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The Grid


There is something that we use all day every day. We couldn’t live as we do without it, yet, for the most part, we never look at it. We just assume it will always be there, until it isn’t, and then we are most unhappy! What is it?

The electrical grid, of course. I’ve had occasion to refer to The Grid in previous posts, but I think that relatively few people really know what it is. So I thought I’d spend a few posts on it. The point will be to give ourselves a basic understanding so we can look at and understand the 2013 Long-Term Reliability Assessment published by NERC. This post describes what The Grid is.

Electricity Grid Schematic by MBizon. Downloaded from wikipedia.org.

Electricity Grid Schematic by MBizon. Downloaded from wikipedia.org.

The Grid is the interconnected network that delivers electricity from suppliers to consumers in the Continental United States, most of Canada, and a small portion of Baja California.

A schematic drawing of The Grid is at right. Electricity is generated in thousands of generating stations across the country. It is gathered together over high voltage lines and stepped-up to ultra-high voltage, which is more efficient to transmit over long distances. It is then transmitted over ultra-high voltage transmission lines until it nears its destination. Then it is stepped down to high voltage and distributed to other step-down stations, where it is converted to low voltage (but still much higher than the voltage used in your home or business). From there it is distributed to still more step-down stations, and then to the transformers we see on utility poles. There it is stepped down one last time to the voltage we use in our homes and distributed to individual customers.

(Click on graphic for larger view.)

All of this, from the door of the generating station to the door of the customer, is properly part of The Grid. However, this series of posts is going to focus on the high voltage and ultra-high voltage transmission system, aka the bulk power system. The map at right shows the network of ultra-high voltage transmission lines in the United States. National Transmission GridThe Grid is denser in the region from Pittsburgh, through Indiana and Illinois, to St. Louis. There is another dense area running from Southern Texas through Oklahoma to Kansas City. The reason for the density in these regions is the need to transmit electricity to industry from remote generating stations.

(Click on map for larger view).

There is also an area of the country where there are very few transmission lines, especially running east-west. The Eastern Interconnection includes everything east of the Rocky Mountains, while the Western Interconnection includes the Rocky Mountains and everything to their west. There are surprisingly few connections between the Eastern and Western Interconnections – in fact, only one is shown on the map. There are a few more lower voltage connections, but the point still holds: these two regions operate largely independently from each other.

Thus, Missouri is part of a big electrical network that includes everything from the Rocky Mountains to the East Coast, from Texas and Florida to the northern edge of Manitoba and Saskatchewan.

Now, in the description above, I noted that The Grid includes generating stations and transmission systems. In some locations, electric utilities own both the generating stations and transmission systems. That is the case with Ameren, Missouri’s largest electric utility. Sometimes, however, they are separate. In these cases, the transmission network is owned by a separate company. If that company operates only in one state, it is called an Independent Service Operator (ISO). If it operates across multiple states, it is called a Regional Transmission Organization (RTO). This will matter when we talk about Missouri’s grid in more detail.

The next post will look a little deeper into how The Grid is organized.


North American Electric Reliability Corporation. 2013. 2013 Long-Term Reliability Assessment. http://www.nerc.com/pa/RAPA/ra/Reliability%20Assessments%20DL/2013_LTRA_FINAL.pdf.

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