FLORIDA SOLAR ENERGY CENTER
®
A Research Institute of the University of Central Florida
Cell Line Checker Correlated to
Infrared Thermography
Neelkanth G. Dhere. Narendra S. Shiradkar, and
Eric Schneller
Florida Solar Energy Center, University of Central Florida
1679 Clearlake Road, Cocoa, FL 32922-5703, USA
E-mail: dhere@fsec.ucf.edu
Outline
Need for modern PV system inspection and
characterization tools
 Principle, operation and capabilities of cell
line checker
 Analysis of field degraded modules using
Cell Line Checker and Infrared
Thermography
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Introduction
The role of reliability is increasingly being
realized as vital for success of any PV
technology.
 PV modules are typically sold with
warranties up to 25 years.
 The IEC qualification tests help in reducing
the infant mortality. However, currently,
there is no test that can estimate the actual
lifetime of PV modules in field.
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Background
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Continuous monitoring of PV performance is useful for
assessing reliability and even for enforcing warranty claims for
PV modules.
However, it does not give any information about modes and
mechanisms of failure and hardly has any value as it regards to a
predictive model for PV module degradation.
Therefore, it is essential to use the modern techniques of PV
module diagnostic and characterization that can not only shed
light on modes and mechanisms of degradation, but can also be
used to predict the performance of PV modules as they undergo
outdoor exposure in field.
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The Importance of Field Data
and System Inspection Tools
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It is important to study the behavior of PV modules in field in order to
identify the actual modes of degradation observed in the field deployed
modules.
When large number of different types of field deployed modules are
studied, it will be possible to identify the signs of inception of
degradation and even predict its progress.
It is necessary to carry out the inspection quickly yet efficiently to
identify and locate failures in PV systems and modules.
Infrared Thermography has been used to identify hot cells,
interconnects and bypass diodes.
Dr. Kazuhiko Kato, National Institute of Advanced Industrial Science
and Technology (AIST) has pioneered the use of “Cell Line Checker” a
tool based on non-contact method to identify and locate electrical
failures in PV modules and systems. (To be Discussed Later)
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PV Module Reliability Studies at
FSEC
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In order to study PV reliability, long term field exposure of
PV modules, high voltage bias testing, and PV module
characterization are routinely carried out at FSEC.
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FSEC Module Characterization
Capabilities- Infrared Imaging
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Outdoor Infrared imaging is carried out to detect hot spots at the cells
formed due to cell mismatch in a module that have compromised
interconnects undergoing excessive heating.
Indoor IR imaging can also be performed in a dark room by forward
biasing the module provides exclusive information about damaged
interconnects that may experience excessive heating in field.
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FSEC Module Characterization
Capabilities- Cell Line Checker
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“Cell Line Checker” - a tool based on non-contact method to identify
and locate electrical failures in PV modules and systems has been
added to this arsenal of various PV module characterization techniques
at FSEC.
Combination of Cell Line Checker and Infrared Thermography has
proved to be extremely beneficial in identifying field failures in PV
modules.
With these two techniques, it is possible to quickly locate the failures or
sites for potential failure in PV systems and identify the causes of
failure.
Such modules then can be taken inside the lab for detailed
characterization using other techniques such as Electroluminescence
Imaging, Insulation resistance testing etc.
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Cell Line CheckerIntroduction and Principle
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The cell line checker works on the
principle of a non-contact method to
locate electrical faults in PV systems or
modules.
It consists of a transmitter to be
connected across the terminals of a
string in PV system and a receiver to be
scanned above the module.
A high frequency (5 KHz) signal is
applied by the transmitter and it is
detected by the receiver using either
electric or magnetic field mode.
Individual strings with voltages up to
1000 V DC can be tested.
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Cell Line Checker- Operation
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As the receiver is scanned above the module, a
beeping sound indicates good electrical
connection.
The number of flashing LEDs on the receiver is
proportional to the intensity of current flowing
through interconnect.
When a bad electrical connection is detected, the
beeping sound stops and LEDs cease to flash. This
corresponds to the location of failure.
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Cell Line Checker- Capabilities
Identifying PV System Configuration:
 If the exact layout and design of PV system
to be tested is not available, cell line checker
can be used to identify modules connected
in the same string.
 This is especially useful when the PV
system is installed on multiple roofs.
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Cell Line Checker-Capabilities
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Identifying bad connection, wiring or module level
fault in a string of modules- just like finding a
faulty light bulb in a string of lights used for
Christmas lightning.
Procedure- The transmitter is connected across the
terminals of a string of PV system and the receiver
is scanned above the modules until the fault
(cessation of beeping) is detected.
Next slide shows a demonstration of this
procedure.
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System Testing
Cell line Checker- Capabilities
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Failed interconnects or bus bars within the module
can also be detected in a similar way.
Faulty (open circuited) bypass diodes can be
detected by cell line checker.
Essentially, cell line checker can be used for
tracing the system level faults to modules or
wiring, and then analyzing the origin of these
faults at module level and narrowing them down to
the interconnect or bus bar level
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Analysis of Field Degradation
of PV Modules
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Severe module degradation was
observed in a rooftop PV system
consisting of c-Si modules
deployed at FSEC after more than
10 years of field exposure in hot
and humid climate of Florida.
Cell line checker combined with
IR Thermography has proved to
be very effective in locating field
failures identifying their root
causes.
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Field Failure #1- Visual
Inspection
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Nonuniform browning was
observed on one cell, with
most degradation occurring
near one of the interconnects.
Discoloration, burn marks
and cracking was observed in
the backsheet at the same
location.
Moderate delamination was
observed at several locations.
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Field Failure #1: Outdoor and
Indoor IR Imaging
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Nonuniform hotspot is
observed in both outdoor
(left) and indoor (right) IR
imaging.
High temperatures(~800C)
observed in both images
indicate that the main cause
of heating is damaged
interconnects.
The hottest region is located
just near the interconnects in
both cases.
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Field Failure #1:
Interconnect Testing
Field Failure #1: EL Imaging
Broken
Interconnect
Current
Crowding
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The EL image taken at ISC shows the
darker region near the faulty
interconnect, and brighter region
where current crowding was predicted
by cell line checker.
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EL at ISC
The darker regions seen in
EL image span larger area
than the visibly brown
region seen on one cell,
indicating that some cell
level degradation has also
taken place.
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Field Failure #1
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Indoor IR
EL at ISC
The inactive region around this cell is
responsible for cell mismatch.
The hotspot identified in the indoor IR
image correlates well with the higher
brightness region in the EL image
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Field Failure #1- Discussion
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The failure of one interconnect resulted in current
crowding in the other interconnect.
This resulted in a hot spot near the other
interconnect, which accelerated EVA browning.
The browning further worsened the cell mismatch
and excessive heating resulted in burnt, discolored
and cracked backsheet.
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Field Failure #2
Field Failure #2- Visual
Inspection
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Approximately uniform
browning on one cell.
Burn marks, discoloration,
blisters and cracks in
backsheet on the same cell.
Moderate delamination at
several locations.
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Field Failure #2- Outdoor and
Indoor IR Imaging
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Outdoor IR
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In Outdoor IR imaging
(left), the brown cell is
clearly seen as an
approximately uniform
hotspot with temperatures
close to 700C.
In the indoor IR imaging
(right), the left interconnect
corresponding to the brown
cell and one below do show
some heating, but the
temperatures are less than
400C.
Indoor IR
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Field Failure #2: Testing using
Cell Line Checker
The cell line checker did not show
discontinuity at any interconnect in the
module.
 This indicates that the interconnects are not
yet broken or severely damaged, despite of
excessive heating.
 Bypass diodes were found to be working
fine.
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Field Failure #2
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Outdoor IR
Indoor IR
EL at ISC
Please note that scales for each
IR image are different, and the
heating observed in the outdoor
IR was much higher than that
seen in Indoor IR imaging
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Field Failure #2: EL Image
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The inactive region around this cell is responsible for cell
mismatch.
The darker regions seen in EL image span larger area than
the visibly brown region seen on one cell, indicating that
some cell level degradation has also taken place.
Although the cell interconnects have not completely failed,
these points are of high resistance which also contribute to
the excessive heating
Backsheet
EL at ISC
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Field Failure #2: Discussion
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The temperature of brown cell observed in outdoor IR imaging
was significantly greater than the temperature observed in indoor
IR imaging, under similar amounts of currents through module.
This indicates that the damage is caused by a hotspot generated
by cell mismatch.
The increased temperatures accelerated the browning of ethylene
vinyl acetate (EVA), which reduced the amount of power
generated by that cell. It further worsened the mismatch and
aggravated the heating due to hotspot.
Eventually, the backsheet of the module was burnt and cracked.
The interconnects have not yet broken, but can be damaged if
this situation of excessive heating persists.
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Predictive Models for Module
Degradation
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Mere monitoring of PV performance over time is of
limited value to the manufacturers as it does not yield
concrete information on how to improve the durability
of product.
Use of modern PV module characterization techniques
should be encouraged in order to understand various
modes and mechanisms of failure.
Once this is carried out for several field deployed
modules of various types, it will become possible to
come up with predictive models for PV module
degradation.
Conclusions
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The use of modern PV system inspection and
characterization techniques is necessary to get insight into
modes and mechanisms of failure.
Infrared Thermography combined with Cell Line Checker
is very effective in quickly identifying modes and
mechanisms of failure in field deployed PV systems.
The information obtained using these two techniques can be
used to develop predictive models for PV module
degradation.
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Questions and Comments
Thank you!
Thank You!
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