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© 2011-2013 Silfab Ontario Inc. - Specifications in this manual are subject to revisions without notice
SAFETY AND INSTALLATION MANUAL FOR
PHOTOVOLTAIC MODULES
8. USE AND MAINTENANCE
8a) Intended Use
SLA/SLG 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 110V
AC - 60Hz 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 (Vp
max
) of the string
•
Operational current at maximum power
output (Ip
max
) of the string
From these above values it is possible to estimate
the voltage at maximum power (Vp
max
) of a
module in the string under review and any non-
uniformity in the voltages (Vp
max
) of multiple
strings connected to the same inverter.
From the Ip
max
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:
•
V
ocSTC
is the open circuit voltage
measured at STC;
•
-0,125V/ °C is the average variation of
V
oc
of a module for a variation in
temperature of 1°C;
•
25°C is the reference temperature of
STC;
•
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
