Sunny Island connection to three phase grid

kargozar
kargozar Registered Users Posts: 7
Hey Guys,

I want to connect a single Sunny Island inverter (120v) between two phases of a three-phase grid (208v). Does anybody know if I can get a 208/120 transformer and put it in between the inverter and the gird? How does this affect the inverter performance. I appreciate if anyone can give me some hints about the feasibility of this solution.

Thanks in advance.
«1

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sunny Island connection to three phase grid

    If this is "Wye" wound power (120/208 sounds like it is)--Then you don't need any transformers. You simply connect the 120 VAC GT inverter to One phase and the Neutral to the center of the Wye connection (center of the "Y").

    Do you want to tell us a little more about what you are doing? Sunny Island connected to a commercial 3 phase utility or a 3 phase genset or what?

    I am not sure that you need to go this way--Sounds kind of expensive for a "simple" grid tied AC inverter system. Assuming this is a US installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kargozar
    kargozar Registered Users Posts: 7
    Re: Sunny Island connection to three phase grid
    BB. wrote: »
    If this is "Wye" wound power (120/208 sounds like it is)--Then you don't need any transformers. You simply connect the 120 VAC GT inverter to One phase and the Neutral to the center of the Wye connection (center of the "Y").

    Do you want to tell us a little more about what you are doing? Sunny Island connected to a commercial 3 phase utility or a 3 phase genset or what?

    I am not sure that you need to go this way--Sounds kind of expensive for a "simple" grid tied AC inverter system. Assuming this is a US installation.

    -Bill

    Hi Bill,

    Thanks for the reply. Yes this is a US installation. We have a three-phase breaker panel (commercial utility connection) where Our PV inverter and some loads are connected between two phases in the panel. So we want to connect Sunny island to the same phases (not between 120v and neutral). The sunny island is intended to only control battery banks for now. Cost is not a big factor but I need to make sure putting a 208/120 transformer between SI and breaker panel will work fine.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid

    Details, details.

    What Bill said about the 3-phase 'Y' windings: one phase would be 120 VAC so no transformer is needed.

    Now, what's going to happen when the grid goes down and the Sunny Island is asked to energize that single phase with 3-phase loads connected to all phases? Unless you can positively isolate that single phase during an outage you could have problems.

    The standard when dealing with 3-phase is to deal with all phases equally, meaning you'd have one SI per phase. It sounds as though you are using two of the phases as a standard 240 VAC split phase arrangement? With the third winding left out or used only for those loads that need 3-phase?

    Details, details.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    Details, details.

    What Bill said about the 3-phase 'Y' windings: one phase would be 120 VAC so no transformer is needed.

    Now, what's going to happen when the grid goes down and the Sunny Island is asked to energize that single phase with 3-phase loads connected to all phases? Unless you can positively isolate that single phase during an outage you could have problems.

    The standard when dealing with 3-phase is to deal with all phases equally, meaning you'd have one SI per phase. It sounds as though you are using two of the phases as a standard 240 VAC split phase arrangement? With the third winding left out or used only for those loads that need 3-phase?

    Details, details.

    He said that he wanted to connect it phase to phase, which would be 208V, not 120V. I see no reason why a transformer wouldn't work. I don't see why he would want to connect it that way, though, when he could just connect it natively phase to neutral.

    Two legs of 208V three phase won't be 240V split phase, either; it's 208V phase to phase. The phases are 120V referenced to neutral, but they are 120 degrees apart, not 180 degrees, so you can't get 240V from it.

    I agree that the "correct" way to do this is with three SI's each on a phase to neutral connection.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid
    ggunn wrote: »
    He said that he wanted to connect it phase to phase, which would be 208V, not 120V. I see no reason why a transformer wouldn't work. I don't see why he would want to connect it that way, though, when he could just connect it natively phase to neutral.

    Two legs of 208V three phase won't be 240V split phase, either; it's 208V phase to phase. The phases are 120V referenced to neutral, but they are 120 degrees apart, not 180 degrees, so you can't get 240V from it.

    I agree that the "correct" way to do this is with three SI's each on a phase to neutral connection.

    Yes; just trying to understand what it is he's got and what it is he's trying to do with it. :p
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sunny Island connection to three phase grid

    There are other ways too... There are "native" 208 and 240 VAC small AC inverters (even down to the Enphase type micro inverter with one inverter per panel).

    Depending on where you you will be installing the system, the utility/PUC requirements, size of the system vs size of the utility service, etc... You can hit various issues...

    Many utilities have limits on how much power you can back feed unbalanced per service (perhaps around 3-10 kWH maximum difference per leg).

    Some Wye systems may only have two transformers on the pole--Meaning that the third phase is a "phantom" transformer, and you have limits about on which 208 leg you can back feed on.

    And, you may have a limit that requires all multi-phase GT Solar AC inverters are shut down if there is a single (Or multiple) phase failure.

    Lastly, check the billing plan for your proposed installation. In California, "net metering" plans for business can offer very poor return on investment (you may only get ~($0.05 per kWH generation credit--I.e., 1/2 the bill is fixed service charges and 1/2 the bill is actual $/kWH charges).

    And why Sunny Island... SMA makes very good inverters and systems. But unless you are planning on putting a battery bank in this system somewhere, not sure that a Sunny Island component is really even needed (there are GT Sunny inverters that can operate in both "standard" GT mode and Sunny Island Mode--depending on installation configuration). But unless you are doing some sort of off grid backup AC power--Sunny Island may not make any sense.

    -Bill "certainly no expert in SMA product line" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    BB. wrote: »

    And why Sunny Island... SMA makes very good inverters and systems. But unless you are planning on putting a battery bank in this system somewhere, not sure that a Sunny Island component is really even needed (there are GT Sunny inverters that can operate in both "standard" GT mode and Sunny Island Mode--depending on installation configuration). But unless you are doing some sort of off grid backup AC power--Sunny Island may not make any sense.

    -Bill "certainly no expert in SMA product line" B.

    SMA has inverters that will power a single 120V outlet when the grid is down as long as the sun is shining, but that's not "Sunny Island mode"; it's still a high voltage inverter where a SI is a low voltage inverter. If you have an application that calls for a Sunny Island, no other SMA inverter can substitute for it. From the OP I assumed he knew what a Sunny Island is and why he wants it, and yes, the SI is for off grid and grid backup. The SI is a battery inverter only; there is no PV input.
  • kargozar
    kargozar Registered Users Posts: 7
    Re: Sunny Island connection to three phase grid

    Thanks for all the replies guys. I'll try to explain my application in more details this time.

    1- The SI will be installed in a research facility to study energy management system so utility bill or the installation cost is not a big factor.

    2- I was looking for a bidirectional inverter/charger which can charge the batteries from the utility and then later discharges that energy back to the grid WITH remote control capabilities. I compared Xantrex, Radian, and SMA sunny island and found out sunny island is our best bet.

    3- The system we have is three-phase 208 volt (no split phase) . In the main breaker panel a solar inverter is connected between L1 and L2. We also have some loads connected between L1 and L2. the system is always grid-tied (no off-grid applications). So although this is a 3 phase system only two phases are connected to pv inverter and loads. Now we want to put a battery inverter/charger between L1 and L2 .I know that I can buy Two SI each for one phase. But we decided to go with one SI and a transformer to boost its 120v output to 208v if that works fine.

    So my main concern which I'm seeking your expert opinions about is that if connecting a sunny island inverter to a transformer with 120/208 ratio and then connecting the tranformer to L1 and L2 of a three phase grid-tied system could disturb the operation of sunny island in terms of charging and discharging the batteries.

    I understand this is probably not the most practical application but your comments are highly appreciated.

    Many thanks
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid

    Exceltech will build custom output inverters, but their design is not grid-interactive. I wonder if SMA might do this? It seems a shame to run an SI with its output transformer into another transformer to get the 208 Volt output. Not efficient and rather redundant. They do build 230 VAC 50 Hz units for overseas, so ... would they be willing to adapt?

    Or is the purpose of this whole project to see if the existing technology can be integrated as-is?

    I don't like piling up transformers, but as far as the AC coupling in this manner is concerned I don't see any major difficulty with it. The SI should have enough Voltage latitude to accept the slight variations that may result.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sunny Island connection to three phase grid
    kargozar wrote: »
    2- I was looking for a bidirectional inverter/charger which can charge the batteries from the utility and then later discharges that energy back to the grid WITH remote control capabilities. I compared Xantrex, Radian, and SMA sunny island and found out sunny island is our best bet.

    If you want to sync the Sunny Island to the grid and "Back Feed" energy into the utility--I am not sure if it capable of "Hybrid" GT operation. If you have confirmed it is--then all is well (I don't know much about the SMA products).
    3- The system we have is three-phase 208 volt (no split phase) . In the main breaker panel a solar inverter is connected between L1 and L2. We also have some loads connected between L1 and L2. the system is always grid-tied (no off-grid applications). So although this is a 3 phase system only two phases are connected to pv inverter and loads. Now we want to put a battery inverter/charger between L1 and L2 .I know that I can buy Two SI each for one phase. But we decided to go with one SI and a transformer to boost its 120v output to 208v if that works fine.

    In general, why would you not have the common 3 phase Wye neutral (you really don't have access to the grounded neutral of the 3 phase utility power??)? You might have some issues with the Sunny Island if it was designed for a grounded neutral (common US 120 VAC configuration)... Don't know that you can float. But if you can run the Sunny Island transformer to boost to 208--Then great.
    So my main concern which I'm seeking your expert opinions about is that if connecting a sunny island inverter to a transformer with 120/208 ratio and then connecting the transformer to L1 and L2 of a three phase grid-tied system could disturb the operation of sunny island in terms of charging and discharging the batteries.

    If the 120:208 transformer is fully isolating--Then you can ground reference the neutral on the 120 VAC side--And the Sunny Island would not know the difference.

    But--Why not just bring the center tap of the Y -- That should be available from any 3 phase 120/208 Wye building mains connection... That is why (as I understand) 120/208 3 phase power is so common for office parks (at least in Northern California)--Lots of 120 VAC connections and "natural" balancing across all 3 phases back to the utility.

    3 Phase 120:240 VAC Delta systems can only ground reference (at a transformer center tap) the Neutral on one of the phases... The other two phases can only supply 240 VAC power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    kargozar wrote: »

    2- I was looking for a bidirectional inverter/charger which can charge the batteries from the utility and then later discharges that energy back to the grid WITH remote control capabilities. I compared Xantrex, Radian, and SMA sunny island and found out sunny island is our best bet.

    I do not think that the Sunny Island will do that. As long as the grid is up and connected to the AC2 of the Sunny Island, the inverter section is off. I am pretty sure that the only way it can feed power to the grid is if you have a PV inverter connected to AC1 and its output exceeds your loads, and that power comes from the PV, not the batteries. You should set your PV inverter to 240V instead of 208V and feed it through an autotransformer to AC1 of the Sunny Island at 120V. That is the way it was designed to interconnect.

    And I still do not understand why you would want to connect it phase to phase when it will natively connect phase to neutral. The net power to/from the grid is calculated on all three phases irrespective of what you have connected to each phase(s). I am pretty sure that it will interconnect through the transformer, but when you use equipment in configurations for which it was not designed, you must prepare yourself for unexpected results and a lack of support from the equipment manufacturer.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid

    If I understand the OP correctly, he's already got GTI across the two phases and wants to add battery back-up ability. Now if the GTI is also SMA then it can communicate directly with an SI and all should be good despite the transformer pile up. Otherwise the SI can operate with non-SMA units but relies on the AC coupling to determine power needs. An SI will not function as a GTI in the hybrid sense like a Xantreex XW would; it is only meant for back-up power.
    when you use equipment in configurations for which it was not designed, you must prepare yourself for unexpected results and a lack of support from the equipment manufacturer.

    That should be carved in stone somewhere.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sunny Island connection to three phase grid

    I am not sure how the Sunny Island works... It does appear that it can live with AC mains driven by a genset (one of the sunny island examples is Egg Island in their brochure). But it does not appear that it can operate in Battery Powered GT mode and drive energy back to the utility (certainly would not want it to back feed a generator).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    If I understand the OP correctly, he's already got GTI across the two phases and wants to add battery back-up ability.
    If that's what he is doing then he is going about it all wrong. The SI does not go in parallel with the existing PV inverter, it goes in series with it with the grid on one side and the PV inverter on the other.

    The way to do this is to change the existing PV inverter from 208V to 240V (easy if it is SMA) and run it through an autotransformer to AC1 of the SI, and then connect AC2 of the SI phase to neutral on one phase of the 208V service. Whatever he wants to back up with battery power from the SI goes into a protected loads panel between the PV inverter and the SI. He cannot feed the grid solely from the SI AFAIK. He may meet with disaster if he tries to make that happen; the Sunny Island is an AC bus master, not a slave like a Sunny Boy.

    Either this guy knows more about the Sunny Island than I do (and I have installed and programmed them) or he is clueless about it; I can't tell which. :D
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Sunny Island connection to three phase grid

    The SI can feed into the grid from the batteries, the same way that the XW does, it's just slightly unusual for the SI to do so, because SMA recommends their AC coupling concept where you'd have the sunny boys feeding the grid.

    Attachment not found.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    stephendv wrote: »
    The SI can feed into the grid from the batteries, the same way that the XW does, it's just slightly unusual for the SI to do so, because SMA recommends their AC coupling concept where you'd have the sunny boys feeding the grid.

    Attachment not found.
    When it says,"This ensures that the Sunny Island can feed into the power distribution grid with a full battery and at full solar irradiation" it's talking about passing power through the Sunny Island from a Sunny Boy (or other PV inverter) on AC1, not from the batteries. Solar irradiation has nothing to do with a Sunny Island.

    Why would you ever want to discharge your batteries into the grid anyway?
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Sunny Island connection to three phase grid

    Then it goes on to explain how you can feed into the distribution grid from the batteries, read the section titled "Feeding into the grid on the DC side".

    I've seen this happen accidentally with my generator when I had the settings wrong, the gen would shutoff, but the SI would back-feed it, and it kept spinning because the midnite classic has the battery voltage above the charging settings of the SI.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid

    I suppose you could have an SI with an array charging its batteries and have it feed surplus to the grid the same as an XW or Radian does, but it is not a typical SMA set-up for sure.

    In this case I think it would be more problematic due to the nature of the 3-phase attached to the AC side. As it is I don't know how the OP is getting away with GT on only two phases (or is it one) which would unbalance the legs if it was of any significant amount.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sunny Island connection to three phase grid

    My confusion is the apparent lack of a the 120/208 Neutral connection for "kargozar's" building/local connection... And then trying to get a 120 VAC Sunny Island into a pure two wire 208 environment.

    I don't know enough to say if the SI can back feed the utility from battery+solar battery bank or not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    stephendv wrote: »
    Then it goes on to explain how you can feed into the distribution grid from the batteries, read the section titled "Feeding into the grid on the DC side".

    I've seen this happen accidentally with my generator when I had the settings wrong, the gen would shutoff, but the SI would back-feed it, and it kept spinning because the midnite classic has the battery voltage above the charging settings of the SI.

    But you don't just have batteries and an SI, you have a CC feeding the batteries from PV, which is sort of like having a SB feeding the AC side - DC instead of AC coupled - so that excess power from your PV is exported to the grid when the batteries cannot take any more. That's cool and I didn't know the SI would do that, but the OP didn't say anything about any other PV other than the inverter(s) he has on two phases of the 208V service.

    And I still don't understand why it's so important to him to connect phase to phase rather than phase to neutral.
  • kargozar
    kargozar Registered Users Posts: 7
    Re: Sunny Island connection to three phase grid

    I have already discussed with SMA engineers and they have confirmed that we can discharge all energy from the battery into the grid. It doesn't need to be the surplus energy from PV or other DC sources. You can just charge the battery from grid (for example at night) and then during the day discharge that energy back into the grid. Outback Radian has the same capability but It can't discharge the battery below 48v into the grid. That's why we opted out Radian for now because we want to discharge the battery well below 48v into the grid.

    As I mentioned this is a research project for soem system-level studies and that's why we are interested to discharge the stored energy into the grid. Also there are no PV charger or critical loads or Generator directly connected to Sunny Island. Only grid on the AC side and battery on the DC side.
  • kargozar
    kargozar Registered Users Posts: 7
    Re: Sunny Island connection to three phase grid
    ggunn wrote: »
    But you don't just have batteries and an SI, you have a CC feeding the batteries from PV, which is sort of like having a SB feeding the AC side - DC instead of AC coupled - so that excess power from your PV is exported to the grid when the batteries cannot take any more. That's cool and I didn't know the SI would do that, but the OP didn't say anything about any other PV other than the inverter(s) he has on two phases of the 208V service.

    And I still don't understand why it's so important to him to connect phase to phase rather than phase to neutral.

    I believe you can export power to the grid from batteries using sunny island without the need to have a PV charger. You ca define the active and reactive power fed into the grid as well and the battery can be discharged deeply.

    The case you mentioned about exporting the excess pv which battery can't take (because it's already fully charged) it's not definitely what I want to do with sunny island because there is no PV power coming to the sunny island anyway.
  • kargozar
    kargozar Registered Users Posts: 7
    Re: Sunny Island connection to three phase grid
    ggunn wrote: »
    And I still don't understand why it's so important to him to connect phase to phase rather than phase to neutral.

    It's because I already have some loads and generations which are connected between L1 and L2. So I want sunny island connected between L1 and L2 as well so that all the power flow occurs on these two phases. It's not that I don't have access to neutral on the three phase system but it's because of the existing connection of the other devices in our system that I want to have a phase to phase connection for sunny island. The only issue is 120/208 conversion and the grounding issue of neutral on sunny island that i'm not sure about.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    kargozar wrote: »
    It's because I already have some loads and generations which are connected between L1 and L2. So I want sunny island connected between L1 and L2 as well so that all the power flow occurs on these two phases.
    Why do you care about that? Utility metering doesn't care which phases source or sink more (or less) power.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid

    The purpose must be to provide battery back-up power for those loads across L1-L2. I can't think of another reason for doing it.

    BTW as I mentioned before connecting it for back-up in this manner can be trouble because of the 3-phase wiring providing a path from the SI's output to loads on other poles. Normally back-up supply is entirely isolated from what you do not want fed, and the utility power is connected via the AC IN of the inverter, not everything on the AC OUT as this would be. I don't see this working properly.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    The purpose must be to provide battery back-up power for those loads across L1-L2. I can't think of another reason for doing it.
    I don't think so. He has explicitly said that he wants to discharge his batteries into the grid. He can do that just as easily phase to neutral as phase to phase and the metering won't know the difference. The fact that he already has a PV inverter phase to phase doesn't have anything to do with anything; they are independent systems.

    As an aside and assuming that all of this is due to TOU charges and net metering, I don't know how the utility will feel about him buying energy from them cheap at night and selling it back to them at a premium during the day.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sunny Island connection to three phase grid
    ggunn wrote: »
    I don't think so. He has explicitly said that he wants to discharge his batteries into the grid. He can do that just as easily phase to neutral as phase to phase and the metering won't know the difference. The fact that he already has a PV inverter phase to phase doesn't have anything to do with anything; they are independent systems.

    As an aside and assuming that all of this is due to TOU charges and net metering, I don't know how the utility will feel about him buying energy from them cheap at night and selling it back to them at a premium during the day.

    Well if it's not for back-up then I agree with you completely; no reason to do it across two poles, or for that matter to do it at all. Time-shifting is not economically sound (I did a calc in that thread about "what the utility fears") and they probably won't like it if they find out. Maybe he wants to try and balance the loads use/non use by charging up when they're off and discharging when they're on?

    Not quite chasing-your-tail but it's close. :roll:
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sunny Island connection to three phase grid
    Well if it's not for back-up then I agree with you completely; no reason to do it across two poles, or for that matter to do it at all. Time-shifting is not economically sound (I did a calc in that thread about "what the utility fears") and they probably won't like it if they find out. Maybe he wants to try and balance the loads use/non use by charging up when they're off and discharging when they're on?

    Not quite chasing-your-tail but it's close. :roll:
    He calls it a "research project", but that could mean just about anything from a bunch of eggheads in a lab at NREL or one guy in a garage somewhere. :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sunny Island connection to three phase grid

    He is doing engineering experimenting/developing (?) -- So he is looking for loads+inverter+battery and he likes the SI because of its computer integration (as I understand).

    This is not a "practical" install.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kargozar
    kargozar Registered Users Posts: 7
    Re: Sunny Island connection to three phase grid
    ggunn wrote: »
    Why do you care about that? Utility metering doesn't care which phases source or sink more (or less) power.

    It's not just about utility metering. I want to regulate the AC current from/into sunny island as well as the loads based on some algorithms. It's much more consistent to have a same base voltage for both these equipments and the load voltage is 208V connected across L1-L2 (same for PV inverter although its current can't be controlled). So I really don't want to mix and math 120v and 208v devices in my system.

    Now what do you think about the transformer connection. Is it gonna work?