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3 phase supply query

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  • 10-06-2010 10:59pm
    #1
    grousedogtom
    Registered Users Posts: 463 ✭✭


    Hi all,
    Iam upgrading a farm supply from single phase to 3 phase at moment,the farm load has exceeded its MIC from ESB so the ESB are going to supply farm now with a 3 phase supply. At the moment there is no 3 phase appliances on the farm so i have split all loads evenly over the 3 phases, my question is about running my mains swa cable from my consumer unit to the new esb 3 phase meter , since there is no 3 phase loads at present on farm as everything was originally wired for single phase do i have to run an extra large size neutral as all loads will be returning through this neutral as every load at moment is single phase. Many thanks


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    #2
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    grousedogtom wrote: »
    Hi all,
    Iam upgrading a farm supply from single phase to 3 phase at moment,the farm load has exceeded its MIC from ESB so the ESB are going to supply farm now with a 3 phase supply. At the moment there is no 3 phase appliances on the farm so i have split all loads evenly over the 3 phases, my question is about running my mains swa cable from my consumer unit to the new esb 3 phase meter , since there is no 3 phase loads at present on farm as everything was originally wired for single phase do i have to run an extra large size neutral as all loads will be returning through this neutral as every load at moment is single phase. Many thanks

    No the neutral will be the same size as the 3 phases. The neutral will never carry any more current than any one phase is carrying, the 3 phase currents do not add up to a bigger neutral current and in actual fact the neutral current reduces. It might not seem right but for electricians its a simple enough principle.

    If your phases all happen to be carrying 50 amps each then the neutral will actually be carrying zero current, if 1 phase has 50, another has 30 and other has 40, then the neutral current will be around 10 amps. If 1 phase is carrying 50amps and the other 2 have nothing then the neutral carry`s 50amps.


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    #3
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    Very helpfull Robbie ,i have my head around it now.


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    #4
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    grousedogtom wrote: »
    Very helpfull Robbie ,i have my head around it now.

    No bother.


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    #5
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    Is there any basic formula for calculating the current in the neutral on 3 phase unbalanced loads? just inquisitive


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    #6
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    grousedogtom wrote: »
    Is there any basic formula for calculating the current in the neutral on 3 phase unbalanced loads? just inquisitive

    There are no simple basic formula`s to calculate it, the easiest way to approximate it is with vector diagrams, look it up in a seach page and you might find something. There are formulae to do it but its quite involved.

    Best way is to measure it with a grip ammeter.


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    #7
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,591 Mod ✭✭✭✭2011


    Is there any basic formula for calculating the current in the neutral on 3 phase unbalanced loads? just inquisitive

    Well there sort of is, triginometery and it can get complicated!

    The ESB supply electricity at 50 Hz. That means that the phase voltage goes from a peak value of about +325 volts to a peak value of -325 volts 50 times a second and if you were to plot this voltage over time it would look very like a sine wave.
    The DC equivelent of this voltage is known as the "RMS" voltage which in the EU is 230 VAC.

    With 3 phase power you get 3 lives (known as phases) each like a sine wave of equal magintude but each one is "out of sync" with the others by the same amount, so they are all equally spaced in terms of time.

    Because a sine wave is a trigonometric function it is often ploted against an angle rather than time. With 3 phase electricity each phase is said to be 120 degrees out of phase with the other 2. At any given point in time the sum of all 3 voltages is equal to zero.

    Just to make things more complicated, for a fixed load the current varies at the same rate as the voltage, i.e. like a sine wave too. When the current for a given phase is in sync with the voltage (no phase shift) the "power factor" is equal to 1. The power factor is the cosine of the angle between the voltage and the current. In reality this is rarley the case. The ESB aim for a power factor of about 0.9 or 0.95 as far as I can remember.

    When all three phases have the same load and each load has the same power factor the resulting current is zero. This is known as a "balanced load". The ESB like balanced loads and so do electrical design engineers. As this makes thing more efficient and saves money!

    If the load is unbalanced, which is normally the case to some extent then the resulting neutral current is the phasor sum of the 3 currents. When the 3 currents have different magnitude and different power factors the sum gets more complicated as the pahse shift is no longer 120 degrees between each current. The simplest way to work out the resulting neutral current is by drawing a phasor diagram, getting a calculator and using a bit of triginometery. Computer simulation programmes is often used for this. I used a free one called "PSpice" quite a bit.

    To make matters worse harmonics can cause distortions to the neutral current that can cause the current to be far larger than any of the phase currents caused neutral cables to burn out! Harmonics can be caused by all sorts of modern electrical equipment.


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    #8
    danjo
    Registered Users Posts: 589 ✭✭✭danjo


    A very good description of three phase supply 2011.


    danjo


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    #9
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,591 Mod ✭✭✭✭2011


    danjo A very good description of three phase supply 2011.
    A small point , i think you have a typo on the first line i.e. -235 instead of -325.

    Thanks.

    I will correct the typo.


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    #10
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    danjo wrote: »
    A very good description of three phase supply 2011.


    danjo

    Well the question was is there any simple formula for calculating the neutral current, i`d still say the answer is no.


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    #11
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    When all three phases have the same load and each load has the same power factor the resulting current is zero. This is known as a "balanced load". The ESB like balanced loads and so do electrical design engineers. As this makes thing more efficient and saves money!

    Large generators would be very inefficient and noisy without a balanced load on them, they probably would`t last long at all, its probably close to perfectly balanced at the generation stage anyway. The further back toward generation stage the more balanced it will get i`d say.


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    #12
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,591 Mod ✭✭✭✭2011


    The load is balanced as much as possible. If it was perfectly balanced there would be no neutral current (as you stated), but in reality this is rarely the case. The secondary side of the ESB traffos normally have some neutral current, although I agree with you that this is minimized as much as possible.


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    #13
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    The load is balanced as much as possible. If it was perfectly balanced there would be no neutral current (as you stated), but in reality this is rarely the case. The secondary side of the ESB traffos normally have some neutral current, although I agree with you that this is minimized as much as possible.

    Yes but the further up the power chain the more balanced the loading should work out over the phases, the primary side of the 10kv traffo`s are delta so there`s no neutal current on that side, i think the 38kv traffo`s are delta star as well, although with the amount of stuff i forgettin these days in the old age i could be wrong again.


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    #14
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,591 Mod ✭✭✭✭2011


    Yes but the further up the power chain the more balanced the loading should work out over the phases
    That is all well and good, but my post was in reply to the OP's question about neutral currents pretty far down the supply chain.
    the primary side of the 10kv traffo`s are delta so there`s no neutal current on that side
    Correct, if there is no neutral cable you can't have a current in it :D
    38kv traffo`s are delta star as well, although with the amount of stuff i forgettin these days in the old age i could be wrong again.
    You are right.


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