TECHNOLOGY I SAFETY
Figure 3: Comparison of arcing (eed trace) and non-arcing (blue) signals from an inverter
present a significant challenge.
While the signal generated from an arcing event is quite easy to detect, the real world is not as friendly as a lab environment. Routing PV cables to minimize coupling from radio transmitters, solar flares, radar, and other potential interferers is usually not considered for system installations. In practice, however, the major source of interference turns out to be the inverter itself.
Inverter interference
In Figure 3 the spectrum of an arcing event is compared to a normal operating system. This spectrum covers the range of 0–125 kHz, and the vertical axis is scaled to 10 dB/div. The roll-off in the lower frequencies is due to the transformer used to couple the signal to the digitizer in the system.
The switching interference from the inverter at 38 kHz is more than 10 dB above the peak amplitude of the arcing signal. A simplistic arc detection system that simply looks at the RMS level in the time domain would probably indicate an arc with this type of interference. Even an arc detection system that performs spectral analysis has to find a region of the spectrum with the largest separation between the arcing and non-arcing signals to minimize errors.
Still, with proper frequency range selection, it should not be too difficult to manage this interference. As long as a system does not rely on the range of 95–103 kHz, there should be no major difficulty. It turns out that this specific inverter generates a very low level of interference. The inverter response shown in Figure 4 contains a bit more interference – at 36 kHz it produces a signal with a peak about 45 dB more than the arcing signature trying to be detected.
This inverter presents a specific challenge to arc detection systems. With interference exceeding 45 dB over the arcing
signature, it is easy to overwhelm the arc signature for which the ADU is searching. During development of the RD-195, we evaluated inverters with interference levels 50 dB greater than the arcing signature. The RD-195 must reliably detect arcing even with such strong interference. This high level of potential interference requires a high-dynamic range for the system – with additional headroom needed on top of the 50 dB interference to ensure that the captured waveform response does not over- range or under-range in the presence of additional interference. It is important to note that the system must never encounter under- range/over-range conditions, as there is no useful information on monitored PV system when this condition occurs. Without that, a safe, non-arcing condition cannot be ensured.
It is necessary to maintain sufficient resolution at the low powers for effective detection. The system dynamic range calculation is:
>3 dB of additional headroom to avoid clipping 50 dB of interference range 40 dB to non-arcing signal floor
Figure 5: An inverter with a spread-spectrum switching interference
This exceeds 90 dB of dynamic range in the band between 20–100 kHz. Many components may meet this dynamic range requirement at 1 kHz, but have a performance roll off so that by 100 kHz, they are below 70 dB of dynamic range and not optimum for arc detection. In comparison, the analog-to-digital converter (ADC) used in the RD-195 is tailored to this application. It has over 100 dB of dynamic range and consumes little power in a small 10-lead MSSOP package.
Inverter switching interference can manifest in other ways. The inverter characteristic shown in Figure 5 has a spread-spectrum type of switching, which results in a more difficult detection challenge.Other inverters can present even more challenging interference characteristics. The inverter in Figure 6 has only a small region of the spectrum where effective detection is possible. Inverter interference has even more complexity. It can vary significantly across different panel configurations as the inverter attempts to extract the maximum power from the array, which can change due to shading or temperature. The inverter start-up and shut down sequences also can be quite different from the normal operation modes.
Protection Figure 4: Higher interference inverter 34
www.solar-uk.net I Issue III 2013
It is clear that an effective real-world solution need to be able to detect an arc in the presence of high levels of interference, as
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