So, where were we in our attempt to bring the inherent virtues of The Grid into focus for the near-sighted non-engineers-or-physicists among us? We had wrapped up our paean to the Transmission half of the grid, and promised to conclude with a stimulating stroll through the world of Distribution.
Once again, remind me: who cares? Isn’t it just a bunch of wires and poles, occasional snowshoe-looking thingies, aerial metal canisters and boxes, and occasional weird complicated gray-metal structures – stringing basically the whole of civilization together (minus a few off-grid eccentrics) and keeping us all alive? OK– so yeah, maybe some of us feel we should know just a tiny bit about it.
We learned in The Grid Part 3 about how power is transmitted over the high voltage lines, and why they have to be high voltage. These lines carry the bulk power from larger generating stations over long distances to where the power is to be used. One of these circuits can carry the power needed for over 1 million typical homes (over 1 million watts) at voltages up to 750,000 volts.
Yes that’s a lot of volts! You wouldn’t want that many volts hopping around your neighborhood and charging into your house. So the high voltages are reduced at substations, which are those roadside conglomerations of metal components that look like a 5-year-old’s erector/lego set invention/engine room/1960’s sci-fi set/appliance graveyard/super scary playground.
But they do good things! The sub-stations reduce those 750,000 volts to somewhere between 5,000 and 25,000 volts. A distribution substation is essentially like an exit ramp for the power. It will tap some of the power off the high voltage line and transform it to a lower voltage to be distributed at a local level.
Distributed how? If you look up at the power lines running along the roadsides you will see a variety of types of lines and voltages. The utility poles also usually carry telephone and cable lines, the heavy thicker lines that are mounted lower down on the poles, carrying perhaps hundreds of telephone wires, cable TV, or fiber optic cable.
The electricity is carried on the somewhat thinner upper wires – those are the LIVE ones! They are generally bare copper or aluminum and can also be distinguished by the ceramic and glass insulators (necessary because they are live) that support them on the poles.
The electricity distributed by these lines is always AC. It turns out that it is very simple to change AC voltage up or down, but much more difficult to change DC voltages. So the AC current is reduced to 240 volts when it enters your home. This is where the transformers come into the distribution picture.
It’s all about transformers, from the wastebasket size transformers you see on the utility pole outside your home to the huge transformers at substations, and even to the tiny little transformers in your cell phone charger. What are they?
They are actually very simple devices. Typically they consist of two coils of wire, a “soft iron” core, and, in larger ones, some coolant, and no moving parts. They are also very efficient (as high as 99.7% for those massive substation transformers) at converting power from one voltage to another. If you follow your power lines from your meter to the pole, you will see that it connects to one of those wastebasket size transformers. This transforms the voltage to from a typical 7200V to 240V. There are three wires going to the meter: two insulated and one bare “ground” wire. There is 240 V between the insulated wires and 120V between each insulated wire and ground. Most of the circuits in your house use the 120 V, but some, such as a water heater, electric dryer, or range use 240V.
Transformers even work automatically in both directions. That is to say, power from a 7,200 volt wire on your street gets converted to 240 volts into your house, but if you have a solar array and are putting excess power onto the grid that same transformer converts your house voltage up to 7,200 volts.
You will also have a device called an “inverter” associated with your grid-connect photovoltaic (PV) array. The inverter takes the DC output of the PVs, converts it to 60 cycle AC and synchronizes the output with the grid-supplied power. This, along with the fact that the transformer out on the utility pole works equally well in both directions, allows power to go in both directions through the meter: into and out of the home. With the net energy billing currently in effect here in Maine, you only pay for the net amount of electricity you use.
Distribution surrounds us. It lines our streets and enters our homes, connecting and animating the myriad lifelines of our modern existence. Some of us like to leave it behind at least periodically, to hark back to our unelectrified roots. But almost all of us return home to our covenant with 21st century power reliability and quantity from The Grid. We expect it. Depend on it.
Might as well have at least a nodding acquaintance with it!
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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