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Grid Futures

Evolutionary directions for electric system architecture.

Fortnightly Magazine - April 2011

of large-scale AC networks, which would be awkward to discover subsequent to a major public funding effort.

Following up on this initial analysis, future research and discussion might flesh out the scenarios in more detail and examine discontinuities in any one independent variable. For example, analysis might focus on the impacts of common-mode failures (what if the advanced metering infrastructure didn’t work?) or killer apps (what if electric vehicles swamp the market?) in each quadrant.

Finally, economic pressure by itself is insufficient to determine which scenario might come to pass. Pressure to minimize the cost of electric energy delivery seems likely to exist regardless of quadrant, while at the same time each scenario would require some significant expenditures. Thus, cost-benefit calculations and contests among technologies would occur within any quadrant and guide tactical investment decisions. But the strategic question of which direction public policy ought to steer the evolution of T&D systems is ultimately not one of minimizing costs, but of maximizing social benefit. To this end, the scenario analysis exercise is intended to provide some useful framing and vocabulary.

 

Endnotes:

1. These scenarios focus specifically on the United States, but some of this discussion may be internationally applicable. The analysis refers to “the system,” singular, as either collective or representative of what can also be seen as multiple AC systems across the continent, assuming the main drivers of evolution to occur at the national level and therefore lead to unified or at least similar development.

2. Although certain portions of T&D systems have always been networked, the overall character of the architecture is best described as a radial, hierarchical structure extending from a central transmission backbone out toward customers along links of successively lower capacity, somewhat like major arteries branching out into capillaries.

3. Arguably, these factors aren’t exogenous to, but are mutually interactive with the development of T&D architecture. That argument may be conveniently avoided because these factors weren’t chosen as drivers for the exercise.

4. There’s also a growing interest in distributed generation sited on customer property ( e.g., rooftop PV) and interconnected at the primary or secondary distribution level. At sufficiently high penetration levels, this distributed generation will impose a different set of pressures on the T&D system, particularly the accommodation of bi-directional power flow with associated requirements for voltage control and protection coordination. The jury is still out on what the critical penetration levels are that would necessitate architectural changes in distribution systems, but they have not yet been reached in the United States; therefore, the dominant pressure at present is to extend transmission.

5. With apologies to mathematical convention.

6. The temporal scale of interest ranges from high-frequency switching at the sub-cycle level, 10-6 seconds, to planning on the order of decades, 109 s—fifteen orders of magnitude! Geographically, the range might be considered from distribution at 101 meters to transmission over 106 m.

7. Ironically, public aversion to siting transmission lines could thereby result in much greater population exposure to 60-Hz electromagnetic fields.

8. The auto mechanics of Cuba come to mind, who are

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