You’ve heard that knowledge is power.  Well, we say knowledge of power is power.

As our energy future comes into focus, we begin to understand that the future is all about electricity.  No more combustion, just channeling the energy of natural forces (wind, sun, water, tides) and distributing it as electrical current.  As we shift away from burning things, everyone, even incurious English majors, will benefit from a basic appreciation of the dynamics of electricity distribution.

It’s certainly easy for anyone born in the 20th or 21st century to lack a relationship to the electricity infrastructure that has surrounded us our entire lives.  When something is part of the visual landscape and requires no response from us, our brains take it for granted and more or less block it out of conscious perception.

This is how average citizens might have absolutely no awareness of the aerial wires that are everywhere, along with transformers, insulators, substations, circuit breakers, and other doodads, and find themselves utterly astonished the day they look around and start noticing the bizarre and ubiquitous spiderweb of electrical infrastructure festooning our inhabited spaces.

Would you like to join the power cognoscenti, and learn to speak truth about power? Read on!

Let’s start with the two different kinds of power lines. Transmission lines are those with the tall towers that you see running through the countryside over hills and dales.  These carry the bulk power from generating stations to substations and from one substation to another. Distribution lines are the ones you see along the roads that bring power from a substation to individual customers. Those poles also usually carry telephone and cable lines–the fat cables that are a few feet below the power lines. Here we’ll discuss transmission lines and save distribution for next time.

The transmission lines are high voltage – power super-highways.  We need high voltage transmission lines because of the inherent problem of power loss over distance – typically 5% to 10% is lost, due mostly to electrical resistance in the wires, which generates heat.  The amount of loss to heating depends on how much current (amperes) there is in the line and the electrical resistance of the wires (ohms). The more current, the more loss. And these losses are not proportional to the current, but rather to the square of the current, so that ten times the current produces a hundred times the loss.

But why such high voltage? The power (watts) transmitted is a product of voltage times the current. (volts x amps).  That means you can transmit the same amount of power with a lower current if the voltage is higher.  Since we are trying to minimize losses, the higher the voltage then the lower the current, and lower the losses.

If you tried to transmit large amounts of power at 240 volts the currents would have to be huge, and the power lines would melt.  Even at 25,000 volts most of the energy would be lost on a long distance line carrying large amounts of power.  But at 250,000 volts a much smaller fraction is lost– 100 times less. The highest voltage lines are over one million volts but are typically between 40,000 and 750,000 volts for transmission lines.

Two kinds of current are involved in the electrical power system: Alternating (AC) and Direct (DC).  To simplify ridiculously, AC voltage oscillates (varies in waveform cyclically) 60 times per second (in the US), while DC voltage is steady.

All distribution lines and almost all transmission lines carry AC.  DC lines are able to transmit power over very long distances with smaller losses than high voltage AC lines.  However AC has a specific advantage over DC.  It is very simple to change AC voltage up or down, but much more difficult and costly to change DC voltages, so AC lines are used for distribution.

DC lines do have some other advantages, but they only pay off when used over very long distances or underground and underwater. They are also ideal for interconnecting separate synchronous grids. DC electric and magnetic fields exert virtually no effects on living things, while the jury is still out on AC’s effects.

The proposed power line from Quebec to Maine would carry 1200 megawatts of DC power at 310,000 Volts. The line would thus carry a current of about 3800 amperes.   Some basic physics calculations tell us that the magnetic field of the line at ground level, directly underneath the line, is roughly equal to the earth’s field—much less than a typical refrigerator magnet—and the static electric field is less than when you pet your cat or pull a sweater out of the dryer.

Contrast this with the AC current that is almost ubiquitous in the modern world (start paying attention to every wire around you — roadside overhead, to your house, around your rooms, throughout your walls) and you can either feel better about the high voltage lines or doomed about the AC in our lives.

Next time we’ll work on getting to know and love our “frenemy” AC, and explore the dynamics of distribution.

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side.  He can be reached at pauls@maine.edu.  Cynthia Stancioff, one-time English major, likes to re-word things. Previous columns can be found at https://paulandcynthiaenergymatters.blogspot.com/

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