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Solving The Crisis In Unscheduled Power

While NAESB and NERC struggle over the issue, North America steadily drifts toward unreliability.
Fortnightly Magazine - August 2004
  1. Subcommittee of the North American Electric Reliability Council. Supporting attachment


  1. Inadvertent interchange is the unscheduled power flowing between BAs. Some of the BA's unscheduled power that is caused by the BA is immediately "countered" by energy the BA is obligated to provide in the opposite direction (called "bias" or a fixed percent share of the combined BAs' scheduling errors at that moment) by instantaneous response by governors that instantaneously change the generator speed, or by instantaneous adjustment by non-resistive loads. This response (called "primary response") limits the size of the frequency error but does not correct the frequency error. The BA's "net inadvertent interchange with the rest of the interconnected system" is the amount by which the BA's scheduling error differs from the BA's instantaneous or "primary" response to the frequency error provided the BA is exactly meeting the BA's primary response obligation. Inadvertent interchange that later corrects the frequency error is called "secondary response." The secondary response normally would be provided by the BA that made the scheduling error, but often never is. When it isn't it is socialized among all the BAs in interconnected systems except the Western in a procedure called "time-error correction," in which performance is measured against "scheduled frequency" that is temporarily shifted away from 60 Hz in the direction of the desired response.
  2. Why is unscheduled power a reliability issue? Because momentary errors in the schedules which balance supply and demand are unavoidable and occur only in electricity, unlike any other commodity. The sum of all the BAs' scheduling errors at any moment is reflected in the deviation of the interconnected system's electrical frequency from target, normally 60 Hz, and is "made up for" by unscheduled power provided to the BAs by themselves or by other BAs. Too much frequency deviation can harm equipment and make blackouts more likely. Scheduling errors used to be more like random noise, but market pricing of scheduled power, while unscheduled power is given/had "for free," makes them occur deliberately and makes them occur more when they contribute to frequency error, than when they correct frequency error.
  3. The NAESB IIPTF has correctly agreed to settlement only of a BA's net inadvertent interchange with the rest of the interconnected system over all the BA's tie-lines, not the flow on an individual tie-line. This means that only inadvertent interchange that a BA is the physical generator of or consumer of is counted for settlement purposes, not inadvertent interchange that is "passed through" from a second party to a third party. In other words, only "source" or "sink" inadvertent interchange is explicitly settled, not unscheduled loop flow, which is more of a transmission usage issue than a frequency control issue but whose cost does get reflected in the energy/congestion part of the price of "source" or "sink" net inadvertent interchange. This both is economical in terms of the number of settlements to consider but also conforms to the inadvertent interchange data collected by NERC.
  4. The annual "energy" value alone is over a third of a billion dollars but the