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three-phase power transmission, circuits, etc.

Discussion in 'Mathematics and Physics' started by PG1995, Dec 10, 2013.

  1. PG1995

    PG1995 Active Member

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  2. JimB

    JimB Super Moderator Most Helpful Member

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  3. Mike odom

    Mike odom Active Member

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    On your question one, that is exactly how power is transmitted (and consumed) in the U.S. Two Lines, 180° out of phase, so if you connect either line to neutral, you get 120V. If you connect line to line, you get 240V. They also tie Neutral to Earth Ground (at the box) so that either line is never more than 120V from earth.
     
  4. dave

    Dave New Member

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  5. rumpfy

    rumpfy Active Member

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    Hi PG,
    I'll try,
    In the US, the first use of electricity commercially was for street lighting and generally was a DC system. Very quickly, the need to transform power from one voltage level to another became a requirement and then AC systems were developed. Again, the US led the way with a single phase system and the system is a single phase system with centre tap.
    With the march of progress, and the increasing use of electric motors, it quickly became apparent the single phase system was inadequate and the maths showed the most economical use of a generator(alternator) was as a 3 phase system. Additionally, the constant power versus rotation, was a big plus for 3 phase power.
    In the US there were a number of methods developed to get a 120 degree system using single phase generators, and there was a love affair with generator windings in quadrature to give a 90 degree phase lag between phases. To further develop this arrangement, there was the so-called 'Scott Connection'.
    mike talks about the US system of 240 volt centre tapped transformers, but the reality is that most energy in the US is supplied to factories using a 480 Volt 3 phase system.
    Most of the rest of the world use a 'multiple earthed neutral' system, where the subscribers supply transformer is wound with star connected windings, the common point being earthed. In the British countries, the supply is 230/240 Volts phase to neutral, with a phase to phase voltage of 415 volt. In most of Europe, the standards are 220 volt/ 380 volt.
    In the US system, with a centre tapped phase winding, the common or star point cannot be connected to earth. Further, the centre tap on each phase, may be be at a different potential to the centre tap on the other windings. The US system gets away with this by not having an earthing system as part of the electricity supply. the whole thing just floats above ground.
    In Australia, and other sparsely populated places, there is a 'rural' supply system, based on a 3 phase generation, where a long rural line is taken from a phase at 22kV, and it runs out to outlying consumers, who have their own 22 Kv to 480 volt centre tap transformer. The return wire is actually the earth. This system is known as 'Single Wire Earth Return (SWER). The big disadvantage of this system is that farmers etc, who want a big electric motor for running pumps etc, cannot use normal 3 phase electric motors. They have to use big expensive single phase motors.
    Your fig 12.3 is the Rolls Royce of power system.
    Most domestic systems are your fig 12.1(a).
    The australian SWER system is like fig 12.2(b) except the phase voltages are 240/480 volt.
    Fig 12.2 is a US system which is possibly not used anymore.
    I probably didnt answer yr question 1 cos its US specific.
    The second question about 48 phases is a special case, and I cannot comment, except too say the there is a big aluminium smelter in australia which gets a three phase supply of 500 Kv at 50 Hz. So if they use say 48 phases, they must be doing the conversion themselves. It is conceivable that because aluminium smelting is SO power intensive, the only way they can rectify the incoming power is to split it into so many phases to enable the electrical components to be reasonably sized.
    Hope this is of interest.
    ps are you in the US?
     
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  6. PG1995

    PG1995 Active Member

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    Thank you, JimB, Mike, and rumpfy.

    rumpfy: Thank you for such a detailed reply. No, I'm not in the US.

    I will get back with my follow-on queries soon. Thanks.

    Regards
    PG
     
  7. PG1995

    PG1995 Active Member

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    Hi

    Q1:
    Suppose that we have a delta 3-phase mains supply driving a delta 3-phase load such as a motor like this. I get it that the system can work without a separate neutral wire. But what if the motor builds a short circuit and someone touches it. In short, how do we ground such a load?

    Q2:
    This picture shows a delta-y 3-phase transformer in theory and this is the picture of an actual such transformer.

    i: What are those connections in yellow highlights?
    ii: What does that dashed line in green highlight represent?

    Please help me. Thank you.

    Regards
    PG
     

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  8. JimB

    JimB Super Moderator Most Helpful Member

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    The associated schematic is something out of a theoretical textbook, safety earthing does not come into it.
    In a practical situation, the metal structure of the motor is connected to the local safety earth.

    The transformer has a high voltage side H1, H2, H3 and a low voltage side X0, X1, X2, X3.
    The low voltage is defined as 208v phase-phase, and 120v phase-neutral.

    The high voltage can be selected connecting the various taps as defined in the table.
    If our incoming high voltage is 480 volts, we connect 2 to 3 in each of the three windings.
    then when there is 480v on the HV winding, there will be 120/208v on the LV winding.

    Not sure, this is going outside my experience. It appears to be trying to represent a "neutral" connection, compare to the X connections to the right of the triangle.

    JimB
     
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  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    As to the question about the 48 phase system...

    If we have a three phase system to start with, we can generate three more phases where each phase angle is half of the previous phase angle. For three phase the angle between first and second phases is 120, so half of that would be 60 degrees, and that is one way we can get the in between phases, because when two phases are added and they both have the same amplitude, the phase angle is exactly one half the angle between them, which is 60 degrees in this case.

    We can start by using the identity:
    sin(t)+sin(t+120)=sin(t+60)

    and with this it is simple to see that 60 degrees is half way between 0 and 120, so that gives us one in between phase. Doing this three times gives us three new in between phases in addition to the original three, so now we have 6 phases. Repeating the process again, we get 6 more phases for a total now of 12 phases, and repeating again we get 24 phases, and repeating once more we get 48 phases.

    The only catch here is that we would have to use transformers, where we wire the primary to the two original phases and one side of the secondary to neutral and the other side of the secondary becomes the new phase, but the other catch is that this subtracts not adds, so we would have to invert each phase first in order to perform the addition.

    So that's one way of getting from 3 to 48, by going from 3 to 6, then 6 to 12, then 12 to 24, then 24 to 48. There could be some simplification too.

    When it comes to diode rectification, the more phases we start with the smoother the DC is even without filtering.
     
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  10. PG1995

    PG1995 Active Member

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    Thank you, JimB, MrAl.

    MrAl, though that query about 48 phases was quite old, your explanation was very good. Thanks.

    This is what someone told me before but I find it hard to understand. Let me explain why.

    Suppose, you have mains supply coming from a single phase generator. There is one live wire and one neutral. The neutral wire is grounded at the grid and also in your home. I'm sure that even the generator itself is also grounded at the power station. At the point of main panel, neutral and ground wires are the same but any distribution inside your home would carry two separate wires for neutral and ground. The ground wire of your wiring gets directly connected to your appliances. If ever any of your appliances gets shorted, the current would be carried toward the main panel directly and you won't be harmed.

    The generator completes its electric connection thru both live and neutral wires. You wouldn't be harmed when your appliance gets shorted because the generator was able to complete electric connection thru ground (which is path of least resistance) and it would spare you the pain because you are not a path of least resistance.

    Now let's talk about 3-phase mains supply. This picture [1] shows four wire three-phase mains supply. It is a y-connected system. Just like your single phase wiring, you would use a separate ground wire. If ever any of appliances develops a short circuit, the connection could be completed thru the ground wire. As far as I can see, a y-connected system is safer compared to delta one because it has a neutral connection which can be used for ground purposes although the system can work without the neutral wire.

    This picture [1] shows a delta-connected mains supply. You can see that there is no neutral connection available on delta system. Even if you ground your three-phase motor, in my humble opinion, it won't do any good because neutral connection is not really a part of delta system. Let me say it differently. You have a three phase generator delta connected which means generator's body cannot be grounded. Three wires run from the generator toward a delta-delta transformer near your home. You can see that there is neutral/ground involved.

    Thank you. This reference [1] also explains it.

    How can it be a neutral connection because there is no neutral in delta connection? Thank you.

    Regards
    PG

    References:
    [1] Electrical studies for trades, Stephen L. Herman
     

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  11. JimB

    JimB Super Moderator Most Helpful Member

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    As far as I see, it cannot.
    I was just making a comparison between the two diagrams.
    To me, the delta connection with the dotted line does not make sense. I do not understand it.

    NEVER, EVER, think of the neutral as a safety earth.

    The neutral carries the "return" current in single phase systems, and the unbalanced current in three phase systems.

    The protective earth should only carry small leakage currents due to capacitive coupling etc, EXCEPT during fault conditions when it must carry the whole fault current until the circuit breaker trips or the fuse blows.

    Neutral and earth are usually connected at some distribution point, but this does not mean that they are interchangeable.

    JimB
     
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  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Wow that was over two years ago already (12/2013), didnt notice that :)
    It was still interesting though.
    Why didnt you start a new thread?
     
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  13. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Aluminum Refineries are like massive arc welders that use 100kA at high voltage and arc at the same phase voltage . In order to even out the power factor, 3 Pots may have its own transformer with Constant Current regulator once arc occurs which changes with boxite concentration added and gas forming high resistance on anode.

    Thus in order to have 48 Phases there must be a way to rotate the secondary phase. This is done by the geometry and phase shift of windings around the 3 phase core.

    In addition there must be multiple taps to set the voltage and adjust for feeder nominal levels which would come from the factories main transformer 500 MW type or more...as PG illustrated here http://www.electro-tech-online.com/attachments/3phase_delta_y_trans-jpg.92138/ in a tiny 150KVA unit. Usually 0.5 to 1.5 MVA units supply Al refineries with cheapest power available with govt subsidies or close to a reactor or cheap hydro power.

    So imagine the 48 choices of 3 phase as different factory chosen variables for each custom transformer and that the coils are continuous circle relative fixed position from primary to secondary done by design, so that 48 Smelter pots arc one at a time with 8 or 16 transformers like spark plugs in an engine to distribute the surge current pulse to the source and not create a huge degraded power factor... Just like a V8 delivers smoother torque under load than a 1 cylinder.



    [​IMG]

    There are also subtle phase shifters in transformers that allow the user to shift the phase by lag or lead using Zig zag phase connections to advance or retard phase to either the load or generator optimize power transfer.
    [​IMG]
     
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  14. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    even if the the thread was old, the concept needed a broader as well as focused reply
     
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  15. PG1995

    PG1995 Active Member

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    Thank you, JimB, MrAl, Tony Stewart.

    MrAl, I always try to combine related topics under one thread. It makes easier for me to refer back to the material.

    Tony Stewart, thank you for your help with that 48 phases query. MrAl also replied to that query but at the moment I won't pursue 48 phases topic any further until I'm done with intuitive understanding of three phase systems.

    Q1:
    i:
    Suppose, you have mains supply coming from a single phase generator. There is one live wire and one neutral. The neutral wire is grounded at the grid and also in your home. I'm sure that the neutral wire at the generator's end is also grounded. Do you agree that the neutral wire is grounded all along its path from the generator to your home?

    ii:
    I assume that you agree that the neutral wire is grounded at many points along its path. Now suppose that the neutral wire isn't grounded anywhere along its path to your home. Do you think that it would help you if your electric appliances are grounded in this case?

    Q2:
    This is the main question.

    This shows you a general picture of Wye three phase system and how its grounding is done. A Y-connection has a 'natural' neutral/ground point.

    Could you please show/tell that how a delta system is grounded? This might be helpful here. Thank you.

    Regards
    PG
     

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  16. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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  17. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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  18. PG1995

    PG1995 Active Member

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    Hi

    The following query(ies) is a follow-on to Q2 which was discussed in posts #6 and #7 above.

    The transformer is a Δ-Y one and it resides within the building; let's call it in-house transformer.. Its input is 480V and output is 208Y/120. "480V" is coming from the mains transformer which is placed on the pole; let's call it mains transformer.

    i: "208Y/120" means that line-to-neutral is 120V and line-to-line voltage is 208V. Right?

    ii: Do you know what such boxes are called?

    iii: The mains transformer is supplying the building with 480V. This box is directly connected to the mains line coming to the building. As you can see it reads 277/480V. I believe that it could mean that the mains transformer has secondary in Y configuration where line-neutral voltage is 277V and line-to-line is 480. Do I have it correct?

    Thank you for helping me with the queries.

    Regards
    PG

    Helpful links:
    1: http://waterheatertimer.org/images/277-480-3-phase-surge-AG48013-a.jpg

    (the following links were added after Tony Stewart's post)
    Z connected phases are also called interconnected-star (zig-zag).
    1: http://www.schneider-electric.co.uk...ang=en&locale=en_US&id=FA133185&redirect=true
    2: http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html

    general information about three phase systems:
    1: phase and line voltages in delta system are the same (video)
    2: http://www.ia470.com/primer/electric.htm
    3: http://waterheatertimer.org/How-to-wire-3-phase-electric.html (very good reference)
    4: http://www.electracool.com/277Volt.htm
    5: good video (think of open delta or open y, wye)
     

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    Last edited: May 9, 2015
  19. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Any time a surge load or short occurs on one line to neutral or line-line , the other phase voltages have overvoltage transients from imbalanced loads. N.B. ok?

    Okie Dokie
     
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  20. PG1995

    PG1995 Active Member

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    Thank you, Tony Stewart.

    Q1:

    Please have a look here.
    i: Out of "Delta" and "Delta+Ground", which one is the more popular?
    ii: As it looks, in this case all the three phases are grounded. What do those line segments in green represent?

    Q2:
    Please have a look on this panel schedule. Loads are connected to the phases in a way so that each phases carries approximately similar load. Why is done so? Suppose that in a region three phase is supplied to all consumers who have single-phase appliances. If all the consumers connected their loads to "A" phase then it would possibly trouble the generators. Think of three phase system being driven by three separate single phase generators. If all the consumers biasedly start connecting to some particular phase (or, generator) then this constitute too much load on a single generator which might damage it and other two generators would be running unloaded. Do I make any sense?

    Q3:
    What does this "100A Max" rating mean on this panel? I don't think that it's maximum current the panel can carry because at present too many breakers are connected to it and the panel has been working fine for years.

    It might be maximum rating for a breaker which can be used on this panel.

    Note to self: Have a look here; it might help you with Q3.

    Thank you for your help.

    Regards
    PG
     

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    Last edited: May 30, 2015
  21. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Q1:
    Most of the delta's i have seen only have an earth ground, which means it does not connect to any winding. It looks like it is totally separate from the windings.
    The delta can be loaded line to line or line to 'fake' neutral. The fake neutral is just the point where the three loads connect together like those three capacitor looking things in your drawing. The voltage then is obviously lower because it would be line to 'neutral' rather than line to line.

    The current and power rating for one phase would be less than for all three phases, obviously. Companies that order three phase lines will tend to use all three phases. If the current ratings are 100A, 100A, and 100A, if they only use one phase they only get 100 amps.
     
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