I'm considering a new idea for calculating DC Incident Energy PPE requirements at solar sites and I'd like to hear your feedback. Even at low fault current values and while using a more detailed calculation method (Ammerman) I regularly see Combiner Boxes exceed HRC 2 or 3 due to long clearing times.

No method for calculating PV array arcing current gives me any degree of confidence in fuse clearing during an arcing fault. The iterative I_arcing calculations have to either treat PV sources as batteries or as voltage-controlled current sources which requires iterations through the V-I curves. And in the end, neither method can predict system pre-fault conditions and produce an arcing current calculation that can confidentially be plotted on a fuse time-current. The event could occur early in the morning, in the winter or on cloudy day and small deviations in arcing current can cause major shifts on the steep section of a fuse curve. The fuse was typically ignored to be on the safe side and FCT was set at 2 seconds.

Instead, I'm not going to calculate what arcing current will be during a fault.

I'm doing this in a spreadsheet that automates the calculation. Here are the steps:

1. Consider an entire a range of currents from 0 to maximum short circuit current.

2. At 10A increments, calculate resultant Heat Flux using Stokes-Oppenlander for V_arcing (gap, working distance and multiplying factor fixed).

5. If Heat Flux is less than 6 cal/cm².s, a 2 second clearing time is assumed. The area under the flux/time curve is stil less than 12cal/cm². (The significance of 12cal/cm² is that it's the minimum arc rated PPE on site)

6. At some current value down the line, the Heat Flux will exceed 6 cal/cm².s. That is the only current value I care about. It's a fixed value unaffected by operating conditions as long as Zg, WD and MF are constant.

7. Only then do I check the fuse curve to see if that current is high enough to burn the fuse in less than 2 seconds. If it is, that point becomes the crest on the flux/time curve. Currents above that value burn the fuse faster and currents below can't accumulate enough IE to exceed minimum PPE. I take into account what portion of that current value would be back-fed through the fuse before checking the TCC.

This should give me a secure way to bring the fuse back into the equation. I don't know what arcing current will be during a fault but it has to be on the left or right side of that crest. This can also help evaluate different fuse models during the design phase. I've seen PV rated fuses with the same continuous rating but extremely different clearing times. The spreadsheet is only equipped with 3 fuse curves for now created using Digitizer4.1.

If fuse clearing time is still greater than 2 seconds at the 6 cal/cm².s mark, then FCT stays fixed at 2 seconds and IE area exceeds 12 cal/cm². I think this should also help with concerns about solar irradiance multipliers, cumulus clouds or NEC multipliers (if you use these).

I attached a few example plots below. One with a fixed FCT = 2 seconds curve. One with a good fuse and one with a slow fuse. All on the same system. The spreadsheet only needs the cells in red to create plot.

Attachment:

No Fuse.png

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Good Fuse.png

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Slow Fuse.png