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Voltage collapse is a critical problem that impacts power system operational security. Timely and accurate assessment of voltage security is necessary to detect post-contingency voltage problems in order to prevent a large scale blackout. This paper presents an online voltage security assessment scheme using synchronized phasor measurements and periodically updated decision trees (DTs). The DTs are first trained offline using detailed voltage security analysis conducted using the past representative and forecasted 24-h ahead operating conditions. The DTs are also updated every hour by including newly predicted system conditions for robustness improvement. The associated synchronized critical attributes are obtained in real time from phasor measurement units (PMUs) and compared with the offline thresholds determined by the DTs to assess security. This approach is tested on the American Electric Power (AEP) system and properly trained DTs perform well in assessing voltage security. Several new ideas to improve DT performance are also introduced. Index Terms--Decision trees, online security assessment, phasor measurement units, voltage collapse.

The advent of advanced sensor technology and the breakthroughs in telecommunication open up several new possibilities for demand-side management. More recently, there has been greater interest from utilities as well as system operators in utilizing load as a system resource through the application of new technologies. With the wider application of demand-side management, there is an increasing emphasis on control of loads with minimum disruption. This paper develops a new framework for designing as well as assessing such an advanced direct load control program with the objective of minimizing end-user discomfort and is formulated as an optimization problem. With a fairly general setup for demand-side management, a simulation-based framework is developed for the stochastic optimization problem. Next, using this framework, insights into the effect of different parameters and constraints in the model on load control are developed. Index Terms--Cold load pickup, demand-side management, direct load control, dynamic programming, minimum disruptive control, Monte Carlo simulations.

This paper presents a diakoptic-based distributed state estimation (SE) algorithm suitable for large-scale power systems. In the proposed approach the large-scale power system is divided into a certain number of subsystems by removing tie line measurements. Based on diakoptic theory, the SE problem is then partitioned into a number of subproblems which are solved for each subsystem using local computational resources. In subsequent steps, the intermediate subsystem SE solutions are sent to a central computer for completing the state estimation process by taking the tie line measurements into consideration. Phasor measurement units are used to make each subproblem solvable and to coordinate the voltage angles of each subsystem SE solution. Test results on the IEEE 14-bus test bed and IEEE 118-bus test bed are provided. Index Terms--Diakoptics, distributed state estimation, measurement tearing, phasor measurement units, state estimation.

Power system oscillatory behavior can be analyzed in terms of modes, expressed as exponentially modulated sinusoids, exhibited in signals measured on the system. These signals are driven by the behavior of a large, nonlinear, time-variant system. In this paper, two modal identification methods are examined comparatively: Prony analysis and a method based on the Hilbert transform. Considerable structural differences exist between the two methods. Prony analysis yields modes which are directly expressed as exponentially modulated sinusoids, whereas the Hilbert method provides a more general solution. Synthetic and measured signals are used in the comparison. Some general conclusions are drawn from the analysis of several signals, including two sets of measured field data. Index Terms--Dynamics, empirical mode decomposition (EMD), Hilbert transforms, Hilbert-Huang method, interarea oscillations, power system dynamic stability, power system measurements, power systems, prony analysis.

Electric power transmission lines face increased threats from malicious attacks and natural disasters. This underscores the need to develop new techniques to ensure safe and reliable transmission of electric power. This paper deals with the development of an online monitoring technique based on mechanical state estimation to determine the sag levels of overhead transmission lines in real time and hence determine if these lines are in normal physical condition or have been damaged or downed. A computational algorithm based on least squares state estimation is applied to the physical transmission line equations to determine the conductor sag levels from measurements of tension, temperature, and other transmission line conductor parameters. The estimated conductor sag levels are used to generate warning signals of vertical clearance violations in the energy management system. These warning signals are displayed to the operator to make appropriate decisions to maintain the line within the prescribed clearance limits and prevent potential cascading failures. Index Terms--Clearance, conductor sag, mechanical state estimation, overhead conductor, ruling span, tension, transmission engineering.

A method of calculating the communication delay (latency) for measurements and control signals in a power system is shown. The basis of the calculation is a dedicated communication channel for control signals. The time delay calculation is examined using a dynamic equivalent of the Western Electricity Coordinating Council transmission system. The impact on control system response is discussed. The application given is a wide area control system for interarea mode damping. Results demonstrate that control signal latency can degrade the performance of controls in a wide area control system. Index Terms--Damping, latency, network delay, power system control, power system stabilizer, queuing, wide area control systems, wide area measurement systems.

This paper describes an online dynamic security assessment scheme for large-scale interconnected power systems using phasor measurements and decision trees. The scheme builds and periodically updates decision trees offline to decide critical attributes as security indicators. Decision trees provide online security assessment and preventive control guidelines based on real-time measurements of the indicators from phasor measurement units. The scheme uses a new classification method involving each whole path of a decision tree instead of only classification results at terminal nodes to provide more reliable security assessment results for changes in system conditions. The approaches developed are tested on a 2100-bus, 2600-line, 240-generator operational model of the Entergy system. The test results demonstrate that the proposed scheme is able to identify key security indicators and give reliable and accurate online dynamic security predictions. Index Terms--Decision trees, online dynamic security assessment, phasor measurements, preventive control, transient stability.

A modeling framework based on normal form theory and singular perturbation techniques is proposed for analyzing the nonlinear behavior of power system models described by nonlinear differential-algebraic equations (DAEs). The method exploits the time scale separation of power system dynamic processes, to avoid reduction of the original DAE model and may therefore be used to assess control effects and network characteristics on system behavior. This approach allows the full potential of the normal form formulation to be reached, and is applicable to a wide variety of nonlinear phenomena described by DAEs. Using a control theory framework, a constructive approach is outlined for transforming a system of DAEs to a state space approximation that is suitable for normal form analysis. By casting the problem in the context of singular perturbation theory, a structure-preserving nonlinear mathematical model of the power system is then established for the study of nonlinear behavior. Criteria for this representation are derived and implementation issues are discussed. The developed procedures are tested on a four-machine, two-area test system. The accuracy of the model is quantified by comparing normal form simulations with those from a commercial stability software. Index Terms--Nonlinear power system behavior, normal form theory, singular perturbation analysis.

An approach based on the empirical mode decomposition (EMD) technique and proper orthogonal decomposition is proposed to examine dynamic trends and phase relationships between key system signals from measured data. Drawing on the EMD approach, and the method of snapshots, a technique based on the notion of proper orthogonal modes, is used to express an ensemble of measured data as a linear combination of basis functions or modes. This approach improves the ability of the EMD technique to capture abrupt changes in the observed data. Analytical criteria to describe the energy relationships in the observed oscillations are derived and a physical interpretation of the system modes is suggested. It is shown that in addition to providing estimates of time dependent mode shapes, the analysis also provides a method to identify the modes with the most energy embedded in the underlying signals. The method is applied to conduct post-mortem analysis of measured data of a real event in northern Mexico and to transient stability data. Index Terms--Empirical eigen functions, inter-area oscillations, mode shape, phasor measurement units.

With the creation of balancing authorities by the North American Reliability Council that span large portions of the North American interconnection, and stringent requirements for real time monitoring of power system evolution, faster and more accurate state estimation algorithms that can efficiently handle systems of very large sizes are needed in the present environment. This paper presents a distributed state estimation algorithm suitable for large-scale power systems. Synchronized phasor measurements are applied to aggregate the voltage phase angles of each decomposed subsystem in the distributed state estimation solution. The aggregated state estimation solution is obtained from the distributed solution using a sensitivity analysis based update at chosen boundary buses. Placement of synchronized phasor measurements in the decomposed subsystems is also investigated in this paper. Test results on the IEEE 118-bus test bed are provided. Index Terms--Distributed state estimation, phasor measurement units, state estimation, synchronized phasor measurements.

Wireless sensor networks are becoming the technology of choice for sensing applications mostly due to their ease of installation and associated lower costs. This paper proposes a novel conceptual design for an application of wireless sensor technology for assessing the structural health of transmission lines and their implementation to improve the observability and reliability of power systems. A two-layer sensor network model is presented for overcoming the communication range limitations of smart sensors, and two operational modes for enhanced energy efficiency are introduced. Simulations integrating the output of the sensor network with an energy-management system were conducted, obtaining improvement in the security of the power system. Index Terms--Energy-management system (EMS), network sensitivity factors, power system security, power transmission lines, wireless sensor networks.

A nonlinear analysis framework based on normal form (NF) theory and center manifold reduction is proposed to determine the mosl effective selection of generating units to be equipped with power system stabilizers. The proposed method takes into account the nonlinear system structure and provides information on the effect of control action on nonlinear behavior and modal interaction. Based on a second-order NF representation of the system, the effect of control action on nonlinear behavior is approximated via suitable modification of the initial conditions in the nonlinear coordinate transformations which relate the physical system to the NF coordinates. On the basis of this model, nonlinear power system stabilizer (PSS) sensitivity indices are then proposed to determine the optimum sites to allocate PSS, and criteria to assess the effect of control action on linear modal interaction are developed. Examples of application of the developed approaches on a two-area four-machine test system are presented to estimate controllers' effects ,on system dynamic performance. The efficacy and accuracy of the method is demonstrated by comparisons to conventional analysis techniques. Index Terms--Nonlinear systems, normal forms, power system small-signal stability.

This paper demonstrates the use of a slow-coherency-based generator grouping algorithm and a graph theoretic approach to form controlled islands as a last resort to prevent cascading outages following large disturbances. The proposed technique is applied to a 30 000-bus, 5000-generator, 2004 summer peak load, Eastern Interconnection data and demonstrated on the August 14, 2003 blackout scenario. Adaptive rate of frequency decline-based load shedding schemes are used in the load rich islands to control frequency. The simulation results presented show the advantage of the proposed method in containing the impact of the disturbance within the islands formed and in preventing the impact of the disturbance from propagating to the rest of the system. This is demonstrated by the significant reduction in line flows in the rest of the system and by improved voltage and relative angle characteristics. Based on the suggestion in the joint U.S.-Canadian task force final report on the blackout, load shedding without any islanding is also performed, and results obtained are compared with the proposed controlled islanding method. The islanding method outperforms the load shedding-only method in reducing the transmission line flows, but both methods have similar effects on voltage and relative angle behavior. Index Terms--Blackout prevention, islanding, load shedding, power system stability, slow coherency.

In the restructured power industry environment, use of flexible ac transmission systems (FACTS) elements for steady-state corrective control has great economic advantage compared with other corrective control strategies, such as generation rescheduling and load shedding. Due to the requirements of both speed and accuracy for online corrective control, a new linear programming (LP)--based optimal power flow (OPF) algorithm for corrective FACTS control is proposed in this paper to relieve overloads and voltage violations caused by system contingencies. The optimization objective is chosen to minimize the average loadability on highly loaded transmission lines. A new set of parameter sensitivities of FACTS devices are derived to include the operational constraints of FACTS devices during optimization. In order to reduce the effect of errors due to linearization, all sensitivities can be updated during the OPF iterations. The algorithm is implemented with MATLAB and tested on the New England 39-bus system and the WECC 179-bus system. The simulation results show that the proposed LP-based OPF algorithm is effective, fast, and accurate in finding the optimal parameter settings for FACTS devices to solve the problem of overloads and voltage violations. Index Terms--Corrective control, flexible ac transmission systems (FACTS), linear programming (LP), optimal power flow (OPF), overloads, unified power flow controller (UPFC), voltage violations.

Controlled islanding refers to the controlled separation of an interconnected power system into electrically isolated regions. The objective of this paper is to develop adaptive controlled islanding as a component of an emergency power system control strategy. There are two primary aspects of controlled islanding: 1) where to island and 2) when to island? Assisted by a decision tree (DT) approach, this paper seeks to address the 'when to island' aspect. A decision tree based tool is proposed to recognize conditions existing in the system that warrant controlled islanding. A 29-generator, 179-bus system is employed to demonstrate the tool. Simulation data are used to train DTs, and the online performance of DTs is then evaluated as part of a controlled islanding strategy. Index Terms--Controlled islanding, decision tree, machine learning, power system stability, R-Rdot relay, self healing, special protection systems.

Characterization of the dynamic phenomena that arise when the system is subjected to a perturbation is important in real-time power system monitoring and analysis. This paper discusses the use of Hilbert spectral analysis to visualize and characterize nonlinear oscillations from synchronized wide-area measurements. The method has the potential to be applied for real-time, wide-area monitoring and analysis. As an illustrative example, synchronized phasor measurements of a real event in northern Mexico are used to examine the potential usefulness of nonlinear time series analysis techniques to characterize the time evolution of nonlinear, nonstationary oscillations and to determine the nature and propagation of the system disturbance. The proposed approach is also compared with Prony analysis. Index Terms--Interarea oscillations, spectral analysis, wide-area measurements.

In this paper, a new gain-scheduling control design approach is applied to a supplementary damping controller (SDC) design for a FACTS device in the IEEE 50-machine system. The power system model with the FACTS device is linearized at different operating points to obtain an LPV open-loop model, whose state-space entries depend continuously on a time-varying parameter vector. A synthesis procedure for an improved LPV controller using multiple parameter-dependent Lyapunov functions and an interpolation scheme is presented and applied to the static VAr compensator (SVC) SDC design. Comparisons from simulation results are presented among the interpolated LPV controller designed using multiple parameter-dependent Lyapunov functions (MLPV SDC), the LPV controller designed using a single parameter-dependent Lyapunov function (SLPV SDC), and a conventional controller designed using the Root-locus method (RL SDC), which show that with an interpolated MLPV SDC for the SVC device, the system can achieve better robustness in an extended operating range. Index Terms--Damping controller, FACTS, gain-scheduling, IEEE 50-machine test system, linear parameter varying (LPV), Lyapunov function,

In this paper, the Linear Parameter Varying (LPV) controller design technique is applied in the design of a supplementary damping controller (SDC) for a Thyristor Controlled Series Capacitor (TCSC). The power system model with TCSC devices is linearized at different operating points to obtain an LPV open loop model, whose state space entries depend continuously on a time varying parameter vector. Appropriate scheduling parameters are chosen and a suitable grid is formed in the entire parameter space. The locations of the TCSCs with their controllers are carefully chosen based on the analysis of mode shapes as well as the calculation of the modal controllability. An LPV damping controller for TCSC is then designed based on a single Lyapunov function under some realistic constraints. Comparisons from simulation resuits are presented between an LPV SDC and an [H.sub.[infinity]] SDC, which show that the LPV SDC can guarantee robustness over a wider operating range as well as allow more flexibility in achieving specific performance objectives at each operating point. Index Terms--Damping controller, IEEE 50-machine test system, linear parameter varying (LPV) controller, thyristor controlled series capacitor (TCSC).

Hilbert spectral analysis (HSA) is used to characterize the time evolution of non-stationary power system oscillations following large perturbations. Using an analytical procedure based on the Hilbert-Huang Technique (HHT), data from transient stability simulations are decomposed into a finite number of time-varying oscillating components that can be associated with different time scales. Hilbert analysis is then utilized to characterize the time evolution of critical components giving rise to the observed oscillations. The objectives of this study are to obtain information of a quantitative nature on nonlinear processes in power system oscillatory phenomena and assess the applicability of the developed procedures to track the evolving dynamics of critical system modes. A six-area, 377-machine power system is analyzed to examine the onset of nonlinear, non-stationary behavior. Examples of the developed procedures to detect and quantify the strength of nonlinear interaction in power system behavior and to estimate the distribution of the non-stationarity are provided. Index Terms--Nonlinear systems, power system dynamic stability, spectral analysis.

This paper is the first of a two-part paper on the application of selective direct control of loads for small-disturbance angle stability enhancement in power systems. The change in the utility business model and developments in enabling technologies open up new possibilities for power system reliability enhancement. This paper proposes preventive modulation of selected loads through direct control for small-disturbance angle stability enhancement. An analysis framework based on the structured singular value theory has been proposed in which the controllable load range at selected buses that satisfies the desired stability robustness performance criterion is determined. Two conceptually different approaches for analysis of load modulation have been presented. Results have been presented on an equivalent of the Western Electric Coordinating Council system in the second part of this paper. Index Terms-[micro]-analysis, direct load control, robust performance, skewed-[micro], small-disturbance angle stability.

In this second part of this paper, numerical simulations and test results have been presented on a 29-generator, 179-bus equivalent of the WECC system for both approaches of load modulation analysis developed in Part I. The accuracy of the robust performance analysis framework as well as the analysis approaches have been established through results characterizing different operating conditions. The robustness of the proposed scheme for small-disturbance angle stability enhancement has also been demonstrated through these results. Index Terms--[mu]-analysis, direct load control, preventive control, robust performance, skewed-#, small-disturbance angle stability.

It is widely known that corrective switching, including transmission line switching, bus-bar switching, and shunt element switching, may change the states of the power systems, and consequently, affect the distribution of power flows, transmission losses, short circuit currents, voltage profiles as well as transient stability of power systems. In this paper, a new algorithm is developed to find the best line and bus-bar switching action for relieving overloads and voltage violations caused by system contingencies based on a sparse inverse technique and fast decoupled power flow with limited iteration count. A general model of bus-bar switching action is also presented such that the new algorithm can simulate any kind of complicated bus-bar switching action. Furthermore, on the basis of a newly proposed voltage distribution factor by multiple iterations in power flow calculation, a novel algorithm for corrective voltage control by shunt switching is developed. These two algorithms are then integrated into a corrective switching algorithm. Simulation results on the WECC 179-bus system indicate that the new corrective switching algorithm proposed in this paper can effectively solve certain problems of line overloads and voltage violations. The computation time required is also satisfactory. Index Terms--Bus-bar switching, corrective switching, fast decoupled power flow, line switching, overload, shunt switching, voltage distribution factor, voltage violation.

Round rotor synchronous generator parameters are identified from online data measured at the terminals of the machine. Parameter estimation results from two generators are presented. Data sets at different operating points are considered to examine the consistency of the proposed method at varying operating conditions. A magnetic saturation model is implemented in the estimation procedure to account for the effect of saturation on generator inductances. Inclusion of saturation in the machine model is found to improve the estimation results. Noise filtering and bad data detection and rejection techniques are employed to increase the reliability of the estimates. An observer is used to estimate the damper winding currents that are otherwise unknown. The estimated parameters are compared to available manufacturer data and the accuracy of the method is assessed. Index Terms--Damper currents, modeling, online measurements, parameter estimation, saturation, state estimation, synchronous generator.

Normal form (NF) theory is used to characterize and quantify nonlinear modal interaction near critical equilibria. This study focuses on the analysis of second-order modal interaction and the study of nonlinear aspects of system behavior which are of interest to the design and location of system controllers. A systematic approach to deriving second-order NF representations in the neighborhood of equilibrium points is presented. On the basis of this model, nonlinear interaction measures are then obtained to assess the extent and distribution of nonlinearity in the system. Analytical criteria are developed to predict the existence of nonlinear modal interactions that significantly affect system dynamic performance. To demonstrate the effect of nonlinear interaction, a case study of locating controllers to damp electromechanical oscillations is developed. Examples of application of the developed approaches on a two-area four-machine test system are presented to determine the strength of nonlinear interactions in the system response, and estimating its effects on system dynamic performance and control design. Index Terms--Nonlinear systems, normal forms, power system small-signal stability.

Slow coherency-based generator grouping in power systems is primarily dependent on the participation of generators in the slow modes. In the literature, generator coherency has been typically obtained at a specific operating point. This paper presents a new approach using the continuation method to trace the loci of the coherency indices of the slow modes in the system with respect to variation in system conditions to obtain the updated coherency information between generators. The approach has been applied to a 10-generator 39-bus New England system, and a 29-generator 179-bus model of the WECC system and the impact of load variation on generator coherency indices has been investigated. Index Terms--Continuation method, generator coherency indices tracing, power system dynamics, slow coherency based grouping.

Higher order spectral analysis techniques are used to identify nonlinear interaction involving the electromechanical modes of oscillation in complex power systems. First, the presence and extent of nonlinear interactions between frequency components in oscillatory processes following large perturbations are identified using bispectrum and bicoherence analysis methods. Then, the strength and distribution of nonlinear couplings between frequency components is investigated using the phase relationships between spectral components. A case study with a 377-generator model of the Mexican interconnected system is used to illustrate nonlinear aspects arising from the nonlinear interaction of the different low-frequency inter-area modes. The results of the numerical simulations show that low-frequency modes may interact nonlinearly producing intermodulation components at the sum and/or difference frequency of the fundamental modes of oscillation. Such an identification can be used as a benchmark for validation of nonlinear analysis methods, and to reduce the burden associated with these methods. Index Terms--Nonlinear systems, power system dynamic stability, spectral analysis, statistics.

In this paper, the power system model is formulated as a finite dimensional linear system whose state-space entries depend continuously on a time varying parameter vector called the scheduling variables. This system is referred to as the linear parameter varying (LPV) system. Although the trajectory of the changing parameters such as load levels and tie line flows is not known in advance, in most situations, they can be measured in real time. The LPV technique is applied to the decentralized design of power system stabilizers (PSS) for large systems. In the approach developed, instead of considering the complete system model with all the interconnections, we develop a decentralized approach where each individual machine is considered separately with arbitrarily changing real and reactive power output in a defined range. These variables are chosen as the scheduling variables. The designed controller automatically adjusts its parameters depending on the scheduling variables to coordinate with change of operating conditions and the dynamics of the rest of the system. The resulting decentralized PSSs give good performance in a large operating range. Design procedures are presented and comparisons are made between the LPV decentralized PSSs and conventionally designed PSSs on the 50-generator IEEE test system. Index Terms--Decentralized control, gain scheduling, LPV, oscillation damping, power system stabilizer.

This paper presents a method to identify synchronous generator parameters from on-line data measured at the terminals of the machine. An observer for estimation of synchronous generator damper currents is designed and implemented. The observer-estimator is used in a Graphic User Interface (GUI) application. Possible internal machine fault conditions can be detected and remedial action can be applied. Noise filtering and bad data detection and rejection are implemented to increase the reliability of the estimates. Saturation of the synchronous generator inductances is also considered. Secondary objectives include calculation of the error characteristics of the estimation, development of an index of confidence, study of the observability of generator parameters, and evaluation of alternative GUI features. Index Terms--Damper currents, filtering, graphic user interface, modeling, observer, on-line measurements, parameter estimation, pseudoinverse, saturation, state estimation, synchronous generator, visualization.

The problem of defining and classifying power system stability has been addressed by several previous CIGRE and IEEE Task Force reports. These earlier efforts, however, do not completely reflect current industry needs, experiences and understanding. In particular, the definitions are not precise and the classifications do not encompass all practical instability scenarios. This report developed by a Task Force, set up jointly by the CIGRE Study Committee 38 and the IEEE Power System Dynamic Performance Committee, addresses the issue of stability definition and classification in power systems from a fundamental viewpoint and closely examines the practical ramifications. The report aims to define power system stability more precisely, provide a systematic basis for its classification, and discuss linkages to related issues such as power system reliability and security. Index Terms--Frequency stability, Lyapunov stability, oscillatory stability, power system stability, small-signal stability, terms and definitions, transient stability, voltage stability.

This paper provides the analytical basis for an application of slow coherency theory to the design of an islanding scheme, which is employed as an important part of a corrective control strategy to deal with large disturbances [1]. The analysis is conducted under varying networks conditions and loading conditions. The results indicate that the slow coherency based grouping is almost insensitive to locations and severity of the initial faults. However, because of the loosely coherent generators and physical constraints the islands formed change slightly based on location and severity of the disturbance, and loading conditions. A detailed description of the procedure to form the islands after having determined the grouping of generators using slow coherency is presented. The verification of the islanding scheme is proven with simulations on a 179-bus, 29-generator test system. Index Terms--Determination of islands, nonlinear verification of grouping, sensitivity to loading conditions and network configuration, slow coherency-based grouping.

This paper provides a summary of the state of electric power engineering education through descriptions of professional society activities, Internet resources, and results of a worldwide survey conducted in March 2003. Index Terms--Engineering education, Internet resources.

This paper describes software implementation for online risk-based security assessment which computes indices based on probabilistic risk for use by operators in the control room to assess system security levels as a function of existing and near-future network conditions. The paper focuses on speed enhancement techniques that are essential for online application and result visualization methods that offer clear and meaningful ways to enhance human assimilation and comprehension of security levels. Results of testing on a series of 1600 bus power flow models retrieved from the energy management system of a large U.S. utility are presented and serve to illustrate the benefits of the software. Index Terms--Cascading, control center, decision-making, operations, overload, probabilistic risk, security assessment, uncertainty, visualization, voltage instability.

A robust power system stabilizer (PSS) is designed using Glover-McFarlane's [H.sub.[infinity]] loop shaping design procedure. Guidance for setting the feedback configuration for loop shaping and [H.sub.[infinity]] synthesis are presented. The resulting PSS ensures the stability of a set of perturbed plants with respect to the nominal system and has good oscillation damping ability. Comparisons are made between the resulting PSS, a conventionally designed PSS, and a controller designed based on the structured singular value theory. Index Terms--Gap metric, loop shaping, oscillation damping, power system stabilizer, structured singular value.

The work described in this paper was motivated by a perceived increase in the frequency at which power system operators are encountering high stress in bulk transmission systems and the corresponding need to improve security monitoring of these networks. Online risk-based security assessment provides rapid online quantification of a security level associated with an existing or forecasted operating condition. One major advantage of this approach over deterministic online security assessment is that it condenses contingency likelihood and severity into indices that reflect probabilistic risk. Use of these indices in control room decision making leads to increased understanding of potential network problems, including overload, cascading overload, low voltages, and voltage instability, resulting in improved security-related decision making. Test results on large-scale transmission models retrieved from the energy-management system of a U.S. utility company are described. Index Terms--Cascading, control center, decision making, operations, overload, probabilistic risk, security assessment, uncertainty, voltage instability.

This paper provides a self-healing strategy to deal with catastrophic events when power system vulnerability analysis indicates that the system is approaching an extreme emergency state. In our approach, the system is adaptively divided into smaller islands with consideration of quick restoration. Then, a load shedding scheme based on the rate of frequency decline is applied. The proposed scheme is tested on a 179-bus, 20-generator sample system and shows very good performance. Index Terms--Islanding, load shedding, self-healing, slow-coherency-based grouping.

The proposed approach to avoiding catastrophic failures in interconnected power systems uses a defensive system based on multiagent system technologies. Due to their dynamic environments, these agents need to automatically adjust their decision criteria. The method of reinforcement learning with temporal differences can provide a suitable basis for the adaptation. An appropriate convergence criterion is derived and an application of the method to load shedding is demonstrated. A dynamic simulation involving a 179-bus power system is used to validate the method. Index Terms--Defense system, intelligent systems, multiagent system, power system vulnerability, reinforcement learning.

Analysis of 162 disturbances from 1979 to 1995 reported by the North American Electric Reliability Council (NERC) indicates the importance of information systems under the regulated and competitive environment. This paper points out the major deficiencies in current communication and information systems and proposes a new power system information architecture aimed at correcting these deficiencies. The proposed architecture includes automation and control systems at all levels, from substation control system to independent system operator (ISO) operating center, taking into account the requirements of real-time data, security, availability, scalability, and appropriate Quality of Service (QoS). It uses multiple communication channels employing a wide variety of technologies to transmit real-time operating data and control signals. The real-time operating and control system is modeled with various redundancy configurations; the reliabilities of different configurations are studied and compared for practical values of component failure rates and repair rates. Index Terms--Communication technology, EMS/SCADA, failure rate, fault-tolerance, information architecture, internet, multi-protocol label switching (MPLS), power system, real-time, reliability, repair rate, virtual private network (VPN).

Special protection systems (SPS) have been widely used to increase the transfer capability of the network by assisting system operators in administering fast corrective actions. Compared with constructing new transmission facilities, SPS can be placed in service relatively quickly and inexpensively. However, increased reliance on SPS results in additional risks to system security. In this paper, based on existing reliability evaluation methods, a generic procedure for risk-based assessment of SPS is proposed. The procedure can help the system operator to identify the risk brought by SPS and to make SPS-related decisions. An illustrative example which uses a generator rejection scheme (GRS) for transient instability is provided. Index Terms--Generator rejection scheme, impact, probability, reliability, risk, special protection systems.

This paper presents a systematic procedure for the design of decentralized controllers for multimachine power systems. The robust performance in terms of the Structured Singular Value (SSV or [Mu]) is used as the measure of control performance. A wide range of operating conditions were used for testing. Simulation results have shown that the resulting [Mu]controllers would effectively enhance the damping torques, providing better robust stability and/or performance characteristics both in the frequency and time-domain compared to conventionally designed power system stabilizers (PSSs). Key words: Robust Performance, Sequential Design, Structured Uncertainty, [Mu]-Synthesis

This paper presents numerical simulations and testing results of the new method which is proposed for stability robustness of multimachine power systems. The approach is based on Structured Singular Value (SSV) tools. The variations of operating conditions are treated as structured uncertainty. Simulation results for a test system have shown excellent accuracy of robust stability assessment for a wide range of operating conditions. Key words: Stability Robustness, Structured Singular Value, Parametric Uncertainty

This paper is Part-I of a two-part paper in which a framework for robust stability assessment of controls in multimachine power systems is proposed. Variations in the operating conditions are modeled and then incorporated into a framework amenable for robust stability analysis. The Structured Singular Value (SSV) tool is then applied in order to determine the stability for given variations in the operating conditions. Keywords: Robust Stability, Structured Uncertainty, Structured Singular Value

This is part II of a two part paper dealing with control design in power systems using the normal forms technique. The companion paper (part I) [1] describes the general theory and control procedure. Also, the proposed procedure was applied to the Base Case (Case 1) of the 50-generator IEEE Test System [5]. This paper depicts the case studies to show the validation of the proposed method and to observe the effects of controller setting changes. In addition the results dealing with the design of the controller are given and verification of the controller design using nonlinear time simulation is provided.

This is Part I of a two part paper dealing with control design in power systems using the method of normal forms. In stressed power systems, due to the presence of increased nonlinearity and the existence of nonlinear modal interactions, there exist some limitation to the use of conventional linear control design techniques. The objective of this work is to understand the effect of the nonlinear modal interaction on control performance and to develop a procedure to design controls incorporating the nonlinear information. Part II presents the numerical results dealing with the design procedure.

This paper describes the philosophy and development of a contingency screening system for the selection and ranking of dynamic security assessment. The most severe cases are identified and ranked high on the contingency list for more exact analysis. The non-severe cases are filtered out of the list. In the proposed system, a cascade of more restrictive filters is developed based on the sparse transient energy function method. Key Words: Dynamic security, Sparse transient energy function, energy margin, filtering, disturbance severity, network robustness, controlling up, synchronizing power coefficient.

This paper presents a systematic scheme to evaluate nonlinear modal interaction in a power system with excitation control and determine the effect of the interaction on control performance using the normal forms of vectors fields. The proposed approach has been applied to a two-area, four-generator system. The results obtained showed that the control modes have significant interaction with the inertial modes, especially with the so-called interarea modes. The nonlinear interaction between modes following the excitation of certain specific modes could result in a detrimental effect on control performance when the controls are designed based on purely linear analysis. Keywords: Modal interaction, interarea mode, excitation control, normal forms, power system stability.

The accuracy of stability assessment provided by the transient energy function (TEF) method depends on the determination of the controlling unstable equilibrium point (UEP). The technique that currently determines the controlling UEP in the TEF method is based on the so-called exit point method and has also been recently labeled the BCU method. The exit point method consists of two basic steps. First, the exit point is approximated by the point [[Theta].sup.egsa], where the first maximum of the potential energy along the fault-on trajectory is encountered. Second, the minimum gradient point [[Theta].sup.mgp] along the trajectory from [[Theta].sup.egsa] is computed. The controlling UEP is then obtained by solving a system of nonlinear algebraic equations with [[Theta].sup.mgp] as an initial guess. It has been observed that this method lacks robustness in the sense that the following two problems may occur. 1) There may be no detection of the minimum gradient point [[Theta].sup.mgp] and hence, no determination of the controlling UEP, 2) if [[Theta].sup.mgp] is found, then based on the definition of the controlling UEP, it may not be in the domain of convergence of the controlling UEP for the particular solving algorithm used. Hence, another equilibrium point, possibly a stable equilibrium point, not the controlling UEP will be located. This results in a flawed transient stability assessment. The result of this research has been the development of a new numerical technique for determining the controlling UEP. With an initial starting point that is close to the exit point this technique efficiently produces a sequence of points. An analytical foundation for this method is given which shows that under certain assumptions this sequence will converge to the controlling UEP. Hence this new technique exhibits a substantial improvement over the exit point method because of the following reasons: (1) the technique does not attempt to detect the point [[Theta].sup.mgp], (2) the technique can produce a point that is close to the controlling UEP thus avoiding a domain of convergence problem. The analytical foundation is provided for the unloaded gradient system, but an application of the technique to the IEEE 50-generator system shows that satisfactory stability assessment is also obtained for more general systems, for which the exit point method fails.

Recent developments in direct transient stability assessment using the Transien Energy Function (TEF) method have included the exit point technique to determin the controlling unstable equilibrium point (uep). In this paper, analytical sensitivity of the energy margin is coupled with the exit point based TEF metho to assess system stability when there is a change in system parameters: plant generation or network configuration. The principal features of this paper include: * introduction of a very fast sensitivity technique to account for network configuration changes. * elimination of the assumption that the mode of disturbance of the controlling uep does not change. * correlation of the sensitivity results with time simulation through swing curves. The technique is tested on the 50-generator IEEE test system and the 161-generator Northern States Power (NSP) system.