This paper discusses conceptual frameworks for actively involving highly distributed loads in power system control actions. The context for load control is established by providing an overview of system control objectives, including economic dispatch, automatic generation control, and spinning reserve. The paper then reviews existing initiatives that seek to develop load control programs for the provision of power system services. We then discuss some of the challenges to achieving a load control scheme that balances device-level objectives with power system-level objectives. One of the central premises of the paper is that, in order to achieve full responsiveness, direct load control (as opposed to price response) is required to enable fast time scale, predictable control opportunities, especially for the provision of ancillary services such as regulation and contingency reserves. Centralized, hierarchical, and distributed control architectures are discussed along with benefits and disadvantages, especially in relation to integration with the legacy power system control architecture. Implications for the supporting communications infrastructure are also considered. Fully responsive load control is illustrated in the context of thermostatically controlled loads and plug-in electric vehicles.

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Achieving Controllability of Electric Loads

An attempt is made in This work to present critical literature review and an up-to-date and exhaustive bibliography on the AGC of power systems. Various control aspects concerning the AGC problem have been highlighted. AGC schemes based on parameters, such as linear and nonlinear power system models, classical and optimal control, and centralized, decentralized, and multilevel control, are discussed. AGC strategies based on digital, self-tuning control, adaptive, VSS systems, and intelligent/soft computing control have been included. Finally, the investigations on AGC systems incorporating BES/SMES, wind turbines, FACTS devices, and PV systems have also been discussed.

Recent philosophies of automatic generation control strategies in power systems

Tapping the energy storage potential in electric loads to deliver load following and regulation, with application to wind energy

The authors describe 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, 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. >

Understanding automatic generation control

This two-part paper surveys the frequency and voltage control ancillary services in power systems from various parts of the world. In this second part, essential economic features that must be taken into account when designing markets for ancillary services are first discussed. The methods adopted in eight different systems (Australia, France, Germany, Great Britain, New Zealand, PJM, Spain, and Sweden) for trading frequency and voltage control ancillary services are then compared. Lastly, new indicators for comparing volumes and costs of ancillary services across systems are proposed and calculated for the systems included in this survey. The companion paper surveys the technical features of these ancillary services

A Survey of Frequency and Voltage Control Ancillary Services—Part I: Technical Features

Extending the theoretical foundations established in Part I(1), this paper further describes this efficient method of computing the real-time load flow solution for a power system network. An efficient numerical solution technique particularly applicable to this problem is outlined. Relative weighting of the measurements is analyzed in terms of the measurement accuracies. A method is given for dynamic determination of the frequency of the state estimate calculations. The effects of real-time changes in the network are discussed.

State Calculation of Power Systems from Line Flow Measurements, Part II

This paper describes methods for using power system readings to compute a real-time power flow solution of the transmission network. The use of redundant data is discussed and procedures are outlined to detect, identify, and correct for data errors. Several examples are presented and the numerical behavior of the methods is discussed.

Techniques for the Real-Time Monitoring of Power System Operations

The software in an automatic generation control (AGC) system is typically in charge of all other elements of the process. The number of AGC input/output variables depends on the number of units under control. At AEP, there are about 200 variables for most system scenarios. Analysis for one half hour of system operation, at the 2-s AGC cycle rate, therefore requires examination of about 180000 data values. Together with findings and some suggested revisions of practices, a methodology for such an analysis is described. The plotted data sets show that many signals need enhancement or calibration. The ability for plotting all important operating data has established a strong bias on which the cause of many unexplained and/or undesired characteristics of the EP AGC can now be explored. >

A comprehensive shakedown of an automatic generation control process

Practices for inadvertant energy and time error correction have been discussed in the literature. A description of these practices is presented from the interconnection point of view by considering the resulting constrained coupling effects between control areas. Partial implementations of decomposition theory are suggested as practical solutions to the iniquities and deficiencies of scheme presented here. An implementation scheme for phasing-in decomposition accounting, which can be voluntary, by area, is suggested and explained. An argument is made for an official and essentially constant annual value of frequency bias to be used for decomposition. The frequency bias used in area control error need not be constant or necessarily equal to the decomposition parameter. >

Louis S. VanSlyck

Papers

Understanding automatic generation control

State Calculation of Power Systems from Line Flow Measurements, Part II

Techniques for the Real-Time Monitoring of Power System Operations

A comprehensive shakedown of an automatic generation control process

Implications of frequency control bias settings on interconnected system operation and inadvertent energy accounting