Unlike conventional electrical applications, most solar photovoltaic (PV) systems cannot contain arcs. Although alternative topologies using micro-inverter technologies are being developed with DC voltages below 80 V and a direct AC voltage output, the vast majority of solar PV systems available today utilize high voltage series DC circuits as they provide a superior cost per watt.
These systems use a central or string topology where many PV panels are connected in series to a central DC to AC inverter (Figure 1), which in a typical residential solar panel system carries 200-600 volts. These high voltages pose potentially serious safety issues including electric shock and fire. If there is faulty wiring or connectors, arcing can occur over high voltage DC lines.
This can lead to the installation being electrified shocking anyone who touches it, or cause a fire that can extensively damage the solar equipment and property. Consequently, arc detection is required between the inverter and the string of panels.
Figure 1: String inverter architecture with single central inverter
With the rapid expansion of solar technology both for small scale residential/commercial installations and large scale power generation using "solar farms," there has been a need to develop safety measures to address the potential dangers of high voltage DC arcing. This has led the solar industry in the USA to develop the UL 1699B photovoltaic arc-fault circuit protection standard to increase personal safety and protect equipment.
Key requirements of UL 1699B
UL 1699B stipulates the requirement for solar technology companies to include arc detection in high-voltage PV systems with a DC bus voltage of =80 V and <1000 V. The full development of UL 1699B is expected to be completed by the end of 2012. A similar standard in Europe is then expected to be implemented. Meeting this standard will undoubtedly pose a challenge for designers of solar inverters, converters, charge controllers, and standalone DC arc-fault interrupters.
Key standard requirements for arc detection systems include:
• An annunciator (i.e. siren or flashing light) to signal when arcing has shut down the system. A wired or wireless communication link may be necessary to enable remote detection e.g. to notify a utility company that an arc event has triggered the shutdown of the system
• A self-test circuit (initiated directly via a switch, or remotely) that is capable of simulating an arc event and shutting down the system if the test fails. The requirement for automated self-testing to verify that the arc detection unit is operating correctly is under consideration in order to ensure testing is carried out routinely
• A manual reset following tripping of the system. This presents a problem for developers who must balance the potential consequences of not detecting an arc (e.g. electrocution, and fires and destruction of equipment) and the cost of false arc detection and the unnecessary shutting down of the system (loss of power and expensive technician visits), particularly if located remotely.