Three phase current

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ggunn
ggunn Solar Expert Posts: 1,973 ✭✭✭
in Advanced Solar Electric Technical Forum #1
Every time I think I understand the basics of three phase power, I run across something that shows me that I don't, and it's frustrating.

Here's my situation: I've got (or will have) a boatload of SMA SB's set to 277V. They will be split into three equal groups, all connected phase to neutral on a 480VAC interconnect. What is the current in the 480V line? Is it the total wattage of the inverters divided by 480V, or do I need to divide by sqrt3 as well?

In a 240V split phase system, line current and phase current are one and the same. Is the same true for three phase systems? It now seems to me that it should be, and it follows that if that is the case, then the line current should be total power divided by (480V)(sqrt3). Looking at the data sheet for an inverter with three phase output bears that out; for a 100kW inverter, 120A is shown as AC Maximum Continuous Current at 480VAC. (100kW)/(480V)(sqrt3) = 120.3A.

This goes back to my earlier voltage drop question, and with the power and distances involved, a factor of sqrt3 makes a huge difference in the cost of the interconnect. I really need to get this right.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: Three phase current

    I have to go right now--but have you found anything useful on the web--such as (perhaps?):

    http://www.3phasepower.org/3phasewiring.htm

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #3
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    Re: Three phase current
    BB. wrote: »
    I have to go right now--but have you found anything useful on the web--such as (perhaps?):

    http://www.3phasepower.org/3phasewiring.htm

    -Bill
    I have been to that website and it has been no help to me.

    EDIT: Never mind. I think I have finally got it straight.

    Three SB7000's @ 277VAC connected phase to neutral on a 480V interconnect:

    Phase current = 7000W/277V = 25.27A

    Line current = 21000W/(480V)(sqrt3) = 25.25A

    Line current = Phase current

    I don't know why it took me so long to get around the bases.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #4
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    Re: Three phase current

    The difference is the neutral on 240 balances out because its 180 degrees out of phase

    3-Phase is 120 degrees , not 180, so that why you get the "thirds" to deal with on the neutral if its unbalanced on the combined load
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
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    Re: Three phase current
    Solar Guppy wrote: »
    The difference is the neutral on 240 balances out because its 180 degrees out of phase

    3-Phase is 120 degrees , not 180, so that why you get the "thirds" to deal with on the neutral if its unbalanced on the combined load

    OK, but are you saying I'm right or wrong? The crux of the matter is whether phase current = line current in three phase power as it is in split phase. The neutral is a non-player in either system if the phases are balanced.

    Let's say, for example, I have 3 SB7000's configured to 277V connected one per phase (phase to neutral) to a 480VAC line. I have it connected to the service in #1 Cu wire (R = 0.154 ohms/1000') and the distance is 100'. What is my voltage drop?

    a) 0.67V
    b) 1.16V
    c) 0.38V
    d) none of the above

    And should it be compared to 277V or to 480V to get the %VD?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #6
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    Re: Three phase current

    Correct, there is no neutral current in the three phase Wye connected system if the phases are balanced.

    The worst case current in the neutral should be the rating of one phase's circuit breaker (i.e., worst case is one phase to neutral current).

    If you have a 100 kW total load running on three phase, then, assuming PF=1.0, each hot leg with carry power*1/voltage*1/sqrt(3) current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #7
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    Re: Three phase current

    There are three operating conditions

    one phase inverter(s) operational
    two phase inverter(s) operational
    three phase inverter(s) operational

    Only if you have balanced loads on all three phases will the neutral balance in 3-phase, which will never happen. I believe Bill is saying, you design for the one out of three operational condition and its 277V to neutral that your calculating the Voltage drop on which should be below the breaker rating for that phase

    Keep in mind, if you lose some % of the generation of one phase, you have to shut down the others to prevent phase imbalance in the source transformer.

    I doubt you will get a sign off from the utility on your truck full of sunnyboys for this reason. ( you can interlink three, but never heard of interlinking sets of three ) This is one of the primary reasons you use a single larger unit, it has the required fault prevention built in.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #8
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    Re: Three phase current
    Solar Guppy wrote: »
    There are three operating conditions

    one phase inverter(s) operational
    two phase inverter(s) operational
    three phase inverter(s) operational

    Only if you have balanced loads on all three phases will the neutral balance in 3-phase, which will never happen. I believe Bill is saying, you design for the one out of three operational condition and its 277V to neutral that your calculating the Voltage drop on which should be below the breaker rating for that phase

    Keep in mind, if you lose some % of the generation of one phase, you have to shut down the others to prevent phase imbalance in the source transformer.

    I doubt you will get a sign off from the utility on your truck full of sunnyboys for this reason. ( you can interlink three, but never heard of interlinking sets of three ) This is one of the primary reasons you use a single larger unit, it has the required fault prevention built in.

    The utility has already signed off on it. I would love to use a central inverter, but decisions were made before I was brought in that are set in stone.

    But let's cut to the chase, shall we? I've got 204kW of inverter (could be central, could be an array of SB's) on a 480/277V line. I've got a 400' run to the interconnect, and I'm held to a 1% voltage drop. What conductors would you use? I get 500kcmil Cu for a voltage drop of 4.38V. Do you concur?
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  • Jburgess
    Jburgess Solar Expert Posts: 130 ✭✭✭
    #9
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    Re: Three phase current

    I get 4.4 volts for a delta connected system on the 480 volt line to line. However your inverters are 277 volt and I get 2.54 volt drop on the 277 volt line, which I think is what is important, and still is less than the 1 % with 500 mcm wire copper.

    The sizing of the neutral is an interesting question. Do you size it for the maximum load and consider for normal operating conditions it does not need to meet the 1% requirement at 350mcm. Even at 500 mcm if phase of the inverters drops off the other banks will not meet the 1% voltage drop.

    To meet the 1% under all conditions you need something like 900 mcm wire for all conductors. (not exact but ballpark).

    Using a 350 amp breaker instead of fuses would decrease the likeliness of loss of only one phase on the customer side.
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