7 Interpreting Measured I-V Curves
7-9
Potential causes associated with the model settings include:
• Incorrect module is selected for the PV model
• Number of PV strings in parallel is not entered correctly in the model
Potential causes associated with irradiance or temperature measurements include:
• Irradiance changed during the short time between irradiance and I-V
measurements
• Irradiance sensor is not oriented in the plane of the array
• Albedo effects (reflection) contribute additional irradiance
• Irradiance is too low, or the sun is too close to the horizon
• Manual irradiance sensor is not accurate
Uniform soiling
The effect of uniform soiling is like pulling a window screen over the PV modules; the
overall shape of the I-V curve is correct, but the current at each voltage is reduced.
Strip shade
Strip shade is a rectangular shading pattern falling on row(s) or column(s). The most
common example is in tilt-up arrays, early and late in the day when the lower row of cells
in a row of modules is partly shaded by the top edge of a neighboring row of modules.
For modules with conventional bypass diode configurations, the response to this shade is
different between portrait and landscape orientations of the shaded module.
In portrait orientation, uniform partial shading across the lower row of cells causes a
reduction in Isc and Imp, just as if there were less light. If the shading is identical across
the whole string of modules, there will be no steps in the I-V curve. If only some modules
in the string are affected, the bypass diodes in those modules will turn on and produce a
step in the I-V curve.
In landscape orientation, the shaded cells are all within the cell group shaded by a single
bypass diode, so the result is a step in the curve, commonly in the ‘horizontal leg’ of the
I-V curve.
Dirt dam
A constant-width band of dirt across an entire string can also reduce current. The most
common example is a low-tilt array with modules in portrait orientation. Over time, a
band of dirt collects at the lower edge of each module. When the band of dirt reaches the
bottom row of cells, it begins to limit current. If the dirt bands are similar enough from
module to module, the effect is like uniform soiling.
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