What determines the max rating of an AC coupled off grid system?

SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
I'm working on the expansion of an AC coupled system.

The loads have increased over the original design. They were supposed to be just under 3000 watts 24/7. The regular loads have crept up and now include some cycling of 3840 Watts of airconditioning . Now the plans call for closer to 4500 24/7 + the AC and the system has gotten way out of balance. Some generator run time is inevitable but at the cost of propane and maintenance at a remote site which is snowed in for more than 6 months out of the year there are some cost offset considerations at work here.

It has 4 Sunny Island 5048u battery based inverters in a split phase system. It currently has two Sunny Boy 5000s and one 6000 feeding PV power into the minigrid. So, 20kW of battery inverter and 16kW of PV inverter.

The existing PV arrays face SE for quick AM recovery. The new PV would face SW to provide afternoon load support and battery finish/float charge current.

The main AC load panel (where the SBs tie in) has a 200amp rating.

Is there any reason you can think of why it would not work to add another 10kW of PV inverters? There would be many summer hours when the PV is throttled down but many more in marginal weather where any additional PV would help.


-Alex Aragon

Comments

  • BB.BB. Posts: 28,090Super Moderators, Administrators admin
    Re: What determines the max rating of an AC coupled off grid system?

    Just the usual question of battery bank AH @ 48 volt capacity...

    The old 100 AH @ 48 volts per 1kW of load/solar array as a starting point (it is possible that the Sunny Boy control inverters/voltage transients better).

    The other question--Why not just go with a MPPT DC Solar Charge controller back to the battery bank instead of the GT to Off Grid inverter (it sounds like you do not really need another Off Grid inverter--If I understand the setup correctly--But if these are Sunny Island, then perhaps you can keep the current off grid inverter and the frequency shifting will keep the GT inverter total output below the OG inverter's capacity).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Posts: 7,971Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    ......

    Is there any reason you can think of why it would not work to add another 10kW of PV inverters? There would be many summer hours when the PV is throttled down but many more in marginal weather where any additional PV would help.......

    More Power is Better
    - Tim Taylor (Tool Time)
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    i had to put my dunce cap on and think about this a bit. i'd say the buses in the main panel would handle another 10kw, but you are looking at more parasitic power (26kw) being fed to the main power which is at 20kw. i admit that i don't know where the line can be drawn, but i would assume you should not have more being piggybacked than is in the base of the system, but even if you could the extra power is being limited to sunny times. others more familiar could chime in on this, but in my opinion more of the battery based power should be added even though it is more inefficient and then add a tad to the mini gt power if you wish. in other words the same ratio of battery based power to gt power that is working for you now is a go imo, but to add it all to the mini gt aspect i don't know if it would work and, even if it did, is it advisable to do it all that way?
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    More info:

    The batteries are 2, 48v stacks of 2100ah VRLA. 201600 rated Wh. The SIs (Sunny Islands) act as regulated chargers to the batteries by using frequency shift modulation to throttle down the incomming PV power. I need to check with SMA but it is my understanding that the SIs will throttle down the PV SBs (Sunny Boys) based on battery current/charge stage, not based on the power on the AC load bus. I'm wondering if they also regulate current to the AC bus based on AC voltage. There are communication wires between the SIs and the PV SBs so it is possible that the system could be tracking and controlling current based on other algorhythms but that does not make logical sense to me. (Not that everything that works makes sense to me):p

    The system was designed 5 years ago, installed 4 years ago, so the option of high voltage CCs was not available yet. The amount of current on the DC bus if this was a DC coupled system is a little daunting too. The higher voltage of the AC bus makes it a preferrable place for the coupling. One of the greatest advantages of the AC coupled system is the high efficiency of the power from PV to the AC loads while the sun is shining. Because of the air conditioner, the greatest power usage will take place, theoretically, while the sun is shining.

    Originally there were to be 6 SIs connected as one system but for some reason UL would not allow it at the time. SMA has since come out with their multicluster box but it is only available for 3Ø. Changing over is not practical at this point. The original loads required a surge + continuous rating that is higher than 4 SIs can deliver so loads were split between a larger 4 SI system and a smaller 2 SI system but that issue has been mitigated and now surge + continuous will not exceede the 4 SIs. The main problem with the 2 seperate systems was that the batteries would not operate at equal SOCs so the generator was not being run as efficiently as would be possible with only one system. It looks like we'll probably be keeping SI 5/ 6 as spares.

    Thanks,

    -Alex
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    If this ac coupled system was not an SMA with frequency shift modulation this would be much trickier.

    -Alex
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    alex,
    maybe you should run it past sma to be sure.
  • mike95490mike95490 Posts: 7,971Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    Maybe the batteries are the limiting component. If at some point all the loads shut off, and the batteries are low, amps are going to dump into them, and you have to regulate battery charge current, to keep the plates healthy.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    mike90045 wrote: »
    Maybe the batteries are the limiting component. If at some point all the loads shut off, and the batteries are low, amps are going to dump into them, and you have to regulate battery charge current, to keep the plates healthy.

    That's in the programming of the Sunny Islands. I wonder how responsive they are. The surge load has an inrush of about 8.6 kw then settles to a running current of about 3.8kW. (in addition to the 24/7, 4500W) The surge would come from the batteries. Would the PV SBs "try" to ramp up and be putting out an extra 4.8kW after the peak of the surge? If the SIs modulate the frequency at a pre-programmed ratio to how much additional current was needed they might "call for" too much current too quickly. On the other hand, if they have a "sweep" system similar to an MPPT algorithm they may not be able to respond "too fast". I'm definitely going to be on the phone to SMA in the AM.

    (Too many "quotes":blush: )

    It would be really tough without frequency shift modulation. In fact, I would not consider doing this with a system of this size without it. The only ways to regulate charging would be to have a tiered system of turning off the GT inverters and a diversion load capable of diverting the "last" GT inverters full capacity. -Nope, forgitaboutit!

    This is part of my reason for starting the thread: "A better diversion load for an AC coupled battery system?"
    http://forum.solar-electric.com/showthread.php?14612-A-better-diversion-for-an-AC-coupled-battery-system


    -Alex
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    Woo-hoo!

    According to Cody at SMA, for off-grid AC coupled Sunny Island systems they can handle a 2:1 power ratio.

    2 x PV SBs : 1 SI

    So I could connect 40kW of Sunny Boy PV GT inverters to the 20kW of Sunny Island battery inverters.


    -Alex Aragon
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    good to know.:D
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    Here are the one lines:

    Attachment not found.
    Attachment not found.

    -Alex
  • RCinFLARCinFLA Posts: 1,280Solar Expert ✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    AC coupling does not expose batteries to ripple current like DC charge controller to batteries.

    Only potential issue is when grid goes down, the solar generated power has to go somewhere, be tapered back, or shut down. For Sunny Island with Sunny boy there is feedback through slewing frequency to cut back solar if AC loads or battery charging doesn't consume the peak generated power.

    There will be some maximum current limit that can be pumped through the inverter AC relay circuit. Usually the limit is the same as the maximum pass through current from grid to ACout.
  • TheBackRoadsTheBackRoads Posts: 274Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    Able to tell us what the site is? I'm guessing remote telco/ cell tower?
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    RCinFLA wrote: »
    AC coupling does not expose batteries to ripple current like DC charge controller to batteries.

    SMA still requests/recommends 100ah of battery for every kW of PV. :confused:

    -Alex
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    Able to tell us what the site is? I'm guessing remote telco/ cell tower?

    Yep. Is this what you do?
  • stephendvstephendv Posts: 1,571Solar Expert ✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    I need to check with SMA but it is my understanding that the SIs will throttle down the PV SBs (Sunny Boys) based on battery current/charge stage, not based on the power on the AC load bus.

    The SI throttle the SB's based only on the battery voltage. As soon as the batts reach absorb voltage, they'll start throttling the SB's down to ensure that the batts stay at absorb voltage.
    I'm wondering if they also regulate current to the AC bus based on AC voltage. There are communication wires between the SIs and the PV SBs so it is possible that the system could be tracking and controlling current based on other algorhythms but that does not make logical sense to me. (Not that everything that works makes sense to me):p

    Nah, the comms lines are there just to reprogram the SB's if they were feeding the grid. I.e. the SB could configured in UL compliant grid feeding mode, then if the grid goes down, the SI reprograms them on the fly to be in off-grid mode where they can throttle back power based on frequency. The actual throttling back of power is only done through the frequency of the AC line.
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    stephendv wrote: »
    The SI throttle the SB's based only on the battery voltage.
    The actual throttling back of power is only done through the frequency of the AC line.

    There are some other factors at work too. I'm really getting a new appreciation for the Sunny Islands.

    Something new I learned from tec support this morning is that an off-grid Sunny Island system cannot regulate the incomming PV while the generator is running. I had suspected that the PV SBs were disabled while the generator was running but that is not the case. The generator power is fed thru the auto transfer switch and dictates the load frequency. If the PV production is greater than the load current the SIs detect a backfeed current and will disconnect from the generator. If it was auto started, the generator then goes into cool-down mode, shuts down, then remains in lock-out mode for an hour before it can restart.

    It seems to me that this could create some issues with intermittent loads or partially cloudy days. In the "perfect storm" the SOC could decrease below the Auto Gen Start setpoint.

    -Alex Aragon
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    Something new I learned from tec support this morning is that an off-grid Sunny Island system cannot regulate the incomming PV while the generator is running. I had suspected that the PV SBs were disabled while the generator was running but that is not the case. The generator power is fed thru the auto transfer switch and dictates the load frequency. If the PV production is greater than the load current the SIs detect a backfeed current and will disconnect from the generator. If it was auto started, the generator then goes into cool-down mode, shuts down, then remains in lock-out mode for an hour before it can restart.

    It seems to me that this could create some issues with intermittent loads or partially cloudy days. In the "perfect storm" the SOC could decrease below the Auto Gen Start setpoint.

    I am confused about this... A few months ago, in another thread, I ranted about my Outback system. My rant was about the lack of integration between the inverter/charger (with generator running) and the charge controller during bulk charging in an off-grid system.

    In particular, I do not want more than 40 amps DC going into my batteries. My inverter/charger is configured to limit the charging current to about 40 amps. Sometimes on a cloudy day I run the generator to use my power tools. If the sun comes out, all of a sudden I have 65 amps going into my batteries (25 additional amps from the charge controller).

    In that other thread Stephendv chimed in to point out that the Sunny Island system was able to regulate bulk charging from multiple charging sources. Alex, are you saying (in the off-grid situation I describe) that if the sun comes out the SI will disconnect from the generator (rather than regulate one or both charging sources)?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    vtmaps wrote: »
    Alex, are you saying (in the off-grid situation I describe) that if the sun comes out the SI will disconnect from the generator (rather than regulate one or both charging sources)?

    The answer is yes if the PV current exceeds the load current while the batteries are in absorb or float. That's a good question about the maximum charging current. I don't know if the SIs will be able to limit the "Bulk" battery current from both the generator and the PV if the max charge current is set below the SI's maximum DC output. It seems like this would be a necessary design criteria otherwise small batteries in grid-tied systems would constantly be getting fried. In "my" system it is not an issue because the max rated output of the SIs (20kW) is ≤ 10% of the battery rating. SMA does require/request a minimum of 100ah battery to 1kW of PV. Perhaps it's for this issue instead of, or in addition to AC ripple?
    :confused:

    If the system is entirely AC coupled (nothing other than the SIs connected to the DC bus) the only DC charging source is the Sunny Islands. If there is more current comming in from the PV than the loads are using, the SIs will "try" to regulate the available current on the AC bus (AC load panel, which is also the connection point of the PV Sunny Boy inverters) by shifting the frequency. If the generator is running and connected to the AC inputs (not the AC load panel) of the Sunny Islands, the SIs will "pass thru" the generator power to the AC bus (AC load panel). Since the SI cannot shift the grenerators frequency, they need to disconnect from it in order to reduce the incomming PV power or else the PV SBs will back-feed power into the generator.

    The thing to understand is that in AC coupled systems, although the battery inverters can regulate the input of voltage/current sources on their AC input connections, they cannot regulate the backfeed from current sources on their AC load connections. If they are the system's voltage source they can throttle down the non-battery PV inverters output by shifting the AC frequency (if programming allows). If the voltage source is supplied by the grid or a generator the battery inverters cannot shift the AC frequency so the non-battey PV inverters will put out all available pv power. Because of this, for grid-tied systems, the maximum allowable connected PV power is limited to 80% of the battery inverters internal transfer switch rating. For off grid systems the PV power should never backfeed through the battery inverters internal transfer switches so the rating of the connected PV is not limited by the transfer switch rating.

    The Sunny Islands have an off-grid maximum ratio of non-battery inverter to battery inverter of 2:1. An AC coupled system which cannot throttle down the PV is limited to a PV rating ≤ the connected diversion loads. Theoretically, a system could be designed with multiple non-battery inverters which could simply disconnect most of PV inverters when the battery voltage reaches the Bulk set point and then use an appropriately sized diversion load to regulate the battery charging from the remaining connected PV inverter(s).

    Clear as mud?

    -Alex Aragon
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    Clear as mud?

    Unfortunately, yes. The grid-tie stuff confuses me since my concern was off-grid regulation of battery charging current during bulk stage of charging. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SolaRevolutionSolaRevolution Posts: 407Solar Expert ✭✭✭✭✭✭
    Re: What determines the max rating of an AC coupled off grid system?
    vtmaps wrote: »
    Unfortunately, yes. The grid-tie stuff confuses me since my concern was off-grid regulation of battery charging current during bulk stage of charging. --vtMaps

    Sorry,

    I was not trying to stray from the thread. I was thinking about the over all logic of AC coupled systems and specifically the design of the Sunny Islands. Since they are designed for off-grid and for gird-tie use it would follow that they have been designed in consideration of both types of systems. There may be design factors required for one situation which would not be part of their design if they did not also need to be able to function in the other.

    Also, when you are AC coupling, you are creating a mini-grid. Many aspects of an AC coupled off-grid system function as they would in a grid-tied application.

    For example:
    - if your non-battery PV inverters are not SMAs programmed for off-grid mode they will be subject to the UL 1741 required 5 minute wait before they reconnect after the "charge regulation circuitry" has called for them to reconnect.
    - ac load centers have maximum current feed ratings which must be taken into consideration the same as for grid-tie.
    - energy conversion efficiency to loads while the sun is shining.

    -Alex Aragon
  • stephendvstephendv Posts: 1,571Solar Expert ✭✭
    Re: What determines the max rating of an AC coupled off grid system?

    vtmaps,

    I think Alex explained it well. I don't recall what I said about the SI's in the previous discussion you mentioned, but the general idea that the SI will take care of the maximum charging current is right- not so much because it synchronises other charging sources, but because in an AC system it IS the only charger.

    In the case of the gen being on and it's frequency being less than 60Hz while under load and the PV producing full power through the SB then all power not used by loads goes to the battery through the SI's charger. So the SI would be fully aware of the charging current. If this exceeds the maximum programmed amount, then I'd imagine that it would disconnect and shutdown the generator because it's the only way it can regulate the power (Not 100% sure of this, but I'd be surprised if they didn't do it).

    Another complication would be when the gen runs at a naturally high frequency when under load, say for example 62Hz. This would mean that while the gen is on, the SB will throttle back their power so you wouldn't have the benefit of PV + gen during bulk. The solution would be to program the frequency setpoints in the SI and SB so that it starts throttling at a higher frequency... or maybe easier: adjust the governor on the gen to operate at or less than 60Hz when under load.
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