Scientific American has an article on expanding the US power grid to enable greater utilisation of solar and wind power - Giving the Power Grid Some Backbone.
A stiff wind blows year-round in North Dakota. In Arizona the sun beats down virtually every day. The U.S. has vast quantities of renewable electricity sources waiting to be tapped in these regions, but what it does not have there are power lines—big power lines that can carry the bountiful energy to distant cities and industries where it is needed.

The same is true beyond the windswept high plains and the sun-baked Mojave Desert: renewable supply and electricity demand are seldom in the same place, and too often the transmission lines needed to connect them are missing. The disparity exists even in New England, where hundreds of miles of high-tension wires supported by thousands of steel towers run neatly through dense areas of settlement. When Gordon Van Wiele, chief executive of ISO New England—in charge of transmission in the six-state region—unfurls a map of the land there, large ovals show the location of the best wind sites: Vermont near the Quebec border and eastern Maine spilling over into New Brunswick. But sure enough, no transmission lines tran­sect them.

The U.S. has the natural resources, the technology and the capital to make a massive shift to renewable energy, a step that would lower emissions of greenhouse gases and smog-forming pollutants from coal-fired power plants while also freeing up natural gas for better uses. Missing is a high-voltage transmission backbone to make that future a reality. In some places, wind power, still in its infancy, is already running up against the grid’s limits. “Most of the potential for renewable resources tends to be in places where we don’t have robust existing transmission infrastructure,” Van Wiele says. Instead, for decades electric companies have built coal, nuclear, natural gas and oil-fired generators close to customers.

That strategy worked reasonably well until recently, when 28 state governments set “renewable portfolio standards” requiring their utilities to supply a certain portion of their electricity using renewables, such as 20 percent by 2020 or even sooner. But as Kurt E. Yeager, former president of the Electric Power Research Institute in Palo Alto, Calif., points out, such standards “aren’t worth the paper they’re written on until we have a power system, a grid, that is capable of assimilating that intermittent energy without having to build large quantities of backup power, fossil-fueled, to enable it.”

In Colorado the utility that serves most of the state, Xcel Energy, is now building a megawatt of natural gas capacity for every megawatt of wind so that it is ready to come online quickly to provide power when the wind tails off. That plan is a carbon improvement but not really a carbon solution. The U.S. needs a new transmission backbone that crisscrosses the country, knitting together many large wind farms, solar-energy fields, geothermal pools, hydroelectric generators and other alternative sources.

One utility company has already unveiled a grand plan for the U.S., and other experts are charting their own backbone schemes. But whichever one might prevail will require a lot of money and a lot of coordination across what are now independent areas of technological and political control. ...

North America is actually covered by four regional grids (three of which serve the U.S.). The largest is the Eastern Interconnection, an extensive complex of transmission lines that stretches from Halifax to New Orleans, with substations that step down the high-voltage electricity to lower levels so that it can be distributed locally along smaller wires. West of the Rockies is the Western Interconnection, from British Columbia to San Diego and a small slice of Mexico. Texas, in an echo of its history as an independent republic, comprises its own grid, now called the Electric Reliability Council of Texas. And Quebec, with its separatist undercurrent, also has its own grid. The high-voltage transmission systems in the four regions comprise about 200,000 miles of power lines, divided among a staggering 500 owners, that carry current from more than 10,000 power plants run by about 6,000 investor-owned utilities, public power systems and co-ops.