Solmetric PVA-1000S, User Manual

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7 Interpreting Measured I-V Curves
7-12
Module Degradation
Degradation of PV module performance with time and environmental stress is normally a
very slow process. Given the number of factors that can affect the height of the I-V curve,
the operator should estimate the impact of these other factors before concluding that the
modules have degraded.
Incorrect PV Module Is Selected for the PV Model
PV modules with similar PV model numbers may have different I
sc
specifications. Check
that the module you selected from the on-board module list matches the nameplate on the
back of the PV modules. If the array is known to have a mix of PV modules of different
types, this can also contribute to changes in I
sc
. Mixed modules can also cause a
mismatch effect, another class of deviation discussed later.
Number of PV Strings in Parallel Is Not Entered Correctly in the Model
The measured value of I
sc
scales directly with the number of strings in parallel. Check
that the correct value is entered into the model.
Irradiance Changed Between Irradiance and I-V Measurements
The time delay between the irradiance measurement and the I-V measurement can
translate into measurement error. The error is greatest when the sky conditions are not
stable (for example, partially cloudy) and a manual irradiance sensor is being used. The
process of orienting the manual sensor, noting the value, and entering the value into the
PVA software takes much more time than the automated process used by SolSensor.
Irradiance Sensor Is Not Oriented in the Plane of Array
The accuracy of the irradiance measurement is very sensitive to the orientation of the
sensor. The PV analyzer’s model assumes that the irradiance sensor is oriented in the
plane of the array. It is difficult to consistently position hand-held sensors in the plane of
the array. To see how much error this can introduce, orient the sensor to match the plane
of the array and note the irradiance value. Then remove the sensor and repeat several
times within a minute, and examine the consistency of the recorded values. This
experiment works only under stable irradiance conditions.
Albedo Effects (reflection) Contribute Additional Irradiance
The energy production of PV modules can be increased by reflection or scattering of light
from nearby buildings, automobiles, and other surfaces (the Albedo effect). If the
reflection seen by the PV modules under test is the same for all modules, the I-V curve
may look normal but I
sc
may be elevated. If the reflection is not uniform from module to
module, the I-V curve may have a mismatch type of shape, discussed later.