The notion of a macrogrid has been steadily moving into discussions of reliability, resilience, and renewables in US and world-wide power grids. A search on the Institute of Electrical and Electronics Engineers (IEEE) Xplore search engine using keywords “macrogrid” and “microgrid” resulted in 40 publications, all of which used the term macrogrid to refer to the existing bulk transmission system to which a microgrid connects and from which a microgrid may separate in times of trouble. A second search on the word “macrogrid” alone turned up the same 40 plus five more, three of which include Dale Osborn (recently retired from the Midcontinent Independent System Operator (MISO)) as a co-author. Indeed, for several years Dale has been a strong promoter of a US macrogrid, and when he or others use the term in this context, we all understand it to refer not to any existing grid but rather to a new network of high-capacity interregional transmission.
The term “interregional transmission” was used in the Federal Energy Regulatory Commission (FERC) Order 1000, where it refers to high-capacity transmission between two or more distinct geographical regions. The geographical regions may be distinct in terms of their distance of separation, whether they are in the same balancing areas or planning regions, and/or whether they are within the same synchronous grid. Most often, a macrogrid implies use of HVDC (high-voltage DC), though some argue there are good reasons for preferring that a “network of high-capacity interregional transmission lines” should be AC. Two other frequently used terms that have meanings very close to that of “macrogrid” are “overlay” and “supergrid,” where the latter generally refers to designs that span one or more continents.
Growing Interest in Macrogrids
Today, macrogrids are of high interest throughout the world for several reasons. First, they enable economies through the sharing of least-cost energy, ancillary services, and capacity while providing access from remote resources—particularly hydro, wind, and solar—to supply load centers. The economies of sharing increase with load diversity; diurnal load diversity increases with east-west distance due to time zone differences, and annual load diversity increases with north-south distance due to temperature differences. And weather diversity, which increases with geographical area, reduces the per-MW variability associated with renewables and therefore the flexibility requirements. This facilitates increased levels of renewables and corresponding emissions reductions from the power sector.
Second, macrogrids enhance reliability and resilience by providing the ability to economically serve load from distant resources when local generation becomes unavailable, either temporarily (due to any of several types of natural disasters) or permanently (usually due to a shift in energy policy). Third, macrogrids are economic stimuli, i.e., they have the effect of attracting investment in new resources, providing for economic development through lowered energy prices and job creation, an effect that may be attractive in combating pandemic-induced recessions.
Economic development has also been a reason why some regions have opposed the development of macrogrids, preferring to develop local resources in order to benefit from the jobs and tax benefits they produce, even when the projected cost of the remote energy (including the macrogrid cost) is lower than that of the locally produced energy. This perspective has been referred to as resource parochialism. Indeed, a commonly studied US macrogrid design, shown on the left in the figure below, omits connections to the northeast United States, in part because northeastern states may prefer to build offshore wind for this reason.
A Reliable Continental Design
An alternative design, shown on the right in the above figure, extends the macrogrid to the east coast to include the offshore wind resources. Why do this? Would the east coast have surplus offshore energy to send westward and southward? Maybe. And of course midwestern wind or southern solar might find its way eastward and northward. But even more important, this design provides a third north-south link—in the Atlantic Ocean! Providing for a third north-south link will increase macrogrid economies of sharing to include those between the US Northeast and Southeast. But equally important is that it will ensure that there are three north-south transmission links, a feature that satisfies the “rule of three” in macrogrid design. This rule establishes that high-capacity interregional transmission is most economically attractive when there are three or more parallel lines, because doing so enables avoidance of large derating imposed by N-1 security. At Iowa State University, we teach a graduate course in power system planning that develops this interesting concept—see Section 3.4 in the link here.
Where To From Here?
In studying the macrogrid designs shown above, it is hard not to think of additional questions—how far south should the Atlantic north-south link be extended? North or South Carolina? Georgia? Florida? Might the macrogrid benefits be enough to motivate interest from Texas in a terminal for the Electric Reliability Council of Texas (ERCOT) region? What about offshore Gulf wind? To what extent might Canada and Mexico participate?
But there are intriguing takeaways… wind from the Midwest, wind from the Atlantic, solar from the South, and (let’s not forget) hydro from the North … a 21st century macrogrid design indeed!
Jim McCalley and Qian Zhang
Iowa State University