TECHNOLOGY I SAFETY
harsh environments, and minimal system maintenance over long periods of time. The DC power generated by PV systems is more hazardous as compared to AC power systems, because DC arcs do not periodically self-extinguish like AC arcs do. In PV systems, the current generated by the PV modules has a maximum level that does not cause the fuse to blow.
Due to the dangers from arcing, in 2011 the United States National Electrical Code included a section requiring the addition of arc prevention circuitry in new PV systems. Article 690.11 of the NEC2011 requires that inverters sold in the US be able to detect and interrupt series arcs in DC PV systems.
The solar industry adopted UL1699B, an extension onto the UL1699 Arc Fault Interruption specifications, as the applicable standard for detection and mitigation of arcing events. The standard applies to PV systems from 80–1000 V. Below 80 V the occurrence of an arc becomes less likely. Most of the damage caused by PV arcing is from fires that result from the energy dissipated by the arc. UL1699B sets 750 Joules (J) as the detection threshold, an amount of energy dissipation capable of igniting many common materials. With a 14 Ampere (A) arc generating 900 Watts of power, 750 J is discharged in approximately 800 ms. This time budget sets a strict schedule for detecting and responding to an arc.
Arc detection system overview To address this need, Texas Instruments designed a reference solution for an arc detection unit, the RD-195 (Figure 1). The goal was to make an effective, economical, and modular arc detection system for use in the real world. The development effort focused on real-world conditions as much as possible – different strings of PV modules and a variety of inverters were used to test the solution rather than a lab setup.
In Figure 2 the RD-195 was connected to a PV system, a string of 180 W modules, and an inverter. The string current was monitored using a current probe. A safe system was run for a week, after which an arcing event was generated. The start of the arcing event is visible as a sharp transition in the current trace followed by an increasingly chaotic signal. The RD- 195 indicates detection 135 ms later. In a worst case situation, a system using an RD-195 still has more than 650 ms to respond to the arcing event. The large amount of margin allows the C2000 to monitor multiple DC lines concurrently.
After evaluating a large number of arcing conditions, a frequency range of 20–100 kHz was settled on as the detection range. This resulted from some straightforward considerations. First, the electrical signal generated by an arc is reduced at higher frequencies. At low frequencies, the electrical interference from line noise becomes more pronounced. In the 20–100 kHz region, the signal we are trying to detect is –60 dBm/Hz, which does not
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