heating water with PV

[winnie]

I'd assume that the inverter has a maximum input voltage rating and a maximum input current rating, and that is what defines the maximum DC/AC ratio.

If there is a separate maximum DC power value which is lower than 'max string voltage times max Isc', I think you'd be hard pressed to exceed that max DC power number without also exceeding one of the other numbers.

 

[ggunn]

I'd assume that the inverter has a maximum input voltage rating and a maximum input current rating, and that is what defines the maximum DC/AC ratio.

If there is a separate maximum DC power value which is lower than 'max string voltage times max Isc', I think you'd be hard pressed to exceed that max DC power number without also exceeding one of the other numbers.

Most (maybe all) the data sheets for the commercial inverters I design with (I haven't done any resi PV work in several years) show a maximum allowable connected STC DC power in addition to maximum DC current and voltage numbers.

 

[wwhitney]

I'd assume that the inverter has a maximum input voltage rating and a maximum input current rating, and that is what defines the maximum DC/AC ratio.

Here's some numbers from the spec sheet of an inverter I selected semi-randomly, the Fronius Primo 6.0-1:

Recommended PV Power 4.8-9.3 kW
Max MPPT input current 2 x 18A (2 inputs)
Max input SCC 2 x 27A
Operating voltage range 80V - 600V
MPPT voltage range 240-480V

But if we take the upper end of the MPPT voltage range (480V) and multiply by the max MPPT input current (18A x 2), we get 17.3 kW, well above the 9.3 kW upper end of the "recommended" PV Power. So the question is what the inverter will do if connected to 2 strings of panels with PTC ratings of say 440V and 16A each, for a PTC power of 14.1 kW, or a DC/AC ratio of 2.35. It clearly shouldn't blow up, but will it successfully make 6 kW during PTC conditions?

Cheers, Wayne

 

[ggunn]

Here's some numbers from the spec sheet of an inverter I selected semi-randomly, the Fronius Primo 6.0-1:

Recommended PV Power 4.8-9.3 kW
Max MPPT input current 2 x 18A (2 inputs)
Max input SCC 2 x 27A
Operating voltage range 80V - 600V
MPPT voltage range 240-480V

But if we take the upper end of the MPPT voltage range (480V) and multiply by the max MPPT input current (18A x 2), we get 17.3 kW, well above the 9.3 kW upper end of the "recommended" PV Power. So the question is what the inverter will do if connected to 2 strings of panels with PTC ratings of say 440V and 16A each, for a PTC power of 14.1 kW, or a DC/AC ratio of 2.35. It clearly shouldn't blow up, but will it successfully make 6 kW during PTC conditions?

Cheers, Wayne

The data sheets I use typically say something like this (from the SE80KUS 80kWAC data sheet):
Maximum DC Power (Module STC) Inverter / Synergy Unit 120000W / 60000W
Notice that it is 150% of 80kW / 40kW; this is typical.

I haven't investigated to see if it is possible to exceed 150% of 80kW while still conforming to their other constraints, but even if it is, I treat it as a hard stop and I will not do it.

 

[winnie]

Wow, thanks for the correction on max power vs max Voc * max Isc. I am surprised.

Now I'm curious about the internal topology of the system.

-Jonathan

 

[wwhitney]

Wow, thanks for the correction on max power vs max Voc * max Isc. I am surprised.

OK, so here's my question informed only by the I vs V graph for a string of PV cells: if the inverter is connected with an unreasonably high DC/AC ratio, like say 2.5, so that the published limit on Pmpp is violated, but the string Voc and Isc are both within spec, why can't the inverter just pick a region of the I vs V curve away from the maximum power point, where the power is within the inverter's spec? Either a lower voltage than Vmp, in the region of the curve where the current is roughly constant, or a lower current than Imp, in a region of the curve when the voltage is roughly constant, approaching Voc.

Maybe @tallgirl could explain.

Cheers, Wayne

 

[electrofelon]

OK, so here's my question informed only by the I vs V graph for a string of PV cells: if the inverter is connected with an unreasonably high DC/AC ratio, like say 2.5, so that the published limit on Pmpp is violated, but the string Voc and Isc are both within spec, why can't the inverter just pick a region of the I vs V curve away from the maximum power point, where the power is within the inverter's spec? Either a lower voltage than Vmp, in the region of the curve where the current is roughly constant, or a lower current than Imp, in a region of the curve when the voltage is roughly constant, approaching Voc.

Maybe @tallgirl could explain.

Cheers, Wayne

My understanding is that is what they do, but perhaps there is a limit to how far the inverter can move on the curve?

 

[electrofelon]

Here's some numbers from the spec sheet of an inverter I selected semi-randomly, the Fronius Primo 6.0-1:

Recommended PV Power 4.8-9.3 kW
Max MPPT input current 2 x 18A (2 inputs)
Max input SCC 2 x 27A
Operating voltage range 80V - 600V
MPPT voltage range 240-480V

But if we take the upper end of the MPPT voltage range (480V) and multiply by the max MPPT input current (18A x 2), we get 17.3 kW, well above the 9.3 kW upper end of the "recommended" PV Power. So the question is what the inverter will do if connected to 2 strings of panels with PTC ratings of say 440V and 16A each, for a PTC power of 14.1 kW, or a DC/AC ratio of 2.35. It clearly shouldn't blow up, but will it successfully make 6 kW during PTC conditions?

Cheers, Wayne

I am looking at my SMA sunny boy manual and for DC current it has the following:

Maximum input current per input - 10A
Maximum short circuit current per input - 18A

I would say the first is just the max the inverter can/will use, not that you must select a module with 10A or less of current. So the latter would be the only thing that provide a limitation. It isnt really clear to me what the purpose of declaring a maximum short circuit current is. Perhaps just to reduce further damage to the inverter if there is a DC fault that cannot be opened?

 

[ggunn]

Maximum short circuit current per input - 18A

For some (most?) inverters they mean 1.25 * Isc.