3-phase Grid-Tie question

Jcpen68Jcpen68 Registered Users Posts: 5
Newbie here, especially with 3-phase. Have a job just in the design phase with a bit of a twist to it. Utilising 6 inverters (SMA SB8000TL), tied into a 208Y 3-phase grid. Initially, we were simply going to combine all inverter outputs into 3-phases, then tie into the grid by way of individual inverter disconnects (each inverter will represent 1/2 of each phase). Now the design approach is to use a summation panel with a 3-phase output. My question is in respect to the inverter output and the panel breakers. the Inverter has two hot outputs, plus one neutral. Can I simply use a 3-phase load center (Eaton 3BR or SQD QO), with 2-pole breakers? Sorry for my ignorance here, I am trying to learn and playing catchup....

Comments

  • BB.BB. Super Moderators, Administrators Posts: 28,167 admin
    Re: 3-phase Grid-Tie question

    You might need to hire a power engineer for the project...

    Many utilities/permitting authorities can require a system to turn off all three phases of generated power, if there is a loss of phase. I am not sure if the specific inverters have an acceptable a loss of phase shutdown, or if you will need to install an automatic disconnect.

    This is a lot larger system than most of us here have experience with. Although we haves some Solar RE folks here that probably have experience with 3 phase systems.

    Would like to hear how everything works out.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: 3-phase Grid-Tie question

    Each inverter will supply half of one phase? That is a rather strange statement.
    Normally the SMA configuration for a 208V wye system is to connect each inverter between to hot lines.
    There is some confusion in nomenclature as to whether the voltage between two line conductors or the voltage between one line conductor and the neutral (wye centerpoint) is called a phase.
    Since the GTI output is line-to-line, you will have to balance them by connecting two inverters to each pair. That could be considered supplying half of a phase, but better as supplying half of the power to that phase.
    Each hot line will have one lead from each of four inverters attached to it, so you could just as easily say that each GTI supplies 1/4 of a phase. :)

    Anyway, the first and foremost requirement is that the breakers (branch and main) in the load center be listed for bi-directional use. This means that they must not have "line" or "load" markings on their terminals.

    Beyond that, you are correct that you can use normal two-pole breakers (single handle, common trip) for the individual inverter connections.

    However, whichever way you do it, if you are subject to the NEC you will have to size the load center using the specific rules for ampacity of the bus bars in the load center.
    To oversimplify, the most important rule, if you have a main breaker and six branch breakers, is that the sum of the branch breaker current ratings (or 125% of the inverter output ratings) must, when added to the current rating of the main breaker, not exceed 120% of the amp rating of the panel bus bars.

    So if you assume a 40A breaker for each inverter (and you may actually be forced to use a 50A breaker!), you could not put all the branch circuits and a 200A main breaker in a load center with a 200A bus.
    The relationship of the branch current to the main breaker current is a little odd at first sight because of the 120 degree phase shift from line to line in a three phase system.
    Two inverters generating 30A of current each, one connected A to B and other connected B to C, will generate 30A each in A and C but only 51 amps in line B.
    SMA SB 3000, old BP panels.
  • Jcpen68Jcpen68 Registered Users Posts: 5
    Re: 3-phase Grid-Tie question
    inetdog wrote: »
    However, whichever way you do it, if you are subject to the NEC you will have to size the load center using the specific rules for ampacity of the bus bars in the load center.
    To oversimplify, the most important rule, if you have a main breaker and six branch breakers, is that the sum of the branch breaker current ratings (or 125% of the inverter output ratings) must, when added to the current rating of the main breaker, not exceed 120% of the amp rating of the panel bus bars.

    So if you assume a 40A breaker for each inverter (and you may actually be forced to use a 50A breaker!), you could not put all the branch circuits and a 200A main breaker in a load center with a 200A bus.
    The relationship of the branch current to the main breaker current is a little odd at first sight because of the 120 degree phase shift from line to line in a three phase system.
    Two inverters generating 30A of current each, one connected A to B and other connected B to C, will generate 30A each in A and C but only 51 amps in line B.

    hmmm, head is spinning a bit here, but that is OK. Actually, this is where I am a quite fuzzy and still researching it. We are working with a master electrician on this and he is throwing numbers and percentages around like crazy. We are subject to the NEC and certainly do not want to make any mistakes. Thanks for the input.
Sign In or Register to comment.