Inverter set up for a three phase 7.25kw system

I have got myself in a bit of a quagmire - my company has just signed a sales agreement for a 7.25 kw system. The small commercial building has three phase 240v main feed. We are trying to figure out what inverter to use for such a system. SolarEdge, the inverter company we use, doesnt make three phase inverters sub 9k. Solaredge recommends buying three 3k single phase inverters but that doesnt sound right to me either. We target 1.3x dc for inverters as it is hot where we install and it is rare to have a perfectly south array. If anyone has ran into this problem before and has a suggestion, I'd really appreciate your suggestion.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,167 admin
    Check with the customer's utility/engineering department... If I recall correctly, our utilty allows a maximum 10 kWatts of "unbalanced" power between phases (at least for smaller installations. Many of the smaller 3 phase installations have only two pole transformers (open Delta)--So, back feeding 3 phases (or the "wrong" phase) may be an issue for the utility (don't know, just something else to watchout for).

    There should be no issue with 2x 3.5 kWatt inverters (single phase). Or even 1x 7.25 kWatt inverter (again single phase).

    You may get into the question of having a loss of phase shutdown for the 1/2 inverter solution (if one phase is lost, then all three phases for GT inverter may need to be shutdown for safety/utility issues). I am sure there is lots of "stuff" out there that can be used (directly on AC output of GT inverters, or possibly an AC Inhibit line into the inverter):

    https://www.eaton.com/us/en-us/catalog/machinery-controls/phase-monitoring-relays.html

    Also, is the 7.5 kWatt the array? In general, the suggested "minimum" size AC inverter would be:
    • 7.5 kWatt * 0.77 panel+inverter deratings = 5.775 kWatt output...
    So, one ~6 kWatt or 2x 3 kWatt GT inverters would be just fine (in general, solar panels get hot, their Pmp drops, specifically Vmp falls, plus a little losses from dust on panels, GT inverter efficiency, etc.). It would be very seldom that the 5.8 kWatt of GT inverters would "clip power" (at at that, you are generally only talking about clipping 100's of watts for less than an hour per day, for a few cool/clear days per year--SWAG).

    -Bill "not a power/utility engineer" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,075 ✭✭✭✭✭
    As long as the solar edge are designed to run 3 phase which I do not know. Ask if they can be run in clusters!

     Clusters is how Schneider XW and Outback Radian battery based hybrids do this. Schneider is about 7KW 3 phase up to 76KW and Radian is 8KW up to near 100KW. If this is in California both Schneider and Outback are Rule 21 compliant. Below are the IEEE requirements. 

    I get that you probably do not want a hybrid solution but the cluster method may be helpful, it may not ;)



    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 30,167 admin
    I should also add, there are 240 VAC 3 phase (Delta) and 208 VAC 3 phase drops (Wye)... So you may need to confirm what the exact drop your customer has (in our area, office parks are typically 3 phase 208 Wye drops).

    And I guessed/assumed that this is a pure Grid Tied system... And not a hybrid/battery based system (backup power for power failure support). If I was wrong, I am sorry.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solar_davesolar_dave Solar Expert Posts: 2,356 ✭✭✭✭
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