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ELECTRICAL PROTECTION + SAFETY
Special Surge Protection measures for dc side
The typical curve characteristics of PV current sources differ substan-
tially from conventional dc current sources. They have a non-linear
characteristic and a different dc arc behaviour.
These differences does not only affect the size of the dc switches
and fuses but it requires surge protection devices that are capable
of coping with these unique dc follow currents. Supplement 5 of IEC
62305-3 [1] requires safe operation of surge protection devices on the
dc side even in the case of overload.
Lightning Current Distribution and Loading of SPDs
The lightning current is mostly dissipated into the ground via a
properly designed earth termination system (grid type) and only
small partial lightning currents dissipate via the PV system`s cables
which are equipotentially bonded to the earth
termination system via SPDs.
This happens for both low resistance and
high resistance earth termination systems. It is
therefore the shape and dimension of the earth
termination that is important, not the earth ter-
mination resistance.
Figure 9: Lightning current distribution and loading
of SPDs.
Size and Placement of SPDs
The most important factor in the selection of the
type and placement of the surge protection meas-
ures, is the type of PV system.
Central Inverter
A great portion of lightning current directly enters
the earthing; and partial lightning currents also
seek a path to the earth via the dc cabling. The as-
sumption that the whole surface of the PV system
can be considered as ‘equipotential surface’ is not
realisable in practice.
The energy system cables work as equipoten-
tial bonding conductors between the ‘local’ earth
termination at the module array where the direct
lightning strike occurs and the ‘distant’ equipoten-
tial surface of the central inverter.
Therefore, in PV ground-mounted systems with a central inverter, the
partial lightning currents flow through the dc conductors between
the generator junction boxes and the dc input of the central inverter.
Type 1 PV-SPDs need to be installed both at the dc-sub-distribution
boards and at the dc-input of the central inverter.