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solar generator (to protect the solar modules) as well as at
the DC input of the inverter (to protect the inverter). SPDs
should be installed close to the PV generator as well as
close to the inverter as soon as the length of cable required
between the PV generator and the inverter extends beyond
10 meters (Figure 2). The standard solution to protect the
AC side, meaning the inverter output and network supply,
must then be achieved by using type 2 SPDs installed at the
inverter output and – in the case of a building installation
with external lightning protection at the mains feed-in point
– equipped with an SPD type 1 surge arrester.
Special characteristics on the DC solar generator side
Until now, protection concepts on the DC side always used
SPDs for normal AC mains voltages, whereby L+ and L-
respectively were wired to earth for protection. This meant
that the SPDs were rated for at least 50 percent of the
maximum solar generator no-load voltage. However, after
a number of years, insulation faults can occur in the PV
generator. As a consequence of this fault in the PV system,
the full PV generator voltage is then applied to the non-faulty
pole in the SPD, and results in an overload event. If the load
on SPDs based on metal-oxide varistors from a continuous
voltage is too high, this can potentially result in their
destruction or trigger the disconnecting device. In particular,
in PV systems with high system voltages, it is not possible
to completely exclude the possibility of a re developing
due to a switching arc that is not extinguished, when the
disconnection device is triggered. Overload protection
elements (fuses) used upstream are not a solution to this
probability, as the short-circuit current of the PV generator
is only slightly higher than that of the rated current. Today,
PV systems with system voltages of approx. 1000 V DC are
increasingly being installed to keep power losses as low as
possible.
To ensure that SPDs can master such high system voltages
the star connection consisting of three varistors has proven
reliable and has become established as a quasi-standard
(Figure 4).
If an insulation fault occurs two varistors in the series still
remain, which effectively prevents the SPD from being
overloaded.
Photovoltaic application example
Figure 1: Schematic diagram from standard IEC 60364-7-712
Figure 2: Building installation with external lightning protection system,
in compliance with air termination distances
Figure 3: Field installation with external lightning protection system
F
Surge protection for photovoltaic systems
F.52028840000