23 / 48
take note
• Earthing of live conductors
• Maximum short-circuit current I
SCPV
The short-circuit current is extremely important due to the special
characteristic of the dc voltage sources within the PV system. The
important thing to consider when selecting an arrester, is to ensure
that the maximum dc short-circuit current of the PV system does not
exceed the short-circuit current rating of the arrester.
According to the EN 50539-11 [1] standard the short-circuit current
rating for the SPD (I
SCPV
value) must be greater than the maximum
short-circuit current of the PV system: I
SCPV
> I
max
PV system. String
inverters are used for all PV systems, ranging from small rooftop
systems all the way through to large solar parks in themulti-megawatt
range. Installation devices for protecting the dc side against surges
(at least type 2 arresters) are required for each MPP input. They are
either connected upstreamof the inverters in generator junction boxes
or they are already integrated in the string inverters. Dc short-circuit
currents typically do not exceed 100 A.
Central inverters with frequent ratings of 1 MW are mainly used
in solar parks. The dc lines from the field are routed in parallel to a
common busbar, a system of electrical conductors in a generating
or receiving station on which power is concentrated for distribution.
This is where several hundred amperes of direct current accumulate.
Overvoltage pulses from the entire area are centrally collected on the
busbar. Correctly dimensioned surge protective devices will protect
the input circuits from damage and increase the life span and avail-
ability of the inverter. The generator junction boxes used for these
systems are interconnected parallel to one other through the central
inverter. Then, the return currents must safely be discharged even
if a surge arrester is overloaded. The resulting short-circuit current
must not exceed the short-circuit current rating ISCPV of the arrester.
Maximum safety and fire protection
SCI is created for surge arresters with a three-step dc switching device.
SPDs have an integrated disconnector that isolates in case of overload.
In order to prevent the formation of an internal arc, the disconnec-
tor is combined with a bypass path. In the case of an overload, the
disconnector is then activated and any arc will be quenched on the
low-resistance bypass path. The integrated fuse interrupts the flow
of current and a safe electrical isolation of the arrester is achieved.
SCI arresters provide protection in case of an overload due to
the combination of both disconnecting and short-circuiting devices,
making it an effective surge protection system that meets the highest
requirements to personal and fire protection
Switching phases
If the disconnector device is activated:
• The PV system’s current changes over to the bypass current path
of the arrester
• Arising arcs will be directly quenched
• The fuse integrated in the bypass interrupts the dc current flow.
Conclusion
The arresters with SCI technology protect photovoltaic systems all
over the world, across all continents and under very different climatic
conditions. The arresters with SCI technology are internationally ap-
proved as they withstand extreme climatic conditions and thus can
be used in any climate zone.
Reference
[1] EN 50539-11: 2013+A1:2014. Low-voltage surge protective de-
vices. Surge Protective Devices (SPDs) for specific application
including dc requirements and tests for SPDs in photovoltaic
applications.
Abbreviations/Acronyms
MPP
– Message Posting Protocol
PV
– Photovoltaic
SCI
– Short-Circuit Interruption
SPD
– Surge Protection Device
Kirk Risch joined the South African Air Force in 1988 as
an apprentice, learning trade skills in radar technologies
and electronics. In 2010, Kirk joined Webb Industries, a
specialist ancillary telecommunications company, becoming
its lightning and surge protection expert. He joined DEHN
Africa as sales and marketing manager in 2013, becoming
sales and marketing director the following year. Enquiries: Email kirk.risch@
dehn-africa.com
• Electrical arcing over HV dc lines is the main cause of solar
PV fires.
• This company is the only company worldwide to provide
surge arresters that include Short-Circuit Interruption (SCI)
technology.
• This patented technology provides maximum safety and is
key to fire prevention.
EARTHING + LIGHTNING PROTECTION
21
January ‘17
Electricity+Control