Unequal PV strings?

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mhspohr
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Posts: 41
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My RE system: 6.6 kW SolarWorld/Enphase and 2.6 kW Sunpower/SMA
Skybox with 10 kWh SLA (used for power outage backup only)
Tesla 2015 Model S 85D charged from the sun.

Unequal PV strings?

Post by mhspohr » Sat Jun 13, 2020 5:13 pm

I'm planning to add solar to my Skybox. There are two PV string inputs. I plan to set up 15 panels which means that I will have a 7 panel string and an 8 panel string. Can the Skybox MPPT handle different size strings?

Mike Curran
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My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Unequal PV strings?

Post by Mike Curran » Sat Jun 13, 2020 6:02 pm

Skybox puts the two string inputs in parallel. There are not two separate MPPT functions, only one, so you'll get a "blend" of the two arrays with the poorer performer dragging the other down with it. Two inputs are provided for the case where one long string would exceed Skybox's 600V input limit, but if you split the long string into two, you'll get best results if both are identical.
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

mhspohr
Forum Whiz
Posts: 41
Joined: Sun Jun 30, 2019 8:56 am
My RE system: 6.6 kW SolarWorld/Enphase and 2.6 kW Sunpower/SMA
Skybox with 10 kWh SLA (used for power outage backup only)
Tesla 2015 Model S 85D charged from the sun.

Re: Unequal PV strings?

Post by mhspohr » Sun Jun 14, 2020 5:52 am

Thanks. That's what I thought. Thinking of going to 18 panels in two strings. Would be about 400v per string which leaves headroom above the 250v turn on.

Mike Curran
Forum Emperor
Posts: 1804
Joined: Sat Aug 28, 2004 6:28 pm
My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Unequal PV strings?

Post by Mike Curran » Sun Jun 14, 2020 6:13 am

Good plan.

Initially I wired mine as you are planning, 2 strings, because I was concerned one long string of 14 panels would be too high voltage (>600V) but found that the 7 string arrays often didn't reach the 250V turn on. Rewiring them as one, 14 module string has been working fine, no problems with overvoltage.
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

mhspohr
Forum Whiz
Posts: 41
Joined: Sun Jun 30, 2019 8:56 am
My RE system: 6.6 kW SolarWorld/Enphase and 2.6 kW Sunpower/SMA
Skybox with 10 kWh SLA (used for power outage backup only)
Tesla 2015 Model S 85D charged from the sun.

Re: Unequal PV strings?

Post by mhspohr » Sun Jun 14, 2020 7:47 am

I could wire the 15 panels as one string and hope that it doesn't overload the system.
I have another string of six monster 72 v panels connected to an SMA inverter. Voltage gets up above 500v and it's happy.

JRHill
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Re: Unequal PV strings?

Post by JRHill » Sun Jun 14, 2020 12:11 pm

500vdc. Brr. makes me shudder.

Mike Curran
Forum Emperor
Posts: 1804
Joined: Sat Aug 28, 2004 6:28 pm
My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Unequal PV strings?

Post by Mike Curran » Sun Jun 14, 2020 12:54 pm

A selling point for Skybox - none of OB's other charge controllers can go over 300V, so longer strings are possible.

JR, if you're shuddering at 500V, check out my highest VOC:
Screenshot_20200614-155106_Samsung Internet.jpg
https://youtu.be/2tcD_dVcXE4
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

Photon Kapture
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Location: Schroeder, Minnesota

Re: Unequal PV strings?

Post by Photon Kapture » Sun Jun 14, 2020 9:49 pm

Mike Curran wrote:
Sun Jun 14, 2020 12:54 pm
A selling point for Skybox - none of OB's other charge controllers can go over 300V, so longer strings are possible.

JR, if you're shuddering at 500V, check out my highest VOC:
Screenshot_20200614-155106_Samsung Internet.jpg

https://youtu.be/2tcD_dVcXE4
Mike was there something unique about that day you achieved your highest voc?

I’m using a single string of (13) 300 watt panels with my Skybox. Actually have 14 on the roof but with temperature coefficient on paper it could in get too close to 600v here in Minnesota during the winter months. So I left one out. Seems like such a waste but Murphy’s law ](*,)

Mike Curran
Forum Emperor
Posts: 1804
Joined: Sat Aug 28, 2004 6:28 pm
My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Unequal PV strings?

Post by Mike Curran » Sun Jun 14, 2020 11:15 pm

Mike was there something unique about that day you achieved your highest voc? 
Photon - My memory isn't that good :smile: - it had to have been sunny and cool. We've had a bunch of those days this spring. It's been a good year for solar here.

Mine are 275 watt panels so not as close to the edge as your 14 300W panels would be. "Prudence is the better part of valor".
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

AaronF
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Skybox RSI

Re: Unequal PV strings?

Post by AaronF » Mon Jun 15, 2020 7:02 am

Mike Curran wrote:
Sun Jun 14, 2020 6:13 am
Initially I wired mine as you are planning, 2 strings, because I was concerned one long string of 14 panels would be too high voltage (>600V) but found that the 7 string arrays often didn't reach the 250V turn on. Rewiring them as one, 14 module string has been working fine, no problems with overvoltage.
Mike, I also have 14 panels in two 7 string arrays. I just wired them up two days ago and since my Skybox is unable to boot at the moment, I don't have any logged data from it yet. However, I finished at 6:30pm under heavy cloud cover and measured a VOC of 305V with a multimeter. (Unexpected because they are 41V panels and so the strings are nominally 287V, but I assume the extra 18V is padding for the manufacturers warranty.) Based on these preliminary numbers, I would be hesitant to put all 14 in series. When you had yours in two strings of 7 panels each, how much power were you producing near the 250V cut-off and what were the weather conditions? VOC and VMPPT drop really slowly as irradiance decreases until low power conditions, so I suspect your power output near or below the cut-off would have been negligible.
Last edited by AaronF on Mon Jun 15, 2020 8:51 am, edited 1 time in total.

Mike Curran
Forum Emperor
Posts: 1804
Joined: Sat Aug 28, 2004 6:28 pm
My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Unequal PV strings?

Post by Mike Curran » Mon Jun 15, 2020 8:46 am

When you had yours in two strings of 7 panels each, how much power were you producing near the 250V cut-off and what were the weather conditions? 
Hey Aaron - It's really hard to answer these questions because it was back in early April when I made the switch to one long 14 module string from 2, 7 module strings. What would appear to happen was the 2 string array would momentarily output over 250V and SkyBox would accept it, and even keep using it when PV voltage dropped below 250 (I have emails with OB Tech support indicating Skybox still taking the DC array output with voltage as low as 211V). But on a less sunny day when PV output started out less than 250V, Skybox would never turn on to use the DC arrays.

Here's a screenshot from a day in early April when I still had 2, 7 module strings in parallel, FWIW. Note the DC array output is indicated by the "Solar" (lighter green bars). The other bars are green because of the combination of the AC-coupled array feeding the grid, plus the DC array.
Screenshot_20200615-114211.jpg
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

AaronF
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My RE system: Skybox
Skybox RSI

Re: Unequal PV strings?

Post by AaronF » Mon Jun 15, 2020 9:17 am

Ouch. Who designed that plot?
(1) Grid and load are almost the same shade of green.
(2) There are no legend colours for negative values.
(3) The order of the legend is solar, grid, load, battery, but in the plot it shows solar, grid, battery, load.
I especially feel sorry for colour-blind users, because there would be no way to differentiate the red and green.
Last edited by AaronF on Mon Jun 15, 2020 10:43 am, edited 2 times in total.

raysun
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Re: Unequal PV strings?

Post by raysun » Mon Jun 15, 2020 9:54 am

AaronF wrote:
Mon Jun 15, 2020 9:17 am
Ouch. Who designed that plot? I feel sorry for colour-blind users.
The data to display are selectable. The color blind, and the slow on the uptake can both follow along without too much trouble.
Screenshot_20200615-065553_Chrome.jpg
A less data dense example.

Mike Curran
Forum Emperor
Posts: 1804
Joined: Sat Aug 28, 2004 6:28 pm
My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Unequal PV strings?

Post by Mike Curran » Mon Jun 15, 2020 12:29 pm

There are no legend colours for negative values.
The order of the legend is solar, grid, load, battery, but in the plot it shows solar, grid, battery, load.
If any value goes negative, it will be some shade of red. Looking at the "all green period" between 10am and 12pm, all are green because solar (DC PV) is positive (always will be), grid was backfed more than it was used in each hour, battery got a net charge each hour, and load was less than the AC-coupled PV input in each hour.

And the order of the legend exactly matches that of the graph.
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

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