Power Lines - To All Sparkies

[tricky4000]

Excuse my ignorance here. While travelling back from a little beach town on the 90 Mile Beach (South East Vic) over the Easter break, I began noticing the power lines along the roads (the little highways in particular). My question to those out there in the know, why do power lines have so many lines? I understand single phase and 3 phase, but why is there sometime 6 lines or 8 lines on a power pole...I even counted 15 on those big power towers leading from the La Trobe Valley power stations.
Why so many wires? Is there more than 3 phases out there?
Sorry if that's a stupid question. Laugh all you will if you need.
Dan.

[juan]

why is there sometime 6 lines or 8 lines on a power pole?

I even counted 15 on those big power towers leading from the La Trobe Valley power stations.

Depending on the circumstance you may have seen multiple voltage transmissions using the same poles or in the case of larger pylons, the use of multiple wires in parallel to increase current carrying capacity. It is not unusual particularly in metro areas to have high voltage(11,000v etc) strung on the same poles as 415v 3 phase. Often the step down transformers are mounted on the same pole.

Is there more than 3 phases out there?

No, 3 is the maximum.

cheers

[chrisp]

Dan,

On the really high-voltage lines if the conductor diameter is too small, the electric field intensity can become too large for air-insulation. That is a corona can form around the conductors.

The way to reduce the electric field strength near the conductor is to increase the diameter of the conductor. But a large conductor can become heavy and expensive - so a trick is done using a number of small conductors to mimic a large conductor and thus reduce the electric field without the full expense and weight of a large conductor.

Have a look at 500kV transmission lines and use will see each of the phase conductor is made up of four conductors spaced to form a square cross-section. You will also notice spaces placed between the conductors to maintain the cross-section through the drops between the pylons. The lower voltage high-tension lines get away with using two conductors to do the same trick.

[thatirwinfella]

...okay, i'm done laughing. just kidding, it's a good question, juan and chrisp has it spot on.

[Smurf]

Also it is common to run more than one circuit on a pole or tower. So the six wires on a transmission line are two (3 phase) circuits.

The ends of those may be in totally different places - the lines may come from totally different directions but be on the same towers at one end where they physically (not necessarily electrically) meet.

Same with poles beside the road. 9 wires could be 3 x high voltage lines which may go in different directions after a point. Or could be different voltages as others have said.

[dan76]

as aobve

[malb]

Answers offered in above posts are valid but sometimes there may be be additional lines at the 240/415 V level on the poles as well, being (I believe) a neutral line for and a common switched line for sensor operated street lighting (multiple lights grouped to a single sensor)

[Smurf]

Surburban line configuration varies a bit between locations but the following is the most common arrangement in Tas.

Top - 3 wires for the 11, 22 or 33kv high voltage.
Next row - 4 wires at 415/240V including a neutral.
Next - Streetlighting pilot wire.

Next - Additional streetlighting pilot under some circumstances.
Bottom - Any other cable, for example communications cables.

In most urban situations only the top 3 rows above would be on the pole. That is, HV, 415/240V, 1 streetlight pilot. The other two (one or both) are less common.

Some short suburban runs (generally limited to a street or two) do not have any HV. In that case it's 415/240V at the top and anything else down below.