Solmetric PVA-600, Руководство пользователя

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5 Interpreting Measured I-V Curves
5-10
Shunt Paths Exist In PV Cells or Modules
Shunt current is current that bypasses the solar cell junction without producing power,
short circuiting a part of a cell or module. Some amount of shunt current within a solar
cell is normal, although higher quality cells will have a higher shunt resistance and hence
lower shunt current. Shunt current can lead to cell heating and hotspots appearing in the
module‘s encapsulant material. Shunt current is typically associated with highly localized
defects within the solar cell, or at cell interconnections. Infrared imaging of the PV
module can usually identify minor shunt current hot spots since a temperature rise of 20º
C or more is common.
A reduced shunt resistance will appear in I-V curves as a steeper (less flat) slope near Isc.
As the cell voltage increases from the short circuit condition, the current flowing in these
shunts increases proportionally, causing the slope of the I-V curve near Isc to become
steeper. The shunt current in a series of modules or within a single module can be
dominated by a single hotspot on a single cell, or may arise from several smaller shunt
paths in several series cells.
Shunts within a module can improve over time, or can degrade until the module is
damaged irreparably. Smaller shunts can self-heal if the high current through the shunt
path causes the small amount of material shorting the cell to self-immolate. Larger shunts
can result in localized temperature rises in the module that can reach the melting point of
encapsulant material or the module backsheet. Modules that have failed in this manner
will tend to show burn spots or other obvious evidence of failure. Bypass diodes in the
PV module are designed to prevent damage due to hotspot, and so failure of the bypass
diode may accompany hotspot damage.
If the I-V measurement of a PV string shows a substantial slope, you can localize the
problem by successively breaking the string into smaller segments and measuring the
segments individually. Be sure to update the model with the reduced number of modules
in series.
Module Isc Mismatch
A reduction in slope of an I-V curve for a series string may have less to do with shunt
resistance, and more to do with small mismatches between the Isc values of each module.
Isc values in a real PV system will have some mismatch, due to slight manufacturing
variations, different installation angles, or partial soiling. The impact on the I-V curve
from Isc mismatch will not be as obvious as a partial shading condition (refer to 4. The I-
V Curve Has Notches or Steps) and may only be visible as a slight change in I-V slope
and the fill factor.