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Grid Futures

Evolutionary directions for electric system architecture.

Fortnightly Magazine - April 2011

function as the legacy grid, providing the capacity to transact increasing volumes of power over long distances. It readily accommodates central-station generation wherever it is desired or economical to build the power plant, so that generation—and not transmission—siting constraints dominate planning decisions. In the absence of transmission constraints, economics would typically favor bulk generation projects, connected in the traditional manner at the transmission voltage level. The penetration level of distributed generation would also be limited by the technical capability to allow safe back-feeding of power.

Operationally, the beefy system works much like before. Especially at the distribution level, very little would change in architecture, since power continues to be fed hierarchically out from substations. Distribution feeders might be added locally to accommodate load growth, but without changing the fundamental radial structure of the system. At the transmission level, incremental changes can be summarized as reducing the impedance between nodes in the grid, by upgrading links or adding lines along new rights-of-way. At the extreme, this scenario might be called the “metallic skies.”

One caveat of this growth in transmission capacity is the potential instability in alternating-current (AC) transmission systems resulting from very large power transfers over long distances. Voltage magnitude and angle oscillations have been increasingly observed in synchronous AC systems spanning large geographic regions, and aren’t very well understood at present. With thermal limits generally increasing due to transmission system build-out, stability constraints can be expected to play an increasingly important role. Without significantly better knowledge of system stability behavior and implementation of phase-angle measurement and control, or the strategic introduction of DC links, brute-force addition of transmission capacity can be expected to run up against this fundamental limitation. Another way to say this is that while more transmission capacity on the one hand adds to security, when fully utilized it also introduces new vulnerabilities. Besides oscillations, a well-known example of this is relay tripping and system separation in response to distant events (such as the proverbial tree limb in Oregon taking down California). A related caveat of reducing impedances between nodes is the associated increase of fault currents. Because of this, interconnectivity of transmission networks might be limited by the capacity of devices to safely interrupt large fault currents. Finally, more transmission links might also increase the potential for potential for other power instabilities, loop flows and larger wide-spread cascading blackouts.

In sum, while the overall T&D operating philosophy wouldn’t change much in the “beefy” scenario, and while the system would be built out incrementally in response to operational needs, operational limits ultimately would arise that can no longer be addressed with just the addition of traditional capacity.

Note that because grid technology plays a small role in this scenario, this discussion of T&D operation assumes that smart grid technology is minimally implemented at the load level (such as meter reading and billing, signal-controlled demand response, or charge scheduling for electric vehicles) and for generation (such as wind resource forecasting). Also it doesn’t assume a particular regulatory or market model for transactions of energy and ancillary services. Where information technology