Page 21 - Issue 4 2023

TECHNICAL

area of concern with the move to high string voltages (>1000 VDC). PID is generally occurs during operation in the
caused by the difference in potential between the cell and other parts of the system, daytime. However, if the panels have
notably the metal frame which is generally at earth potential. Often, PID is related to been subject to PID for a long time,
a negative voltage potential to earth, with the more negative panel being the panel at the regeneration will last half of the
most risk. The PID effect is caused by the system’s exposure to an external potential. degradation period.
The current-voltage (IV) curve
There are two types of power degradation: of a panel gives an indication of the
a) Reversible (polarization) performance of the panel under
b) Irreversible (electro-corrosion) different levels of current. Figure 5
shows a typical IV curve of a healthy
When a PV module has a high negative voltage between the cells of the module and panel.
the frame, which is usually grounded for safety reasons, current flows through the The IV curve of PV modules contain
grounded frame, resulting in a leakage current. This current disturbs the photovoltaic a lot of information about their health.
effect (Figure 4). An IV curve tracer will test a panel
It seems that the accelerator of this phenomenon is humidity and high from open circuit to short circuit and
temperature, combined with applying negative voltage. Almost all PV systems use all points in between under load. This
p-type crystalline or thin film modules at the moment. provides the most accurate indication
PID countermeasures can be applied by the smart inverter. There are several into the health and performance of a PV
methods of which the most common is to “regenerate” the modules at night by module. Figure 6 shows examples of IV
submitting them to a high positive potential. This reverses the ion migration which curves of panels under test.
During manufacturing, modules
are tested under standard conditions
(STC) at an irradiance of 1000 W/
m , a temperature of 25°C and air
2
mass of 1,5. Field measurements
using natural sunlight are taken under
different conditions of irradiation and
temperature, and measurements of
irradiance and temperature captured
at the same time as the IV curve data
can be used to convert field IV curve
measurements to STC. Corrected
measurements can then be used for
a direct comparison with the rating
plate figures.

Individual module testing
Figure 4: PID mechanism (Omron) In this method a portable test unit is
connected to the output connector
of an individual panel. Access to
panel wiring is required, which is not
normally available on rooftop systems.

In-string testing
Ideally, individual panels should be
tested but it has been found that
the IV characteristics of a string of
panels can indicate faults in the string.
This testing can be done by smart
inverters. The in-string test may not
be able to identify individual panels
but will certainly identify strings that
Figure 5: IV curve of a healthy panel (Huawei) contain faulty panels.

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