difficulty stringing 96 cell panels

[electrofelon]

We have a 14.625KW ground mount, 45 325W 96 cell panasonic modules. We have two sunny boy 7.7 inverters (6 MPPT inputs total) Long story short, the original modules planned were destroyed in shipment, and this is what we have now. The problem is the temperature corrected VOC allows only 7 modules per string, so that is 42 modules. We discussed, and decided to get 3 sunny boy 5.0 inverters, however there were delays in those and we jut found out they havnt even shipped yet. We dont have much time to finish this system so have scrapped that idea. One option we are looking at is 3 strings of 7 to one inverter, and 4 strings of 6 connected to the other inverter with two of those connected in parallel. The other idea is to use a third 7.7 inverter (which we do have) and do 9 strings of 5. Obviously there is a cost to providing a 3rd inverter for a system originally designed and priced for 2. I minor advantage of the 3, besides the increased number of MPPT's, is stringing is real nice because the array is 9 rows of 5 so all connections can be at the top end. Any thoughts on these two options or anything else that maybe I havnt thought of?

 

[wwhitney]

I have zero intuition for the relative lifetime energy production vs total installed cost of any of these choices, but a couple more ideas just brainstorming:

- Delete 3 panels with an according price change.
- Run 9 strings of 5 and just wire one 7.7 kW inverter with 2 strings of 5 on each input. (Or on 2 inputs of one inverter and 1 input of the other). Then depending on the comparative data between the two 7.7 kW inverters when it is up and running, it could be worth coming back to swap the 2 * 7.7 kW for the delayed 3 * 5 kW inverters.

Cheers, Wayne

 

[electrofelon]

I have zero intuition for the relative lifetime energy production vs total installed cost of any of these choices, but a couple more ideas just brainstorming:

- Delete 3 panels with an according price change.
- Run 9 strings of 5 and just wire one 7.7 kW inverter with 2 strings of 5 on each input. (Or on 2 inputs of one inverter and 1 input of the other). Then depending on the comparative data between the two 7.7 kW inverters when it is up and running, it could be worth coming back to swap the 2 * 7.7 kW for the delayed 3 * 5 kW inverters.

Cheers, Wayne

Wayne,

I dont think using 42 module is an option. Tempting, yes. Using to two inverters with paralleling but using the simpler-for-stringing 9 strings of 5 is a good idea to consider, even though it is two more strings total than my original parallel plan.

Another wrench in the consideration bag is that we bought these 7.7 inverters some time ago and we paid less for them what a 5.0 would cost now. Of course in theory we could and would sell them in future systems for our cost to replace them not what we paid for them....so not sure what that means

 

[electrofelon]

I am doing this system with a friend of mine, and basically the deal with this one is that it is "his" job and I am getting paid for my labor. He is managing the job, doing the excavating, and mounting the modules. I am doing the electrical work which involves disconnecting the service lateral from the pole transformer and rerouting it to a new meter pedestal (using my 230.40 exception #3 method as usual), hooking up a new lateral to the pole transformer, and the service and other AC wiring at the ground mount. I guess that was unnecessary detail, but the point is it is ultimately his decision and money and he says "there is money in there" for the third inverter. We both kinda like the simplicty and symetry of the 3 inverters with 3 strings of 5 each, but I told him there is nothing wrong with paralleling two strings. So what say you, is the two inverter and paralleling method kinda a bastard unprofessional system? OR totally fine?

 

[wwhitney]

So what say you, is the two inverter and paralleling method kinda a bastard unprofessional system? OR totally fine?

The usual suspects surely have more informed opinions, but I would say all that matters is the projected lifetime kWh produced. If paralleling strings gives the same production but is easier to wire or to make work with the equipment available, that's a win.

[I'm unclear on the details of how an inverter internally deals with multiple MPPTs. Will a 7.7 kW inverter with 3 MPPT inputs limit each input to 2.6 kW? Or will each MPPT input have a higher limit, with the 7.7 kW as an additional overall limit?

And for that matter, if an MPPT input is limited to say 10A and 400V, can those necessarily be achieved simultaneously, allowing up to 4 kW of power, or can there be an additional power limit lower than 4 kW?]

Cheers, Wayne

 

[jaggedben]

... So what say you, is the two inverter and paralleling method kinda a bastard unprofessional system? OR totally fine?

My gut says totally fine. Definitely not unprofessional. Your low DC to AC ratio is what's unprofessional, especially with the three inverter option. The only thing I don't think you've mentioned is whether there's a current limit on the MPPs that will clip the paralleled strings, but if that checks out...

The only advantage I see to a third inverter is if he ever wants to expand he can avoid reworking the AC side or worrying about any code limits there.

 

[electrofelon]

[I'm unclear on the details of how an inverter internally deals with multiple MPPTs. Will a 7.7 kW inverter with 3 MPPT inputs limit each input to 2.6 kW? Or will each MPPT input have a higher limit, with the 7.7 kW as an additional overall limit?

And for that matter, if an MPPT input is limited to say 10A and 400V, can those necessarily be achieved simultaneously, allowing up to 4 kW of power, or can there be an additional power limit lower than 4 kW?]

Cheers, Wayne

The only thing I don't think you've mentioned is whether there's a current limit on the MPPs that will clip the paralleled strings, but if that checks out...

That brings up a good point. Looking at the SB manual, it looks like each input is limited to 10A. Imp is 5.5A so I guess that blows tha parallel idea...or at least I would have to try and evaluate the clipping losses.

 

[electrofelon]

My gut says totally fine. Definitely not unprofessional. Your low DC to AC ratio is what's unprofessional, especially with the three inverter option.

I was paranoid someone who knows PV would see this system with the parallel option and think "who the heck designed this, I wouldnt have done it that way" but yeah having 23.1Kw of inverters on a 14.6KW system would probably elicit the same response

 

[ggunn]

I was paranoid someone who knows PV would see this system with the parallel option and think "who the heck designed this, I wouldnt have done it that way" but yeah having 23.1Kw of inverters on a 14.6KW system would probably elicit the same response

Do you mean 23.1kW of modules on 14.6kW of inverter power? The SMA specs have limits on cold rated string Voc, maximum Isc per MPPT, and maximum DC:AC ratio. I would think a 1.58 DC:AC would be out of their specs and would void their warranty.

 

[electrofelon]

Do you mean 23.1kW of modules on 14.6kW of inverter power? The SMA specs have limits on cold rated string Voc, maximum Isc per MPPT, and maximum DC:AC ratio. I would think a 1.58 DC:AC would be out of their specs and would void their warranty.

No you have it backwards. This was one of those learning moments for me, but these high cell count modules are hard to use on string inverters.

 

[jaggedben]

That brings up a good point. Looking at the SB manual, it looks like each input is limited to 10A. Imp is 5.5A so I guess that blows tha parallel idea...or at least I would have to try and evaluate the clipping losses.

That's not much clipping, assuming it's just what the inverter will limit input to and not something that will void the warranty. (Sorry, it's been a few years since I worked with SMA.)

I would run it through the SMA software, IIRC it will estimate for you what the clipping will be.

 

[electrofelon]

That's not much clipping, assuming it's just what the inverter will limit input to and not something that will void the warranty. (Sorry, it's been a few years since I worked with SMA.)

I would run it through the SMA software, IIRC it will estimate for you what the clipping will be.

Yeah it's not really clear in the destruction manual whether I can connect more than 10 amps to an input and it will clip as necessary, or I must not connect more than 10 amps. I would guess the former.

So with these high voltage modules on string inverters, basically you will need multiple smaller inverters or another option might be use fronius inverters which do 1000 volt strings (although those only have two mppt inputs so you might be in the same boat without doing the math).

 

[solarken]

Yeah it's not really clear in the destruction manual whether I can connect more than 10 amps to an input and it will clip as necessary, or I must not connect more than 10 amps. I would guess the former.

So with these high voltage modules on string inverters, basically you will need multiple smaller inverters or another option might be use fronius inverters which do 1000 volt strings (although those only have two mppt inputs so you might be in the same boat without doing the math).

Are these inverters the SB-7.7-US-41 model? If so, the datasheet shows 3 MPPT inputs, and "1 string per MPPT tracker", which suggests no paralleling. Not sure why they would specify that limit on a datasheet but they do. Maybe to keep the current below the 10A max limit? But there are a few modern panels that exceed 10A Ipm anyway. Or maybe to just convey there are only one set of terminals? Your discussion here is why I quit using SMA residential inverters a long time ago. They no longer make much sense. They have three MPPT inputs, for a 5kW, 6kW, 7kW, and 7.7kW inverter model. Why???? Probably because they were designed when 220W modules were common. They should have come out with a larger inverter long ago.

I would have spec'd SolarEdge for that project. One single 11.4kW Energy Hub Inverter, three strings of 15. Clipping would likely be minimal in NY or WA area.

I would just use up the two SB 7.7 inverters and the available third one, since you kind of are limited based on the replacement modules.

 

[electrofelon]

Are these inverters the SB-7.7-US-41 model? If so, the datasheet shows 3 MPPT inputs, and "1 string per MPPT tracker", which suggests no paralleling. Not sure why they would specify that limit on a datasheet but they do. Maybe to keep the current below the 10A max limit? But there are a few modern panels that exceed 10A Ipm anyway. Or maybe to just convey there are only one set of terminals? Your discussion here is why I quit using SMA residential inverters a long time ago. They no longer make much sense. They have three MPPT inputs, for a 5kW, 6kW, 7kW, and 7.7kW inverter model. Why???? Probably because they were designed when 220W modules were common. They should have come out with a larger inverter long ago.

I would have spec'd SolarEdge for that project. One single 11.4kW Energy Hub Inverter, three strings of 15. Clipping would likely be minimal in NY or WA area.

I would just use up the two SB 7.7 inverters and the available third one, since you kind of are limited based on the replacement modules.

Yes SB-7.7-US-41. IMO by "1 string per tracker" they mean 1 physical connection terminal

Yes I agree it is annoying they don't make a larger inverter.

So I worked on it today and we just used the third inverter, 9 strings of 5. The deed is done.

 

[Designer101]

My gut says totally fine. Definitely not unprofessional. Your low DC to AC ratio is what's unprofessional, especially with the three inverter option. The only thing I don't think you've mentioned is whether there's a current limit on the MPPs that will clip the paralleled strings, but if that checks out...

The only advantage I see to a third inverter is if he ever wants to expand he can avoid reworking the AC side or worrying about any code limits there.

one question , we used those inverter once and i thought each mppt has 10A max limit, if we parallel two string's depending upon amps from each ,modules what if the input current exceeds the rated mmpt input??

 

[electrofelon]

one question , we used those inverter once and i thought each mppt has 10A max limit, if we parallel two string's depending upon amps from each ,modules what if the input current exceeds the rated mmpt input??

The extra current just gets clipped, no different than running a DC AC ratio greater than one - At least that is what I am assuming . I ran it through Sunny design and it says it is an acceptable configuration.