Z200 PV Analyzer
User Manual
resistance that should ideally be close to zero in order to minimize power loss. It is the sum of
junction diffusion loss in the PV cells and of all series losses in cables, connectors and bus-bars
in the PV modules. An illuminated healthy PV string (with irradiation
>
100
W
m
2
) will only have
an impedance represented by R
S
, since the photo diode is fully turned ON by the photovoltaic
voltage, and thus effectively shorting C
d
and R
P
.
Solar module string - equivalent circuit. This model is roughly equivalent to the solar module
string, and a basis for understanding the impedance measurement.
1. I
LIGHT
is the current generated by light on the modules
2. I
D
is the diode current
3. C
d
is the diffusion capacitance
4. R
P
is the parallel (shunting) resistance
5. R
S
is the series resistance
R
S
should be estimated in the frequency range
f
= 100 Hz to
f
= 10 kHz where the simple
model is most accurate (at higher frequencies e.g. effects of cable inductance become a fac-
tor and cause the impedance to increase). At EmaZys, a study on more than 500 commercial
modules representing various PV cell technologies and sizes, has been conducted. A worst-case
relationship concerning the dependence of R
S
on I
SC0
(short- circuit current at 1000
W
m
2
irradia-
tion) and V
OC
(open-circuit voltage) has been
R
S
<
30Ω
A
·
V
OC
100
V
·
1000
W
m
2
Irr.
/I
SC
0
(1)
Example: Let us see what this means by a concrete example. An operator is in the field
conducting measurements on modules having I
SC0
= 10A, string open circuit voltage is 500 V and
the irradiation has been measured at 100
W
m
2
. The R
S
on a healthy string should be less than 150
Ω
according to the above formula. Let us insert the values an check:
R
S
<
30Ω
A
·
500
V
100
V
·
1000
W
m
2
100
W
m
2
/
10
A
⇓
R
S
<
30Ω
A
·
5
·
10
/
10
A
⇓
R
S
<
30Ω
A
·
5
1
A
⇓
19