Z200 PV Analyzer
User Manual
6.3
String Tester
APPLICATION.
The string tester performs a range of impedance measure-
ments. The overall result is an assessment of the health
condition of the solar PV module string under test.
The String tester is a diagnostic tool for checking the overall health condition of the PV mod-
ule string. The basic content of the string test, is to measure an impedance spectrum using the
Z200 PV Analyzer pre-programmed measurement routine. The open circuit voltage is also mea-
sured, and an optional R
ISO
measurement may also be included.
The impedance spectrum is recorded by measuring the alternating current flow in the PV mod-
ule string, when the string is subjected to an alternating voltage test signal, transmitted by the
Z200. The impedance is then found by dividing the AC voltage with the AC current, accord-
ing to
Ohm’s law
. The impedance is most often denoted as a complex number, and note that
impedance accounts for both ordinary ”Ohmic” resistance, as well as reactance i.e. capacitance
and inductance. The mathematical theory of impedance spectroscopy, is beyond the scope of
this manual, and the reader is referred to textbooks on the subject matter e.g.
Electrochem-
ical Impedance Spectroscopy
by authors Mark E. Orazem, Bernard Tribollet. However, a full
understanding of such theory is not a prerequisite to work with the Z200 PV Analyzer.
Connect the instrument to the PV string and
press the
Measure
button to collect a spec-
trum. The measurement will take 30-60 sec-
onds. In the example shown to the right, R
S
is
16.9
Ω
as is the low frequency norm at a voltage
of close to 410V, which indicate a healthy string
of 12 x 250 W modules with a decent sun irradi-
ation. With a higher irradiation Rs could go as
low as 10
Ω
.
The in-depth interpretation of solar PV module impedance spectra requires some knowl-
edge about the physics of a PV string/module. Below is a diagram of a simple so-called three
parameter PV string model. It contains the three component parameters R
P
the shunting (or
parallel) resistance, R
S
the series resistance and C
d
the diffusion capacitance. The remaining
components in the model are the light current generator I
LIGHT
and shunt diode with current
I
D
. The current source models the current delivered by the PV string when short-circuited and
the diode characteristics determine the open circuit voltage of the string. R
S
is the string series
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