Short circuit on Module / Source circuit

[Grouch1980]

If you have a short circuit condition on the wires interconnecting the solar PV modules... on the source circuit... you would have short circuit current coming in from the other PV source circuits (if I remember correctly, I haven't looked at this in awhile). Does short circuit current also come in from the utility, on the AC side of the inverter?

 

[Grouch1980]

And a follow up question... say multiple inverters connect to an AC combiner panel, and then that combiner panel connects to the utility on the line side of the service disconnecting means. if you have a short circuit on the wires between one of the inverters and the combiner panel (on the AC side), would you be pulling in short circuit from the utility AND the solar PV modules? Or only from the utility?

 

[pv_n00b]

If you have a short circuit condition on the wires interconnecting the solar PV modules... on the source circuit... you would have short circuit current coming in from the other PV source circuits (if I remember correctly, I haven't looked at this in awhile). Does short circuit current also come in from the utility, on the AC side of the inverter?

If you have a short on the string wiring you will have current from the string. If the string is paralleled with other strings you will have current from all the other strings, up to the point where the fuse protecting the string opens.

And a follow up question... say multiple inverters connect to an AC combiner panel, and then that combiner panel connects to the utility on the line side of the service disconnecting means. if you have a short circuit on the wires between one of the inverters and the combiner panel (on the AC side), would you be pulling in short circuit from the utility AND the solar PV modules? Or only from the utility?

You will have fault current from the inverters and the utility.

 

[jaggedben]

Does short circuit current also come in from the utility, on the AC side of the inverter?

Maybe. It depends on the design and safeguards in the inverter. I've seen a couple cases where I thought inverters failed because of DC side faults.

if you have a short circuit on the wires between one of the inverters and the combiner panel (on the AC side), would you be pulling in short circuit from the utility AND the solar PV modules? Or only from the utility?

The available fault current from the utility is typically much higher for a residential or small commercial installation. For example on a residence you might have up to 22,000A of available utility fault current on a 200A service, and 60A of available fault current from a 32A max continous output PV system. In this example the PV is more or less a rounding error in any available fault current calculation, although it certainly exists in some amount. When dealing with larger PV systems the PV fault current component may be less negligible.

 

[Grouch1980]

Thanks. How do you calculate the available fault current from the inverter? Is it found in the inverter specs or do you calculate it? Understood that it may be negligible when compared to the utility fault current, just curious though.

 

[jaggedben]

Thanks. How do you calculate the available fault current from the inverter? Is it found in the inverter specs or do you calculate it? Understood that it may be negligible when compared to the utility fault current, just curious though.

Yes it's in the specs. Actually it will depend on irradiance but specs will give a max.

 

[Grouch1980]

Yes it's in the specs. Actually it will depend on irradiance but specs will give a max.

Got it, thanks.

 

[ggunn]

Thanks. How do you calculate the available fault current from the inverter? Is it found in the inverter specs or do you calculate it? Understood that it may be negligible when compared to the utility fault current, just curious though.

It's in the specs but you may have to dig for it. For SMA inverters, see https://files.sma.de/downloads/Iscpv-TI-en-21.pdf.

 

[Grouch1980]

Yeah I realized that... i'm looking through a spec for a Solaredge inverter, and it does not mention the available short circuit current. Will have to dig.

https://www.solaredge.com/sites/default/files/se-three-phase-inverter-with-synergy-technology-208v-datasheet-na.pdf

 

[ggunn]

Yeah I realized that... i'm looking through a spec for a Solaredge inverter, and it does not mention the available short circuit current. Will have to dig.

https://www.solaredge.com/sites/default/files/se-three-phase-inverter-with-synergy-technology-208v-datasheet-na.pdf

It won't be on the data sheet, but see https://www.solaredge.com/sites/def...-currents-three-phase-inverters-tech-note.pdf

 

[Grouch1980]

It won't be on the data sheet, but see https://www.solaredge.com/sites/def...-currents-three-phase-inverters-tech-note.pdf

Thanks!

 

[pv_n00b]

Thanks. How do you calculate the available fault current from the inverter? Is it found in the inverter specs or do you calculate it? Understood that it may be negligible when compared to the utility fault current, just curious though.

If you can't find it from the manufacturer a safe range is 1.2-1.5 times the max AC output current.

 

[Grouch1980]

Maybe. It depends on the design and safeguards in the inverter. I've seen a couple cases where I thought inverters failed because of DC side faults.

Came back to my post . Now that I'm rereading it all... if the inverter has safeguards against the utility short circuit current coming in, wouldn't the inverter have the same / similar safeguards to prevent short circuit current coming in from the other parallel inverters as well?

 

[ggunn]

Came back to my post . Now that I'm rereading it all... if the inverter has safeguards against the utility short circuit current coming in, wouldn't the inverter have the same / similar safeguards to prevent short circuit current coming in from the other parallel inverters as well?

Inverters do not have them, but every inverter should be connected to an OCPD between it and the rest of the system. They aren't to protect them from other inverters, though, they are to protect the conductors from fault current coming from the service. Inverters are current limited by design.

I don't know what you are driving at, though; in the event of a short circuit any fault current from an inverter goes to feed the short for a small interval of time until it shuts down, not other inverters.

 

[jaggedben]

Came back to my post . Now that I'm rereading it all... if the inverter has safeguards against the utility short circuit current coming in, wouldn't the inverter have the same / similar safeguards to prevent short circuit current coming in from the other parallel inverters as well?

Yes. I mean, that's mostly a distinction without a difference. In the parallel wiring system there is just voltage and total available fault current from all sources mixed together, utility and other inverters. In most cases, outside of very large PV systems with lots and lots of large paralleled inverter, the additional available fault current from parallelel inverters will be just a rounding error compared to the utility.