8
FuturaSun srl – The specifications in this manual are subject to revisions without further notice
8. Use and Maintenance
8a) Intended Use
FU modules are designed for use in grid-
connected systems. They are therefore linked in
series/parallel combinations to feed a dedicated
inverter with a DC input and an AC output of 230V
AC - 50Hz to provide energy to the local electricity
grid.
8b) Operational Measurements
The only two electrical parameters of output from
a PV module, measurable with conventional
instrumentation, are the V
oc
and I
sc
.
When the PV modules are instead connected in
series/parallel configuration to an inverter, from its
display it is possible to read:
•
Operational voltage at maximum power
output (Vmpp) of the string
•
Operational current at maximum power
output (Impp) of the string
From these above values, it is possible to estimate
the voltage at maximum power (Vmpp) of a module
in the string under review and any non-uniformity
in the voltages (Vmpp) of multiple strings
connected to the same inverter.
From the Impp for the string it is also possible to
verify whether there are obvious differences
between one string and another. When a uniform
condition is detected, it can be assumed that all the
modules are working properly.
The following measures serve to collect
preliminary information on the operational status of
the PV modules in a PV system.
If there is a need to perform direct measurements
on individual modules using conventional
instrumentation, the following action should be
taken.
To measure the open circuit voltage (V
oc
):
•
Note: even in the presence of an insolation
average of 500 W/m
2
, a module exposed to the
rays of the sun presents at its poles (+ and –)
a V
oc
very close to the nominal value at STC
(as shown in Fig.3).
•
When taking the temperature at which the
module is working at that moment into account,
the open circuit voltage module (V
ocmod
) will be
approximately equal to:
V
ocmod
= V
ocSTC
– [(T
mod
– 25°C) x 0.125V]
Where:
o
V
ocSTC
is the open circuit voltage measured
at STC;
o
-0,125V/ °C is the average variation of V
oc
of a module for a variation in temperature
of 1°C;
o
25°C is the reference temperature of STC;
o
In the case of good solar radiation and at
the ambient temperature (T
amb
), one can
estimate the temperature of the module as
follows:
T
mod
= T
amb
+ 30°C
•
Using the calculations above, it is possible
when measuring with a multi-meter, to verify
V
oc
meets the standard shown in the module
datasheet.
•
In a case that the V
oc
to the connectors is
decidedly lower than the standard values (75%
or less) this could represent a condition of
anomaly which should be investigated more
thoroughly.
To measure the short-circuit current (I
sc
):
•
A PV module exposed to the south, inclined
perpendicularly to the rays of the sun, in the
middle of the day (about 12:00 to 1:00 PM ) and
in conditions of good weather, presents a value
of I
sc
similar to the rated values at STC, as
measurable with an amp-meter in continuous
current.
•
By measuring the solar radiation (E) effective
at the moment with a solarmeter/pyranometer
in W/m
2
the short circuit current of the module
at the moment I
scmod
should be very close to the
following value:
I
scmod
= I
scSTC
x E/1000
Where:
o
I
scSTC
is the short circuit current
measured at STC;
o
1000 W/m² is the radiation at STC.
•
The measurement of the Isc is executable with
precision
only
when
using
a
solarmeter/pyranometer which gives exact
information on the conditions of solar radiation
at the moment, otherwise it is not reliable.
•
In the case of the unavailability of a
solarmeter/pyranometer, it will only be possible
to have an estimate of the functionality of the
module by comparing the value of I
scmod
measured in relation to those of the other
modules of the PV system, measured under
the same conditions of irradiation.
The acknowledgment of any obvious
discrepancies of I
sc
in the modules thus serves
to identify anomalies.
•
The maximum module configuration when
connected in series is 19 modules size 60 cell
Fig. 16
