Series, parallel PV. math for VOC and AMP safely into Sol-Ark 12k, with temperature correction.

[Sun_Dried_Toad]

We are planning for our system upgrade.

Sol-Ark 12k has dual MPPT inputs. it can handle 6500w per input. if done in series only, it can go up to 500v input, but that would be without temperature correction, I assume.

12 panels X 41 VOC = 492v
if panels get cold (it gets FREEZING here during winter) that would go over the 500V input, again, assuming that the 500v input they list is non-temp-corrected. That tells me I need to run them parallel string of 6 series, to another 6 panels in series.

when adding up the Amps for the parallel calculation, do I use ISC (short circuit current) or do I use Max Power Current (IMPP)?
ISC = 10.92A
IMPP = 10.34A

Since I will (probably) be using Mission Solar 345w panels, here is my concern:

sol-ark input max:
500v @ 18Amp (non temp corrected, I think)
or
450v @ 20Amp (again, non corrected, I think)

The Sol-Ark does say that the DC (per MPPT) input is self limiting to 20A, so I'm assuming that if my parallel amperage (whether it be 10.92 ISC Orr 10,34 IMPP) it should be ok to be slightly over 20 amps total, allowing me to run 2 parallel strings of 6 panels wired in parallel to another string of 6 panels?

Do I need to fuse between panels or install micro-inverters if I do the above scenario?

I am hoping to run 12 total Mission Solar 345w panels into each MPPT, eventually, for a total of 24 panels into the Sol-Ark 12k.
For now, I would like to run 12 Mission Solar 345's into one MPPT and use my existing 8 qcell 305w panels into the second MPPT.

Finally, If I can run series, parallel, 6 and 6 into the MPPT, will I need to increase the size of the wires from the array to the controller due to the increased Amperage? I am currently using "standard PV" wire (that's what it is marked, I did not see a AWG label) its the "standard" size that fits into the MC4 connectors.

I am attaching two photos. Specification Info for the Sol-Ark 12k and one for the Mission Solar 345w panels.

 

[RCinFLA]

Voc can peak at 500v but it is close to damage limit and cannot go greater than 425v running at Vmp. Better if you run lower for safety margin.

500v @ 18amp will be death to inverter. Voc is zero current from panels.

Voltage on PV panel input cannot exceed internal HV DC point that runs PWM IGBT switches. It uses 500 vdc rated electrolytic caps.

 

[Nobodybusiness]

We are planning for our system upgrade.

Sol-Ark 12k has dual MPPT inputs. it can handle 6500w per input. if done in series only, it can go up to 500v input, but that would be without temperature correction, I assume.

12 panels X 41 VOC = 492v
if panels get cold (it gets FREEZING here during winter) that would go over the 500V input, again, assuming that the 500v input they list is non-temp-corrected. That tells me I need to run them parallel string of 6 series, to another 6 panels in series.

when adding up the Amps for the parallel calculation, do I use ISC (short circuit voltage) or do I use Max Power Current (IMPP)?
ISC = 10.92A
IMPP = 10.34A

Since I will (probably) be using Mission Solar 345w panels, here is my concern:

sol-ark input max:
500v @ 18Amp (non temp corrected, I think)
or
450v @ 20Amp (again, non corrected, I think)

The Sol-Ark does say that the DC (per MPPT) input is self limiting to 20A, so I'm assuming that if my parallel amperage (whether it be 10.92 ISC Orr 10,34 IMPP) it should be ok to be slightly over 20 amps total, allowing me to run 2 parallel strings of 6 panels wired in parallel to another string of 6 panels?

Do I need to fuse between panels or install micro-inverters if I do the above scenario?

I am hoping to run 12 total Mission Solar 345w panels into each MPPT, eventually, for a total of 24 panels into the Sol-Ark 12k.
For now, I would like to run 12 Mission Solar 345's into one MPPT and use my existing 8 Cell 305w panels into the second MPPT.

Finally, If I can run series, parallel, 6 and 6 into the MPPT, will I need to increase the size of the wires from the array to the controller due to the increased Amperage? I am currently using "standard PV" wire (that's what it is marked, I did not see a AWG label) its the "standard" size that fits into the MC4 connectors.

I am attaching two photos. Specification Info for the Sol-Ark 12k and one for the Mission Solar 345w panels.

Go to

Sol-ark.com/Solar-panel-sizing/solarpanelsizing.html

Put in panels and Inverter into calculator and it will tell you exact what you need to include wire.

 

[Sun_Dried_Toad]

Ok, went to www.Sol-ark.com/Solar-panel-sizing/solarpanelsizing.html

here is what I got:





According to this max "number of modules in series" per MPPT is 9.
but then is says "number of strings" per MPPT is 2.

9 is not divisible by 2. obviously, you cannot have 2 parallel strings using 9 panels. likewise, you cannot run 2 strings of 9 panels, unless you want a BLAZE of glory.

does this mean that I can only run 8 panels per MPPT, 2 stings of 4 panels in S,P?

Or can I run 10 panels per MPPT, in 2 strings of 5 each, parallel? that would come out to 410v total at ISC = 10.92A or IMPP = 10.34A (still unclear on which I have to go by)

When using 20% as temp correction for freezing temps:
if I run one string of 10 that will give me 410v with a temp correction of 82v for a total of 492v
if I run 2 strings of 5, parallel together, that would give me 205v with temp correct of 41v = total 246v
I am still unsure of what the amperage would be in the parallel 5 and 5. either 21.84A or 20.68 depending on using ISC or IMP.
likewise, by my calculations, if I did 2 strings of 6 panels, parallel together, I come up with 246v with a temp correct of 49.2v = 295.2v with a current rating of either 21.84A or 20.68 depending on using ISC or IMP.

the calculator is a neat tool, especially since it's on the Sol-Ark website, but I question its accuracy, since it seems to spit out exact numbers, not taking into account requiring pairing of equal number of panels in series for parallel connections.

 

[rmaddy]

According to this max "number of modules in series" per MPPT is 9.
but then is says "number of strings" per MPPT is 2.

9 is not divisible by 2. obviously, you cannot have 2 parallel strings using 9 panels. likewise, you cannot run 2 strings of 9 panels, unless you want a BLAZE of glory.

I'm pretty sure it's saying you can do up to 9S2P (18 panels) per MPPT. It says 9 in series and it says 2 strings. That's 2 strings of 9 each. 18 x 345W = 6210W per MPPT which is less than the 6500W max per MPPT. Not sure why you say that 2 strings of 9 would be a "BLAZE of glory".

Since you want to do 12 panels per MPPT is would seem you can do 6S2P per MPPT. This would be 6 x 41V = 246V. Your 2P would be clipped to 20A so it doesn't matter whether you use Imp or Isc.

When using 20% as temp correction for freezing temps

A 20% temp correction takes you down to around -40º. You entered a minimum temperature of 0ºC. That's a HUGE difference.

You have the exact temperature coefficient of -0.262%/ºC. You can calculate the exact voltage at a given temperature. No need to use the "20%" factor. People start with a 20% factor as a quick test. If you are still safe at 20% then the exact result doesn't really matter unless you live where it gets colder than -40º. If the 20% factor puts you over a bit then you use the real temperature coefficient to calculate the real result for your actual lowest possible temperature so you can be sure if it will work or not.

 

[Nobodybusiness]

Ok, went to www.Sol-ark.com/Solar-panel-sizing/solarpanelsizing.html

here is what I got:


View attachment 91714


According to this max "number of modules in series" per MPPT is 9.
but then is says "number of strings" per MPPT is 2.

9 is not divisible by 2. obviously, you cannot have 2 parallel strings using 9 panels. likewise, you cannot run 2 strings of 9 panels, unless you want a BLAZE of glory.

does this mean that I can only run 8 panels per MPPT, 2 stings of 4 panels in S,P?

Or can I run 10 panels per MPPT, in 2 strings of 5 each, parallel? that would come out to 410v total at ISC = 10.92A or IMPP = 10.34A (still unclear on which I have to go by)

When using 20% as temp correction for freezing temps:
if I run one string of 10 that will give me 410v with a temp correction of 82v for a total of 492v
if I run 2 strings of 5, parallel together, that would give me 205v with temp correct of 41v = total 246v
I am still unsure of what the amperage would be in the parallel 5 and 5. either 21.84A or 20.68 depending on using ISC or IMP.
likewise, by my calculations, if I did 2 strings of 6 panels, parallel together, I come up with 246v with a temp correct of 49.2v = 295.2v with a current rating of either 21.84A or 20.68 depending on using ISC or IMP.

the calculator is a neat tool, especially since it's on the Sol-Ark website, but I question its accuracy, since it seems to spit out exact numbers, not taking into account requiring pairing of equal number of panels in series for parallel connections.

It’s says 9 in a string with 2 strings in parallel (18 panel) per MPPT controller.

That’s also using 10 awg wire and 100 feet.

I used the settings from here for mine and they were spot on.

Drop one panel per string if you think it will ever get that cold.

 

[RCinFLA]

The two PV converters feed HV DC to the HV DC node between battery to HV DC converter output and PWM Sinewave AC generation IGBT H-bridge input.

The only way you can consume more than 9.6kW of PV power is to have the power from HV DC supplied from PV SCC's split with some power going forward to AC output (9.6kW max) limited by IGBT PWM switchers, plus 8kW max of battery charging going backwards to battery for battery charging (8kW) limited by battery to HV DC converter. It also creates too much heat within the SCC heat sink, therefore the maximum combined PV power delivered spec of 12kW. The 12kW of PV power is possible if some is going to AC output and some going to battery charging, within the limits of 9.6 kW max going to AC output and 8 kW max going to battery charging.

The 9.6 kW push to grid maximum represents the maximum AC power flowing through the second stage PWM sinewave IGBT output switches. The maximum spec of 8kW off grid represents the maximum power through the first stage battery to HV DC converter. The 8.25kW max on each SCC represents the peak PV power from each PV controller but when both SCC's are fed the combined power cannot be over 12kW or SCC heat sinking will overheat.

500 vdc is maximum allowed DC input voltage but not for PV power transfer. It represents the maximum PV DC input voltage that gets too close to internal 500 vdc HV DC supply. PV Voc is open circuit voltage of panels where all illumination current is consumed in panels. There is no current output from panels at Voc, so 500vdc max and 500 vdc at 18A makes no sense as a PV array producing 500 vdc at 18A would have greater than 500 vdc actual Voc. Voc for such an array would be about 600 vdc.

SCC MPPT search needs some overhead wiggle room to maximum 500 vdc input voltage limit to allow it to search out MPPT point. Therefore the maximum MPPT voltage spec of 425v. Since Vmp for silicon panels is typically 0.81 to 0.85 x Voc, and Voc max allowed is 500 vdc then max Vmp is more like 405 vdc in practical terms.

Having grid connected further mucks up the power spec's. The battery to HV DC converter and Sinewave PWM converter limits remain the same but the amount of AC current flowing through connect relay to AC input is limited to 50 amps when pulled from AC input. Back push would also be 50 amps maximum but since PWM sinewave IGBT can only produce 9.6 kW maximum (40 amps) any additional current would have to come from a separate GT inverter AC coupled to output of inverter.

SolArc is actually two 120 vac inverters, series connected, to provide 240vac.

 

[Sun_Dried_Toad]

Not sure why you say that 2 strings of 9 would be a "BLAZE of glory".

Since you want to do 12 panels per MPPT is would seem you can do 6S2P per MPPT. This would be 6 x 41V = 246V. Your 2P would be clipped to 20A so it doesn't matter whether you use Imp or Isc.

thank you. my math was off. I forgot to cut the VOC in half when I calculated 9S2P.

I thought the unit would limit the current at 20A, but I was not certain. thanks, again.

I did not realize 20% correction factor was taking me down to -40° (f? c?). a cold day in hell here in my area might get down to 15°f (-10°c)

 

[Sun_Dried_Toad]

The two PV converters feed HV DC to the HV DC node between battery to HV DC converter output and PWM Sinewave AC generation IGBT H-bridge input.

The only way you can consume more than 9.6kW of PV power is to have the power from HV DC supplied from PV SCC's split with some power going forward to AC output (9.6kW max) limited by IGBT PWM switchers, plus 8kW max of battery charging going backwards to battery for battery charging (8kW) limited by battery to HV DC converter. It also creates too much heat within the SCC heat sink, therefore the maximum combined PV power delivered spec of 12kW. The 12kW of PV power is possible if some is going to AC output and some going to battery charging, within the limits of 9.6 kW max going to AC output and 8 kW max going to battery charging.

The 9.6 kW push to grid maximum represents the maximum AC power flowing through the second stage PWM sinewave IGBT output switches. The maximum spec of 8kW off grid represents the maximum power through the first stage battery to HV DC converter. The 8.25kW max on each SCC represents the peak PV power from each PV controller but when both SCC's are fed the combined power cannot be over 12kW or SCC heat sinking will overheat.

500 vdc is maximum allowed DC input voltage but not for PV power transfer. It represents the maximum PV DC input voltage that gets too close to internal 500 vdc HV DC supply. PV Voc is open circuit voltage of panels where all illumination current is consumed in panels. There is no current output from panels at Voc, so 500vdc max and 500 vdc at 18A makes no sense as a PV array producing 500 vdc at 18A would have greater than 500 vdc actual Voc. Voc for such an array would be about 600 vdc.

SCC MPPT search needs some overhead wiggle room to maximum 500 vdc input voltage limit to allow it to search out MPPT point. Therefore the maximum MPPT voltage spec of 425v. Since Vmp for silicon panels is typically 0.81 to 0.85 x Voc, and Voc max allowed is 500 vdc then max Vmp is more like 405 vdc in practical terms.

Having grid connected further mucks up the power spec's. The battery to HV DC converter and Sinewave PWM converter limits remain the same but the amount of AC current flowing through connect relay to AC input is limited to 50 amps when pulled from AC input. Back push would also be 50 amps maximum but since PWM sinewave IGBT can only produce 9.6 kW maximum (40 amps) any additional current would have to come from a separate GT inverter AC coupled to output of inverter.

SolArc is actually two 120 vac inverters, series connected, to provide 240vac.

thank you.
I will need to read that several times to understand, but I already see a lot of valuable info and stats there.

much appreciated.

 

[rmaddy]

I did not realize 20% correction factor was taking me down to -40° (f? c?). a cold day in hell here in my area might get down to 15°f (-10°c)

FYI: -40ºF and -40ºC is the same temperature which is why I simply wrote -40º.

20% will get most panels to -40º but that assumes a more typical temperature coefficient of -0.3%/ºC. Your panels have a better coefficient of -0.262%/ºC so 20% actually gets your panels down to -60ºF/-51ºC.

If your record cold is 15ºF/-10C then you should enter that into the online calculator. For most SCCs it is very bad for the Voc to exceed the max PV input voltage.

I'm not sure why that online calculator only shows 9 panels in series. With a Voc of 41V, a max Voc of 450V, and a temperature coefficient of -0.262%/ºC, my math shows that 10 panels would reach 450V at 10ºF/-12ºC. 9 panels would be good down to -73ºF/-58ºC.

 

[Snowynorth]

Here we get occasional temps below -45c. I used -60c to calculate, (better safe than sorry) and I am staying well below the max voc at -60. Your inverter specs should have the max pv Isc (mine didn't but I found it). Don't go above that. The inverter will clip the excess amps. For your 12k Sol-Ark I don't think the majority of panels will be any issue. Mine is 7.6k deye. 18+18a mppt, I am going to run 425w panels 6s2p. I think your 9s2p will be fine.