Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Neutral wire in electric power distribution between substations

  1. Aug 12, 2017 #1
    I had learnt from a book in a lower class that electric power distribution from the generating station is done in phases. Like in India, we have three phases of power transmission: the red, green and yellow colours distinguish the three phases. I believe the same is the case in any other country, irrespective of their voltage supply or frequency of AC.

    Wikipedia has an article on this. I learnt from this article that the live is brought from the generating station via three different cables, and no neutral wire is there. The AC in the three wires are of the same frequency and voltage, but differ in phase by one-third a time period. I've also understood the necessity of differentiating into phases so that more power can be supplied. But can you explain why is there a phase difference? What difference does it make if I removed the phase difference completely?

    Moreover, there is no neutral wire. I have recently learnt from a previous question of mine in this forum, the difference between the live and the neutral wires from and local transformer. I understood that in the neutral us and ways at 0V as it is earthed somewhere. When the voltage of the live wire changes to a negative half cycle, then the current flows through the neutral wire as 0>-ve. But we never get a shock because we ourselves being earthed, we and the neutral are at same potential.

    Now the question arises as to how the return path of the current is completed in the absence of a neutral wire during power dustribution. Is it that one output of the dynamo is connected to the ground, and one wire of the city substation is also earthed, so that there is a transfer of electrons through the earth?
     
  2. jcsd
  3. Aug 12, 2017 #2

    cnh1995

    User Avatar
    Homework Helper

    The three phases are 120 degrees apart. If you do phasor analysis, you'll see that their resultant is zero. They are like three equal vectors 120 degrees apart whose resultant is zero.
    In absence of the neutral, return path for one phase is via the other two phases.

    Plus, apart from the fact that more power can be transferred, three phase power is time-independent unlike single phase ac power. So, for a given value of voltage (rms), current and power factor, the three phase power is constant w.r.t. time i.e. it is not "pulsating" like single phase power.
     
  4. Aug 12, 2017 #3
    Also, another cool aspect, when you use two phase wires for a circuit instead of one phase wire against ground, the subtraction of the sine waves comes out that you get another sine wave of 1/sqr(2) the amplitude. E.g. in Europe a household receives 360V 3-phase, but from there you can get, without transformers, 3 separate 220V circuits.
     
  5. Aug 12, 2017 #4
    Can you elaborate a bit?
     
  6. Aug 12, 2017 #5

    cnh1995

    User Avatar
    Homework Helper

    There is a neutral wire in the three-pase distribution network. In India, we have 3-phase-4-wire, 400V ac distribution. The household single phase supply is obtained by tapping the three-phase network at any one phase and the common neutral. Thus, you get 230V single phase ac at your home (400/√3).
    Neutral is absent in the three-phase-three-wire system, which is used in high voltage transmission.
     
  7. Aug 12, 2017 #6
    It's just an outcome of mathematics. The difference of two sine waves that are phased 120 degrees apart will be another sine wave with 1/sqrt(2) the height of the original ones.
     
  8. Aug 12, 2017 #7

    Nugatory

    User Avatar

    Staff: Mentor

    I think you and @Wrichik Basu are saying the same thing: three phase conductors and no neutral for the high-voltage power distribution system; and then at the local terminus a common neutral connected to a local earth ground allows us to take 230V single-phase out from any one of three phases.
     
  9. Aug 12, 2017 #8
    That's the case in India. What about the no neutral case mentioned in the article of Wikipedia? How does current return?
     
  10. Aug 12, 2017 #9

    cnh1995

    User Avatar
    Homework Helper

  11. Aug 12, 2017 #10
    OK, I've understood the process of return through another phase.

    Thank you everyone for taking your time here. :partytime::partytime::partytime:
     
  12. Aug 13, 2017 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Not just through "another Phase". Both of the other phases are involved. The three phases constantly balance out, throughout the cycle.

    There are two possible connections: Wye and Delta. (See this link) Any imbalance in the three loads will cause 1. A neutral current in the Neutral of the WYE circuit or 2. A mean shift of Potential of the three circuits with respect to ground in the Delta circuit. It is not too hard to understand at an arm waving level if you choose not to get into the trig of the thing. :smile: If you want to progress it a bit further, there are thousands of links at various levels on AC Theory.
     
  13. Aug 13, 2017 #12
    Hopefully it won't be too much thread drift if I ask this question that has been puzzling me for a while. My neighborhood is at the end of the power distribution network - there is one hot phase at the top of the power poles, and a neutral wire a few feet below. Each house has its own transformer that takes a primary connection from the hot phase, and on the secondary side has two hot legs 180 degrees out of phase, and a neutral, all going to the house. Now on most of the transformers the secondary, or house, neutral wire is connected to the neutral wire on the pole, but on a few this connection is omitted. Why would this be the case? It seems either one scheme or the other would be preferred, if not required by electrical code, so why two ways of doing it?
     
  14. Aug 13, 2017 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You would first need to confirm what part of the world you live. There are several alternative arrangements and the people who live under each regime will tell you it's the best and all the rest are rubbish. From what you say, it would probably the US? But, from what I read here, the US distribution is usually three phase all over and one phase taken off for each consumer with a pole mounted transformer (rural and semi-rural). You seem to have the three phases in a 'star' distribution.
    Connecting the secondary centre tap of your transformer to the supply neutral is fairly optional. You need a local safety ' Earth' so that your can connect all your large metal appliances and pipes etc. to reduce the likelihood of shock. There have been countless threads on PF about how best to do this and the view can either be to keep the neutral connected to a local `earth', to have it separate or to use the company neutral as an Earth.
    Actually this is a bit of an extreme migration. Perhaps a new thread would be best?
     
  15. Aug 13, 2017 #14
    Thanks for your reply, and my apologies for going off-topic. Edit: I do live in the US.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Neutral wire in electric power distribution between substations
  1. Neutral Wire (Replies: 16)

Loading...