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Electrical Utility Neutral Return Wire

  1. Jan 14, 2015 #1
    I have questions based on my understanding on the home electrical system (USA). I am not a student, this is for my personal understanding.

    My understanding (please bear with me):

    Electrical power to the house Mains Panel comes from the secondary of the utility company transformer in the form of 3 wires: 2 phase (hot wires) and 1 neutral return wire (center tapped on the secondary and grounded). Inside the house Mains Panel the 2 separate phase wires are connected to the 2 separate hot buses and the neutral return wire is connected to the Mains Panel neutral bus. At the house Mains Panel this neutral bus is also bonded to ground.
    In the house Mains Panel the branch circuit hot wires are connected to the buses through circuit breakers and the branch circuit return neutral wires are connected to the neutral bus.
    Within the house Mains Panel branch circuits the current flow alternates from phase to phase for 240v and from phase to phase through the neutral bus for 120v. If the phase to phase current flow is balanced then virtually no current flows back to the transformer on the utility neutral return wire. If there is an unbalanced current flow then only this unbalanced current flows back to the transformer.

    My questions:

    1. Does this mean the return current from the house Mains Panel neutral bus to the utility transformer flows only in one direction and does not alternate (on the neutral return wire) as the current in the phase wires does?

    2. If it is a one way flow, then, is this unbalanced current returned to ground at the transformer center tap ground point or is it somehow reintroduced into the phase wiring?

    3. If the return current on the utility return neutral wire alternates, what keeps it from impeding the current flow of one of the phase wires? If the return current alternates, it seems like it would be "out of phase" with one of the 2 phase wire?

    If you have made it this far, thank you for your patience.

    Mike
     
  2. jcsd
  3. Jan 14, 2015 #2

    jim hardy

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  4. Jan 14, 2015 #3
    Looks to me nobody explained there (or I missed it) what happens with voltages in 3-phase isolated systems when one phase shorts to the ground?
    It's not just that asymmetry in the network present problem to proper operation of 3-phase motors and other devices , but also phase voltages of 2 healthy phases rise to line voltage (rise by 73%). That's not all. Voltage with respect to grounded parts may rise many times higher. Fault phase in many cases generates spark of relatively weak rms current due to presence of distributed capacity in the system. Behaviour of the spark is that it acts like nonlinear switch and charges whole the system as current repetitively turns on and turns off. The phenomenon is called "escalation of the voltage" and it doesn't have to be explained how bad and destructive that can be.
     
  5. Jan 14, 2015 #4

    jim hardy

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    Hmmmm that's a good point. I'm so used to industrial systems where you never "float" anything i didn't think about it.


    That's what brought about IEEE 142, the grounding standard.

    Ferroresonance between distributed capacitance and inductance of equipment will raise the voltage enough to pierce insulation . It can wreck all the motors in a factory.
    That's why one sizes the earthing resistor not more than Z of that distributed capacitance, to keep down Q..
     
  6. Jan 14, 2015 #5
    Ferroresonance is another "fish in the pond" (also bad & dangerous nonlinear phenomenon which can lead to overvoltages). However, I was talking just about 3-phase ungrounded system where only nonlinear element is a spark between fault phase and ground. This is enough for escalation of the voltage to occur. Higher the nominal voltage of the system the voltage escalation more dangerous. It is interesting that in Europe, on rare occasions, one can still find cases of ungrounded 10 kV distribution networks! However, most of MV networks of today are earthed systems (resonantly earthed sometimes). And networks above 69 kV are not resonantly earthed. I've read once there were cases in North America of 138 kV network (or maybe 230 kV?) with resonantly earthed neutrals. That surprised me quite becouse overvoltages are also significant in phase-to ground faults in resonantly earthed networks.
     
  7. Jan 14, 2015 #6

    jim hardy

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    You're ahead of me on that one.

    This was my first time hearing of "Resonant Earthing"

    very clever !
    http://www.oriongroup.co.nz/downloads/Technical_information.pdf [Broken]
     
    Last edited by a moderator: May 7, 2017
  8. Jan 14, 2015 #7
     
  9. Jan 14, 2015 #8
    Mr. Hardy,

    Thank you for your time and your information. Following your links, I found another response of yours on a similar topic that I believe helps me more clearly "see" this return current flow to the transformer and that it is a one way return to the transformer (supply) and on to ground. I was trying to get a clear picture in my mind to explain this "flow" to my grandson. Again, thank you.

    Mike
     
  10. Jan 14, 2015 #9

    jim hardy

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    Thanks for the kind words - makes an old fella feel less useless !


    Us Grampas gotta stick together.....
    old jim
     
  11. Jan 15, 2015 #10
    Principles of resonant earthing and Petersen coils are known for a long time. For example, see:
    http://www.hvpower.co.nz/TechnicalLibrary/RE+DS/Petersen Coils Basic Principle and Application.pdf
    That works very well in MV networks. Hence I was surprised to find some 10 and 20 kV networks are still left floating today (no, they are not delta-delta). As always, pros and cons to be considered. Generally and technically, it is easier to deal with higher or smaller fault current than with TOV in the system so any grounding is better than no grounding. OTOH, there is financial aspect vs risk factor to be considered as well.
     
    Last edited by a moderator: May 7, 2017
  12. Jan 15, 2015 #11

    anorlunda

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    In three phase utility power transmission lines and distribution lines, there is no neutral wire. In a wye connected circuit, the wye is grounded. In delta connected circuits, there is no earth or neutral at all. Since there are many ways to wire transmission, and since distribution circuits split from three phase to multiple single phase circuits, there is no single scheme for fault protection.

    Protective relays detect faults by various means including combinations of overcurrent, undervoltage, apparent impedance, and unbalance.

    To dig deeper, google "The Art And Science Of Protective Relaying", it is a free publication from General Electric that you can download.
     
  13. Jan 15, 2015 #12
    Hello ToDot - nice OP. Sorry if the thread is getting off topic with the 3 phase, transmission and distribution issues. I believe your question has to do with the basic nature of AC power as much as anything.

    In typical US households the 240 / 120 is really split phase, by that I mean the 120 and 240 Circuits are in-phase with one another and there is no phase shift in the AC voltages supplied.

    What I believe confuses most people is the idea of "flow" in general - they think of current only. Really we are dealing with POWER which requires a Voltage and Current - the direction of power flow, is determined by the relative polarity of the voltage and the current. So only talking about current can be difficult to understand especially when thinking about AC - "Wait the current flows "in" and then it flows "out" ? - How does that do anything?"

    One way to help - think of your feet on the pedals of your bike - a basic bike rider pushes down. makes sense.. but a serious cyclist uses toe-clips and pushes and pulls... but the power delivered is always from their body to the bike. ( Foot pressure downward, pedal motion down = Delivered Power, with both "polarities" reversed foot pulls up, pedal moves up = delivered power.)
     
  14. Jan 15, 2015 #13

    jim hardy

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    Maybe that depends on where one lives.
    I grew up in South Florida, the lightning capital of the world.
    There the neutral is run on top of the transmission towers. It acts both as neutral and as a lightning rod .
    That's fairly typical in US, this image claims to be in Indiana.
    http://www.betaengineering.com/en-us/projects.aspx [Broken]
    http://www.betaengineering.com/Portals/0/projects/Transmission%20Line%20Indiana.jpg [Broken]

    In my plant the delta fed medium voltage buses were impedance grounded through a wye transformer that made a
    "virtual neutral".

    IEEE 142 describes recommended practices for earthing, search on "IEEE Green Book".
    University curricula ought to include a course on grounding. It is widely misunderstood.
     
    Last edited by a moderator: May 7, 2017
  15. Jan 15, 2015 #14

    jim hardy

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  16. Jan 15, 2015 #15
    Windadct,

    Thank you for your comment. I appreciate your time. I believe I understand the basics about the "power" flow, although, as a non EE, the nuances of all the science involved in system designs is over my head.

    Mike
     
  17. Jan 16, 2015 #16

    anorlunda

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    Not quite. Those wires on top are called shield wires. They are grounded and their purpose is lightning protection. But they have no connection to the power conductors and can not act as a neutral for the power circuits in case of a fault.

    By analogy, compare these shield wires on a transmission line to lightning rods on the roof of a building. Lightning rods are never connected to the power wiring of the building.
     
    Last edited by a moderator: May 7, 2017
  18. Jan 16, 2015 #17

    jim hardy

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    Call it what you like
    but it's earthed at same place as stepup transformer neutral
    as well as at every tower along the way
     
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