Abstruct -This paper describes what automatic generation control (AGC) might be expected to do, and what may not be possible or expedient for it to do. The purposes and objectives of AGC are limited by physical elements involved in the process and, hence, the relevant characteristics of these elements are described. For reasons given in this paper, it is desired that AGC act slowly and deliberately over tens of seconds or a few minutes. From a perspective of utility operations, there is no particular economic or control purpose served by speeding up the AGC action.By this Task Force paper, the System Control Subcommittee is providing a resource to the power engineering community to help guide research into topics related to AGC.

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The authors whole-heartedly agree that voltage sags are of critical concern to certain customers, and STSs are candidate solutions for voltage sag mitigation. In fact, since this paper 1 has been submitted, the authors have integrated comprehensive voltage sag analysis capabilities to their predictive reliability assessment tool. This includes the voltage sag impact of STSs.To model voltage sags, each customer is assigned a customized set of voltage sag levels (e.g., sag to 80%, sag to 70%, and sag to 60%). When a fault occurs on a component, the voltage profile is computed for the entire system (three-phase bolted faults are assumed). If a customer sags below a specified level and is not subsequently interrupted, that customer's "sag index" for that voltage level is incremented by the fault frequency. The sags results for a customer would look something like: 12.7 sags below 80%, 4.6 sags below 70%, and 2.8 sags below 60%.The impact of STSs on sags is handled in the same manner as interruptions. The only twist is that the voltage level at the STS secondary must be above a specified threshold for the transfer to occur. This provides a reasonable model of the STSs impact on a customer's complete voltage sag profile.As the discussers have correctly identified, a fault on a feeder will cause a voltage sag not only on itself, but on other feeders connected to the same bus at the substation. This means that all feeders on a low voltage bus need to be included to accurately model voltage sags. In addition, substation transformers must be modeled so that voltage sags can be computed accurately.

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George Gross and Jeong Woo Lee [University of Illinois at Urbana-Champaign]: We welcome the comments and questions of Jaleeli and VanSlyck and appreciate their interest in our work. We begin our response by reiterating the importance with which we view the pioneering work of the discussers [6], [7] and the key role this work played in the research reported in this paper. At the outset, we also need to emphasize that the aim of our paper was to provide a firm analytical basis for the formulation, analysis, and evaluation of load frequency control (LFC) performance criteria. We constructed a solid analytical basis by effectively exploiting the concepts of random processes and probabilistic models to develop the general criteria for LFC performance. The general LFC criterion we formulated using our analytical framework has as special cases the NERC CPS1 and CPS2 criteria. As a Manuscript

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