This report has (1) volatile references and (2) art work copied without legal release. Author mux fix. - RBL
|Fig. 1: The three major electrical interconnections of North America (the Québec Interconnection is considered part of the Eastern Interconnection). Source: Wikimedia Commons.|
An evolving patchwork quilt, the US electric grid today is more a product of history than planning. Today the electrical regions of North America are overseen by the North American Electrical Reliability Corporation (NERC), a non-profit corporation consisting of industry expert volunteers with the power to make and enforce mandatory industry standards. At its grossest level the electric grid of the continental United States is serviced by three regional interconnections - the Western Interconnection, the Eastern Interconnection, and the Texas Interconnection.  An interconnection, also known as a wide area synchronous grid, is a region of interconnected AC power systems operating at the same frequency and phase with one another, though not with other interconnections. While all of the North American power systems operate at an average frequency of 60Hz they are not all in phase. Each interconnection in turn consists of from one to several regional Reliability Councils overseeing at least part of the total interconnection. These Reliability Councils are comprised of individual utility companies, power producers and distributors as well as larger groups/networks of such entities. Since the beginning of the 20th century this system has been evolving and growing as small scale producers and distributors have been competing, absorbing, and expanding into greater areas until we're left with the mottled infrastructure of the present.
Although all of the North American interconnections operate at the same average frequency, the individual interconnections are not in synch with one another and therefore cannot be directly connected through AC transmission lines. Instead, High-Voltage Direct Current (HVDC) transmission systems are employed to connect between the various regional systems. These systems require a rectifier to convert from one region's AC into DC where HVDC lines then transmit to the next region where an inverter converts the DC back to AC in the new region. Six DC ties connect the Western Interconnection with the Eastern Interconnection within the US, with one additional tie in Canada.  The Texas Interconnection is linked to the Eastern Interconnection by two DC ties, though has no direct links with the Western Interconnection.
A major factor in building a national electric grid will be the ability to fuel peak demand in one region with idle power in another. Power plants in a cooler part of the country could supply electricity for air conditioners in a hotter one for instance. In addition the 24 hour cycle of societal activity creates variable demand for power. As the rate of consumption of electricity peaks during the day and lulls into the night, power plants in time zones prior to or past peak will be able to transmit and sell power at a premium in zones where it is currently peaking. As peak and off peak prices can vary by as much as 6 cents per kWh, it would be an exceptional use of idle capacity in one region to provide higher priced power to another. 
With the expansion of renewable energy markets, especially wind power, there is growing need for transmission systems capable of transmitting power from its source to where it's needed, often many hundreds of miles away.  Although wind power is dependent on an obviously unpredictable source (the weather) it is still estimated that overall, if completely integrated, around 33% of wind energy could be used as a reliable baseload power source.  Essentially, if many wind farms are all connected to the electric grid, if the wind isn't blowing strongly for one, there's a good chance that it is for another. As can be seen in Fig. 2, our existing grid lies largely unconnected east to west, while a great deal of wind energy lies in the middle of the country, isolated both from the grid itself and from the population centers that could utilize it. Various proposals and projects currently exist to begin connecting these disparate sources of power to the US's energy marketplace. It has been estimated that around 12,900 miles of new lines will be required in order to integrate emerging renewable energy sources with the existing grid, costing around $37 billion. 
|Fig. 2: Mapped wind resources and existing transmission lines in the continental US. Source: NREL.|
One proposed expansion to the regional interconnections is the Tres Amigas Project, a 22.5 square mile site, slated to begin construction in 2012 to be finished in 2014.  This project would be a 3-way triangular link between the Western Interconnection, Eastern Interconnection and Texas Interconnection, located in Clovis, New Mexico. Each interlink would be a 5GW DC super conducting transmission line connecting between the respective AC/DC conversion station of each interconnect. This project would allow future solar production in the desert southwest to be sold in eastern markets, wind energy from Texas and Oklahoma to be sold in western markets, and excess power generated conventionally to be shuttled between markets as needed. Regardless of the form of energy being exchanged, the project would be the first power market hub allowing for the buying and selling of electricity directly between the three interconnections.
Although Tres Amigas or a similar project will enhance inter-grid transmission, what will ultimately be required is a federally promoted national electric grid similar to the federal interstate highway system. Due to the hodgepodge nature of our electric grid where state, private and local utilities all control and vie for different regions, federal authority will be required in order to promote and create a truly interconnected power distribution system.  Like the interstate highway system a national electric grid would consist of energy corridors criss-crossing the country linking the various regions and forming a "backbone" of power transmission lines. The extent of this backbone network is on the order of 10,000 miles of new transmission lines, costing approximately $39 billion.  A backbone grid of this nature would be capable of moving a quarter of America's electricity across the country at a cost of only 0.5 cents/kWh moved.  These backbone grids would likely be made up of 765 kV AC lines, as these lines possess the greatest power to cost ratio per mile of line. When compared to 345 kV transmission lines, a 765 kV line is about the equivalent of a 6-circuit 345 kV (read as 3 double-circuit lines) costing about $2.6 million per mile for 765 kV versus $4.5 million per mile for the equivalent 345 kV lines. In addition 765 kV experience half of the power loss in transmitting as the equivalent 345 kV lines.  HVDC converter stations will also need to be placed at the regional boundaries of the various synchronous grids in order to transmit power between them.
© Jesse Sherer. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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