D.A. Pierre

An application of Prony methods in PSS design for multimachine systems

A method for obtaining transfer functions for power system stabilizer (PSS) design is presented. It results in an accurate reduced-order model of a generator connected to a large system. The method is based on Prony signal analysis and incorporates both local and interarea electromechanical oscillatory modes. Two PSS units are designed using the proposed method along with root-locus and sequential decentralized control techniques. The design procedure and results are illustrated using computer simulation of a 27 bus system with 16 generators. The test system was developed to exhibit interarea modes that are comparable to the western North American power system. This provides a new perspective on damping interarea oscillatory modes using PSS units. >

A Perspective on Adaptive Control of Power Systems

Potential applications of adaptive control theory to electrical power systems control are explored. In the first part of the paper, the status of adaptive control theory is reviewed. Current mechanisms and methods of electrical power systems control are then examined. The two areas in adaptive power system control that have received the most attention are adaptive generator exciter control and adaptive load-frequency control. Nine recent papers on adaptive exciter control are described, as also are three papers on adaptive load-frequency control. Potential applications of adaptive control to other power system control problems are noted.

Identifying linear reduced-order models for systems with arbitrary initial conditions using Prony signal analysis

A method of identifying reduced-order linear models for systems operating in the neighborhood of an equilibrium point is presented. The method is based on Prony signal analysis, which has recently received considerable attention in the study of power system electromechanical oscillations. Prior to the application of the input test signal, the system can be in a transient state. The system input test signal is piecewise continuous and allows several Prony analyses to be performed during a transient, with each analysis conducted between input discontinuities. Results of these Prony analyses can be combined in various ways to obtain system eigenvalues, transfer-function residues, and initial condition residues. Two examples are given to illustrate the use of the method. >

An enhanced LQ adaptive VAr unit controller for power system damping

Static VAR compensators have been installed in power systems primarily to function in the steady state regulation of voltage levels or reactive power flows. More recently however there has been much interest in utilizing these devices to improve the dynamic performance of power systems. This paper presents an adaptive linear quadratic Gaussian control strategy for static var systems to enhance power system damping and stability. The control strategy uses only local information to dampen oscillatory modes present in the network. The controller calculates an appropriate value of VAr unit susceptance to present to the network at each sampling instant. The calculation of the appropriate susceptance value is based on a reduced-order model of the power system which is obtained on-line by a least squares identification procedure. The controller consists of three main components: an identifier, an adaptive observer, adn an adaptive LQG regulator. The identifier users a recursive least squares type of algorithm to fit a linear, discrete transfer function model to a sequence of input and output signals obtained from the power system. This results in a reduced-order approximation to the actual power system. For this study, VAr unit susceptance is used as the input signal and bus frequency deviation is used as the output signal. The coefficients of the identified transfer function are then sent to both the adaptive observer and the adaptive regulator. The observer is an observable-cannonical representation of the system and it calculates a state vector representing system dynamics.

A supplementary adaptive VAr unit controller for power system damping

A multivariable adaptive LQ control strategy for static VAr compensators is described which can be added to an existing fixed controller to enhance the robustness and effectiveness of the VAr unit in damping power system oscillations. The control strategy uses only local network information to damp oscillations between machines in the network. Simulation results are presented for a nine-bus network. The controller is effective in reducing local machine oscillations even in cases where the oscillatory disturbance is not controllable. >

Coordination of multiple adaptive PSS units using a decentralized control scheme

The authors present a method of coordinating multiple adaptive power system stabilizer (PSS) units in a power system. The method is based on a decentralized adaptive control scheme. Self-tuning adaptive controllers are used as PSS units on given generators. The generators that tend to strongly dynamically interact are coordinated by communicating the controlled inputs between them. The communicated information is used in such a way that the controllers are robust to any communication failures. Simulation results that compare noncoordinated controllers with coordinated ones are presented for a 17-machine system. Two different system operating points were tested. It is shown that better system damping is obtained if the adaptive PSS units on the strongly coupled generators were coordinated. >

Robust power system controller design based on measured models

This paper presents combined power system identification and controller design methods to dampen low-frequency oscillations in multimachine power systems. An iterative closed-loop identification method is used to find a linear model for the power system. Linear quadratic Gaussian controller design with loop transfer recovery (LQG/LTR), based on a generalized technique for the nonminimum phase (NMP) power system model, is used to design the controllers. Simulation results are presented to demonstrate the robustness of these controllers based on closed-loop identified plant models and the amount of loop transfer recovery that is possible for NMP plant models.

Digital controller design — Alternative emulation approaches

Abstract The emulation approach to digital controller design involves two steps: (1) the design of a continuous controller to satisfy performance and robustness objectives, and (2) the conversion of the continuous controller into a digital controller with the objective of obtaining comparable closed-loop system properties. This paper addresses the second step of the emulation approach. Previously developed methods are discussed, and several new methods are shown to be effective in accounting for computational delay and for sample-and-hold delay in closed-loop control applications. When the step response of the digitally controlled system is required to be closed to that of the corresponding continuous system, the selection of the sampling period T and the emulation performance are related in a fundamental way. This is illustrated in a closed-loop example.

General formulation of a Prony based method for simultaneous identification of transfer functions and initial conditions

Previous results on transfer function identification using Prony signal analysis methods are extended. The class of allowed input signals is expanded: the signals can exhibit jump discontinuities and can be characterized by a finite number of eigenvalues between discontinuities. The advantage of this more general input form is that it can be tailored to excite system modes in frequency ranges of interest. In addition to generating system eigenvalues and transfer function residues, the approach can be used to obtain a feedthrough gain and initial condition residues. The approach makes maximum use of available input-output data during each phase of the solution. For the first phase of the solution, in which system eigenvalues are estimated, several alternative approaches are described. For the second phase of the solution, all available data are used in determining estimates of transfer function residues and initial condition residues. Methods of model order selection also are described, and a detailed example is given to illustrate the approach.<<ETX>>

Nonlinear VAr control for damping multivariable swing equations

Idealized swing equations are used in the development of local control strategies for VAr (reactive volt-ampere) units, with the objective of providing damping to all oscillatory modes of multimachine power systems. A one-machine infinite-bus system is used to introduce and motivate the method. The control strategy provides damping even during severe transients involving reverse power swings. For systems containing n machines, simulation results indicate that n-1 locally controlled VAr units, using only local output feedback, can be placed to dampen all oscillatory modes of the system, provided that a stable equilibrium point exists and that the initial disturbance is limited in severity. >