Current Observed on Array Series Strings after Opening the Circuit

[forresthuynh]

I work in utility scale solar and have observed the following situation frequently on both new and old sites and need clarity on how, why, and any safety procedures related to the topic:

Situation
After opening the disconnect switch on an array level combiner box (and thus breaking the circuit from the inverter aka load) I have amp clamped the module series string input conductors going into the combiner box and observed currents ranging from low values (1-2A) to full array IMP. This current is present while the combiner box input fuses are closed in (the fuses are non-load break rated), but coworkers have confirmed that opening them will not draw an arc and will make the current disappear via the meter.


I have heard that "differential voltage potential" is the cause of this presence of current on an open circuit. The thought being that if different voltages are present (from broken/defective modules, soiling, ect) on the combiner box input strings, a circuit can develop in the module series strings that have higher or lower voltages.

Note: I always zero my meter before amp clamping, this occurrence has been seen with the inverter/DC Recombiner/CB open, ground faults were not present on these occurrences.

Can this situation please be clarified for me?

thanks,

 

[jaggedben]

A guess...in layman terms...

Solar modules are semiconductors with a 'forward' and 'backward' resistance. With all the strings paralleled together, any one string can carry current backward if that string's backward resistance is overcome by the voltage present on the other strings. A salient example would be a mismatched string size. A short string (mistakenly wired) could be easily overcome by the voltage of the other strings and pass current in the opposite direction it's supposed to, especially if the internal diodes break down. Or another example, a string that is accidentally reversed, will complete the circuit of the other strings.

If you are seeing full array IMP that is probably a shorted, miswired, or damaged string passing the rest of the strings' current. That will almost certainly arc when disconnected, too. Not sure how it wouldn't blow the fuses though.

If it's just an amp, that may be benign differences between string voltages.

I have to believe that the currents are essentially zero-sum. That is, if you can figure out which are forward and which are backward, they will cancel out.

If Golddigger sees this thread he may be able answer in more scientifically robust terms.

 

[GoldDigger]

I think you did just fine...

Sent from my XT1585 using Tapatalk

 

[BandGap1.1eV]

I've seen this on occasion. For the instances where you are seeing full string current, this is likely a case of swapped polarity. The installer goofed during install and you now have a high voltage ~10 amp circuit feeding the others. Exercise lots of caution, this will arc when you try to open the fuse.

Leakage current is a known nuisance for PV arrays depending on the quality of the module. I would not been too concerned with an amp here and there if the fuses in the combiner are still closed. If the fuses were open and you are still reading current, I would be concerned of a ground fault.

 

[Zee]

I work in utility scale solar ..............observed currents ranging from low values (1-2A) to full array IMP.

"full ARRAY current"? Like several strings worth? That seems huge.

 

[pv_n00b]

Leakage current will get you a few amps in a grounded array. If the modules are frameless thinfilm the leakage is usually significantly higher than mono or poly modules. But, getting Imp is a sign something is wrong.