DC Arc-Fault Circuit Protection for PV Systems
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Article 690.11 is a new proposal for the 2011 National Electrical Code that would require the use of listed dc arc-fault circuit protection devices in grid-tied PV systems, as well as in most stand-alone PV systems. This raises many questions. How does the dc arc-fault circuit protection requirement differ from existing GFDI requirements for PV systems? Where is the best place in the PV power circuit to locate a dc arc-fault circuit protection device? And, since this is a new category of equipment, what are the potential challenges to the successful adoption and implementation of dc arc-fault circuit protection?
Article 690.11 Arc-Fault Circuit Protection (dc). PV systems with dc source and/ or output circuits on or penetrating a building operating at a PV system maximum system voltage of 80 volts or greater shall be protected by a listed (dc) arc-fault circuit interrupter, PV type, or other system components listed to provide equivalent protection. The PV arc-fault protection means shall comply with the following requirements:
(1) The system shall detect and interrupt arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or other system component in the direct current PV source and output circuits.
(2) The system shall disable or disconnect one of the following:
(3) The system shall require that the disabled or disconnected equipment be manually restarted.
(4) The system shall have an annunciator that must be manually disabled.
Arc-Fault Detection Could Improve Safety
Marv Dargatz / Enphase Energy / Petaluma, CA / enphaseenergy.com
The proposed requirements for dc arcfault detection are necessary and prudent. Specific implementation details of arc-fault detection and mitigation are yet to be determined, however. The reduction of arc-fault risk requires some new thinking in terms of PV system design practices. The proposed change to the NEC would require dc arc-fault detection and mitigation for series arc faults in PV systems with system voltages greater than 80 Vdc. Notice that the requirement applies to series arc faults only. There are three basic types of arc faults, defined by their placement and mode of operation in the circuit: series, parallel and to ground.
A series arc occurs when a connection is pulled apart while the PV is producing current. Any intermittent connection in the dc circuit could produce a dc arc fault. These connections may include soldered joints within the module, compression type wire connections or the connectors that are commonly used on the wire leads attached to PV modules.
A parallel arc occurs when an insulation system suffers a breakdown. 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 freezethaw cycles.
A fault to ground requires only the failure of one insulation system. While the GFDI provides some measure of protection against this fault, there have been cases of faults to ground that failed to trip the GFDI device, yet created an arc.
Detection of the arc is the first challenge. It is paramount that arcs are reliably detected without raising false alarms. Many different techniques can be employed. Most of these techniques rely on voltage, current, radiated energy or a combination of these.