7 Interpreting Measured I-V Curves
7-16
6. Less steep slope in vertical leg
An example of this type of deviation is shown below.
Figure 66. Measurement with less steep slope in the vertical leg of the I-V curve.
The slope of the vertical leg of the I-V curve is affected by the amount of series
resistance internal to the PV modules and in the array wiring. Increased resistance
reduces the steepness of the slope and decreases the fill factor.
Potential causes are summarized below, and then discussed in more detail.
Potential causes located in the array include:
• PV wiring has excess resistance or is insufficiently sized
• Electrical interconnections in the array are resistive
• Series resistance of PV modules has increased
PV Wiring Has Excess Resistance or Is Insufficiently Sized
The electrical resistance of the PV modules and their connecting cords are accounted for
in the models stored in the PV analyzer module database. If the PV output conductors
(for example, from string to combiner box) are very long, or the wire gauge unusually
small, or both, the PV model can be adjusted to account for that extra resistance.
To see the effect of wire resistance on the predicted I-V curve, enter 500 feet (1-way) of
#10 wire. This will add approximately 1 ohm of series resistance. Notice the change of
slope in the I-V curve near V
oc
.
The resistance of the primary test leads of the PV analyzer is extremely low and can be
neglected. The resistance of the Solmetric Test Lead kit can also be neglected. Using
smaller-gauge test leads can add significant resistance and corresponding measurement
error.
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