

desired

performance

criterion

thereby

irnproving

the

efficiency of the overall system. ACKNOWLEDGMENT

The authors wish to thanl