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usbr/wapa As a control area operator, Western "regulates" the transmission system within a prescribed geographic area. Western is required to react to moment-by-moment changes in electrical demand within this area. Regulation means that "automatic generation control" will be used to adjust the power output of electric generators within a prescribed area in response to changes in the system frequency, time error, and tie-line loading, to maintain the scheduled level of generation in accordance with prescribed NERC criteria. The "record" used to calculate the degree to which Western is responding to these change on the transmission system is called the "ACE" - Area Control Error. Hydro facilities such as Glen Canyon have an inherent design that allows them to respond rapidly to changes in power system demands. Other control area operators, across the nation, that do not have hydropower plants must either build small units (natural gas or oil-fired being the most likely) or add this capability to the design of larger units. The transmission system that Western distributes power through is dynamic. Load requirements are constantly changing as a result of eitheT the demands of the customers connected to it or changes that occur in other interconnected power systems. Western maintains the ACE signal to record its response to the fluctuations in system "loading"; (an effort to maintain a balance between power being consumed and power being generated - described above). Ifmore demand is placed on the transmission system than is being generated, the resulting ACE is negative. Generators automatically respond to this condition by increasing generation. If demand is less than generation, ACE is positive. Generators automatically respond to this condition by reducing generation. The targeted ACE is zero. The ACE signal that is sent to Glen Canyon where it is effectively added to, or subtracted from, the existing scheduled hourly generation base point. Therefore, at any moment during the clay or night, Glen Canyon might be producing more or less power than the current hourly megawatt schedule. The ACE signal is transmitted to Glen Canyon Dam every four seconds. The NERC requirement for regulation is that the ACE must "cross" the zero target every 10 minutes. The frequent "swings" in generation are described in the MOU signed by Reclamation and Western': "These changes which occur many times during the hour are both positive and negative in relation to the schedule. The resulting output from Glen Canyon generators is an envelope of generation swings that are frequent, small in 4 USBR/W AP A Interagency Agreement - July 7, 1997 4