Solmetric PVA-600, Руководство пользователя

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5 Interpreting Measured I-V Curves
5-15
Array Is Partially Shaded
Partial shading of a PV cell reduces the current that can be generated by that cell, which
in turn reduces the maximum current that can be produced by other series connected
cells. For example, slightly shading one cell in a 72 cell module that has 3 bypass diodes
will slightly reduce the current in 24 cells. Bypass diodes are present to prevent that
section of cells from going into reverse bias. If the PV module is supplying a load and the
current demanded by the load is above the (reduced) current provided by the partially
shaded cells, the bypass diode will begin conducting and short them out. Without the
bypass diode present, the cells would be reverse biased, which can generate potentially
damaging reverse breakdown voltage and hotspot failure, as discussed in 2. The Slope of
the Curve near Isc Does Not Match the Prediction. The impact of partial shading on the I-
V curve is to create a distortion or notch, as shown in Figure 53. In a single PV string, the
vertical height or current at which the notch appears is equal to the reduced short-circuit
current of the partially shaded cells. The horizontal or voltage distance from Voc to the
notch is related to the number of cell strings within modules that have been bypassed.
PV Cells Are Damaged
In a cracked cell, a portion of the cell may be electrically isolated. This has the same
effect on the I-V curve as shading of an equivalent area of a normal cell. A notched I-V
curve can result depending on the severity of the PV cell damage.
Cell String Conductor Is Short Circuited
As described in 2. The Slope of the Curve near Isc Does Not Match the Prediction, a
localized hot-spot can also effectively short out a particular cell. When this happens, the
bypass diode spanning that cell string can turn on, causing a notched I-V profile. This I-V
profile would be similar to a completely shaded PV module, but with a lesser voltage
reduction corresponding to the loss of a single cell string.