Electricity + Control January 2017

EARTHING + LIGHTNING PROTECTION

MPP

– Message Posting Protocol

PV

– Photovoltaic

SCI SPD

– Short-Circuit Interruption – Surge Protection Device

Abbreviations/Acronyms

• Earthing of live conductors • Maximum short-circuit current I SCPV

• 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.

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.

take note

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.

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

January ‘17 Electricity+Control

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