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Flexible ac Transmission Systems (FACTS) play an important role in improving the transfer capability and stability of a power system. In the application of voltage-sourced converter (VSC)-based FACTS controllers, it is important to study how a VSC impacts the flows in a power system. In this paper, we investigate this flow control problem using two sensitivity approaches, one using an injected voltage source formulation and the other an equivalent impedance formulation. The applications of sensitivity analysis for line active power redispatch and for new series VSC siting in a 1673-bus system are presented. Index Terms--Equivalent impedance, FACTS controllers, sensitivity analysis, voltage source model, voltage-sourced converter.

Due to their ability to provide reactive power support and control active power flow, voltage-sourced-converter (VSC)-based flexible ac transmission system controllers can be used to improve the power transfer capability of congested transmission lines. A converter-based controller structure has the inherent feature to be used to provide shunt or series compensations. This inherent feature is utilized in the convertible static compensator installed at the New York Power Authority's Marcy substation, which can operate in 11 configurations of shunt and series compensations. Moreover, each configuration can be operated in different control modes. The paper presents a novel approach to model converter-based transmission controllers for load-flow calculations. The paper focuses on modeling converter-based controllers when two or more VSCs are coupled to a dc link (e.g., unified power-flow controller (UPFC), interline power-flow controller, and a generalized unified power-flow controller). This approach also allows efficient implementation of various VSC operating limits, where one or more VSCs are loaded to their rated capacity. A computer program incorporating this approach is developed to illustrate the maximum dispatchability of UPFC and IPFC in a large power system. Index Terms--Flexible ac transmission systems (FACTS), interline power-flow controllers (IPFCs), Newton-Raphson algorithm, rated-capacity operation, unified power-flow controllers (UPFCs), voltage-sourced converters (VSCs).

This paper develops a measurement-based method for estimating a two-machine reduced model to represent the interarea dynamics of a radial, multimachine power system. The method uses synchronized bus voltage phasor measurements at two buses and the line current on the power transfer path. The innovation is the application of the interarea oscillation components in the voltage variables resulting from disturbances for extrapolating system impedances and inertias beyond the measured buses. Expressions for the amplitudes of the bus voltage and bus frequency oscillations as functions of the location on the transmission path are derived from a small-signal perturbation approach. The reduced model provides approximate response to disturbances on the transfer path and offers an alternative to model reduction techniques based on detailed system models and data. Index Terms--inertia extrapolation, interarea oscillations, phasor measurement, power system model reduction, power transfer interface, reactance extrapolation.

Many large interconnected power systems such as the U.S. eastern interconnection and the U.S. western power system are characterized by many power transfer paths or interfaces with high loading. Disruptions of these transfer paths frequently lead to increased loading on neighboring transfer paths, which themselves will become less secure and could cause further disruptions. State estimators have limited performance under large system disruptions, because of low sampling rates and potentially poor solution quality due to topology errors. Furthermore, disruptions in external power systems cannot be readily seen by control room operators because most state estimators only use reduced models for external systems. A system of well-placed phasor measurement units (PMUs) that provide voltage and current magnitude and phase at a high sampling rate can provide useful system dynamic security information. In this paper we apply energy function analysis using phasor data to monitor the dynamic status of power transfer paths. The ideas will be illustrated using actual data captured by several PMUs in the U.S. western power system. Index Terms--Energy function, power system stability, power transfer path, synchronized phasor measurement unit (PMU).

For a system with a unified power-flow controller (UPFC), maximum power transfer capability is often achieved when the UPFC is operated at its rated capacity and conventional voltage and line-flow setpoint regulation is no longer possible. This paper uses injected voltage sources to directly model a UPFC and impose the rating limits in a Newton-Raphson load-flow algorithm. A dispatch strategy is proposed for a UPFC operating at rated capacity in which the power circulation between the shunt and series converters is used as the parameter to optimize the power transfer. Voltage-stability curves for two test systems are shown to illustrate the effectiveness of this proposed strategy. Index Terms--Flexible ac transmission system (FACTS) controllers, Newton-Raphson algorithm, unified power-flow controllers (UPFCs), voltage-source converters (VSCs), voltage stability.

The overspeed deflector control of two Pelton turbine hydro units in the Alaska Railbelt power system has caused many incidents of oscillatory instability. Among the causes is the high controller gains applied to the deflectors, which are essential for load rejection operations but destabilize the power swings. In this paper, the overspeed control is achieved by reducing the feedback gains derived from frequency-domain analysis and using an adaptive feedforward control to counter the transient overfrequency condition. Simulation studies show the improved performance of the proposed control design. Index Terms--Adaptive feedforward control, deflector bias, deflector control, Pelton turbine, PI regulation, swing modes.

This paper discusses the use of injected shunt and series voltage sources to model voltage-sourced converter (VSC)-based flexible ac transmission systems (FACTS) controllers such as the unified power-flow controller (UPFC) and the interline power-flow controller (IPFC). Compared to traditional power network models including only shunt voltage injections, the inclusion of series voltage injections provides a new capability of power system models and offers a common modeling framework in which a Newton-Raphson load-flow solution and a network sensitivity analysis can be readily developed. The sensitivities can be used for optimal dispatch of FACTS controllers. Index Terms--Flexible ac transmission system controllers, interline power flow controller, Newton-Raphson algorithm, optimal dispatch, sensitivity analysis, unified power-flow controllers, voltage-sourced converters.

This paper develops optimal pumped-storage unit bidding strategies in a competitive electricity market. Starting from a weekly forecasted market clearing price curve, an algorithm to maximize the profit of a pumped-storage unit considering reserve bids is developed. A comparison between the optimal bidding strategy and a fixed-schedule weekly generating and pumping strategy is provided. Index Terms--Electricity market deregulation, fixed weekly schedule optimization, generator bidding strategies, optimal pumped-storage unit scheduling, reserve bids.

The use of power system stabilizers (PSS) to damp power system swing mode oscillations is of practical importance. The design of PSS is taught in graduate level courses on power system dynamics and control, and has been the topic of numerous M.S. and Ph.D. theses. This paper discusses the experience in assigning PSS projects in an undergraduate control design course to provide students with a challenging design problem using three different techniques and to expose them to power system engineering. The details of the PSS design projects using root-locus, frequency-domain, and state-space methods are provided. Index Terms--Frequency-domain compensation, power system stabilizers, root-locus techniques, state-space methods, undergraduate design projects.

This paper describes the results of a study to evaluate the performance of three identification methods for the study of low frequency electromechanical oscillations. The three identification methods considered are: the Steiglitz-McBride Algorithm, the Eigensystem Realization Algorithm, and the Prony method. The identification methods are used to identify low order linear systems of power systems modeled in standard transient stability programs. This is accomplished by processing the system response to a simple probing pulse. The frequency domain characteristics of several identified systems are compared using three power systems with lightly damped electromechanical modes. Keywords - System identification, modal analysis, Steiglitz-McBride algorithm, Prony analysis, Eigensystem Realization Algorithm, ERA.

This paper presents an integrated approach for designing damping controllers for large power systems. First, a low-order model is identified from a time simulation of the nonlinear power system model. Then an exact multi-modal decomposition is used to develop design indices for selecting an appropriate input signal to the controller. The method is illustrated with the design of thyristor-controlled series compensator damping controllers in two test systems. Keywords - Power system stability, flexible ac transmission systems (FACTS), damping control design, modal decomposition, system identification methods.

Time-optimal control of switchable series or shunt capacitors requiring only a single switching has been proposed for damping power system swings resulting from large disturbances. This strategy is useful for a switchable series capacitor with a high compensation rating relative to the transmission system. In this paper, the general case of time-optimal control of series capacitors requiring multiple switchings is investigated. The strategy is applicable to a switchable series capacitor with a low compensation rating relative to the transmission system. It is illustrated for a single-machine infinite-bus system and an interconnected system. Suboptimal capacitor-switching control schemes are also investigated. Keywords - time-optimal control, switchable capacitor control, multiple-switching bang-bang control, co-state equations, trajectory approximation

Controllable series capacitors have been proposed to damp power swings resulting from large transient disturbances in systems with two machines. In this paper we propose a time-optimal series capacitor control to damp interarea modes in multi-machine systems. To find the switching line, we apply the slow coherency transformation to the full system and project a slow subsystem trajectory found from the trajectory reversing method onto the predominant interarea mode. The method is illustrated by the time simulation of a six-machine power system with two interarea modes and is shown to give the desired result. Keywords - transient stability, time-optimal control, series capacitor, interarea modes, switching line

This paper describes the application of a reduction method to power systems to obtain simplified models that facilitate the design of damping controllers. The reduced system retains the lightly damped swing of interest and exhibits modal characteristics similar to the unreduced system in the frequency range associated with swing modes. A power system damping controller is designed using a reduced system. The modal characteristics of the reduced and unreduced system, with the damping controller included, are compared. The validity of the control design is assessed via nonlinear simulations. The reduction method is simple to implement and is based on the computation of the observability and controllability Gramians. Keywords - Power system reduction, interarea oscillations, singular values, observability Gramian, controllability Gramian, balanced realization.

The design of controllers sited in the transmission network for damping interarea power oscillations requires several types of analytical tools and field verification methods. Probably the most important aspect of such control design is the selection of proper feedback measurements from the network. This paper describes concepts which provide design engineers with the insight to control performance and the understanding needed to ensure the secure operation of the bulk transmission system. Specific attention is directed to procedures for selecting feedback signals. Keywords - Power system stability, Flexible AC Transmission Systems (FACTS), Control design, Modal decomposition

This paper presents new aggregation algorithms for obtaining reduced order power networks when coherent generators are aggregated. The generation terminal bus aggregation algorithm in the EPRI DYNRED software tends to stiffen the reduced order network during the aggregation process, thus increasing the frequencies of inter-area modes. The inertial and slow coherency aggregations will decrease the stiffening effect and produce, for the same coherent machine groups, aggregate networks with improved inter-area mode approximations. This paper contains new procedures to construct these aggregate networks and demonstrates the benefits of these new aggregate networks on a 48-machine system using eigenvalues and nonlinear simulations. Keywords Model reduction, two-time-scale systems, coherency, inertial aggregation, slow coherency aggregation

The paper investigates the use of sensitivity methods to identify the parameters of reduced transformer models. The reduced model retains the structure of the original model. The identification is performed in both the time domain and the frequency domain. The technique is applied to a lumped-linear model of an experimental coil, and the results are promising. Keywords: Transformer modeling, frequency-domain identification, time-domain identification, parameter sensitivities

Decentralized stable factors (DSF) are used to extend the stable factorization approach to the design of decentralized controllers. The DSF allow a parameterization of all the stabilizing decentralized controllers in the form of a Youla parameterization. A decentralized controller serves as the central controller, and the Youla parameter has a particular structure and must satisfy an interaction constraint. This parameterization encompasses several other parameterization results on decentralized control systems.

The paper investigates the use of trajectory sensitivity techniques to identify nonlinear excitation system models. The technique can identify nonlinearities such as limits and saturations. It also allows the use of singular values to provide information on the identifiability of the parameters. The technique is applied to two types of excitation systems. The identifiability of parameters in the presence of noise is also studied.

This paper discusses the design concept and usage of the Power System Toolbox, a Matlab-power system dynamics simulation and control design package. The motivation for developing the package is to provide a flexible environment for teaching power system simulation techniques and control design concepts to advanced undergraduate and graduate students, and for graduate students to perform research and development on power systems. The package can be executed without modification on all platforms that Matlab supports, including IBM PC, Macintosh and many workstations.

This paper proposes a new approach to identify a data‐based power system linear model by means of frequency interpolations, aiming to obtain a suitable system representation for selective‐modal analysis purposes. The key idea behind the identification process is the Loewner‐based frequency interpolation carried out by the Loewner matrices. The proposed approach demonstrates that the Loewner‐based frequency interpolation is able to fit a linear model that can be used for small‐signal analysis studies, since it provides the state‐space representation, the frequency response, and modal information (frequency, damping, and mode‐shape). Then, a selective modal analysis is accomplished over two test cases (Kundur and New England–New York power grids) by employing the identified linear model provided by the Loewner‐based frequency interpolation method. The attained results confirm the outstanding performance of the proposal which is validated against the small‐signal analysis and compared with the eigensystem realisation algorithm, overcoming the absolute error of the model identified with the traditional eigensystem realisation algorithm approach by at least 37 times, and properly capturing the modal information in a frequency band of concern. [ABSTRACT FROM AUTHOR]