Operation and installation manual
Page 15 of 96
(PVI-STRINGCOMB (-S, -MC, -S-MC) - Rev:1.0)
3 LARGE PHV FIELDS AND PARALLEL STRINGS.
When speaking of large photovoltaic fields, the immediate image is one of large
extensions of solar panels.
Since a field (array) is composed of several parallel strings, it is clear that to
obtain high power the number must be increased. In this case, particular
precautions must be taken in the project / installation phase, otherwise the strings
may be damaged due to breakdowns in the strings themselves or to incorrect
connections or dimensioning.
3.1 Why use PVI-STRINGCOMB(-S)?
Connecting strings in parallel does not entail particular problems as long as the
"Uoc" voltage (Open Circuit Voltage) for each string is the same as the other
strings. If not, e.g. because of considerable differences in the length of the strings
or short circuits in one or more of the solar modules in a string, there can be a
considerable difference in the Uoc voltage which can cause an inverted current
on the string with the lower voltage (i.e. made up of a lower number of modules
connected in series). Contrary to popular opinion, shading has no significant
influence on this effect; this is because Bypass diodes have been adopted and
they are now standard on all panels.
If the inverted current is greater than the maximum permitted for the panel, it can
overheat and/or even be irreparably damaged (Figure 3-1).
Diodes can be inserted in series to each string (string blocking diodes) to
prevent this problem. Using these diodes has two disadvantages: 1) The
string current pass through the diode and consequently the loss of power
is not negligible. 2) If there is a short circuit in the diode, there is no
protection and the phenomenon in Figure 3-1 can occur.
The best solution is to use a fuse for every string.
In the event of excessive inverted current, the fuse opens and protects
the string itself (Figure 3-2). If the fuse status is monitored, it is also now
possible to remotely check the status of the strings which make up the
photovoltaic generator. Moreover, a fuse's power losses are considerably
less than a diode's. The PVI-STRINGCOMB has fuses both on the
positive pole and on the negative pole of each string.
