Message-Id: <v01540b16b1ee4ecb8e56@[192.174.2.173]>
Date: Wed, 5 Aug 1998 10:04:39 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: power line info
I recently asked EPRI for information on high voltage transmission lines.
I got back this wonderful infromation
Paul D
>
> 1. The first thing you can notice about high voltage transmission
> lines is that the wires come in multiples of three. This is because
> we generate electricity at what is called three phase. You can have a
> single circuit transmission line with three phase conductors. These
> usually have the wires in a horizontal configuration. Occasionally
> you may see a single circuit line where the wires are in a triangular
> or delta (named after the Greek letter) configuration. Sometimes you
> have a double circuit transmission line where two circuits are on
> opposite sides of a tower in a vertical configuration.
>
> 1a. You can tell the difference between an AC and DC line by the fact
> that an AC line has three conductors and a DC line has only two -- one
> for the plus side and the other for the minus side of the circuit.
> There are only two places around here where you can get a good view of
> a DC line. On is on Interstate 80 east of Reno before you get to
> Lovelock. The other is in the Los Angeles Area near the town of
> Sylmar. Now one must be careful because distribution lines often
> come with just two wires. We are talking about high voltage bulk
> power cross country transmission lines, not the lines that run up and
> down the streets on wood poles by our homes!
>
> 2. Lightning protection is achieved by placing one or two small wires
> on the very tip top of the towers. These wires are not insulated from
> the tower but connected directly to it. In that way these wires
> intercept the lightning stroke and safely guide it to ground. In
> California lightning is an infrequent phenomenon. Here in the Bay
> Area we have between one and maybe five lightning storms a year. In
> the mid west there may be as many as 30 to 50 lightning storms per
> year. In Tampa, Florida, where there is the most intense lightning in
> America they may have 90 to 100 lightning storms a year. EPRI has
> developed a lightning detection network.
>
> 3. The voltage of a line can be determined by counting the number of
> insulators. Lines operate at a constant voltage which in the case of
> PG&E are 115,000 volts, 230,000 volts and 500,000 volts. Generally
> 115kV lines have about 6 insulators, 230 kV lines have 10 or 12
> insulators, and 500 kV lines have 24 insulators. BUT in the Bay Area
> because we live by the sea and there is contamination on insulators
> from salt spray and fog, PG&E generally "double insulates" some of the
> lines meaning they use twice as many insulators as would be needed in
> locations away from the sea or they use special fog insulators which
> are larger than the normal insulators used on transmission lines.
> This may be a subject of interest to the kids. Can you spot fog
> Insulators on any lines around here?
>
> Insulators are also designed to be in I or V strings. An I string is
> a single string on insulators that just hangs down from a cross arm.
> A V string consists of two strings of insulators that are in the shape
> of a V. V strings are usually on the highest voltage lines to
> constrain movement of the conductors in the wind and reduce the size
> of the towers for economic reasons.
>
> 4. Going back to conductors or phase wires. Vibration is a problem
> on many transmission lines. To prevent vibration engineers install
> dampers called Stockbridge dampers on the lines near the towers.
> There are devices that look sort of like dumbbells. Many lines in the
> Bay Area have these dampers installed on them.
>
> Bundled conductors are another things you can observe about
> transmission lines. Many lines just have one conductor per phase, but
> some, to increase the current carrying capacity of the line, have two
> or four conductors (a few lines in the eastern USA have three
> conductor bundles but not here to my knowledge). These are closely
> spaced wires and generally there are spacers placed along the span
> between the towers to prevent the wires in the bundle from clashing
> together. There are very good examples of this on I-680 behind
> Fremont. These lines come over the hill and head toward the bay.
> Several circuits have bundled conductors.
>
> 5. Occasionally you will see a transmission line with bright orange
> balls installed on the ground wires. These balls are to warn
> airplanes that the line is there.
>
> 6. Transmission lines may be on lattice structures made of small thin
> elements like an Erector set if kids play with that kind of toy any
> more. Other lines are on "tubular" structures. Going down I-280 from
> Los Altos to San Jose you can see a line running parallel to the
> highway that has both kind of structures.
>
> 7. Finally kids should notice the shape of the wires between the
> towers. This shape is known as a catenary and can be defined very
> exactly in mathematics. Engineers talk about the sag of the line
> which is the distance from an imaginary straight line drawn between
> the towers and where the lowest point of the line actually comes.
> Lines are under tension; the wires are not just hanging there.
>
> I don't know what he means by "braiding" the wires unless he is
> talking about transposing the lines to reduce impedance unbalance.
> PG&E loves to transpose their lines because they tend to be rather
> long. A transposition tower is a rather large structure where the
> position of the wires is changed. The top wire is moved to the bottom
> and the other two are moved up. You can see a number of transposition
> towers in the Bay Area.
>
> Now you know everything to look for when you see a transmission line.
> Probably more than you wanted to know