TECHNICAL ARTICLE
Arcs in Photovoltaics (PV)
Over the last few years, the number of PV system installations has grown enormously in certain parts of
the world. As a result, safety and quality standards have been continuously updated in order to ensure
that today's systems are as safe and reliable as possible. It is vital that these standards continue to
evolve and that any potential system weaknesses continue to be identified and resolved. One current
topic of discussion is the occurrence of arcs in PV systems and the ways to eliminate this risk. Read on
to find out more...
Arc Characteristics
Like all electrical systems, PV systems are subject to the risk of arc faults. These occur when a sufficiently high
voltage is generated between two electrodes located a certain distance apart. The electrical voltage causes the
gas between the electrodes (generally air) to ionize. In other words electrons are "forced out" of the gas
molecules and atoms, creating a plasma of positive ions and electrons. This plasma can heat up to several
1000°C and create a flow of current between the electrodes, causing a characteristically bright arc to appear.
The high arc temperatures can cause severe damage to the system and in extreme cases even cause fire.
Occurrence of Arcs
If a wire in a circuit is cut, the two ends of the wire act in the same way as the electrodes, separated by air,
mentioned above. Depending on the voltage and the distance between the two wire ends, an arc may therefore
occur (see Figure 1). Various studies have shown that arcs can occur at just 20 V and with very small
amperages. Moreover arcs can occur in circuits with either an AC or a DC voltage source supplying the voltage
between the two electrodes. The main difference between AC and DC systems is that arcs in AC systems are
easier to extinguish. The periodically varying voltage means that the arc is extinguished upon each voltage zero
crossing. The arc may be subsequently reignited but ultimately this type of arc is less stable than those in DC
systems because it is continually extinguished.
Figure 1: Occurrence of a serial arc
Types of Arc Occurring in Photovoltaic Systems
Two main types of arc can be distinguished depending on their position in the PV system (see Figure 2).
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Parallel Arcs
Parallel arcs are those that occur either between the positive and negative pole or between one of the two live
wires and the ground in grounded systems.
Typical causes of parallel arcs in PV systems are:
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Short circuits (between + and -) in DC receivers; and
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Defective insulation on the main DC wires ( + to - or live wire to ground).
Serial Arcs
This type of arc typically occurs when the contacts in the circuit are opened.
Typical causes of serial arcs in PV systems are:
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Poorly crimped/assembled DC plug;
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Defective module junction boxes;
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High resistance solder connections between the cells of a module;
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Poorly connected cable on the DC side; and
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String fuses that are the wrong size or not permitted for PV applications.
Figure 2: Serial and parallel arcs in PV systems
Preventing Arcs in Photovoltaic Systems
Serial and parallel arcs have different causes and therefore require different prevention measures. Generally
speaking, parallel arcs are very unlikely to occur.
Parallel Arcs
Thanks to the comprehensive combination of protective measures prescribed in the applicable standards, this
type of arc is very unlikely to occur. The PV generator comprises a circuit with protection class 2 (double
insulation on the DC cable, plus safely grounded and short-circuit-proof wiring). In the majority of cases, this
prevents short circuits and thus arcs from occurring. The inverters also provide additional safety features in the
form of insulation monitoring and fault current monitoring. These features ensure that defects (such as defective
insulation on a live wire) are always identified as soon as they occur, even if they do not pose any immediate
danger. Fixing any defects right at the start prevents further defects from developing that could cause a parallel
arc.
Serial Arcs
The key to preventing serial arcs is to ensure that the connections between individual wires in the PV system
are completely secure and thus cannot become disconnected unintentionally. There are two fundamental
requirements for secure connections: Premium components and meticulous and correct installation.
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High quality processes for the production of module junction boxes and the solder connections in the module
are vital for ensuring that connections remain secure in the long term. Likewise, the electricians responsible for
installing the system have a key role to play. In addition to selecting the correct components (e.g., string fuses
that are suitable for PV systems), they must also ensure the system is correctly installed. This includes
tightening the screw terminals to the correct torque and crimping and assembling the DC plug exactly as per the
instructions.
Nonetheless, even if the PV system meets all of the above requirements, there is always a risk of an arc fault
occurring due to the high thermal load the system is subjected to every day, year-in-year-out, as well as the
impact of weather conditions such as rain, snow, and ice. In order to eliminate this risk, researchers are
currently looking into the option of using an arc detector.
Arc Detector
The principal behind the arc detector is that the occurrence of an arc in the PV generator causes significant
changes in the current and voltage signals for the PV system (see Figure 3). An electronic device with
evaluation capability can be used to monitor these changes and switch off the system by means of a suitable
control mechanism, where necessary, in order to extinguish the arc. This device can either be integrated into
the inverter or installed as a separate unit on the DC side of the PV system.
New Challenges
Even though this type of detector is already in use in other industrial sectors, its use in photovoltaics poses new
challenges for manufacturers. The high voltages, long wires and diverse module technologies (all with different
electrical systems) make it extremely difficult to detect arcs. Intensive research is currently being conducted in
order to find ways to overcome these challenges and arc detection products are expected to be launched on the
market in the next few years. These will help make PV systems even safer and even higher quality.
Figure 3: Current and voltage curves when an arc fault occurs
Summary
As with any electric circuit, defects in PV systems can give rise to arc faults. The installation and production
standards currently in force and the components produced in accordance with these standards help to prevent
arcs and increase safety standards to a large extent. However, it is also vital that the systems are installed by
experts with due care and attention. In order to eliminate any residual risk, researchers are currently looking to
develop arc detectors that meet the requirements of PV systems. Arc detectors that make PV systems even
safer and higher quality are expected to be launched on the market in the next few years.
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Images by: Fronius International GmbH (reprint free of charge).
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Fronius International is an Austrian company with headquarters in Pettenbach and other sites in Wels, Thalheim
and Sattledt. With 3,250 employees worldwide, the company is active in the fields of battery charging systems,
welding technology and solar electronics. Around 95% of its products are exported through 17 international
Fronius subsidiaries and sales partners/representatives in over 60 countries. In fiscal year 2010, the company
achieved total sales of EUR 499 million. With its outstanding products and services and 737 active patents,
Fronius is a world technology leader. 392 employees work in research and development.
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Technical press:
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Andrea Schartner, +43 664 88536765, schartner.andrea@fronius.com,
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