Where To Combine PV Strings

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jkelsch
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Joined: Mon Aug 24, 2020 7:57 pm

Where To Combine PV Strings

Post by jkelsch » Mon Aug 24, 2020 8:15 pm

Hi All,

I currently have (40) 265W Panels connected to (2) SolarEdge Grid Tie inverters. I am considering changing from my SolarEdge Inverters to a stacked SkyBox setup so I can install a battery.

My panels are installed about 250 feet from my house. I am considering putting 20 Panels on Each Skybox (2 Strings of 10 panels = 5300W). I know 5300W is more than is usable for the SkyBox, but I rarely ever get 5300W, so I assume that will be fine. According to the Sizing Tool, 5300W shows up in the caution colored cells.

Can I combine the 2 strings out by the panels and only run only 1 line into the SkyBox?

I only have 3 sets of lines coming from the panels to the house and I doubt that I can pull another set of wires through the burried conduit.

Also, I see that the Skybox can use the Tigo TS4-S-R for rapid shutdown. Will the Tigo TS4-A-O work with the Skybox so I can have optimization on the panels?

Thanks for any thoughts.

Ampster
Forum Guru
Posts: 119
Joined: Sat Apr 11, 2015 5:14 pm
My RE system: 5.7K solar system with Solaredge inverter. Heat pump water heater.
28 kWh pack of LFP batteries powering a Skybox which is used for load shifting and backup.
Location: Sonoma, Northern California

Re: Where To Combine PV Strings

Post by Ampster » Tue Aug 25, 2020 6:25 pm

jkelsch wrote:
Mon Aug 24, 2020 8:15 pm
.......
Can I combine the 2 strings out by the panels and only run only 1 line into the SkyBox?
......
Also, I see that the Skybox can use the Tigo TS4-S-R for rapid shutdown. Will the Tigo TS4-A-O work with the Skybox so I can have optimization on the panels?
I can try to answer most of your questions but to give you a more informed answer I have a few clarifying questions:
What size are your existing Solaredge inverters?
What are your loads in kWhs during the day and overnight?
What size battery pack are you considering and how long do you think that will last normally?
Is your desire for just backup or do you want to be more self sufficient and consume more of the energy that you produce?
Have you considered AC coupling?
Drive Tesla Model X and Model 3 which are charged at Super Off Peak rates.

pss
Forum Czar
Posts: 637
Joined: Mon Jul 24, 2017 8:40 am
My RE system: 8330 watts in three strings, Flexmax 60 x 3, Radian 8048A, GSLC load center, Mate 3S, Hub 10.3, FN-DC and 900 Amp, 48V Trojan T105-RE battery bank.

Re: Where To Combine PV Strings

Post by pss » Tue Aug 25, 2020 8:01 pm

The skybox has a charge controller whose MPPT function requires 250 - 600 volts of PV input and a maximum of 20 amps of DC current. Your 265 watt panels most likely produce about 8.5 amps of peak current and a Voc of about 38. 600 divided by 38 = 15.78 and 250 divided by 38 = 6.57. So you can connect anywhere from 7 to 15 panels in series and be within the MPPT parameters of the Skybox charger. 15 in series would be 38 volts x 15 = 570 x 8.5 amps = 4845 watts. Two strings in parallel would = 9690 watts, up to 570 volts and 17 amps. Remember, in colder weather Voc is increased and decreased in hot weather. So if you get below 50 degrees routinely in the wintertime, you may want to reduce to 14 panels in series. You have 40 x 265 watts of panels which is 10,600 watts of faceplate value. if you put in 2 parallel strings of 14 panels each, that would be 7420 watts of PV to the Skybox.

Now, as to how to connect, you will make all of your series connections at the array, then bring each connection to a combiner box that is grounded at the array and has a DC circuit breaker rated for 1000 volts DC and 20 amps on each string. Then when the 2 strings are wired into the combiner box, run the largest gauge wire that you can into a DC disconnect which is fused at or very near to the combiner box. Your combiner box should have a DC lightening arrestor installed. There should be a grounding rod in the earth at the array. The array and combiner box should be grounded. From the DC disconnect switch, run the largest wire that you can (this means the largest gauge that will screw onto the terminals of the charge controller or terminal of the Skybox), THHN 1000 volts, in the proper sized electrical conduit. Since you have a 250 foot run, run in 100 feet, 100 feet and 50 feet into your access point of the Skybox. By using 100 foot lengths of wire and installing a grey electrical connection round box point in the ground, you will create a splice point that will serve as a future service point. If there is a failure along the way in the future, you can easily pin point it to either the combiner box, disconnect switch, the first 100 foot wire pull, the second 100 foot wire pull or the last 50 feet wire pull or in the Skybox. Pulling replacement wire 100 feet is a lot more doable than 250 feet, believe me. Make your wire splices with copper splice blocks brought just above the ground, waterproofed and wrapped in electrical insulater tape.

This should get you to where you want to be, just double check my calculations for the amount of PV you want to direct into the Skybox.

Ampster
Forum Guru
Posts: 119
Joined: Sat Apr 11, 2015 5:14 pm
My RE system: 5.7K solar system with Solaredge inverter. Heat pump water heater.
28 kWh pack of LFP batteries powering a Skybox which is used for load shifting and backup.
Location: Sonoma, Northern California

Re: Where To Combine PV Strings

Post by Ampster » Wed Aug 26, 2020 11:43 am

@jkelsch

Now that you know you can probably do what you want let me focus on my last question to see if it might offer a cost effective alternative. Perhaps you know the answers to the earlier questions about load and battery capacity or like me you are able to make a rough estimate. I found out that I could get by with one Skybox but that I needed a lot more battery capacity than I had originally estimated. That wasn't all that bad because the cost of DIY Lithium packs has come down.

One alternative to all the rewiring and adding RSD devices to your existing panels would be to buy one Skybox and AC couple it to one of the existing Solaredge inverters which already has all the RSD functionality built in. The only assumption is that the Skybox is limited to 7kW of GT inverter that it can AC couple to. AC coupling is not yet implemented in multiple Skyboxes so if your inverter fits that maximum it would only cost you one Skybox. The AC coupling is simple since all you have to do is wire the one Solaredge inverter to the essential loads panel that the Skybox is powering. RSD functionality is accomplished by wiring the internal contacts to an external RSD switch which will shut down the Skybox and the Solaredge inverter with its native RSD built in. Both GT inverters will work as before, when the grid is up. When the grid is down only the AC coupled Solaredge will work. That could be Phase One of a plan that would get you some back up power at a reasonalble cost and you would have the infrastructure for adding capacity as you found you needed it. You would already have an essential loads panel.

I assume Outback is working on a plan to AC couple multiple inverters so that is an option for the other half of your system in the future as Phase two. f
Drive Tesla Model X and Model 3 which are charged at Super Off Peak rates.

jkelsch
Forum Junior Member
Posts: 3
Joined: Mon Aug 24, 2020 7:57 pm

Re: Where To Combine PV Strings

Post by jkelsch » Fri Sep 11, 2020 7:20 pm

@Ampster

I considered AC Coupled. I was originally looking at the Radian Inverter to AC Couple, but was nervous to buy the Radian, and find out it won't AC couple very well. Then I was stuck trying to buy the FlexMax chargers and trying to figure out how to wire them to the Radian. At least the Skybox had everything built-in, in the event AC Coupling didn't work very well.

I was not sure how well AC Coupling works, since the Inverter is trying to "trick" the SolarEdge into working.

Do you have expericence with AC coupling?

If my well pump (takes about 3500 Watts running, not sure for startup) starts up and for a split second makes the inverter "brown out", will that variation in power make the SolarEdge shutdown and wait the 5 minutes to start up again?

I have a cousin that is trying to AC Couple his Micro-inverter strings with a Sol-ark 12K and it seems like the "browns" keep shutting down his micro-inverters, and to date, he is unable to get the AC Coupling to work for more than just a few minutes.

Also, have you heard from Outback if there are plans to make AC Coupling work with Stacked SkyBoxes? I would ultimately like to have 10Kw of Inverter power.

I have been considering the "start with 1 skybox" approach. I guess I could buy the 1 Skybox and try to AC Couple it, and then buy the RSD and rewire the panels if I have difficulty getting AC Coupling to work well.

jkelsch
Forum Junior Member
Posts: 3
Joined: Mon Aug 24, 2020 7:57 pm

Re: Where To Combine PV Strings

Post by jkelsch » Fri Sep 11, 2020 7:41 pm

@pss
pss wrote:
Tue Aug 25, 2020 8:01 pm
2 parallel strings of 14 panels each, that would be 7420 watts of PV to the Skybox.
Is that 7420 watts if I am using 2 skyboxes stacked. Basically, 14 panels per Skybox? According to the below link, the Skybox has max DC output of 5000 watts. So if a single Skybox could take 7420 watts would we be losing the 2470 watts of power and not selling it back to the grid?

http://www.outbackpower.com/downloads/d ... p_note.pdf

Ampster
Forum Guru
Posts: 119
Joined: Sat Apr 11, 2015 5:14 pm
My RE system: 5.7K solar system with Solaredge inverter. Heat pump water heater.
28 kWh pack of LFP batteries powering a Skybox which is used for load shifting and backup.
Location: Sonoma, Northern California

Re: Where To Combine PV Strings

Post by Ampster » Fri Sep 18, 2020 10:43 pm

jkelsch wrote:
Fri Sep 11, 2020 7:20 pm
I considered AC Coupled....

I was not sure how well AC Coupling works, since the Inverter is trying to "trick" the SolarEdge into working.

Do you have expericence with AC coupling?
Yes i have had very good luck with AC coupling. I am AC coupled to a Solaredge 3.8 kW inverter and 4 Enphase IQ7s.
If my well pump (takes about 3500 Watts running, not sure for startup) starts up and for a split second makes the inverter "brown out", will that variation in power make the SolarEdge shutdown and wait the 5 minutes to start up again?
If your Solaredge is browning out your grid power must be weak. If it takes 3500 Watts running, the surge has to be several times that. That will be your biggest challenge is finding an inverter that can handle the surge. It would be woth buying or borrowing an Ammeter that can measure surge. There are soft start inverter driven pumps that might be considered. It will be a trade off between a new pump and less expensive inverter or a bigger inverter or two. You need surge data to inform your decision making.
I have a cousin that is trying to AC Couple his Micro-inverter strings with a Sol-ark 12K and it seems like the "browns" keep shutting down his micro-inverters, and to date, he is unable to get the AC Coupling to work for more than just a few minutes.
There are some limits to the SolArk AC coupling capacity but i don't know the details. The other thing could be that he has older micros that do not modulate and are either on or off. Sounds like he should split some of the micros off AC coupling until he finds the right combo.
Also, have you heard from Outback if there are plans to make AC Coupling work with Stacked SkyBoxes? I would ultimately like to have 10Kw of Inverter power.

I have been considering the "start with 1 skybox" approach. I guess I could buy the 1 Skybox and try to AC Couple it, and then buy the RSD and rewire the panels if I have difficulty getting AC Coupling to work well.
I have no info about AC coupling and stacked Skyboxes. I bought mine before AC coupling was available and they delivered tbe firware as promised. I know with the Enersys purchase some things have changed and i would make some inquiries to confirm their commitment.
Sorry for the late reply. I got distracted with all the smoke in tbe Sonoma Valley.
Drive Tesla Model X and Model 3 which are charged at Super Off Peak rates.

Ampster
Forum Guru
Posts: 119
Joined: Sat Apr 11, 2015 5:14 pm
My RE system: 5.7K solar system with Solaredge inverter. Heat pump water heater.
28 kWh pack of LFP batteries powering a Skybox which is used for load shifting and backup.
Location: Sonoma, Northern California

Re: Where To Combine PV Strings

Post by Ampster » Fri Sep 18, 2020 10:44 pm

jkelsch wrote:
Fri Sep 11, 2020 7:20 pm
I considered AC Coupled....

I was not sure how well AC Coupling works, since the Inverter is trying to "trick" the SolarEdge into working.

Do you have expericence with AC coupling?
Yes i have had very good luck with AC coupling. I am AC coupled to a Solaredge 3.8 kW inverter and 4 Enphase IQ7s.
If my well pump (takes about 3500 Watts running, not sure for startup) starts up and for a split second makes the inverter "brown out", will that variation in power make the SolarEdge shutdown and wait the 5 minutes to start up again?
If your Solaredge is browning out your grid power must be weak. If it takes 3500 Watts running, the surge has to be several times that. That will be your biggest challenge is finding an inverter that can handle the surge. It would be woth buying or borrowing an Ammeter that can measure surge. There are soft start inverter driven pumps that might be considered. It will be a trade off between a new pump and less expensive inverter or a bigger inverter or two. You need surge data to inform your decision making.
I have a cousin that is trying to AC Couple his Micro-inverter strings with a Sol-ark 12K and it seems like the "browns" keep shutting down his micro-inverters, and to date, he is unable to get the AC Coupling to work for more than just a few minutes.
There are some limits to the SolArk AC coupling capacity but i don't know the details. The other thing could be that he has older micros that do not modulate and are either on or off. Sounds like he should split some of the micros off AC coupling until he finds the right combo.
Also, have you heard from Outback if there are plans to make AC Coupling work with Stacked SkyBoxes? I would ultimately like to have 10Kw of Inverter power.

I have been considering the "start with 1 skybox" approach. I guess I could buy the 1 Skybox and try to AC Couple it, and then buy the RSD and rewire the panels if I have difficulty getting AC Coupling to work well.
I have no info about AC coupling and stacked Skyboxes. I bought mine before AC coupling was available and they delivered tbe firware as promised. I know with the Enersys purchase some things have changed and i would make some inquiries to confirm their commitment.
Sorry for the late reply. I got distracted with all the smoke in tbe Sonoma Valley.
Drive Tesla Model X and Model 3 which are charged at Super Off Peak rates.

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