In an earlier project, we analyzed data on total system load as well as the loads of eight large industrial customers (Kirby and Hirst 2000). We analyzed these data in terms of system and customer-specific requirements for two real-power ancillary services, regulation and load following. We conducted these analyses using 12 days of data from February 1999 plus 12 days of data from August and September 1999. The project discussed here focused on the supply side (provision) of these two services. Specifically, we examined the output of this control area generation resources, in aggregate and by unit. We analyzed the performance of these generating units in two ways. First, we analyzed the contribution of these generators to overall system performance (generally relative to the North American Electric Reliability Council (NERC) standards). Second, we analyzed performance relative to what the control center requested of the generators. The two services examined here are briefly defined below. See also Hirst and Kirby (1998) and Interconnected Operations Services Working Group (1997). (1) Regulation is the use of online generating units that are equipped with automatic generation control (AGC) and that can change output quickly (MW/min) to track the moment-to-moment fluctuations in customer loads and to correct for unintended fluctuations in generation. Regulation helps to maintain Interconnection frequency, manage differences between actual and scheduled power flows among control areas, and match generation to load within the control area. This service can be provided by any appropriately equipped generator that is connected to the grid and electrically close enough to the local control area that physical and economic transmission limitations do not prevent the importation of this power. (2) Load following is the use of online generation equipment to track the intra- and inter-hour changes in customer loads. Load following differs from regulation in three important respects. First, it occurs over longer time intervals than does regulation--10 min or more rather than minute to minute. Second, the load-following patterns of individual customers are highly correlated with each other, whereas the regulation patterns are largely uncorrelated. Third, load-following changes are often predictable (e.g., because of the weather dependence of many loads) and have similar day-to-day patterns. Even when load following is not predictable by the control-area operator, the customer can inform the control center of impending changes in its electricity use.
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