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33kV 3phase fault causing transmission line entanglement

  1. Jul 20, 2011 #1
    Good morning Gentlemen (and Ladies),

    I'm looking for some help from you HV experts out there.

    I have had an event where it is believed that a flooded transformer connection box induced a fault on to a 33kV 3phase transmission line, and the fault current was enough for the repulsive forces generated were enough to twist the 3 lines together.

    The initial event was for 2 cycles (50hz) and did not cause protection to trip.

    The fault current induces was capures on a trace and lasted 300mS, showing phases A & B initial contact and phase C approx 180mS into the event.

    Criteria:
    • 33kV 3 phase no neutral
    • Fault current - measured at approx 10kA per phase
    • Initial fault - across all 3 phases 1.5 - 2 cycles
    • Secondary fault - approx 180mS between A & B and C joined in for a further 120mS before trip
    • Duration between fault 1 and 2 - 1.7 seconds
    • Conductors: approx 100 meters in length (over 2 spans - 1 centre pole - centre pole is around a bend and the line translates from horizontal to vertical to horizontal at this centre pole), 16mm steel core aluminium, ACB at each end.
    • Weather - Cold (12 deg C), Medium rain, Minimal wind
    • No other external influences in the lines can be discovered.

    Now we have spent the last couple of days doing calcs based on every theory and formulae we can think of, and have results that range form induced forces of 30kg/m2 to over 1300N per linear meter.

    My gut feeling says that this isnt enough force over a 300mS period to cause this to happen, but I need to prove it - one way or another

    And none of us have ever heard of this type of event ever occuring in the real world (although we could find evidence of very heavy duty DC cables in an arc furnace snaking on the floor and having to be tied down).

    So what I'm asking is:

    Has anyone out there ever experienced this phenomenon?

    Do you have any reports / evidence / pics / etc?

    Can anyone provide a definate answer to the induced forces? And provde the maths behind it..... :)

    I can post pics if I can work out how, but I am happy to send to anyone who asks.

    Thanks in advance.

    Shane
     
  2. jcsd
  3. Jul 28, 2011 #2
    OK, I take it from the astounding interest and the pure number of replies that;

    Nobody knows - or they arent enough genuine propellerheads outthere with the math skills to work it out (which is good, because then I dont feel quite so stupid), or

    Nobody cares (which makes this whole thing abit pointless....)
     
  4. Jul 29, 2011 #3
    Obviously, it was not a "repulsive force" that twisted the lines together.

    Do you know what is the attractive electric force between two lines (or the total electric force on any one line) under normal operation?
     
  5. Jul 29, 2011 #4
    Sorry to disagree, but an "attractive" force might have caused then to touch, it would not have caused 1 line to swing over the ther two and wrap around them.

    And the field creates between 2 conductors, 50hz, 120Deg phase seperation (3 phase) would be 2/3rds repulsive.......
     
  6. Jul 29, 2011 #5
    hmm, you may be right. If one line is at 10 kV, then the two other lines are at - 5 kV. I'd think in this situation the -5 kV lines would repulse from each other and attract to the 10 kV line. When, 0.005 s later, the first line becomes 0 V, the other two would be at around -8.66 kV and +8.66 kV, so then those lines might be attracting.
     
    Last edited: Jul 29, 2011
  7. Jul 29, 2011 #6

    FOIWATER

    User Avatar
    Gold Member

    Don't know any maths for the attractive forces here... however if I had to guess I would say it's possible.. what kind of machines are supplying fault current? synch. generator sets cause low subtransient reactances (<5cycles of the fault) because of armature reaction, with such a low reactance its possible for fault currents (which are high...) to cause such an attraction..... I'm really not sure...

    Any sketches of the system on per unit or anything?

    Is it possible an arc caused the bolted fault?
     
  8. Sep 3, 2011 #7
    When a conductor carries a current it creates a magnetic field which interacts with any other magnetic field present to produce a force. When the currents flowing in two adjacent conductors are in the same direction the force is one of attraction, and when the currents are in opposite directions a repulsive force is produced.
    In most systems the current-carrying conductors are usually parallel to one another. The force produced by the two conductors is proportional to the products of their currents. Normally in most conditions the forces are very small and can be neglected, but under short-circuit conditions, they become large and must be taken into account together with the conductor material fibre stresses when designing the conductor insulator and its associated supports to ensure adequate safety factors.
    The factors to be taken into account may be summarised as follows:
    a) stresses due to direct lateral attractive and repulsive forces.
    b) Vibrational stresses.
    c) Longitudinal stresses resulting from lateral deflection.
    d) Twisting moments due to lateral deflection.
    In most cases the forces due to short-circuits are applied very suddenly. Direct currents give rise to unidirectional forces while alternating currents produce vibrational forces.
    In a three-phase system a short-circuit between two phases is almost identical to the single-phase case and although the phase currents are normally displaced by 120°, under short-circuit conditions the phase currents of the two phases are almost 180° out of phase. The effect of the third phase can be neglected. Therefore short circuit forces shall be repulsive force in two phase fault and it may be vary in three phase fault situation.
    In a balanced three-phase short-circuit, the resultant forces on any one of the three phases is less than in the single-phase case and is dependent on the relative physical positions of the three phases. In the case of a single-phase short-circuit, the forces produced are unidirectional and are therefore more severe than those due to a three-phase short-circuit, which alternate in direction.
    In the International Standard IEC 60865-1 a simplified method is stated for the calculation of maximum values of the
    – short-circuit tensile force Ft during or at the end of the short-circuit current flow,
    – drop force Ff when the span falls down from its highest point of movement,

    – pinch force Fpi in the case of bundled conductors, when the sub-conductors clash or reduce their distance without clashing,
    – horizontal displacement bh during the swing out of the span.




    For more information you can refer to:

    THE MECHANICAL EFFECTS OF SHORT-CIRCUIT CURRENTS
    IN OPEN AIR SUBSTATIONS
    A companion book of the CIGRE brochure 105


    Overhead distribution conductor motion due to short-circuit forces

    Power Delivery, IEEE Transactions on
    Issue Date: Oct. 2003
    Volume: 18 Issue: 4
    On page(s): 1534 - 1538
    ISSN: 0885-8977
    Cited by: 1
    INSPEC Accession Number: 7766847
    Digital Object Identifier: 10.1109/TPWRD.2003.817818
    Date of Current Version: 2003-10-14 10:45:07.0


    Movements of Overhead Line Conductors During Short Circuits

    American Institute of Electrical Engineers, Transactions of the
    Issue Date: Jan. 1929
    Volume: 48 Issue: 1
    On page(s): 67 - 90
    ISSN: 0096-3860
    Digital Object Identifier: 10.1109/T-AIEE.1929.5055178
    Date of Current Version: 2009-06-02 13:38:12.0

    --------------------------------
    Creative thinking is breezy, Then think about your surrounding things and other thought products. http://electrical-riddles.com
     
  9. Sep 3, 2011 #8

    jim hardy

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    Science Advisor
    Gold Member
    2016 Award

    It's a freshman physics exercise to calculate the attractive force between two conductors carrying constant current in opposite directions - in fact didn't they at one time use that phenomenon to define the amp?
    Anyhow alternating current alternates in all the lines so the forces probably continue to be attractive.

    i can tell you that in our three phase switchgear the copper bus bar was stoutly braced for precisely the reason you cite - fault current tends to pull the conductors together. We had fast breakers that cleared in about one and a half cycles.
    That your wires twisted together tells me something failed to clear the fault quickly.


    So i guess i'm not really much help except moral support to say i think you're on the right track.

    old jim
     
  10. Sep 3, 2011 #9
    In ac regime we can say:
    1-Induce forces always are atractive when phase difference between two faulted phase is zero.
    2-Induce forces always are impulsive when phase difference between two faulted phase is 180 degree.
    3-generally the average of time variable induce force is related to definite integral of F=K.i1.i2 and it may be attractive or impulsive regarding amount of phase difference, in 120 degree phase difference it is impulsive. Of course as you know in mechanical effects study the first impulse is very important, therefore everything will relate to instant of fault initiation.
     
  11. Sep 5, 2011 #10
    related diagram
     

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