What is I-V Curve
Power P is given by P=V*I and Solar Modules in any given operational
conditions have a single operating point where the values of the current (I)
and voltage (V) of the cell results in maximum power output. An I-V curve is a
graphical representation of the relationship between the voltage applied across
an electrical device and the current flowing through it.
I-V curve is one of the simplest and common
method to determine the functioning of an electronic device. The shape and
details of the curve provides greater insights into the functioning of the
electronic device.
What is I-V Curve Testing
I-V curve testing is performed to the PV
Modules and Strings to determine the performance of the array in a PV system.
I-V curve testing reveals about the PV Module performance and condition better
than other conventional testing methods.
I-V testing is performed by applying a
series of voltages to the device. The current flowing through the device is
measured at each point of the voltage. The current is measured by an ammeter
connected in series and the supplied voltage is measured by a voltmeter
connected in parallel to the device. A source measuring unit a device capable
of simultaneously supplying the voltage and measuring the current with high
accuracy can also be used for measuring the I-V curve.
The I-V curve (current versus voltage)
represents all possible operating points of a PV module, string or array at
existing environmental conditions. The curve starts at the short circuit
current and ends at the open circuit voltage
as shown in Figure-2. The maximum power point, located at the knee of
the I-V curve, is the operating point that delivers the highest output power.
It is the job of the inverter’s maximum power point tracking circuit (MPPT) to
find that point as irradiance and temperature change. The P-V curve (power
versus voltage) is zero at either end or a maximum at the knee of the I-V
curve.
Any impairment that affects the shape of
the I-V curve as shown in Figure-3 will reduce the peak power value and
diminish the value of the array. The impact of a mismatch impairment on output
power is shown in Figure 4 below
What is the Requirement of I-V Curve Testing
I-V curve testing helps in identifying the
anomalies and pinpoint the root cause of underperforming PV Modules or Strings.
Requirement or the benefits of the I-V
curve measurement are as tabulated below.
|
Requirement/ Benefit
|
Description
|
|
Reduced
Test Time
|
I-V
curve tracing measures array performance with a single electrical connection
at each combiner box, and asingle measurement per string.
|
|
No
need to bring the inverter on-line to test PV string performance
|
Traditional test methods
required the inverter to be brought on-line in order to measure the operating
current ofeach string under load
|
|
Reduced
start-up and commissioning risk
|
Testing
the array before the inverter is brought on-line means less risk of
array-side problems showing up duringstart-up or commissioning.
|
|
More
detailed measurement results
|
I-V curve tracing is the
most comprehensive test possible for PV arrays. In addition to measuring the
short circuitcurrent and open circuit voltage, it uniquely measures the
maximum power point of each string.
The PV Analyzer measures
100 points along the I-V curve. With this resolution, the measurement can
detect more physical effects that may be degrading the performance of the
system.
|
|
Efficient
data management
|
I-V
curve measurement data is saved electronically, eliminating data recording errors.
|
|
Detailed
performance baseline
|
PV arrays are extremely
robust and reliable, but performance does gradually degrade. Occasionally a
module will fail. I-V curves captured and stored at start-up or commissioning
time give you a detailed baseline against which to compare future
measurements over the life cycle of the PV system.
|
|
More
efficient troubleshooting
|
Curve
tracing is an invaluable tool when troubleshooting PV strings, detecting
excess series resistance, low shuntresistance, module mismatch, or shorted
bypass diodes directly from the shape of the I-V curve.
|
Which Stage I V Curve Can Carryout
| Stage-1 | Stage-2 | Stage-3 |
|---|
| -During Manufacturing -To Check the Quality Before Dispatch | -Time of Pre-commissioning -To Check the Performance Before Commissioning | -Performance Testing -To Check the Performance Periodically |
Expected Results of an I-V Curve Testing
The expected results of an I-V curve are as
tabulated below
|
Type of Losses
|
Probable Cause
|
|
Series Losses (Losses due
to excess series resistance show up in the I-V curve as a decreased slope, or
inward tilt, of the curve near Voc.)
|
Corroded or poorly
connected array wiring
|
|
Shunt Losses (Losses due
to shunt resistance show up in the I-V curve as an increased slope, or
downward tilt, of the curve near Isc.)
|
Cracked cell or damage Cell at the time of
Manufacturing, Transportation, installation
|
|
Mismatch Losses
(Substantial Mismatch effect shows up as Notch or Kinks in the I-V curve.)
|
Shading, Uneven soiling,
cracked PV cells, Shorted bypass diodes and mismatch module/Cell
|
|
Reduced Current (Reduction
in the Height of the I-V Curve can be
caused by Uniform Soiling, Edge Soiling or Weather condition that reduce the
Irradiance.)
|
Uniform soiling, edge soiling, PV
module degradation or weather condition
|
|
Reduced Voltage (The Width
of the I-V Curve is affected by Module Temperature. Poor Air-Circulation.)
|
Module temperature, Poor
air circulation and issue with the heat
dissipation
|
|
Series Losses (Losses due
to excess series resistance show up in the I-V curve as a decreased slope, or
inward tilt, of the curve near Voc.)
|
Corroded or poorly connected array wiring
|
|
Shunt Losses (Losses due
to shunt resistance show up in the I-V curve as an increased slope, or
downward tilt, of the curve near Isc.)
|
Cracked cell or damage
Cell at the time of Manufacturing, Transportation, installation
|
Standards of I-V Curve Testing
Applicable standards of I-V curve testing
are tabulated below
|
IEC
Standard
|
Description
|
|
IEC 61829:2015
|
Photovoltaic (PV) array -
On-site measurement of current-voltage characteristics
|
|
IEC 62446-1:2016
|
Photovoltaic (PV) systems - Requirements for
testing, documentation, and maintenance - Part 1: Grid connected systems -
Documentation, commissioning tests and inspection
|
|
IEC 60891:2009
|
Photovoltaic devices -
procedures for temperature and irradiance corrections to measured I-V
characteristics.
|
Limitations of I-V Curve Testing
General limitations of I-V Curve tracing
are as tabulated below
|
Limitations
|
Description
|
|
Cannot be used as real-time monitoring
|
I-V curve testing reports data from a
single point in time and is heavily dependent on environmental conditions.
This cannot be used for real-time monitoring of the Module performance.
|
|
Diagnostic Information is confined to
that reference time stamp
|
Defects can go undetected for longer periods of time until the
next I-V curve tracing activity has been conducted. Hence periodic I-V curve
tracing activity must be planned every 6 months to 1 year for better
diagnostics.
|
|
Module Level Analysis
|
Performing I-V curve testing to
strings would not bring out anomalies or defects of neighbouring Modules
which are underperforming. However, if the neighbouring Modules exhibit
considerable degradation it can be found during String I-V measurement
activity.
|
|
Requires costly labour & equipment
|
Searching for the problem by the technicians in the site is a
time instinctive process and without the support of the real time and
continuous diagnostics this exercise becomes more challenging.
However, this problem can be avoided by interacting with the
O&M team and identifying the low performing blocks/areas for the
measurement and performing the measurement in those blocks/areas.
|
|
Cannot be used for raising Warranty
Claims
|
The I-V curve testing only provides
the information about probable degradation and cannot confirm the same. The
Modules have to be sent to the Third-Party labs for ascertain the degradation.
However, I-V curve can help in
identifying the defective Modules and minimize the financial risk of losing the warranty claim which may arise
if in case the Modules are selected randomly without performing I-V curve
testing.
|
Conclusion
I-V curve testing is an affordable,
compact, fast, and easy to use measurement technique. The shape of I-V curve
provides critical information about potential causes of the performance problems
in the Modules and provide us with preliminary data to infer the probable cause
of underperformance in the PV Modules. However, only I-V testing would not be
enough to determine the root cause of the performance degradation. The
combination of Thermography, I-V curve and EL testing would be required for
effective and reliable results.
Source URL: https://elegrow.com/i-v-curve-analysis-for-photovoltaic-system/
