33kV 3phase fault causing transmission line entanglement

[jackdaniel66]

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 isn't 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 occurring 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

 

[jackdaniel66]

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 don't feel quite so stupid), or

Nobody cares (which makes this whole thing abit pointless...)

 

[MisterX]

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 anyone line) under normal operation?

 

[jackdaniel66]

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 separation (3 phase) would be 2/3rds repulsive...

 

[MisterX]

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.

 

[FOIWATER]

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?

 

[jim hardy]

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

 

[m.s.j]

Anyhow alternating current alternates in all the lines so the forces probably continue to be attractive.

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.

 

[m.s.j]

related diagram