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Questions about three-phase four-wire distribution system

Discussion in 'Power Electronics' started by max_torch, Jun 22, 2020.

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  1. max_torch

    max_torch

    101
    1
    Feb 9, 2014
    Here's a diagram of the common three-phase four-wire distribution system, taken from google, just to make sure we're all on the same page: [​IMG]
    First Question:
    In actual practice if the load has a neutral terminal provided do you always provide a wire connecting source and load neutrals? Or can you leave it unconnected to save costs?

    Second Question:
    From p.507 of Electrical Circuits 2nd Ed by Charles Siskind I quote:
    "Since the single-phase loads in the three phases between lines A and N, B and N, and C and N, are completely independent of one another, the line-to-neutral voltages EAN, EBN, and ECN are maintained in balance no matter how unbalanced these loads may be. Moreover, the line-to-line voltages are kept equal by the source, which means that the three-phase loads, generally balanced, are not affected by any degree of unbalance in the three-phase loads."

    I guess this means one need not concern oneself with distributing loads evenly across all three of the line-to-neutral terminals?
    Can anyone help confirm agreement or disagreement?

    A guy here says that he only picks the line-to-neutral terminal that has the lowest voltage (the most unbalance coming from the electrical company) and uses that for the whole single-phase loading.
    Another guy here says that you are supposed to balance across loading across all the line-to-neutrals.
     
  2. Bluejets

    Bluejets

    4,056
    847
    Oct 5, 2014
    Question 1 does not make sense.
    If the neutral is required then it is connected, nothing to do with cost.

    Question 2....the supply line voltage levels are maintained by the supply authority within certain limits and have nothing to do with load in-balance.
    It is up to the contractor to ensure near as balanced loads are installed.
    As it is up to the contractor to ensure voltage drop is maintained within limits all throughout the entire installation.

    As for the rest.....already answered that.

    Drawing is not necessarily just a sub-station transformer, it could be any of the distribution network pole top units, although, in many instances the HV is 3.3Kv to 400 but that depends which country you are in.
    As an example your 400v single phase load down under is referred to as 2 phase and 415V (voltage levels for the present at least)
    Just as phase colours down under are Red, White and Blue with the neutral Black and earth yellow/green stripe.
     
    Last edited: Jun 22, 2020
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    The one who picks the line with the lowest voltage probably picks the one that is already loaded the most and adds additional load to it. This will increase the unbalance and is counter productive.
    The other guy is right about balancing (as good as possible - perfect balance is impossible).
     
    Martaine2005 likes this.
  4. ramussons

    ramussons

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    66
    Jun 10, 2014
    I'll re frame the question: In actual practice if a 3 Phase load has a neutral terminal provided do you always provide a wire connecting source and load neutrals? Or can you leave it unconnected to save costs?

    A 3 Phase load can be either 3 terminal (Delta connected) or 4 terminal (Star connected).
    Your question will be with reference to the second.
    If the Load is balanced, like a Motor or a Star-Delta transformer, you can get away without the Neutral.
     
    Martaine2005 likes this.
  5. Tha fios agaibh

    Tha fios agaibh

    2,096
    703
    Aug 11, 2014
    It depends on the load.
    The Neutral carries the unbalanced load between phases.
    Some applications such as a Star (Wye) connected motor only need the 3 phase wires and an equipment ground for safety.
    It doesn't require a neutral connection.
    Obviously if there's no neutral load you don't need to bring a neutral to the load.
    At the transformer source, the neutral (xo) terminal is usually intentionally grounded for safety and to stabilize the system voltage.

    A Star system used for something like lighting in a building (480/277) would use the neutral because it connects the 277v lighting load between phase and neutral.

    Balancing loads on phases is important to prevent overloading of the neutral.
    Even if theres no line to neutral loads, its wise to ballance loads as evenly as possible.

    Choosing the lower phase voltage isn't wise as Herald mentioned. The more its loaded, the more the voltage will dip.
     
    Martaine2005 likes this.
  6. Bluejets

    Bluejets

    4,056
    847
    Oct 5, 2014
    Don't assume that motors draw a balanced load.
    It may not require a neutral but the phases can draw different current values.
    Perhaps not much but different all the same.
    If the wiring was to provide a 5 pin plug for a 3 phase motor, one would normally provide a neutral as one never knows when it is needed in the future or if the motor control is 277v.
    In many instances the initial cost for a run of 2 x twin cable plus earth as opposed to 1 x twin plus 1 x single plus earth is neither here nor there considering what it would be if one has to come back and run the extra at a later date.
    There are tight ass leckys that will do the el-cheapo install but they never last very long in the industry.
    The customer may not remember the cheaper initial price but they sure as hell will acknowledge the fact that it will cost them dearly later on and exactly who that el-cheapo drongo was.
     
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