Solmetric PVA-600, Руководство пользователя

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5 Interpreting Measured I-V Curves
5-7
PV Array Is Soiled
The effect of uniform soiling is like pulling a window screen over the entire array, or
reducing the actual irradiance; the overall shape of the I-V curve is correct, but the
current at each voltage is reduced. Non-uniform soiling can also have this effect. The
most common example is a low-tilt array with modules in portrait mode. Over time, a
band of dirt extends upward from the lower edge of each module. When the band of dirt
reaches the bottom row of cells, the height of the I-V curve is reduced.
PV Modules Are Degraded
Degradation of PV module performance with time and environmental stress is normally a
very slow process. Given the number of factors – for instance, soiling or irradiance
measurement accuracy – 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 Model
PV modules with similar model numbers may have different Isc 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 Isc.
Number of PV Strings in Parallel Is Not Entered Correctly in the Model
The measured value of Isc 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
When measuring irradiance with an external irradiance sensor, the time delay between
the irradiance measurement and the I-V measurement can translate into measurement
error.
Irradiance Sensor Is Oriented Incorrectly
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 reliably hold 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 reading. Then tilt the sensor slightly and notice how much the
reading changes.