transmission line tower

So that the wires will stay apart.

Cost materials needed to build tower higher to hold the same distance between lines then building the middle arms longer.

Mr Looflirpa say it is to prevent big birds (Ostriches etc) from bumping their heads against the cable above when they sit on the wire below.

Designed to maintain the mechanical requirement to prevent arching between conductors while maintaining a tower height that is manageable, and of course preventing head injuries to birds, a huge problem with different designs

I will have to check all three arms at the first opportunity. My assumption is that the arms are of different links to prevent a broken upper line from falling on one or more of the phase lines below.

Electrical reason rather than mechanical / structural.

In my view it is to prevent a broken upper line from falling on one or more of the phase lines below and this special design gives strength to the tower.

Reducing one arm to the minimum clearance with the pole will mean that material is wasted to put the other 2 at an excessive distance.

I actually haven't seen any of those but must admit that I don't even look for them. I thought the transmission line across the street from me were spaced in a straight horizontal line but they are spaced in a triangle.

Think:

Dissipation of lateral/rotational wind loads impose on the primary tower structure accomplished by triangular relationship of conductors.........and the GROUND DISTANCE/ ark distance issue cover in other posts.

MR. GUY

Mr. Guy, the wind loading would be the same regardless of the configuration on the tower, as the highest loading is at 90 degrees to the wire direction. The conductors are free swinging and therefore do not play a role in the structrul stability other than adding weight. This tower is built in a vertical configuration.

I would like to have information on line Material of transmission Line

Not necessary middle arm is always longer.

1. The clearance from other phase is a criteria.

2. Mutual inductance minimisation is another criteria.

3. Preventing droplet of water/ice to fall on bottom contor is another criteria.

Check with your TL vendor.

Ice was a huge issue here about a decade back, seems the transmission lines can't carry the load of an eight inch (8) coating of ice, go figure. Tends to bring down the lines and the towers.

If memory serves the conductor is dependent on voltage, google electrical transmission lines. There is an absolute maximum, which can be seen on transmission lines with several conductors on each line, normally four (4).

I figure I spent far too long in the electrical utility business to remember all this useless information, at least to me.

That isn't always the case. It depends upon the system voltage, tower type, terrain, span length, conductor bundle geometry, etc. In your example, the primary reason is electrical; to maintain sufficient phase to phase and phase to ground clearances under all static and dynamic loading conditions (including ice and wind loading) and to reduce the possibility of phase to phase faults. Those clearances are important at the towers as well as in the conductor spans between support structures; tangents, angles, and deadends.

You also don't mention if the tower is single or double circuit.

Another consideration is lightning shielding. Chances are the tower you noticed is also supporting one or more overhead shield wires located above the phase conductors. Although overhead shield wires often perform secondary functions; such as communications, data acquisition, monitoring and control (and may include optical fibers for those purposes) the primary function is to intercept lightning strokes and discharge them to ground thereby preventing direct strokes to the phase conductors. The shielding angle between the shield wires and phase conductors is critical to minimizing shielding failures (when lightning strokes bypasses the shield wire and strike a phase conductor).

If you have the opportunity, take a look at the EPRI Red Book (Transmission Line Reference Book 345 kV and Above) or any number of similar reference and handbooks to get an idea of the different types of lattice, guyed lattice, vee, guyed vee and other tower and pole geometries.

I was trying to keep it simple, having said that you are absolutely correct, there are a vast number of environmental considerations, circuit configurations and load factors. Not to mention the budget restraints, but that is another thread. Then there is the dreaded environmental assessment and the not in my back yard mentality. I remember a project that had to have tower choice changed because a well connected lobby group didn't like to look and wanted something that was less distracting, uninterrupted power supply was of course never a consideration for them, heaven forbid the lights should go out or the pool filtration system shuts off...

<Rant interrupt>

Hi Lineman380, strickly for mutual inductance. You will notice that there are three transpostions in the line. (if any type of distance) Keeps the the phases balanced. Crazy things start happening when you start playing with the higher voltages 240kV ac and above.

There is a 240 kV line in Northern Alberta with flat spacing that runs for 450 km (280 mi) and has a problem with capacitive inductance from the earth. When first turn on, the input voltage was 240 kV. Thinking at the time that the output at the far end, with line loss, would be be about 220 kV. Wrong, turned out the output would climb to 270-280kV with an increased load. They found out because the three phases of this line ran parallel to the earth for such a distance, the capacitance of the earth induced into the line would cause this problem. The overvoltage relays would keep tripping.

You need a triangle of the phases, to lower this type of problem. Hence, the longer arm on an "L" tower.

The geometry is not strickly (nor strictly) for mutual inductance. In fact that's not even a major consideration. As you correctly note, phase transpositions are used on long ac transmission lines to achieve balance and reduce line losses but that doesn't require a triangular circuit arrangement. Transpositions are employed on all types of line construction. Incidently, the poster would generally have no way of noticing "that there are three transpositions in the line" because transpositions are not made that frequently.

The overvoltage problems on the long 240 kV line in Northern Alberta were almost certainly due to power factor but the coupled capacitance of the flat (h-frame) construction was only one component. Electrical transmission lines are fundamentally capacitive but loads are predominantly inductive. I suspect the loaded overvoltage problems were solved by the addition of series capacitance.

Actually thanks to CR4 and to every body joint to answer my question, and really these answers gives us more learn and its open our thinking for many ideas.

Thanks again

from what u mention its is double circuit line.Normally middle arm is longer than top and bottom which are of same size.Under certain conditions vibrations can set of galloping of conductors and clearance under these conditions in vertical configuration becomes less.There fore its a standard practice to have longer middle cross arms in double circuit towers having conductors in vertical configuration.If u see single circuit towers with conductor in vertical formation where two cross arms on one side and one on another side or a bridge tower where conductors are in horizontal formation you won't find this.