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Perspective

Part II: Making Tomorrow’s Electric Transmission More Resilient Requires ‘Current’ Planning

In this second installment of a two-part series focusing on power transmission, Black & Veatch examines the issues of direct and alternating currents as factors in the resilience of tomorrow’s grid.

Part II: Making Tomorrow’s Electric Transmission More Resilient Requires ‘Current’ Planning

The U.S. is served by three major North American grids – the Western Interconnection, the Electric Reliability Council of Texas and the Eastern Interconnection, the last of which came together in the early 1960s. Prior to widescale deployment of large-scale renewable plants, electricity was generated close to its point of use, though in times of peak summer or winter demand interconnects existed which facilitated increased reliability of systems through enablement of power flows between regions within each interconnection.

The three separate interconnects are like a line of dancers, kicking in unison in what defines a synchronous grid. As such, thousands of generators can add power while thousands of substations can withdraw it for local distribution. But the three interconnects aren’t in sync with each other, and they’re barely connected.

A recent Department of Energy study by the National Renewable Energy Laboratory (NREL) found that while the Eastern Interconnection has about 700,000 megawatts (MW) of capacity and the Western Interconnection has 250,000 MW, the connections between them only can handle about 1,320 MW.

Connection of the interconnections is made via high-voltage direct current (HVDC) lines and associated converter stations. The converter stations at either end of the HVDC links and their associated control systems allow for the interconnection of asynchronous power systems.

Along with the controllability of power-flow, which is afforded with HVDC converter stations, HVDC lines lose very little energy.  With all of the benefits of HVDC technology it may seem odd that our grids are largely AC.  The underlying reason for this is the expense of equipment to convert DC from AC and back again (very expensive). 

Furthermore, voltage transformations which are necessary in a power system are done easily with transformers in an AC power system.  HVDC systems are sometimes compared to interstate highways, which pass through cities without interacting with neighborhood streets. They are well-suited to long-distance transmission and the only option for connecting asynchronous grids.

The NREL study predicted that while a national campaign for better linkage would run into the tens of billions of dollars, the benefits would be above $1.25 for each dollar spent. And the linkage would spur the development of zero-carbon wind and solar.

 

High-Voltage Transmission Offers Opportunity

Any new transmission, AC or DC, would need to be high voltage. A 765-kilovolt (kV) AC line – the highest voltage in use in the U.S. – needs a right of way 200 feet wide, versus 150 feet for a 345-kV AC line. But the 765-kV AC line can carry six times as much energy. The 765 kV AC line carrying 1,000 MW of power loses about 0.6 percent of its power over a 100-mile distance; a 345-kV AC line would lose about 4.2 percent. But high-voltage DC lines have 30 to 50 percent lower losses of AC lines. In addition, they do not require towers of the same scale as AC, use less conductor in an overhead configuration and can even be buried or laid in marine environments for long distances through HV cable.

China, which has huge electricity transport needs has generally found it easier to move electricity around the country than to move coal, operates transmission lines at up to 1,100 kV DC.

North America has, however, begun to make its grid “smarter.” Some of the improvements are fairly simple. For example, the actual capacity of most transmission lines is limited by their temperature, and that is usually estimated from factors like power flow, ambient air temperature and wind speed and direction. 

At any installation location these factors have in the past been analyzed from a worse case contingency to drive a single line rating.  With recent adjustments in regulation, line ratings are now adjusted to account for improved transfer capability due to observed meteorological conditions or through direct measurements taken with sensors on the transmission lines.  This technology is known as dynamic line ratings and allows for increased power flow without building more transmission lines.

Another improvement is “phase angle measurement,” which can be thought of as watching for a misstep by the dancing electrons. HVAC voltage can be portrayed as a sine wave, a snaking line that rises and falls regularly. The relative angle difference that the line forms at different points on the grid is a measure of power system stability.

Through enhancements in measurement technology, communication systems and GPS time synchronization it is now possible to gain visibility of this angular measurement throughout the grid. This enhanced measurement can improve the capacity of existing wires, and the overall reliability of the system.

Another area for growth is offshore wind, a focus of the Biden administration. Offshore winds are strong at predictable times of day, including around sunset often when demand peaks. But the grid is weak at the shoreline, which is traditionally where the grid comes to an end. Moving to offshore wind means use of submarine cables (AC or DC), and careful management of environmental, aesthetic and safety concerns at the shoreline.

 

Regulatory Barriers Remain in Play

But transmission in the U.S. is not simply a technical and financial issue; it faces substantial regulatory barriers. Transmission siting is regulated by the states, not the federal government, even though almost all of the electric system is organized into multi-state groups. Grid expansion faces three categories of opponents: people who object to transmission lines in their backyards, people who object to lines in nobody’s back yard but in rural areas where the lines can damage animal habitats, and companies that own generating assets in congested places, and thus have wholesale markets with higher-than-average prices.

In contrast, siting of the natural gas and petroleum pipeline systems is federally regulated and better developed. In the present grid, natural gas transmission has been a substitute for electric transmission, allowing fuel to flow to places where it can be burned to provide local generation. That option will start phasing out as we move to a zero-carbon system.

Renewables have another transmission problem: a developer is never certain whether the line to connect a particular project actually will get built and, if so, when. There is a demonstrated solution to this problem. Ironically, it is in Texas, which has mostly isolated itself form the rest of the U.S. electrically speaking. But Texas is unique in the U.S. for having its own grid, governed by a single public utilities commission and a single legislature. That gives it more ability to plan on an unusually broad scale.

In 2005, the legislature directed the Public Utility Commission to draw up a broad plan to bring wind from West Texas and the Texas Panhandle to the load centers on the coast. The 3,500-mile system was completed in 2013 and brought with it massive wind development. It cost $7 billion and added several dollars a month to the typical household electricity bill, but it lowered energy costs and fossil fuel burn.  The new system which was constructed is known as a Competitive Renewable Energy Zone (CREZ) and there is renewed interest in evaluating whether this concept can be deployed more broadly with Federal oversight in states with tremendous renewable assets but little electrical consumption.

Even Texas, though, has hesitation about transmission. Better interconnections would ameliorate or eliminate problems like the February 2021 blackouts caused by a cold snap that disabled generators and gas wells. It also would allow exports of excess wind energy, which often reduces the wholesale value of electricity on that isolated grid to zero or below. (Prices go below zero because wind turbines earn tax credits in addition to money from energy sales, and thus have an incentive to produce even when prices go negative.) But  the lack of interconnections with grids in other states avoids interstate commerce and federal oversight which ensures independent decision making by authorities in Texas.

In the long run, if the nation moves to zero emissions, there are many roads to that goal. But all roads will lead to more roads – that is, transmission.  A transmission system consisting of a major backbone of HVDC line and enhanced HVAC lines is being evaluated by many decarbonization proponents and in some form is likely to lead the way in efforts to both decarbonize the electricity sector and enhance the electricity flow which will be needed as energy consumption continues to increase in the electricity sector through transportation system electrification and similar initiatives.

Catch up with Part I: What We Know – And Don’t Know – About Tomorrow’s Grid, published here.

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