By Thomas Muhlberger
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.
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).
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:
-Short circuits (between + and -) in DC receivers, and
-Defective insulation on the main DC wires (+ to - or live wire to ground).
This type of arc typically occurs when the contacts in the circuit are opened.
Typical causes of serial arcs in PV systems are:
-Poorly crimped/assembled DC plug,
-Defective module junction boxes,
-High resistance solder connections between the cells of a module,
-Poorly connected cable on the DC side, and
-String fuses that are the wrong size or not permitted for PV applications.
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.
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.
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.
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.
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.
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.
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.
Thomas Muhlberger is Area Manager PV System Technology, Fronius International GmbH (www.fronius.com), Wels, Austria. He is a member of various standardization committees (IEC TC 82, CLC TC 82, etc.). He studies Technical Physics in University of Linz.
For more information, please send your e-mails to firstname.lastname@example.org.
ⓒ2011 www.interpv.net All rights reserved.