Showing papers on "Electric power system published in 1982"

Selective Modal Analysis with Applications to Electric Power Systems, PART I: Heuristic Introduction

Abstract: Selective Modal Analysis (or SMA) is a physically motivated framework for understanding, simplifying and analyzing complicated linear time-invariant models of dynamic systems, see [1,2]. SMA can accurately and efficiently focus on selected portions of the structure and behavior of the system. the part of the model that is relevant to the dynamics of interest is singled out in a direct manner, and. the remainder of the model is collapsed in a way that leaves the selected structure and behavior Intact. The paper heuristically presents basic concepts and results of SMA and lays the foundations for their application to a number of problems in analysis of power system dynamics. The approach is illustrated with several examples, including a 60-machine model of a dynamic instability occurrence in an actual power system. A companion paper [3] elaborates on specific applications of SMA to power systems, particularly to the Dynamic Stability problem.

Load Representation in Power System Stability Studies

Abstract: Proper representation of load is important in power system stability studies, but it is a difficult task. Load modeling is qualitatively different from generator modeling in many aspects. Composition of load needs to be reliably estimated, models of different components must be combined to obtain a reasonably manageable overall system model, and field measurements are not at all easy.

Steady-State Security Regions of Power Systems

Abstract: A steady-state security region is a set of real and reactive power injections (load demands and power generations) for which the power flow equations and the security constraints imposed by equipment operating limits are satisfied. The problem of determining steady-state security regions is formulated as one of finding sufficient conditions for the existence of solutions to the power flow map within the security constraint set. Explicit limits on real and reactive power injections at each bus are obtained, such that if each injection lies within the corresponding limits, the system is guaranteed to operate with security constraints satisfied.

Solution of Large-Scale Optimal Unit Commitment Problems

Abstract: This paper is concerned with the solution of large-scale unit commitment problems. An optimization model has been developed for these problems that incorporates minimum up and down time constraints, demand and reserve constraints, cooling-time dependent startup-costs, and time varying shutdown costs, as well as other practical considerations. A solution methodology has been developed for the optimization model that has two unique features. First, computational requirements grow only linearly with the number of units. Second, performance of the algorithm can be shown (rigorously) to actually improve as the number of units increases. With a preliminary computer implementation of the algorithm, we have been able to reliably solve problems with 250 units over 12 (2-hour) time periods, and we expect to be able to easily double these numbers.

Variable Structure Control of Electric Power Generation

Abstract: A control scheme to reduce the mismatch in power generation and consumption of electric power systems is developed using the variable structure systems concept. This scheme is introduced to refine the dynamic properties of the presently used integral controller which has been originated in accordance with a steady state concept. The new controller changes its structure according to a certain logic which results in distinct advantageous properties.

Reactive Power Considerations in Automatic Contingency Selection

Abstract: On-line steady-state security analysis requires the evaluation of the effects of a large number of possible contingencies on a power system. Almost all contingency analysis programs have had to rely on a list of selected contingencies to be studied. The brute force approach of testing all contingencies every time an evaluation of the system performance is needed, becomes prohibitive due to its computational burden. A particularly attractive technique is to utilize a fast, approximate method to select the few contingencies that actually affect the security of the power system and then analyze only these cases in more detail. The contingency ranking and selection methods presented in the literature have all ranked outage cases on the basis of transmission line overloads. This limits the usefulness of these technqiues as contingencies that cause only voltage problems are totally overlooked. This paper shows that there is little correlation between the contingencies that produce line overloads and those that result in unacceptable voltage profiles, further establishing the need for a contingency selection method that is also voltage sensitive. Such a method, using the first iteration of an AC power flow, is presented. It is shown that the method meets both the speed and accuracy requirements for on-line use.

Dynamics and Stability of Wind Turbine Generators

Abstract: This paper describes the dynamic and stability properties of wind turbine generators connected to power systems. Both synchronous and induction generators are considered. A comparison is made between wind turbines, steam, and hydro units. The unusual phenomena associated with wind turbines are emphasized. The general control requirements are discussed, as well as various schemes for torsional damping such as speed sensitive stabilizer and blade pitch control. Interaction between adjacent wind turbines in a "wind farm" is also considered.

Large Scale Optimal Power Flow

Abstract: A new optimization method is applied to optimal power flow analysis. The method is shown to be well suited to large scale (500 buses or more) power systems in that it is computationally efficient and is particularly effective with infeasible starting points. The optimization approach is based on transforming the original problem to that of solving a sequence of linearly constrained subproblems using an augmented Lagrangian type objective function. A fundamental feature of this algorithm (developed by Murtagh and Saunders) is that the solution converges quadratically on the nonlinear power flow constraints, rather than being forced to satisfy the constraints throughout the iterative process. To demonstrate the performance of this algorithm, a set of descent directions, which includes quasi-Newton (variable metric), conjugate directions, and steepest descent, are compared on the basis of convergence and computational effort for a 118 bus and a 600 bus power system.

A New FFT-Based Digital Frequency Relay for Load Shedding

Abstract: A power system disturbance, or operating abnormality, can produce a severe generation and load imbalance, resulting in a rapid frequency decline. The possibility of such a disturbance has led to increased interest in the application of automatic under-frequency protection schemes to restore load/generation balance and to prevent equipment damage. In this paper a new method for detecting changes in the power system frequency, by relating it to a leakage coefficient in the FFT, is presented. The algorithm computes the deviation of the power system frequency from the fundamental component and the best estimate of its rate of change. The algorithm then predicts the percentage generation and load imbalance, and finally trips the appropriate amounts of load at the appropriate time delays.

Geometric critical point analysis of lossless power system models

Abstract: Electric utility analysts today face an increasingly difficult task of formulating both long and short term operating plans which will provide at the same time efficient and economical operation while delivering reliable and uninterrupted service to electricity users. One of the key ingredients in this planning is a set of large scale simulations of the steady-state network performance under various anticipated operating conditions. Central to these analyses are the classical "load flow" equations which are the equilibrium equations for the "swing equations" which are a physically based model of the dynamic operation of an n -node power system. Despite the long standing and widespread use of these equations, there remain a number of very basic open questions: What are the number and nature of the equilibria of the swing equations? How many stable equilibrium operating points are there in an n -node electric power grid? In this paper some powerful analytical tools from topology and geometry are used to answer certain of these questions. It is well documented that the load flow equations comprise a formidable large scale system but what is interesting, and perhaps surprising, is that even for a small number of buses, these equations possess a rather rich and intricate qualitative behavior which has heretofor been only partially understood. Indeed, until now there was no complete statement in the literature concerning the number of load flows in a general three-bus network. In Theorem 2.7, we state that for the "generic" three-bus network there are, for sufficiently small power injections, either four or six real load flows and that, in either case, exactly one of those load flows is stable. This is a special case of the results we derive for a general n -bus powergrid. Our method consists in first transforming the load flow equations for a lossless electric power network by trigonometric substitutions into algebraic equations. This makes it possible to apply some deep and powerful results from algebraic geometry and intersection theory to study these equations. An obvious tool for determining the number of solutions is provided by the classical theorem of Bezout, but it is shown that for systems describing an n -machine network with n \geq 4 , this result cannot be directly applied because the solutions contain solution components of positive dimension "at infinity." A major result in this paper is a modified Bezout technique which allows us to compute the number of complex (and a fortiori an upper bound on the number of real) solutions to the load flow equations. Combining this with the classical Morse inequalities, we obtain very explicit results regarding the number of stable load flows for a given network topology and set of power injections. The cases of three and four machine networks are considered in detail.

Power System Voltage Stability

Abstract: Power system voltage stability is characterized as being capable of maintaining load voltage magnitudes within specified operating limits under steady state conditions. In this paper, the first order delay model of a load admittance change is introduced. Then, using this model, a set of linearized dynamic equations is derived and stability conditions are obtained. An earlier result in the literature is shown to agree with that in this paper. The stability conditions are tested and verified in a 2-load, 2- power source system and a 13-node, 4-power source system.

Stored power system for vehicle accessories

Abstract: A stored electric power system selectively operates automotive accessories independent of the motive drive force generated by the vehicle heat engine so as to reduce fuel consumption, reduce the generation of pollutants and improve the vehicle performance. The electric power system is charged from sources external to the engine drive.

Phase Shifter and Power Flow Control

Abstract: In this paper a practical model of phase shifter has been described. By using this model a symmetrical Y-bus matrix can be preserved in the load flow calculation. On this basis a simplified real power flow control algorithm by phase shifter is described. This model has also been used to solve the problems of overloads in contingency analysis and minimizing line losses in transmission systems.

Power System State Estimation with Measurement Deficiency: An Algorithm that Determines the Maximal Observable Subnetwork

Abstract: Measurement system failures may make it impossible for the power system state estimator to estimate bus voltage angles and magnitudes at all buses in the power system. When confronted with such a measurement deficiency, the state estimator can estimate bus voltages over only a portion of the entire network. This paper presents an algorithm that determines the largest region where bus voltages may be estimated-the maximum observable subnetwork. The theoretical basis of the algorithm is developed, and the underlying concepts are illustrated on an example network.

On-Line Load Forecasting for Energy Control Center Application

Abstract: An important component of a comprehensive real-time control center for power systems is a short-term internal system load forecast. Short-term forecasts are necessary for on-line commitment, maintenance scheduling, security analysis, and on-line load flow solutions. It enables the economic dispatch to coordinate forecasted load changes with rate-of- response of generating units.

Eigenvalue Analysis of Synchronizing Power Flow Oscillations in Large Electric Power Systems

Abstract: A new computer program is described which permits eigenvalue analysis of the oscillations associated with synchronizing power flow in large electric power systems. The program has been given the acronym AESOPS, for the Analysis of Essentially Spontaneous Oscillations in Power Systems. The program is the principal product of a research project funded by the Electric Power Research Institute. This project was recommended by the Westinghouse Advanced Systems Technology Division and the MAPP/MARCA Dynamic Device Operating Task Force.

Eigenvalue Analysis of Synchronizing Power Flow Oscillations in Large Electric Power Systems

Abstract: A new computer program is described which permits eigenvalue analysis of the oscillations associated with synchronizing power flow in large electric power systems. The program has been given the acronym AESOPS, for the Analysis of Essentially Spontaneous Oscillations in Power Systems. The program is the principal product of a research project funded by the Electric Power Research Institute. This project was recommended by the Westinghouse Advanced Systems Technology Division and the MAPP/MARCA Dynamic Device Operating Task Force.

Dynamic Load Models Derived from Data Acquired During System Transients

Abstract: For several years power system load responses have been recorded during system transients by means of a remote data acquisition system developed by Ontario Hydro. Using these data, models for the loads are developed in the paper and parameters for the models are obtained. Conventional static load models are shown to represent adequately the characteristics of a residential/commercial feeder load. For large rotating industrial loads two models are developed. The first is a transfer function model which relates the small signal power and reactive power outputs to input changes in voltage and frequency. The second is an induction motor model with shunt static load. Recorded voltage is considered as an input to the model and the output real and reactive power are compared with the measured output. The weighted squared error between the measured output and the model output is minimized by obtaining optimum model parameters. Recorded and simulated responses of the paper mill loads are compared.

Developments in Optimal Power Flow

Abstract: This paper presents an advanced power flow methodology for optimally dispatching all active and reactive power in a power system. Two major obstacles impede the success of most optimization algorithms: (1) computational inefficiences associated with large system and (2) the problems associated with handling functional inequality, constraints. In view of these basic problems, techniques are presented to improve the solution algorithm, the handling of penalty functions, and the power solution optimization methodologies. Additionally, new algorithms are provided for the determination of an optimal step length and for scaling of control variable gradients. These improvements and innovations computing techniques have been incorporated into a computer program and demonstrated on practical size power systems.

Dynamic security regions of power systems

Abstract: A method for deriving dynamic security regions of power systems is developed. A power system operating state is defined to be dynamically secure with respect to a given disturbance if the system, starting in that state maintains transient stability after experiencing the disturbance. Specifically, these are regions of prefault angles such that the post-fault system is asymptotically stable. The proposed approach is to construct affine approximations to the nonlinearities in the transient stability model and then derive quadratic bounds on the errors between the nonlinearities and their approximation. These are then used to derive sufficient conditions for a polytope of operating states to be dynamically secure.

A Method for Analyzing Harmonic Distribution in A.C. Power Systems

Abstract: This paper presents a method for a analyzing'charActeristic harmonic current propagation into an AC system. Frequency dependent models of AC network elements and loads have been developed for the appropriate range of frequencies. This methodology is applied to two different systems and the responsesof individual elements and the system as a whole were observed. The harmonic current flows in the systems appear to be related-to certain characteristics of the system and its elements. Input impedance at harmonic frequencies, the magnitude. of shunt capacitances on transmission lines, the size and location of the converters, and the representation of system loads are all found to be factors in cdetermining the magnitude of harmonic current flows in system elements.

Analysis of Utility Protection Problems Associated with Small Wind Turbine Interconnections

Abstract: This paper is an analysis of several protection coordination problems that may result from the integration of small wind turbines (less than 100 kVA) into the electric distribution system. Such problems include the characteristic contributions of fault current, fault detection ability, effects of increased short-circuit capacities, interaction with line reclosers, and islanding of dispersed generators. Examples are shown using actual utility line and equipment data. The wind turbines considered include small synchronous and induction generators as well as generation sources utilizing line-commutated or force-commutated inverter interfaces.

Universal Machine Modeling for the Representation of Rotating Electric Machinery in an Electromagnetic Transients Program

Abstract: This paper describes the theory, development and practical application of a new universal machine module, which can be used for the transient analysis of motor/generator phenomena in large and complex power networks. In the general case, doubly-fed electric machines with an arbitrary number of coils and magnetic saturation can be represented. Included are all of the major classes which are of common industrial interest: synchronous and induction machines of 1, 2, or 3 phases, and dc machines. A procedure for maximizing the flexibility of the machine-network interface is explained. Computer implementation has been variably- dimensioned, so that any number of motors or generators can be interconnected with mechanical systems and electric networks of arbitrary size and configuration. Universal machine modeling is now a standard feature of the BPA Electromagnetic Transients Program (EMTP), which is widely used on numerous different computers.

Stability analysis of multimachine power networks with linear frequency dependent loads

Abstract: This paper presents a more complete stability analysis of a new power system model which was presented in [1]. The essential feature of the model is the assumption of frequency dependent loads. This facilitates a dynamic representation directly in terms of the network structure. Consequently, concepts and results from circuit theory can play a strong role in the stability analysis of the model. The multivariable Popov criterion is used to obtain general Lure-Postnikov type Lyapunov functions which rigorously allow for the presence of (frequency dependent) real power loads. This has not been possible with the previously used model. Results are given for both local (dynamic) stability and for determination of regions of asymptotic (transient) stability.

Generalized Method of Analysis of Simultaneous Faults in Electric Power System

Abstract: This paper describes two methods to analyze any combination of simultaneous balanced and unbalanced faults in a power system The first method can be used to calculate the symmetrical components and phase components for any bus voltage and branch current of a faulted power system Using the second method, an equivalent network connected to the positive sequence network can be obtained

Control Algorithm for a Static Phase Shifting Transformer to Enhance Transient and Dynamic Stability of Large Power Systems

Abstract: A robust control algorithm for the control of a static phase shifting or quadrature boost transformer has been developed and tested. This algorithm utilizes the inherent stabilizing ability of a phase shifter to rapidly return the power system to a stable operating point following severe transients. Under normal operating conditions, the controller minimizes power flow deviations and slip fluctuations due to random power demand changes. The control commands are generated using only local measurements (voltage and complex power at the transformer), specified power flow, and imprecise values of equivalent impedances of the power system.