Reliabilty Study of PV Systems

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Comparative study of different PV modulesconfiguration reliability

W.M. Rohoumaa*, I.M. Molokhia b, A.H. Esuri b aCenter for Solar Energy Studies, Tripoli, Libya

b EE. Department, Alfateh University, Tripoli, Libyaemail: [email protected]

Abstract

One of the most promising source of renewable energy is the direct conversion of solar energy owing to the present state of achieved technology in manufacturing PV modules, their comprehensive cost and the highdepletion in non-renewable energy sources. The reliability of stand alone PV systems becomes one of the major trends in the present design of such systems. The system configuration plays an important factor determining theoverall system reliability. This paper emphasizes the existing manufactured modules and illustrates the reliability

analysis of different system configurations. AC bus level connection using module integrated inverters vs. DC buslevel connection, cabling losses shading effects are also examined. A case study is selected to calculate and compare the reliability of different system configuration using the analytical approach.

Keywords: PV system; Reliability; Inverter topology; Module integrated inverter

1. Introduction

PV electricity is a viable and cost-effective

option in many remote site applications wherethe cost of grid extension or maintenance of

conventional power supply systems would be

prohibitive [1].

A stand alone photovoltaic systems consist

of photovoltaic modules, charge controllers,

batteries and inverters, the interconnection and

configuration of these component has an impor-

tant impact on the overall system reliability and

performance. There are many failures occurred

during the PV systems operating life, the majorityof system failures may be attributed to inverter

failures [2].

In this paper, a study has been made for

quantifying the effects of inverter configuration

on PV system performance and investigate the

several configurations to demonstrate system

performance and reliability. The failure rates

(λ ), of the photovoltaic system components are*Corresponding author.

Desalination 209 (2007) 122–128

0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

The Ninth Arab International Conference on Solar Energy (AICSE-9), Kingdom of Bahrain

doi:10.1016/j.desal.2007.04.020



W.M. Rohouma et al. / Desalination 209 (2007) 122–128 123

assumed to be constant, and the exponential

distribution is employed when a constant failure

rates adequately describe the behavior of the

system component, and the reliability becomes

and the MTTF (mean time to failure) is the

average useful life = 1/λ .

2. Reliability

Reliability analysis is the determination of a

mathematical expression that describes the reli-

ability function of the system, expressed in termsof the reliabilities of its components. Once the

system reliability function has been determined,

other calculations can then be performed to

obtain system probability density function (pdf),

system average useful life and system failure rate.

The following system configurations are

considered in the reliability analysis:

a) Central inverter system.

b) String inverter system.

c) Module integrated inverter system.

2.1. Central inverter system configuration

The central inverter configuration system

consists of PV modules arranged in series and

parallel to get the desired current and voltage,

charge controller, battery bank and DC to AC

Inverter, as shown in Fig. 1, large number of

such classical configuration already in operation

allover the world.

Central inverter systems requires DC wiring

which increases the cost and decreases safety,

and due to the large size, a modular, flexible/

extensible design is harder to realize with suchconfiguration. And the system reliability could

be quantified by multiplying the reliability of

each component in the system:

Rsys = Rarray × RBattery × Rcharger × Rinverter

The reliability of PV module Rm with failure

rate l m is

The reliability of PV array Rarray with n series

and m parallel modules is

.

The reliability of batteries R B, with failure rate

l B is

The reliability of charge controllers Rc, withfailure rate l c is

The reliability of inverters Ri, with failure

rate l i is

and the system reliability for central inverter configuration is

(1)

2.2. String inverter system configuration

In this configuration instead of one string

system (central inverter) it is more practical to

divide the system into k parallel subsystems, and

R t t ( ) = −e λ

Fig. 1. Central inverter system.

Rmm te= − ×λ

Rarraym1 (1 e n t)= − − − × ×λ

RBB te= − ×λ

ecc t R = − ×λ

Rii= − ×e tλ

R isysn m t1 1 e m e ( c B ) t= − −( )⎡

⎣⎢⎤⎦⎥

× + + ×− × ×λ λ λ λ



124 W.M. Rohouma et al. / Desalination 209 (2007) 122–128

interconnect theses subsystem on AC side, as

shown in Fig. 2.

The sub-string reliability is

and for the overall system, the reliability is

(2)

2.3. Module integrated or AC module

An AC solar module is in fact a standard

solar module, combined with a module mounted

inverter, the AC module produce alternating

voltage 220 V at the line frequency. The inverter

is connected to its module using short run DC

cable, then the module is connected to the AC

bus using AC cable. The AC module provide

power to the load and charge the battery in daytime, and the battery will supply the load at

night time via bidirectional inverter (Fig. 3).

The AC modules and the battery set should be

both up for the system to work. So from reliability

point of view the AC module and the battery set

are in series. So the reliability of the system is

For AC module the reliability is

And for the battery with the bidirectional

inverter is

where N = number of parallel modules; M =number of parallel batteries and the overall

system reliability is

(3)

Once the equation of the reliability obtained the

pdf of the system could be obtained, by differen-

tiating the reliability function

(4)

and the average useful life MTTF of the system

(5)

3. School electrification in Beer Almeerhan

village

3.1. Introduction

Beer Almeerhan is a small rural village, lat.:

12° 21’ 16.5 E, long.: 31° 05’ 17.1 N, located in

Rsub-sysn m t1 1 e m e ( c B i) t= − −( )⎡

⎣⎢⎤⎦⎥

× − + + ×− × ×λ λ λ λ

Fig. 2. String inverter.

Fig. 3. Module integrated inverter.

R Rsys sub-sys

k

1 1= − −( )

Rsys n m t

m

( c + B + i ) t

= 1 1 1 1 e

e

− − − −( )⎛ ⎝ )⎡⎣⎢⎛ ⎝ ⎜

× ⎤⎦⎥⎞

− × ×

− ×

λ

λ λ λ

⎠ ⎠ ⎟ k

R R Rsys m B= ×

Rm( m i) t N1 1 e= − −( )− + ×λ λ

RB( B i) t

M

= 1 1 e− −( )− + ×λ λ

Rsys

( m i) t N

( B+ i) tM

= 1 1 e

1 1 e

− −( )⎡⎣⎢

⎤⎦⎥

× − −( )⎡⎣⎢

⎤⎦

− + ×

− ×

λ λ

λ λ

⎥⎥

pdf f t t R t = = −( )

d

d ( )

MTTF d = ⋅∞

∫ R t t ( )0




the south GURIAN city. There are some buildings

in the village which is powered by photovoltaic

system, such as a public building, a school,

tents, etc

In this case the reliability analysis of the

photovoltaic system installed in the school is

considered. And different interconnection strat-

egies are compared to get better system perfor-

mance. The current system configuration is

shown in Fig. 4.

System configuration: (Central inverter system)

Using the above equations it is possible to

calculate the system reliability, pdf and MTTF

as shown below:

The MTTF = MTTR (mean time to repair)

+ MTBF (mean time between failure)

The MTTR is assumed to be very small compared

with MTTF so the MTTF = MTBF

The failure rate l is given by

l m = 0.04 Failure/year [6]

l C = 0.125 Failure/year [6]

l B = 0.1 Failure/year [6]

l i = 0.1 Failure/year [5]

3.2. Central inverter configuration

The system reliability of the central inverter

(existing system) is calculated using Eq. (1)

Using Eq. (4) the system pdf is

System voltage = 48 VSolar power = 3600 Wp

Battery type STG 1560 No. of series 24

Module type SP75

Module power = 75 W

No. of parallel 1

Battery voltage 2 V

No. of series

modules = 4

No. of parallel

modules = 12

Total no. of

modules = 48

Capacity at 100 hr rate

1560 Ah/cell

Battery bank capacity

1560 Ah 48V

System load 1220 W

Fig. 4. School exciting system configuration.

Fig. 5. Central Inverter system pdf.

λ = 1/ MTBF

Fig. 6. Central inverter reliability function.

Rsys0.325 t 0.325 t 0.16 t

12

= e e 1 e− × − × − ×− × −( )

f t ( ) system pdf = 0. 325e

+1.92e 1 e

0

325t

0.485t 0.16t11

=

−( )

−

−

− −

..325e 1 e0.325t 0.16t12− −−( )




MTTF of the system using Eq. (5) = 3.0

years (Figs. 5 and 6).

3.3. String inverter configuration

Using Eq. (2) The reliability of the PV sys-

tem using 4 string systems where K = 4, N = 4,

M = 3 are shown:

Using Eq. (4) the system pdf is

From Eq. (5) The MTTF of the System = 5.5

years (Figs. 7 and 8).

3.4. Module integrated inverter system

configuration

Using Eq. (3), using 48 module integrated

inverter, and 8 batery 12 V, 200 Ah each, with

bidirectional inverter, so the system reliability are

From Eq. (4) the system pdf f (t )

The system MTTF from Eq. (5) = 13.3 years

(Figs. 9 and 10).

Fig. 7. String inverter reliability function.

Fig. 8. String inverter pdf.

Rsys0.16 t

30.325 t

4

1 1 1 1 e e

1 1

= − − − −( )⎛ ⎝ ⎜

⎞ ⎠ ⎟ ×

⎡

⎣⎢

⎤

⎦⎥

⎛

⎝ ⎜

⎞

⎠ ⎟

= − −

− × − ×

ee e 1 e0.325 t 0.325 t 0.16 t3

4

− × − × − ×− × −( )⎡

⎣⎢

⎤

⎦⎥

⎛

⎝ ⎜⎞

⎠ ⎟

= 4 1 e 1 1 e

0.325e 0.

0.325t 0.16t3

3

0.325t

× − − −( )⎛ ⎝ )⎡

⎣⎢⎤⎦⎥

× −

− −

− 3325e 1 e

+ 0.48e 1 e

0.325t 0.16t3

0.485t 0.16t2

− −

− −

× −( )⎡⎣⎢

−( ) ⎤⎦⎥

Fig. 9. Reliability of the system using module integrated

inverter.

Fig. 10. Pdf of module integrated inverter system.

Rsys0.14 t

480.2 t

8

1 1 e 1 1 e= − −( )⎡⎣⎢

⎤⎦⎥

× − −( )⎡⎣⎢

⎤⎦⎥

− × − ×

f t ( ) = 1.6e 1 e 1 1 e

+ 6.

0.2t 0.2t7

0.14t48− − −−( ) × − −( )⎛

⎝ ⎜⎞ ⎠ ⎟

⎡

⎣⎢

⎤

⎦⎥

772e 1 e 1 1 e0.14t 0.14t47

0.2t8− − −−( ) − −( )⎛

⎝ ⎜⎞ ⎠ ⎟

⎡

⎣⎢

⎤

⎦⎥




Fig. 11 shows the comparison between the

different module configurations, and it is obviousthat the module integrated inverter has much more

reliability that the other system configurations.

4. Cabling losses

In a typical central-inverter PV system,

modules are connected to one another and to a

centrally-located inverter using DC cables. These

DC cables tend to be quite long and the resulting

losses relatively high. The exact level of DC

cabling loss will vary considerably between dif-

ferent system configurations. Australian design

guidelines recommend that peak DC cabling

losses be kept below a maximum of 5% [4].

In a typical module-inverter system, each

module connects to a nearby inverter via a short

run of DC cable, and the inverters are connected

to AC buss and as results the DC cabling losses

are reduced to around 0.1%. Module-inverter

systems are subject to additional AC cabling

losses compared to central-inverter systems.Total peak cabling losses (DC and AC) for

module-inverter systems are therefore likely to

be considerably less than for central-inverter

systems, and it is around 2% [4].

5. Shading and mismatch

Partial shading of a PV array can signifi-

cantly reduce its output, since heavily shaded

cells will limit the output of other cells with

which they are connected in series trees, build-

ings, television aerials, roof structures, leaves

and bird droppings can partially shade a PVarray and result in a substantial reduction in

system performance.

Central-inverter systems are more susceptible

to power loss from shading and mismatch than

module- inverter systems.

In a module-inverter system, there is no

series interconnection between PV modules.

Losses from shading of a single PV module are

therefore limited to that module; any un shaded

modules nearby are unaffected. Hence lossesfrom shading in a module-inverter system are

usually much less than for an equivalent central-

inverter system and tend to be more proportional

to the degree of shading [4].

6. Conclusion

From the results presented in this paper the

following concluding remarks can be made:

1) A module integrated inverter system poten-

tially has much higher reliability than theother configurations, multiple inverters pro-

vide in-built redundancy failure of one PV

module or inverter in a system with many

inverters will have only an incremental effect

on overall system performance.

2) The average useful life of the module inte-

grated inverter is very long comparing with

the other configurations.

3) Cable losses are reduced to the minimum

using module integrated inverter, because inmodule integrated system the inverters are

mounted on the back of each individual

panel.

4) In module integrated system the impact of

shading using module integrated inverter is

lower.

5) Module integrated inverter system also pro-

vides enhanced modularity, new panels can be

added easily at any time.

Fig. 11. Comparison between the different module

configuration.




References

[1] Friedrich sick and Thomas Erge, Photovoltaics in

building, a design handbook/james & james Publ.

Ltd, 35–37 William road, London NW1 3ER, UK.

[2] Aleksandar Pregelj, Miroslav Begovic and Ajeet

Rohatgi, Impact Of Inverter Configuration On PV

System Reliability And Energy Production, 29th

IEEE PVSC, 17–24–2002, Neworleaus-USA.

[3] Ljubisav Stamenic, Technology Center, British

Colombia Institute of Technology, Research and

development of the first AC BIPV Installation in

Canada, ISES Solar World Congress, Jerusalem

(1999).

[4] D.M. Roche, Economic Comparison of CentralVersus Module Inverters in Residential Roof-

top Photovoltaic Systems. Greenwatt Consult-

ing, 31 The Ridge, Helensburgh NSW 2508,

Australia.

[5] http://www.antares.co.uk

[6] http://www.sollatek.co.uk

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