Expanding Array in an AC Coupled System

This is a project that quite a few different people have had their hands on (original grid tied installer, generator installer and 1 or 2 consultants) and now me. Homeowner wants to expand the array. I'm looking for some input.

Current system (AC Coupled):
18 Samsung LPC244 / Imp 8.07 / Isc 30.3 / 244w / 4.4kw / mono (2 strings of 9)
ABB Aurora PVI5000 GT inverter
Outback Radian GS8048
12 Outback 200re AGM Batteries / 534ah @ 48v
17kw NG Generator
Ground mount array 200ft from GT Inverter.

We will add 12 modules for a total of 7.3kw (3 strings of 10)
Possible well matched modules:
Canadian Solar 245W CS6P-245M / Imp 8.09 / Isc 30.3 / 245w / mono
Hyundai Solar HiS-S255MG 255W / Imp 8.3 / Isc 30.83 / 255w / mono
Fronius IG Plus 7.5 Inverter
Appropriate fusing as we go from 2 to 3 strings

My questions...
Is there an advantage in moving the inverter out to the array instead of near the service panel? GT Inverter is currently mounted on the outside of the home. And the Radian is inside.
#6 conductor from the array to the inverter.

I have looked at Enphase for this system, but I can't see enough advantage to recommend to the homeowner, an additional $2k-3k over a string inverter. Am I missing something?

How do I figure the Max AC input for Radian? Radian spec sheet says AC Input is 50amps at 240v (I should be just under 30 coming from the new GT inverter), but I read in HomePower that the Radian can handle 6kw of PV.

Is it better to mix 6 old modules and 4 new modules in each string OR go 10 new modules all in one string and 1 new, 9 old in the other 2 stings?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Expanding Array in an AC Coupled System

    Welcome to the forum.

    I hate to say it but this is a mess and should be straightened out by a professional engineer who knows what they are doing. That could be difficult to find. You not only have technical issues here, but legal ones as well (complying with code in respect to GT systems and service panel capacity).

    If you've already got the 5kW GTI AC coupled to the 8kW Radian you can not add the 7.5 kW Fronius as well. It is doubtful that the AHJ will allow it at all, because the backfeed rating of the service panel will need to including the output of all the inverters which can feed the grid. With 8 & 5 kW already I suspect that rating is already exceeded, unless they are going by the Radian's AC IN current rating limit which they should because there are no panels feeding that unit's batteries, correct? (AHJ's sometimes have trouble understanding this).

    You've already got 4kW of PV back-feeding the Radian which has 534 Amp hours of battery. I would not increase that as it is just about the right balance for charging. The AC input rating of the Radian refers to AC IN, not to back-feeding it. In theory it could take up to 8kW AC coupled. I don't think Outback would advise doing this though; ask them. They are pretty good about answering questions.

    There is no advantage to moving the Radian. It should be as close to the batteries as possible, as that is the low Voltage/high current connection for it. The AC lines are @ 240 Volt so represent no significant V-drop potential.

    Any additional GTI will have to be tied to the service distribution, not the Radian. You may already be at the 120% limit there depending on what the service rating is and how back-feed was calculated (should be based on the one 5kW GTI's output from its slightly undersized array but may be based on the Radian's 8kW capacity).

    In theory you could add PV to the 5kW GTI to bring it up to maximum potential, but the panels would have to match the existing ones.

    Frankly I would not have done this system the way it is.

    Oh, and that's about $5,000 worth of advice you just got for free. :D
  • BB.BB. Super Moderators, Administrators Posts: 28,163 admin
    Re: Expanding Array in an AC Coupled System
    radiant wrote: »
    This is a project that quite a few different people have had their hands on (original grid tied installer, generator installer and 1 or 2 consultants) and now me. Homeowner wants to expand the array. I'm looking for some input.

    I guess you are "in the business"? Be very careful of getting out of your comfort zone--With so many people touching the system before you--It may be difficult for you to make any changes/improvements without major changes to the existing system (bad choices, choices you would not have recommended, etc.).
    Current system (AC Coupled):
    18 Samsung LPC244 / Imp 8.07 / Isc 30.3 / 244w / 4.4kw / mono (2 strings of 9)
    ABB Aurora PVI5000 GT inverter
    Outback Radian GS8048
    12 Outback 200re AGM Batteries / 534ah @ 48v
    17kw NG Generator
    Ground mount array 200ft from GT Inverter.

    We will add 12 modules for a total of 7.3kw (3 strings of 10)
    Possible well matched modules:
    Canadian Solar 245W CS6P-245M / Imp 8.09 / Isc 30.3 / 245w / mono
    Hyundai Solar HiS-S255MG 255W / Imp 8.3 / Isc 30.83 / 255w / mono
    Fronius IG Plus 7.5 Inverter
    Appropriate fusing as we go from 2 to 3 strings

    In general, if the GT+Solar Charge controller energy to the battery bank, you would not want to exceed ~1kWatt of solar charging (or rated output from Off Grid inverter) per 100 AH @ 48 volt battery bank.

    Of course, AGM batteries with appropriate wiring/breakers/fuses can exceed the 1kW per 100 AH @ 48 volt maximum rated power (vs flooded cell deep cycle storage batteries), I would be concerned with exceeding the 1 kWatt of solar power per 100 AH @ 48 volt battery bank. AGM batteries (depending on brand/model) tend to go pretty high "resistance" when 100% charged. When you hit too much charging current to an already 100% full battery bank, it has been seen that the battery bus voltage can exceed 72 VDC (on a Xantrex/Schneider XW system).

    A lot of this depends on how the charging system is designed. Many MPPT controllers will dump 100% current into the battery bank during MPPT "sweeps" (for a few seconds +/-????) which can take the battery bus voltage way higher than normal (when battery bank is full).

    If your customer wants to exceed the 1kW per 100 AH @ 48 volt battery bank configuration, would talk with Outback's engineering department to ensure they are OK with that... I don't have any experience--Just what I understand from various issues/discussions on the forum.
    My questions...
    Is there an advantage in moving the inverter out to the array instead of near the service panel? GT Inverter is currently mounted on the outside of the home. And the Radian is inside.
    #6 conductor from the array to the inverter.

    Here are my questions/hot buttons... How you choose to address them is probably best made by you (and consulting with your supplier's engineers) as you are on site and have more information than I. In no particular order:
    • Weather and Temperature for Inverter/Electronics/Battery Bank: Obviously keeping things dry (rain/snow/leaks). And temperature--The rule of thumb is for every 10C (18F) increase in temperature (over ~25C ambient), is reduction in life by 1/2. And every 10C drop in temperature is a corresponding 2x increase in life. Installing Electronics in Cool/Dry location big help.
    • GT inverters--Generally, their input is "high voltage" DC of around 300-400 VDC--Wiring efficiency would slightly favor having the "long run" at 400 VDC vs 240 VAC. Also, with GT inverters on the AC output--You can "voltage rise" problems. Long wire runs (and minimum wire gauge) can cause "increased" voltage at the GT inverter's VAC output. If your AC mains run "hot" (250-260 VAC during parts of the year), the 3-12 volt "voltage rise" in the 240 VAC power run can push the voltage >~264 VAC +/- a few volts UL cutoff for the GT inverter. So--If you have high line voltage, placing the GT inverter closer to the main panel can be a better idea.
    • Sounds like a Grid Tied home/business, but needs serious backup power. GT inverters are certainly the choice for "cheap and reliable" power. However, obviously, GT inverters (in general) do not provide any backup emergency power (SMA does have a 120 VAC "day time only" backup output for their new GT inverters--But not the same as battery backed off grid AC inverter power). I would seriously look at a "balanced" array connected directly to the AGM battery bank with an Outback or Midnite (or other) MPPT charge controller. With a 534 AH @ 48 volt battery bank, that array would max out around (534 AH * 58 volts charging * 1/0.77 panel+controller derating * 0.13 charging rate =) ~5,229 Watt array. If you wanted to go as high as C/5 or 20% rate of charge, possibly as high as 8,045 Watt array (more "bleeding edge" in my humble opinion).
    • If there is lightning in the area--I would pay a lot of attention to grounding/surge suppression. The Midnite surge suppressors are (more than likely) a better choice vs the Delta. Put them at each end of the "outdoor" wire run (at the array, where it enters the battery/power shed, etc.). AC and DC sides. Grounding can be another long discussion (especially when adding genset, utility power, large AC off grid inverters).
    I have looked at Enphase for this system, but I can't see enough advantage to recommend to the homeowner, an additional $2k-3k over a string inverter. Am I missing something?

    I am not a huge fan of AC coupled solar array through a Hybrid/Off Grid type inverter... Charging is usually "controlled" by the OG Inverter varying frequency by >+/-0.5 Hz to "turn off" the GT inverters. This is a "bang bang" type controller--And while it can charge battery banks--It is not ideal for long term off grid operation. I would hang "enough array" (i.e., ~1kWatt array per 100 AH @ 48 volt battery bank) on a solar charge controller connected to the battery bank. And "forget" running a "huge" GT Array onto the protected sub-panel output of the Radian inverter.

    Again, less bleeding edge.
    How do I figure the Max AC input for Radian? Radian spec sheet says AC Input is 50amps at 240v (I should be just under 30 coming from the new GT inverter), but I read in HomePower that the Radian can handle 6kw of PV.

    I do not know anything specifically about the Radian--So this is just general starting point from my experiences.

    The spec. sheet says 50 AAC input maximum. The AC input to the Radian is the Grid+Generator input.--Presumably driving AC loads (with poor power factor) + AC internal battery charger. The minimum AC branch circuit would be rated at:
    • 50 amps * 1.25 NEC branch circuit derating = 62.5 Amps minimum (round up to next standard breaker/wire gauge).
    Of course, the Radian (I believe) has a programmable maximum AC input branch circuit rating--So you can program that down if needed.

    Then you have the protected sub-panel (backup power output) and the 6 kWatt array (presumably AC Coupled GT inverters) limit.

    The 6 kWatt array limit on the sub-panel output--May be the results of some other limits (you have the 8kWatt output of the inverter + the 6 kWatt of the solar array -- That is a lot of current that the output bus needs to manage). The 6kWatt of solar power should actually be a current limit (at 120 or 240 VAC, per leg, etc.)--not a wattage.

    A quick look through the Outback documentation did not clear this up for me.

    Again, I would take the size of the battery bank into account when connecting GT solar to the Radian output--And my choice would be an MPPT charge controller+solar array connected directly to the DC Battery bank (traditional configuration). The Radian is already a hybrid inverter and capable of GT operation with 90% efficiency (vs the 95%+ efficiency of a standard GT inverter).

    Is the intent to run the 17kWatt generator a lot during power fail/off grid operations? 534 AH @ 48 volt battery bank is a "bit small" for a full offgrid operation with 2 days of backup power and 50% maximum discharge on an 8kWatt AC inverter (I would be suggesting 800 AH minimum battery bank in general if there really is going to be near 8 kWatt of loads and ~800ah*48v*0.85eff*1/4=8,160WH per day average load).

    Have the off grid/emergency power loads been well defined yet?
    Is it better to mix 6 old modules and 4 new modules in each string OR go 10 new modules all in one string and 1 new, 9 old in the other 2 stings?

    Mixing old/new panels is not an issue (other than if/when any failures occure due to age). The bigger issue is matching Vmp (parallel connections) and Imp (series connections). If you are over 10% (5% or less is better) different between panel specifications (Imp or Vmp for parallel/series connections), then a separate MPPT/GT controller/Enphase type micro controller would be needed.

    Your thoughts?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Expanding Array in an AC Coupled System

    I believe that AC coupled systems designed from the ground up with compatible components may not suffer from the "bang-bang" control problem.

    As I understand it the SMA SunnyIsland/SunnyBoy AC coupled combination actually has a proportional response from the SunnyBoy GTI to frequency increase from the SunnyIsland rather than just a total shut down from loss of "grid" synch.
    Other GTI components may not have this capability.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Expanding Array in an AC Coupled System
    inetdog wrote: »
    I believe that AC coupled systems designed from the ground up with compatible components may not suffer from the "bang-bang" control problem.

    As I understand it the SMA SunnyIsland/SunnyBoy AC coupled combination actually has a proportional response from the SunnyBoy GTI to frequency increase from the SunnyIsland rather than just a total shut down from loss of "grid" synch.
    Other GTI components may not have this capability.

    That is correct. SMA's system combining the Sunny Island with Sunny Boy GTI's is very smoothly controlled.

    But for the mixtures of brands such as the OP is faced with ...
  • radiantradiant Registered Users Posts: 4
    Re: Expanding Array in an AC Coupled System
    Welcome to the forum.

    Thank you! It's been a great resource...I hope I can contribute.
    If you've already got the 5kW GTI AC coupled to the 8kW Radian you can not add the 7.5 kW Fronius as well. It is doubtful that the AHJ will allow it at all, because the backfeed rating of the service panel will need to including the output of all the inverters which can feed the grid. With 8 & 5 kW already I suspect that rating is already exceeded, unless they are going by the Radian's AC IN current rating limit which they should because there are no panels feeding that unit's batteries, correct? (AHJ's sometimes have trouble understanding this).

    I'm suggesting replacing the 5kw GTI, not adding to it. But your point is well taken on the service panel - you may be right. Correct, no panels feeding the batteries, just the grid.
    You've already got 4kW of PV back-feeding the Radian which has 534 Amp hours of battery. I would not increase that as it is just about the right balance for charging. The AC input rating of the Radian refers to AC IN, not to back-feeding it. In theory it could take up to 8kW AC coupled. I don't think Outback would advise doing this though; ask them. They are pretty good about answering questions.

    I tried Outback on Friday and was hoping to hear back before the long weekend. Will try again Tuesday.
    There is no advantage to moving the Radian. It should be as close to the batteries as possible, as that is the low Voltage/high current connection for it. The AC lines are @ 240 Volt so represent no significant V-drop potential.

    I should have been more clear. I was asking about moving the GTI to the array, not the Radian.
    In theory you could add PV to the 5kW GTI to bring it up to maximum potential, but the panels would have to match the existing ones.

    That may be what we do, but I need to figure out the service panel Radian specs first.
    Frankly I would not have done this system the way it is.

    You and me both. But that ship has sailed.
    Oh, and that's about $5,000 worth of advice you just got for free. :D

    Where do I send the check? :p I do really appreciate the thoughtful response. Thanks.
  • radiantradiant Registered Users Posts: 4
    Re: Expanding Array in an AC Coupled System
    BB. wrote: »
    I guess you are "in the business"? Be very careful of getting out of your comfort zone--With so many people touching the system before you--It may be difficult for you to make any changes/improvements without major changes to the existing system (bad choices, choices you would not have recommended, etc.).
    Yes - just. But this is a friend and I'm not needing to make money on it. I'm trying to help him out and get a better understanding of a complex system. If it's too far beyond me, I'm not afraid to say so.
    BB. wrote: »
    [*]Sounds like a Grid Tied home/business, but needs serious backup power.

    Exactly what it is. But got "sold" serious backup power. Not sure how "needed" it was...
    BB. wrote: »
    GT inverters are certainly the choice for "cheap and reliable" power. However, obviously, GT inverters (in general) do not provide any backup emergency power (SMA does have a 120 VAC "day time only" backup output for their new GT inverters--But not the same as battery backed off grid AC inverter power). I would seriously look at a "balanced" array connected directly to the AGM battery bank with an Outback or Midnite (or other) MPPT charge controller. With a 534 AH @ 48 volt battery bank, that array would max out around (534 AH * 58 volts charging * 1/0.77 panel+controller derating * 0.13 charging rate =) ~5,229 Watt array. If you wanted to go as high as C/5 or 20% rate of charge, possibly as high as 8,045 Watt array (more "bleeding edge" in my humble opinion).

    This was my first approach, but the conductors from the array is 200' from the house. The amount of copper needed got me looking in a different direction.
    BB. wrote: »
    Is the intent to run the 17kWatt generator a lot during power fail/off grid operations? 534 AH @ 48 volt battery bank is a "bit small" for a full offgrid operation with 2 days of backup power and 50% maximum discharge on an 8kWatt AC inverter (I would be suggesting 800 AH minimum battery bank in general if there really is going to be near 8 kWatt of loads and ~800ah*48v*0.85eff*1/4=8,160WH per day average load).

    Customer would really like to minimize the use of the generator when the grid is down. He's expecting 2 days on the batteries and is fine with that. I would like to see the BB a bit larger as well.
    BB. wrote: »
    Have the off grid/emergency power loads been well defined yet?

    Of course not. And I can't really get him to really define them either. The company that installed the generator moved almost all circuits to a backed-up loads panel except the AC and DW. The homeowner "understands" (we'll see) that they can't use many loads when on batteries.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Expanding Array in an AC Coupled System
    radiant wrote: »
    I'm suggesting replacing the 5kw GTI, not adding to it. But your point is well taken on the service panel - you may be right. Correct, no panels feeding the batteries, just the grid.

    Okay different circumstances but similar results both for back-feeding and for the Radian's batteries being able to handle the charging. Going up from roughly 4kW of PV to near the maximum would require a larger bank to smooth things out.
    I should have been more clear. I was asking about moving the GTI to the array, not the Radian.

    On most central inverters the DC side is higher Voltage than the AC side, so the long wire runs are preferred there. Off the top of my head I can't think of any advantage to moving it closer to the array.
  • radiantradiant Registered Users Posts: 4
    Re: Expanding Array in an AC Coupled System

    I came across this document from Outback, that gives a good explanation on why the Radian's max PV input is only 6kw. From page nine...

    Guideline Number Two: The OutBack inverter power rating should be 1.25% of the GT inverter power rating. This
    guideline ensures that the GT inverter does not overwhelm the charging circuitry in the OutBack inverter if the
    load demand goes to zero and all available GT inverter power is flowing to the OutBack inverter. While
    admittedly an unlikely scenario, for safety and equipment protection it’s best to follow this guideline. For
    example, the 8 kW rating of the Radian inverter would dictate a GT inverter no bigger than 6 kW.

    Perhaps this will help others who want to better understand the Radian.
  • BB.BB. Super Moderators, Administrators Posts: 28,163 admin
    Re: Expanding Array in an AC Coupled System

    Must be a typo in the Manual... Not 1.25% but a factor of 1.25 ....

    8 kW / 1.25 = 6.4 kW array

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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