Showing papers on "Electric power system published in 1976"

Reactive Compensation and Harmonic Suppression for Industrial Power Systems Using Thyristor Converters

Abstract: This paper discusses the problems and solutions of applying reactive compensation, either static or dynamic, to industrial power systems supplying large blocks of dc power from diode or thyristor converters. The resonance between power capacitors and system reactance can produce high harmonic voltages caused by the harmonic currents generated by converters. The interaction between these harmonic voltages and regulating systems can be minimized by the use of suitable filters. The design of these filters should eliminate the interaction between power system and load and reduce harmonic current flow. Different methods of reactive power control are also discussed.

Three phase load flow program

Abstract: The paper describes a general three phase load flow program developed for the steady state analysis of any electric transmission network under all possible imbalance conditions such as untransposed EHV lines, single phase loading, single pole switching, etc. Input data preparation and output data interpretation are made easy for program users by using both phase and symmetrical components quantities; however, the load flow problem formulation and solution are all done in phase quantities (A, B, C). The method of solution is the Newton-Raphson method, using optimally ordered triangular factorization to take advantage of the sparsity of network equations. The program has several new features to simulate real three phase operation of power systems and imbalance conditions. For example, the internal induced voltages of generators are balanced, while generator terminal voltages depend on internal machine impedances and the imbalance in machine currents. Also, the steady state performance of the voltage regulator is simulated to maintain some function of the machine terminal phase voltages constant, depending on the actual design of the voltage measurement. Another feature of the program is that it can handle all common transformer connections having two or three windings that are used in HV transmission networks.

Systems Engineering - Theory and Practice

Abstract: One interesting area of systems engineering in which increasing use is being made of analogue and digital controls is the electrical power system. Electrical power systems fulfil the requirements often said to be characteristic of all systems which may be considered examples of systems engineering. They are a complex grouping of human beings and machines, comprising individual subsystems interacting together, each with its own objective but all having to work towards a common overall objective. The modern boiler is a most complex mechanical and electrical system which has to maintain required energy output at designed temperatures and pressures as load demand varies, while subject to a vast array of disturbing and constraining factors. Boiler-turbine systems are now generally controlled by an analogue system, with digital monitoring, logging and supervision, although the trend is to more digital control. Turbine governors and generator excitation systems again have their own controls, which have become very complex and are the subject of much research both in the power companies and at University level.

A fast decoupled static state-estimator for electric power systems

Abstract: A new algorithm is described to solve the static, time-invariant weighted least-square state-estimation problem for large-scale electric power systems. The solution is obtained through P-θ and Q-V decoupling and alternately iterating the active and reactive equations using fixed, simplified submatrices of the information matrix. Thus, a much faster algorithm is obtained yielding the exact solution and requiring little computer storage. The new method is compared with the basic "Weighted-least-square" and the "Line-Only" algorithms on a practical HV network.

A digital computer system for EHV substations: Analysis and field tests

Abstract: This paper describes the research and development work being done at the AEP Service Corp. on digital computer applications in EHV substations. Relaying of transmission lines and other power system equipment, station alarm monitoring and data logging, oscillography, and supervisory control functions are considered for possible computer implementation. A new algorithm for computing the impedance as seen from the terminals of a faulted transmission line is presented. Results obtained with the new algorithm for faults simulated with a model transmission line are included in the paper. Field experience obtained from operating a minicomputer in an EHV substation is also described.

The optimal decentralized control of a large power system: Load and frequency control

Abstract: The load and frequency control of a multi-area interconnected power system is studied. In this problem, the system is assumed to be subject to unknown constant disturbances, and it is desired to obtain, if possible, robust decentralized controllers so that the frequency and tieline/net-area power flow of the power system are regulated. The problem is solved by using some structural results recently obtained in decentralized control, in conjunction with a parameter optimization method which minimizes the dominant eigenvalue of the closed-loop system. A class of minimum order robust decentralized controllers which solves this general multi-area load and frequency control problem is obtained. Application of these results is then made to solve the load and frequency control problem for a power system consisting of nine synchronous machines (described by a 119th-order system). It is shown that the load and frequency controller obtained in this case is not likely to be significantly improved by using more complex controllers; in particular, it is shown that the conventional controller, used in regulating the net-area power flow of a system, is not likely to be significantly improved upon by using more complex controllers.

Small-signal modulation of the Pacific HVDC intertie

Abstract: Recent undamped oscillations in the Western U.S. power system have indicated the need to improve system damping. The development of a control algorithm for small-signal modulation of the Pacific HVDC Intertie is described. The control signal which would be applied to the current regulator at the sending end of the dc intertie is based on rate of change of ac-intertie power. Results from simulation studies are presented which show that modulation will produce considerable damping.

Numerical techniques in probabilistic load flow problems

Abstract: This paper describes numerical techniques which have been developed in order to solve the load flow problem of a power system network probabilistically instead of using normal deterministic methods. The aim in the development of these techniques has been to reduce the computational time and storage commensurate with maintaining a high degree of accuracy.

Estimating the domain of attraction for nonlinear feedback systems

Abstract: An iterative procedure is developed for estimating the domain of attraction of a class of nonlinear systems. The procedure uses an approximation of the system by a Carleman linearization and is suitable for automatic computations. The algorithm is applied to the transient stability problem in electrical power systems and a favorable comparison with previous criteria is obtained.

Digital simulation of faulted e.h.v. transmission lines with particular reference to very-high-speed protection

Abstract: Increasing demands for ultralow fault clearance times in electrical power systems require that measurements be made from information derived from power lines during a very short interval after fault inception. For research and development purposes, the precise nature of this information is required, and in this paper modern transmission-line theory is used to predict the fault-transient behaviour of multiconductor overhead lines to a greater degree of accuracy than has hitherto been possible. The nonlinear nature of simulating fault-transient phenomena has been overcome, and it is shown that even on lines which have traditionally been regarded as electrically short, very severe transient conditions can exist. The necessity for an accurate representation of the frequency variance of line parameters is established, and the various factors which influence fault transient phenomena are considered. The paper concludes with a presentation of the results of studies relating to a very long transmission line.

Probabilistic power-flow techniques extended and applied to operational decision making

Abstract: For effective control and operational decision making related to any power system, techniques are required to evaluate system insecurity risks. One such technique is to establish probabilistic load flows in the power system. The paper describes methods for extending the previously published probability and convolution techniques to powerflow studies in which the consumer demands are not totally independent (i.e. there exists some linear dependence between demands). The methods by which this can be achieved for appropriate probability density functions are described. An analysis of a power system, assuming in turn that demands are totally independent (uncorrelated), totally correlated, and partially correlated, is included and discussed, and the results for each formulation are compared. The potential use of this evaluation for risk analysis is discussed.

Practical application of eigenvalue techniques in the analysis of power system dynamic stability problems

Abstract: A digital computer program for the analysis of small signal dynamic stability of power systems is described. The program uses the state space approach and determines the stability by computing the eigenvalues of the coefficient matrix of the linearized systems. Results of some of the investigations carried out using the program are presented. The examples considered demonstrate how the program has been used as a useful analytical tool by an electric utility for solving a variety of stability problems.

Eigenvalue sensitivities of power systems including network and shaft dynamics

Abstract: A practical digital computer method for obtaining eigenvalue sensitivities of interconnected power systems is presented. The method is especially suited to the analysis of detailed modelled synchronous machine systems where shaft and stator-network dynamic effects are significant. A unified structure in the construction of the state space equations permits the inclusion of a wide variety of control equipment and varying degrees of modelling complexity. Sample results are given for typical cases illustrating the significance of eigenvalue sensitivities. Specifically results are presented for a 4 generator hydroelectric system and for a 2 generator system exibiting subsynchronous torsional resonance.

Torsional protective device for power system stabilizer

Abstract: Power system stabilizers are used in the excitation systems of large shaft driven synchronous dynamoelectric machines for the purpose of enhancing the dynamic stability of power generating systems. Such stabilizers using speed, power or frequency as an input signal when applied with high initial response excitation systems are effective in damping low-frequency local or inter-tie mode oscillations which may occur in the system. The power system stabilizer may adversely affect torsional damping at higher frequencies. The adverse effects of a power system stabilizer at shaft torsional frequencies are obviated by providing a torsional oscillation protection device comprising a composite band-reject filter of the present invention which attenuates the power system stabilizer output at the shaft natural torsional frequencies without introducing an excessive phase lag at the local or inter-tie mode frequencies. The invention is applicable at a universal shaft pickup location and means are provided for detecting any failure in the torsional protective device whereby the power system stabilizer output may be discontinued in the event there is a failure in torsional protection.

Real-time power system simulation for automatic generation control

Abstract: A digital simulation program used for the design of improved automatic generation control algorithms is described. The program can also be used for study of long-term system dynamics following large load or generation changes. Since the simulation runs faster than real-time, the program, can be used as a real-time power system simulator for control system checkout and operator training. The simulation represents the major Pacific Northwest powerplants plus equivalents for the interconnected power system. Included is a newly developed Kaplan hydro turbine model. Automatic generation Control models include non-linear digital filtering and recently developed digital powerplant controllers.For stochastic load changes, close statistical comparison was obtained between simulation and actual system records.

Dynamic equivalents using operating data and stochastic modeling

Abstract: A method is developed to determine equivalents for extensive power systems external to an internal system equipped with a central control computer. These equivalents are needed for different system studies where the internal system is represented in detail, and the external system is represented by the equivalent to simulate the interaction effects of the external system on the internal system for disturbances originating in the internal system. The method uses a generalized Thevenin's theory for representing the quasi-steady-state performance of the external system and stochastic linear difference equations to represent the dynamic changes in the internal equivalent voltage magnitudes and phase angles. The methods are demonstrated on two test systems.

Weather sensitive electric demand and energy analysis on a large geographically diverse power system ߞApplication to short term hourly electric demand forecasting

Abstract: A method of analyzing weather sensitive electric loads and energy is presented which can be utilized in long- and short-term forecast applications. The method has application to power systems containing geographically diverse climate areas and can be utilized in all seasons of the year. The method is well adapted for utilization in on-line system operation and production scheduling. On-line experience with the method has been favorable, producing a forecasting error of approximately 2.0 percent during the highly weather sensitive summer load period.

Control system for hydroelectric power station system

Abstract: In a control system for a hydroelectric power station system including upper and lower water reservoirs connected through a water channel, an intermediate water reservoir having a smaller storage capacity than those of the upper and lower reservoirs and provided on the water channel in cascade with the upper and lower reservoirs, and two power stations disposed between the upper and intermediate reservoirs and between the intermediate and lower reservoirs, respectively, the control system is arranged such that the water level of the intermediate reservoir is detected continuously and the detected value of the water level is used to correct the load instructions which are applied to the power stations for controlling outputs of the power stations whereby the water level of the intermediate reservoir is maintained substantially constant, while the power generation of the power stations is controlled so as to satisfactorily meet with the load instruction applied to the power system.

Long-term stability solution of interconnected power systems

Abstract: This paper describes solution and simulation techniques which result in an efficient and practical approach to long-term stability studies on digital computers. These efficiencies are derived from two algorithms used alternately to simulate large power systems. A technique for algebraization of differential equations, including the simultaneous solution of all network equations, is discussed. The results of testing the solution technique on large systems are included.

Static shunt compensation for voltage flicker reduction and power factor correction

Abstract: It is shown that a static VAR generator comprising a capacitive filter network and a thyristor-controlled inductor can effectively compensate for rapid and unbalanced load variations and is able to maintain system voltage and power factor, even under extremely adverse load conditions. The performance of such a static compensating system is, however, dependent on the principles of control employed, the adequacy of the VAR rating, and the electrical characteristics of the filter network. In arc furnace applications, the VAR generator must be rated for unbalanced load conditions and the capacitive filter network must be designed to be compatible with the ac system impedance, and with the spectrum of the furnace current that contains both odd and even harmonics.

Decentralized Stabilization and Regulation in Large Multivariable Systems

Abstract: A general description of the decentralized control problem , with particular emphasis on large scale systems, is made. In this problem, constraints on the structure of the information flow between the manipulated inputs of the system and measured outputs of the system are imposed. Typically, it is desired to find a controller for this system (subject to the above constraints), so that stability of the resultant closed loop system is obtained, and so that regulation/tracking of the outputs of the system occurs, independent of any input disturbances occurring in the system. A motivation for dealing with the decentralized problem is made, classical ways of solving the problem are described, and recent results obtained on the problem are surveyed. Some numerical examples are included, in particular, a power system consisting of three interconnected synchronous machines; it is shown in this power system that there is no real advantage is using a (more complex) centralized control system over the coventional (and more simple) decentralized control system which is normally applied.

Apparatus and method for transient free energization and deenergization of static VAR generators

Abstract: A static VAR generator is taught for use with a source of electrical power and a rapidly varying load. The static VAR generator comprises thyristor switched inductors connected across a capacitor. The gate of the thyristor switch is fired each half cycle in accordance with a predetermined firing angle to provide sufficient inductive current to cancel a portion of the capacitive current to thus provide a net reactive current for the system served. This is done on a half cycle by half cycle basis. A main disconnect switch is connected in parallel with the series combination of a secondary disconnect switch, a diode and a large resistor. The latter combination is connected in series with the parallel combination of the compensating inductor and the compensating capacitor. The secondary disconnect switch is utilized to charge the capacitor to the peak voltage between lines of the electrical system prior to connecting the static VAR generator to the system or prior to disconnecting the static VAR generator from the system. Prior to a disconnecting or connecting operation an oscillating current is introduced between the compensating capacitor and the compensating inductor. This current is found to have the same frequency phase, wave shape and amplitude as the voltage between the lines to which the static VAR generator is connected. The gate control for the thyristor is utilized to fix the phase difference between the oscillating current and the alternating voltage of the system at or near zero. The oscillating voltage caused by the oscillating current is thus equal in phase, frequency, wave shape and magnitude to the alternating voltage of the system. Consequently the main disconnect switch can be opened or closed as desired with zero electrical potential thereacross.

Estimating peak and off-peak marginal costs for an electric power system: an ex ante approach

Abstract: This paper proposes and implements an ex ante approach to estimating thermal electric power system long-run average and peak and off-peak marginal costs subject to pollution emission constraints. Mixed-integer programming is used as a framework for a static, cost-minimizing model of system capacity and operating costs incurred in meeting loads that vary with time and geographic location. The model assumes multiple generating technologies can be used to meet system loads. Transmission and losses are explicitly considered in this transshipment mathematical program. The model is implemented for the system of the New York State Electric and Gas Corp. Results show that peak marginal costs at a particular location in the system are three to four times system average costs and can be more than ten times as great as base load marginal costs. Marginal costs during a particular demand period vary by as much as 30 percent from one load center to another in the system. System average total costs increase (with output) when new steam plants are all coal-fired, but decrease when they are nuclear-fueled. When the most restrictive pollution standards are imposed, system average total costs are approximately 25 percent higher than when no standards are imposed. These results provide substantial empirical evidence in support of the case for time-specific rates based on marginal costs.

Evaluating alternative models for power system dynamic stability studies

Abstract: A dynamic stability model for detailed stability studies on synchronous power systems is presented in such a form that it can be systematically reduced to models of reduced complexity. The complete model includes representation of stator and network transient effects as well as shaft torsional oscillation effects. The family of models of reduced cemplexity span the range of models found in the literature. The general approach is applied to test cases where the effects of modelling complexity are investigated.

An approach to on-line power dispatch with ambient air pollution constraints

Abstract: An on-line optimal environmental/economic dispatch methodology for electric power generation is developed in this paper. Aside from the conventional economic dispatch, constraints on air quality (such as those specified by the U.S. Environmental Protection Agency) are added to the minimum fuel cost problem. Using the integrated Gaussian puff model based on the statistical turbulent theory, rapid dynamic features of pollutant dispersion and its forecast surrounding the plants are emphasized. By applying a convex programming algorithm repeatedly, a set of marginal environmental imposts for the power plants at different times are obtained. Such imposts are incorporated with the fuel cost in the ordinary short-term economic dispatching program to indirectly account for the environmental impact of power generation on the quality of the ambient air. The approach is specifically taken to have little modification for existing economic dispatch programs and be implemented for real power networks. The proposed approach has been simulated in a power system with three plants and three monitoring points.

Optimal output feedback control of power systems with high-speed excitation systems

Abstract: A method of designing a linear, optimal, constant, feedback control system based on quantities easily measurable at the synchronous generator location is presented for power system equipped with fast-acting excitation systems. The method is applied to a single-machine infinite-bus system, and the performance of the system to large disturbances, created by applying three phase faults, is studied. To permit meaningful comparisons between the proposed control scheme and conventionally used schemes, the performance of an 'optimized' supplementary excitation control system is used as a base against which to judge the relative merits of the new scheme. It is found that the proposed control system design, when evaluated over a wide operating range, consistently results in improvements in system behaviour as compared to the conventional control system. To improve system reliability, the optimal control system design is carried out without assuming that the supplementary stabilizer is disconnected. Thus if, for any reason, the new control is not available, the system still has the benefit of stabilizing action provided by the supplementary stabilizer.

Electric power transmission and distribution systems: Costs and their allocation

Abstract: The costs derived from installing, operating, and maintaining the transmission and distribution system have historically comprised about 2/3 the total costs of producing and delivering electricity to residential-commercial customers, and over 1/3 the total costs of supplying electricity to large industrial customers. This paper estimates the costs of transmission and distribution for nine regions of the United States for the above two customer classes. These costs are detailed for six categories of equipment used in the transmission and distribution system and the contribution to the total cost of each equipment category is determined.