Unbalanced 3phase system

Discussion in 'Homework Help' started by Cerkit, Oct 13, 2010.

  1. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Hi.Can someone explaine what it means to have an unbalanced 3phase system. If that is the case how exactly does the neutral serve to carry away excess current which results from such a system?
     
  2. R!f@@

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    Apr 2, 2009
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    Unbalanced means that the load supplied to a 3 phase supply are not equal.
    This happens only when the phases area loaded by separate single phase appliance connected across a typical 3 phase input.

    A 440VAC 3 phase can have any no. of single 220VAC phase loads. These type of loads shud be connected equally across the R,Y & B phase. If the phase current are not equal, In balance occurs. And are never good for 3 phase Gensets or Power companies. Which always results in Dynamo Stator burnout

    Incorrect and correct phase balanced return currents are always returned via Neutral Conductor. But this is not the case for a 3 wire 3 Phase Power system
     
  3. bertus

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    Apr 5, 2008
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  4. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    When inbalance occurs what exactly happens that is bad for the power companys' generator sets??
     
  5. R!f@@

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    That overloaded phase coil eventually burns
     
  6. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    So essentially it isn't realy the inbalance that is an issue it's simply a result of overloading a particular phase by demanding more than the rated source current..??
     
  7. R!f@@

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    No. Rated Current shud not exceed under any circumstances.

    Imbalance is a real problem under light loads too. But light load does not occur in a three phase system with 4 wires.

    I donno the theory behind burn out, if u asking me tht, I cannot answer. U'll need a professor to answer tht.

    What I know is from practical experience. I know dynamo will burn out
     
  8. DonQ

    Active Member

    May 6, 2009
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    The electric company equipment, from the generator, through all the lines and transformers, has to be able to provide the power required by the highest loaded phase. It still costs them money for all the other unused capacity on unbalanced systems. Using all phases equally maximizes efficiency of the system.

    Also, in Y systems, balanced loads mean zero current in the neutral/center wire. This allows this line to be a smaller gauge wire. Imbalance makes a current flow in this wire. With a very unbalanced load, it can cause a significant voltage drop across this lead, and other problems.
     
  9. subtech

    Senior Member

    Nov 21, 2006
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    There are many reasons why an unbalanced system load is undesirable. Several reasons have been covered here.
    However, from the aspect of a large synchronous generator one of the most significant problems created by an unbalanced system is that of an induced rotating magnetic field which opposes that of normal rotation. This reverse sequence rotating field causes additional heating in the stator windings, in the iron parts of the stator structure, and in the iron parts of the rotor structure as well. If allowed to persist in sufficient quantities, these negative sequence effects can cause severe damage to the generator. Usually, there are protective devices which will disconnect the generator from the system when such an unbalance persists.
    If you are interested in learning more about three phase power systems, and in particular aspects of unbalanced power systems, a search for "symmetrical components" will open a whole new world to you. :D
     
  10. zgozvrm

    Member

    Oct 24, 2009
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    Actually, the neutral conductor (in a wye-connected system) only carries current when there is an imbalance. When the system is balanced, there is no neutral current!
     
  11. DonQ

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    May 6, 2009
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    Yeah, I've read that somewhere before...
     
  12. R!f@@

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    I beg to differ
     
  13. eblc1388

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    Nov 28, 2008
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    Can't argue with mathematical proof.

    [​IMG]
     
  14. R!f@@

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    tht's not actually what I have in mind.
    I was thinking more of a practical approach.

    I have to settle down to write it though. I'll get back on this later
     
    Last edited: Oct 31, 2010
  15. zgozvrm

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    Oct 24, 2009
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    Good luck with that...
     
  16. R!f@@

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    OK..first of all, guys, this is not so much of a big thing to get so cooked up about. I just said I have different experience with it.
    And I am not trying to prove what is written down ages ago wrong.

    I donno abt most of u, but I have worked with 3 phase gensets and loads.
    What I have here I think, are almost same ones u have in ur areas.

    For a three phase system, there can be 3 wire 3 phase and 4 wire 3 phase.
    The 4th wire is the neutral conductor.

    When talking about phase imbalance. It is hard to load the 3 wire 3 phase differently with a 3 phase load such as a motor or transformer. In this case a bad load is definitely the culprit. So this scheme is ruled out.

    For a 3 phase with a neutral conductor results in the 4 wire type

    Any 3 phase load will load the phases equally even to this scheme. Even here a 3 phase motor without a neutral can be used. So even at fault no neutral current occurs. A fault in a motor reflect to the source as imbalance load. Am I wrong here ?


    Now take the scenario we have here. To building here, we are supplied with 440VAC 3 phase 4 wire. So we can get 220VAC from each phase if we wanted it. And it is in this case phase balance is a must. The 220V loads should be equally loaded to all the phases. Any change here will result in imbalance loads.

    So, balanced or not, load current is handled by the neutral through out all the phases here. Even though 220V is used we are loading 3 phases separately and equally.

    This is what I was thinking of. I donno how it is done in other countries
     
  17. zgozvrm

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    Oct 24, 2009
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    If you've truly worked with 3 phase systems, then you'd know that you would NOT get 220V from each phase,
    but rather 254V, since 254 * √3 = 440


    Although the single phase currents are all handled by the neutral. That doesn't mean that they're equal to each other.

    For instance, a typical 4-wire wye distribution in the U.S. is 208/120. That is, line-to-line voltages measure 208V,
    whereas line-to-neutral voltages measure 120V. This is often used in industrial lighting. You could have several 120V lights connected from Phase A to neutral, several from Phase B to neutral and several from Phase C to neutral. It is entirely possible (and likely) that the current in this situation would not be the same on each phase, therefore there would be a current imbalance.

    If you could connect appropriate loads so that each leg was drawing the same amount of current, then the system would be balanced. If you where to measure the current on each phase, you might have 30 amps each, but each of those currents is 120° out of phase from the other 2 currents and they cancel each other out. A current measurement of the neutral wire would verify this.

    The same holds true for a typical (in the U.S.) 240/120 split phase system used in households. The neutral is common to both legs. If each leg draws the same amount of current, the currents on the neutral cancel each other out.
     
  18. R!f@@

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    This might be true for mathematical proof. But as for me, I have measured line to line of 400VAC thru 440VAC here, and a line of voltage of 200VAC to 230VAC.

    Seems to me u are the one with limited practical knowledge.
    I believe what I see, and no one can convince me otherwise

    This all may be true for u all.
     
  19. gschultens

    New Member

    Oct 23, 2010
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    Phase imbalance (not in balance or inbalance) is also a problem for 3-phase electric motors. It results in lower power, but higher current as the motor works harder to drive the mechanical load. Therefore, there will be higher currents in the motor resulting in the risk of overheating, or at least tripping the electrical overload protection.

    Basically, you want all your phase voltages to be equal, and if not, you have a problem that must be resolved.
     
  20. zgozvrm

    Member

    Oct 24, 2009
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    So, you're saying mathematics is incorrect?


    I don't know what measuring line-to-line voltages has to do with line-to-neutral voltages.


    I have extensive practical knowledge. I'm an industrial electrician and have been working with, and studying 3 phase power systems nearly 30 years.


    So it's true for everyone but you? Seems to me you're the one having difficulty understanding!



    Note that there are two kinds of 3 phase distribution systems: wye and delta. For systems where a neutral is used, the wye is more common since it allows for the possibility of a balanced system, whereas a grounded delta system (be it corner-grounded or center-grounded) will be unbalanced by design.

    In the case of a center-grounded delta system, the line-to-neutral voltage readings from 2 of the phases will result in half the line-to-line voltage (that is, 240V line-to-neutral on a 480V system). However, the 3rd leg will measure approximately 208V to neutral.

    On a grounded 480V wye system, the line-to-line voltages will, of course, measure 480V. But measure any one of the phases to neutral, and you will see that it is 277V, not 240V.


    If you're getting something other than these readings, you're either doing something wrong, or you have a bad meter.
     
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