Isc and Max Input Current on Inverter

[dansk]

Im getting myself all confused and want some help.

Inverter is 20A max input current.

I have the following:

Isc 5.72 per string x 3 = 17.16 for the array at stc, inverter is is 20A max DC input, but when i apply 1.25 factor this gives 21.45, so is this inverter not the correct one? or will it be ok?

 

[SolarCity]

I'd check with the manufacturer. I'd be surprised if the 1.25 factor needed to be applied to the inverter but it's best to check.

 

[dansk]

Well what im getting confused about - is the array Isc is 17.16 at stc, but to make sure the DC main cables are rated properly we apply 1.25, so does this mean the DC input from the array could be as high as 21.45A? in which case the inverter input is 20A so do i have this right?

Another way to look at it - is the Isc affected by tempreture like the voltage?

 

[SolarCity]

The 1.25 factor is oversizing to allow for carrying fault current (because fault current is not much higher than normal operating current, other forms of overprotection aren't suitable. I don't think this factor needs applying to inverters.... although I could be wrong.

 

[dansk]

Ok - So the Isc is the max current that will flow into the inverter. So if the inverter is rated at 20amp and the array Isc is 17.16 im ok then.

 

[SolarCity]

As far as I am aware, that is correct. We have never applied the 1.25 factor to the inverter. Of course, we could well be wrong.......!

 

[JulianC]

The 1.25 multiplication factor is applied to the array short circuit current, Isc, and gives you a theoretical maximum figure that you could expect to see here in the UK. (Isc is specified at standard test conditions by manufacturers and in reality, under certain conditions, can go higher).

You need to multiply your Isc value by 1.25 and ensure that this is within the max. current handling capacity of the inverter.

You do not multiply the inverter rating by 1.25.

 

[dansk]

"You do not multiply the inverter rating by 1.25" - understood. It was the 1.25 on the array Isc, so my 17.16 x 1.25 = 21.45A and my Inverter is 20A max - so in this case its not going to work.

 

[JulianC]

Correct

 

[dansk]

bummer. its getting a bit late in the day now to be swapping inverters.

Thats something you need to watch on PV-SOL ive just learned.......its says its ok, but in reality its not!!

 

[JulianC]

Not used PV-SOL but does it set the array default operating temperature range to -10 to +60 degrees? If so, that may be why it's showing the design is OK. You need to use the temperature range -15 to +80 degrees here in the UK (that's where the multiplication factors of 1.25 on current and 1.15 on voltage come from)

 

[dansk]

Mines set to -10 to +70 but thats only for the Voltage, when checking current it only checks at stc. it doesnt use any correction factor - but i will.

Whats the worst that could happen - i guess the inverter will shut down is theres an over current.

 

[JulianC]

I suppose the inverter will either shut down or go pop. Assuming the array current ever gets that high in the first place. Don't know how likely that is to be honest.

Correction to my earlier post: it is the Voc value that is primarily affected by temperature, which is where the 1.15 factor comes in. The biggest effect on current is insolation. stc values are at 1000W/m2 but we have to account for levels up to 1250W/m2. Since the Isc output of a module is pretty much proportional to insolation, this is where the 1.25 factor comes in (1250/1000).

Man, I need to stop thinking about this stuff and watch some mindless crap on the tele.

 

[dansk]

lol - its ok for you ive had this on my mind allday! and im still at it...what a saddo i am.

 

[SolarCity]

Is it not impossible to get a short circuit current at the inverter? It's either short circuited, and therefore bypassing the inverter or connected to the inverter as a load?