If you have electrical questions you’d like answered in a future edition of this column, send them to the Editor at northshorejournal@gmail.com, or email John directly at john@clovervalleyelectric.com.
From the Generator to Your Meter
Last time, we sorted out a reader question about volts, amps, watts, and ohms, which are the fundamental units used to describe electricity from a scientific perspective. This week I want to put those units to use and follow the energy on its journey from the utility generator to the customer meter.
Electricity can be converted from all kinds of other energy forms, including falling or flowing water, wind, sunlight, hydrocarbon fuel, nuclear reactions, and many more. However, electricity begins its journey roughly all the same way: as alternating current (AC) at a fairly “modest” voltage, generally a few hundred volts to a few thousand depending on the type of source generator or inverter. One common example is a natural gas combustion turbine power plant, which burns gas to spin a turbine much like a jet engine on an airplane. That turbine spins a generator, and a spinning generator produces AC electricity. A solar farm works differently, since its panels make direct current (DC) from interactions between photons from the sun and millions of tiny gates on a semiconductor-type wafer. The DC electricity is then converted to AC by an inverter, often at somewhere between a few hundred and a few thousand volts. That voltage is plenty for the equipment and utilization close by, but nowhere near enough to transmit from, say, Duluth to Grand Marais.
Moving electric power a long distance is challenging due to the resistance, or impedance, of the wires used in the process. While a foot or two of large aluminum conductor might have a very small impedance, over 50 or 100 miles that becomes significant. The impedance of the wires, expressed in ohms, causes electricity to convert to heat and dissipate, resulting in the loss of electric power. The amount of power lost in moving it through wires is based on the current squared times the impedance. To put it plainly, if you have twice the amps flowing on a wire, you have quadruple the losses.
The solution developed by engineers was to raise the voltage used to transfer power between remote generators and users. Say you need to deliver one million watts of electricity down a fictional transmission line from Duluth to Grand Marais. At 1,000 volts, that takes 1,000 amps, and the heat lost in the wire would be enormous. Raise the voltage to 10,000 – ten times higher – and you only need 100 amps to move the same million watts. Because loss depends on current squared, cutting the current to one-tenth cuts the loss to one-hundredth. Raise the voltage by ten again, to 100,000 volts, and the loss drops by another factor of a hundred. This may be the single most important physics trick in the entire grid: push the voltage way up so the current, and the waste, significantly decreases.
Going back to the generator, after the power is produced, it goes through a step-up transformer that raises the voltage and reduces the current, with a small amount of loss in the process. The solar farm’s few hundred volts and the turbine’s roughly twenty thousand volts both get stepped up to a “transmission” voltage level, typically somewhere between 115,000 and 345,000 volts here in northern Minnesota. At those voltages, a tall wooden or steel transmission line can carry hundreds to thousands of megawatts of power across the state while losing only a small percentage along the way. On the North Shore, the majority of our transmission lines – operated by Great River Energy and Minnesota Power – carry power at 115,000 volts.
However, running 115,000-volt lines down a residential street would be a terrible idea. Voltage that high requires significant physical clearance for safety, which is why those towers hold the lines far apart and far off the ground. High voltage is wonderful for crossing a county but impractical for crossing a backyard. To solve this, engineers build substations near population centers to reduce, or step down, the voltage to what we call distribution levels – maybe 7,200 to 34,500 volts. That’s the level of the smaller lines we see running through town and back into the woods, usually on the wooden poles most of us see day to day. It’s high enough to move power efficiently across the last few miles, but low enough to run overhead along a road without the giant clearances transmission demands.
Finally, the local utility places one last transformer – on the pole outside, or in the green box on the ground – that takes those 7,200 to 34,500 volts and steps them down one final time to the 240/120-volt service your home uses. From there it runs through your meter, into your panel, and out to the space heater and the well pump we talked about last week.
In a simple version, that’s the whole path electricity takes from the generator to your home: stepped up at the generator, sent down long transmission lines, stepped down at a substation for the trip through town, and stepped down once more at your transformer for the house. Every step is volts traded for amps, or amps traded for volts, all to move watts from where they’re made to where they’re used while wasting as little as possible along the way.
When we return to this series in a future column, I’ll get into what all this costs – namely the markets and pricing mechanisms – that turn those kilowatt-hours into the number at the bottom of your bill.
John Christensen is a licensed Master Electrician in Minnesota and has a bachelor’s degree in electrical engineering from the University of Minnesota – Duluth. If you have electrical questions you’d like answered in a future edition of this column, send them to the editor, or email John directly at john@clovervalleyelectric.com
The advice provided in this column is intended for general informational purposes only. If you have specific concerns or a situation requiring professional assistance, you should consult with a qualified professional for advice or service tailored to your individual circumstances. The author, this newspaper, and publisher are not responsible for the outcomes or results of following any advice from this column. You are solely responsible for your actions.
