It occurs to me that the subject of today’s post – the national power grid - is a delicate artery of modern society. Considering our reliance on electricity, a energy hemorrhage caused by a severe weather event, a breakdown in infrastructure, a solar event of just medium size, or just a few cleverly placed acts of terror could bring us to our knees. Do you know why? Do you know how electricity comes to your house? Let’s look at the national grid and see why it's vulnerable.
We stated last week that the majority electricity is produced by generators attached to turbines – the turbines being spun by steam or water. In 2012, there were more than 19,000 generators operating at 7000 power plants in the United States (those producing 1 megawatt or more). That’s one power plant for every 45,000 people in the U.S, or one for every 550 square miles in the continental U.S., on average.
You can see that the loss of one generating station could have an effect on the distribution of power, and a few going down at the same time could cause a significant problem. Luckily, there are backup generators that can be brought on line in case of problems or if more power is needed, but the point is still made.
Turbine blades spin a shaft. The shaft is attached to
a magnet that then also spins. It passes by coils of
wire; this generates electricity.
When alternating current is generated it switches from one direction to the other and back, reaching maximums on either side of a middle line. If you graph it out, it looks like a sine wave (see picture below). Energy is lowest when it is near the zero line, either going down or coming back up. By having three lines of electricity generated, each 120 degrees out of phase, there is almost always a line near a maximum value. This makes for efficient power generation. Four lines isn't much more efficient than three, but three is much more efficient than two – draw it out and see.
Each coil set in a generator produces a current that
is 120 degrees out of phase with the others, in total
it produces a much more even current for high
The three lines of electricity produced by the generators are inserted into the national electric grid. This is the interconnected network of lines and stations that move electricity from the generating stations to the consumers. Every electrical line you see, from the one entering your house, to the high tension lines that tower over the farm fields, is part of the electrical grid. There are also the lines you can’t see because they are buried – so the grid is even bigger than you think.
When electricity leaves the generating station it is stepped up to a much higher voltage (155,000 to 765,000 volts) so that it can be transferred long distances along the transmission network in a smaller number of wires and with lower energy loss. These high-tension lines travel to the local power company that will distribute it to customers via a local distribution network that it owns.
At power substations, the voltage is stepped back down (around 10,000 volts), but not down to the level at which it will enter your house. From here, the electricity is passed through a bus that splits it to many directions, and through the distribution network. In many bus splits, there will be higher voltage and lower voltage lines, depending on the distance and customer need.
The three lines (still three phase power) move out into the neighborhoods, and get stepped down to the customer usage level, 240 amps. There are buses on the line that split electricity off to each house, this is the first time that they don’t travel in their group of three. The single line enters your house via your electrical meter which records the amount of electricity that is delivered over time in kilowatt hours.
We are moving toward smart grids. They are smaller than
a national grid, use different source of generating power,
and can move in two directions, so customers can generate
power and contribute to the grid.
The grid has some built in protections against black outs, mostly through redundancy. Any part of the entire grid can’t really be described as grid-like unless it is redundant. Power must come from more than one source, so that electricity can be subverted to areas of higher need, and so that a loss of some part of the grid can be compensated for by other sources. Notice that when a tree falls across a line, it is a specific area that goes black. This represents the part of the distribution that is not redundant.
If that were to occur in the transmission or distribution networks (although it would have to be an awfully tall tree), then the redundancy would pull power from another source and through other grid lines. The vulnerability lies in taking out several high tension lines that are the redundancy for a large area, or more likely, in taking out the high voltage transformers.
This is a main power step up transformer, the type that is
vulnerable to terrorist attack. This one doesn’t have lines
connected to it since it is under construction. You can see
how it would be costly and time consuming to replace.
A report by the Congressional Research Service in mid-2014 stated that the high voltage, step up transformers are a likely target for terrorists because they serve large areas, are expensive and hard to replace, and because hitting only a few could cripple a large population. In addition, the transmission lines are just out there in the open. There's no way that they can be protected from terrorist attacks. See now why the grid is so vulnerable?
You notice that there are no dumping grounds for excess energy produced here. The electricity made goes directly into the lines. Copper wires can’t hold electricity, they just transmit it. But what happens if you make too much - is it wasted? Next time we will look at the burgeoning field of large-scale energy storage.
Contributed by Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules
Bashan, A., Berezin, Y., Buldyrev, S., & Havlin, S. (2013). The extreme vulnerability of interdependent spatially embedded networks Nature Physics, 9 (10), 667-672 DOI: 10.1038/nphys2727