This is not an insignificant problem and arc detection requires complex algorithms and the simultaneous evaluation of multiple filters to ensure false detections are prevented. In order to stop catastrophic arc events (fire and electrocution) the circuit must be broken immediately after the arc, which requires that the algorithms are executed as rapidly as possible.
Arcing events cause spectral noise in the nominal power signature of a string inverter over specific frequency bands (40-100 KHz), compared with when there is no arcing in the system (Figure 2). The spectral noise in the nominal power is determined from the digital signal which is converted from the DC voltage. A simple way of arc detection is to establish a baseline nominal power value for the system and then measure when the spectral noise level suddenly exceeds it. The major drawback of this approach, however, is that during power up arcs cannot be detected before the baseline has been established. As this is a requirement of UL 1699B, a detection algorithm should be included that performs without a baseline nominal power measurement.
Figure 2: Spectral noise in the nominal power in an arcing versus non-arcing system Click on image to enlarge
Furthermore, the AC inverters that tend to be used in string inverter topology produce a pattern of noise that is very similar to the pattern generated during an arc event (Figure 3). It is, therefore, difficult to differentiate between normal operating conditions and an arc event.
Figure 3: AC inverters generate similar noise levels during normal operation and when there is an arc event Click on image to enlarge
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