Arc fault events are a pain point in the solar industry for the threat they pose to PV systems and, maybe more so, for the detection headaches they cause.
What is a DC Arc Fault?
An arc fault is the flow of electrical energy through an air gap by way of ionized gas molecules. Whilst air is normally regarded as a non-conducting medium, a high potential difference (voltage) between two conductors in close proximity can cause the air molecules to break down into their ionized constituents (called a plasma), which can then carry a charge from one electrode to the other.
The temperature of an electrical arc depends on a number of factors, such as the level of current flow, but on a typical PV system, it is easily hot enough to melt glass, copper, and aluminum, and to initiate the combustion of surrounding materials.
DC arc faults may occur on the DC cabling of PV solar systems.
Faults can occur anywhere on the DC cabling that runs from the solar modules on the roof to the string inverter which is usually installed adjacent to the main switchboard. There are approximately 30-40 connections in the DC circuit of a typical residential solar system (5kW) which are affected on-site by the solar installer. All of these connections undertaken by the solar installer are a potential point of failure.
There are also additional connections inside the solar modules, DC isolators, and the string inverter which are potential points of failure.
There are three common types of DC arc faults:
Series – A series arc occurs when a connection is broken while the PV is producing current. Any intermittent connection in the DC circuit has the potential for producing a DC arc fault. These connections may include soldered joints within the module, compression-type wire connections, connectors used on the wire leads attached to PV modules, connections in DC isolators, connections in the inverter, any DC circuitry in the inverter, or any of the DC cabling in the string circuit.
Parallel – Parallel arcs occur when there is a breakdown in the insulation system and current flows between positive and negative. Two conductors of opposite polarity in the same DC circuit are often run in close proximity to each other. The insulation between the two wires can become ineffective due to animals chewing on them, UV breakdown, embrittlement, cracking, moisture ingress, and mechanical damage. Parallel arc faults can continue along the conductors towards the array burning materials along the way.
To Ground – This fault only requires the failure of one insulation system to ground. This can be the solar module frame, the solar array racking, the roof, or any other grounded surface.
What can trigger a DC arc fault?
Breakdown of conductor continuity due to:
· Loose joints due to poor installation
· Loose joints due to poor quality connections
· Corrosion of joints over time
Can DC arc faults be prevented?
There are a variety of solar technologies that have been available for more than years which eliminate the risk associated with DC arc faults. These include microinverters manufactured by companies such as Enphase Energy, and also some DC optimized systems such as Huawei FusionSolar or SolarEdge, which have a safe DC mode.
Do string inverters with built-in DC Arc Fault protection provide protection for all types of DC arc faults?
There are some string inverters available now with built-in arc fault detection, such as from Huawei. Inverters with built-in arc detection identity a DC arc-fault using noise on the DC cabling produced by the arc. Once an arc is detected the DC circuit at the inverter will be isolated. This will extinguish a ‘series’ arc but WILL NOT extinguish a ‘parallel’ or ‘ground’ fault arc. The DC cabling under the solar array and down to the inverter will remain live when the DC circuit is isolated at the inverter. A parallel or earth fault will continue to arc on solar systems using string inverters with DC arc fault protection.
The Smart Power optimizer is a DC/DC converter located at the PV modules. Once an arc is detected, the optimizer stops production instantly. This is Huawei’s 0V output technology and is commonly referred to as“module-level shutdown”. Optimizer outputs are connected in series to build a DC output circuit that connects to the inverter which also stops production when an arc is detected.
