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Treebeard's Stumper Answer
12 January 2001

Power Lines

It's easy to take electric power for granted, unless the power goes out or it's time to pay the bills. Lately we've all been more aware of our power use because of California utility company financial problems and shortages. I always watch the power lines on my way to school because they are such a prominent perch for hawks and osprey. Where does our local power come from? Why are there so many different types of power poles with different numbers of wires? How can birds perch on power lines without getting electrocuted? Could we do the same?


Birds and utility workers are safe if they only touch one wire and nothing else. It's not voltage, but voltage difference that's dangerous, just as it's not more risky to step off a curb in Denver despite the mile-high elevation. Large birds like eagles are at risk of spanning two wires, and many die needlessly. It's more efficient to transfer electricity at high voltage and reduce the level before it enters our homes. Different designs for power lines reflect the different loads they carry. Hannes Faul's natural gas plant in Los Alamos is closest, but our power really comes from "The Grid" of power plants across the West.

Notes:

Power is expensive, so it's important to minimize loss when moving it around. You can put your thumb over the end of a hose to make a jet, but it will take longer to fill a bucket because there's less water at greater pressure. A match flame is 2000 °C hot, but you can't boil a pot of water with a match because there's not enough heat. So too, electric power depends on both voltage and current.

  The power loss through a resistance is:
        Power = current x voltage
            P = IV

  By Ohm's law,
      Voltage = current x resistance
            V = IR

  Combining with simple algebra gives:
            P = IV = I(IR) 
              = I2R
Power loss depends on voltage and resistance and the square of current, so it's more efficient to transmit power over long distances at high voltage and low current. Transmission power lines move fewer electrons (current), but they move them with more energy (voltage). High voltage is dangerous because it can jump between conductors, so high voltage power lines must be widely separated. That's effecient, but expensive. This is true even on a micro level. Computer CPU chips like the Z-80 originally operated at 5 volts (or more). One step in creating faster and denser CPU chips was to reduce the core operating voltage to 1.65 volts to allow narrower traces on the chip.

High-voltage low-current power must be converted to a low-voltage high-current form for household use. This conversion to moving more electrons with less energy is done in several steps with transformers, which vary from "wall wart" power packs to the waste basket-size gray cans we see on power poles and the huge monoliths visible behind the fence in power substations.

Transformers only work with changing AC (alternating current) electricity. Thomas Edison promoted DC (direct current) power distribution a century ago, and successfully fought the AC system promoted by Nikola Tesla and his backer George Westinghouse. (There's a fine PBS documentary and web site about the Edison/Tesla AC/DC fight.) Now the power grid is (almost) all AC because of the ease with which voltage and current can be traded off using transformers.

Electric power is generated and transported at different voltage/current levels, and the different designs for power lines reflect the different loads they carry. Efficiency effects profits, so I assume there's a reason for all the variation. Here are a few photos of the different power line configurations I've found trying to trace the power distribution system to my home and work in Santa Barbara County, California. I pay my home power bills to Southern California Edison (SCE), but my school in Los Olivos 25 miles away gets power from Pacific Gas and Electric (PG&E). You can trace your own power distribution by looking around, but it's not easy. This is not complete!


The Mandalay plant at Oxnard Shores. SCE
recently sold this plant to Reliant Energy.


The huge towers of the PG&E Morro Bay
power plant are visible in this aerial shot.

Diablo Canyon Nuclear Power Plant, operated by PG&E.
It's hidden away, but the site of many protests.

There are no large power plants in Santa Barbara County! The closest plants are at Oxnard Shores in Ventura County to the south, and Morro Bay and the Diablo Canyon nuclear plant in San Luis Obispo County to the north. I guess that's the influence of wealth, and a successful NIMBY (Not In My Backyard) strategy.

We used to surf in the warm water from the Mandalay/Oxnard Shores power plant when I was a kid. That was great fun at night because they had huge spotlights pointing out at the waves, but it was tough if you lost your board in the dark!

(Power plant photos from the California Coastal Conservancy.)


Hannes Faul showing his natural gas
power plant to DMS students.
Dunn Middle School parent Hannes Faul runs this independent power plant as Los Alamos Power. He burns waste natural gas from nearby oil wells. This is the closest power plant that I know of, apart from the (unused?) wind generator at Laurel Springs Ranch on San Marcos Pass.

There's an interesting philosophical contrast between these small environmentally-friendly "backyard" power generators and the massive power plants favored by utility companies.

Local power doesn't come from a single source, like water from a well. It comes from "The Grid" of power plants across the West, but every little bit helps, and it's all accounted for.

Power comes to Santa Barbara through these high-voltage transmission lines. The towers march from Ventura to the east along the north side of the Santa Ynez Mountains (left), across San Marcos Pass to the mountain saddle above Bear Creek past the Winchester Gun Club. Then they return back east across the foothills (right) to Santa Barbara. These are high-voltage distribution lines at 155,000-765,000 volts. You can hear the power sizzle as a 120 hz buzz!

The picture on the top-left is the tower on Fremont Ridge on the north side of San Marcos Pass. It has six wires in pairs plus an extra static line on the top (look close) to divert lightning strikes. The picture on the top-right shows the same transmission line where it crosses Highway 154 on the city-side of the mountains. Somehow it becomes nine wires on two towers, with no grounding wire on top. I don't understand this.

High voltage power lines have three wires that carry high AC voltage from 155,000 to 765,000 volts, but slightly out-of-sync as 3-phase AC at 60 hz. The benefit is that one wire is always near the max. All transmission lines are 3-phase before they enter our homes.

These are pictures of the SCE San Marcos Substation on Foothill Road near the bottom of Highway !54. A high-voltage tap comes from the power lines shown above. The first photo shows the huge transformers that convert the 155,000+ volt high voltage power to three-phase power at 7-13 kilovolts for distribution through town. That lower voltage is not as efficient, but it's not as dangerous, and it allows for closer wires on the poles. I think the second photo is a distribution bus that divides that great power into separate power circuits for different neighborhoods. There's three wires in, but six wires out for two separate neighborhoods. This is confusing since we also have three wires in our homes: hot, neutral, and ground, but that's different!


These poles come directly from the power substation shown above with six wires in pairs on top, and another crossbeam that might be a regulator bank. I guess they split into two three-wire lines for different neighborhoods. There are more wires lower down that are probably telephone lines and cable TV, and maybe a ground wire(?).


These double poles come up San Marcos Pass from Santa Barbara to serve residents on the Pass like me. I guess they are a higher voltage than normal neighborhood lines, but less than the transmission towers. Note the wide spacing between the wires going through the picture. The wires on the left branch that go to Hidden Valley are closer.

There are many styles of neighborhood power distribution lines beyond the substation. Most carry three-phase power at 7,000 - 13,000 volts on three separate lines. I haven't noticed an extra ground/neutral wire, though many poles are grounded through a separate wire that runs down each pole. (From where?) Many utility poles also carry telephone lines and cable TV lower down the pole.

The three-phase, moderately-high voltage lines carry electric power close to home, but there's one more transformer to reduce it to household level. There might be a separate two-line power tap from the main before the transformer. The transformer is the can on the power pole or that green box on the lawn. These transformers usually tap any two of the three three-phase lines and deliver three lines to our homes. Our usual line voltage is 120 volts (nominal), but we actually get 240 volts in two separate live wires in two phases and a neutral wire, so we can get 240 volts if we need it. This is confusing since we also see three holes in every electrical socket, but the usual 120 volt hot, neutral, and ground is only half the story.

What a trip from the power plant to my wall socket! It's easy to take this all for granted, but there are problems with this system.

I'm out of time this weekend! I have more to say about:

Check back soon, tomorrow night I hope!
The real stumper is for you to trace your own power to its source!

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Copyright © 2001 by Marc Kummel / mkummel@rain.org