Z200 PV Analyzer
User Manual
within the solar cell PN junction. In this way a low value of R
sh
indicates degradation in the
module i.e. a condition where generated current is not harvested externally. Especially a
gradually falling value of module-R
sh
toward a string terminal is an indication of Potential
Induced Degradation (PID).
3.
Impedance under operation
The low frequency impedance norm, is measured while loading the string with a weak load
i.e impedance is measured while a small electric current is allowed to flow in the string.
When shading a module, while the instrument is transmitting the test signals (during a
current flow) it is thus possible to determine, if the module bypass diodes function as in-
tended. If the diodes do not ”open”, the instrument will measure a much raised impedance
value. The impedance shows up in the measurement, because the test signal must pass
shaded solar cells. If the impedance does not change, the current flows in the diodes, and
the instrument will conclude that there is no risk.
The result of the measurement is a Module Risk Factor (MRF), that is assigned to each
module. The higher the MRF is for a module, the more likely it is, that electric power will
be dissipated in the module in case of longterm shading or internal cell damages. This
will in most cases lead to so-called ”hot spots” and burn marks, which causes significant
irreversible damage to the system.
NOTICE.
•
It has been seen in experiments that the impedance
from even adjacent shaded PV cells can vary an or-
der of magnitude. In order to reduce the risk of erro-
neous measurements it is therefore recommended
to use as big a shade as possible i.e. preferably shad-
ing the entire module.
•
When using the Module test, make sure that the irra-
diation is not varying. A low irradiation is not prob-
lematic, but constantly shifting cloud cover, will to
some extend lead to variations in module voltages.
If the irradiation changes during testing, make sure
to recalibrate as many times as needed, to get the
best possible quality of test results.
•
The Module test is the most advanced test within the
Z200 PV Analyzer
range of methods. The method
relies on a basic understanding of solar PV module
physics, to get the best interpretation of the test re-
sult.
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