Showing papers on "Electric power system published in 1989"

Optimal sizing of capacitors placed on a radial distribution system

Abstract: A capacitor sizing problem for capacitors placed on a radial distribution system is formulated as a nonlinear programming problem, and a solution algorithm is developed. The object is to find the optimal size of the capacitors so that the power losses will be minimized for a given load profile while considering the cost of the capacitors. The formulation also incorporates the AC power flow model for the system and the voltage constraints. The solution algorithm developed for the capacitor sizing problem is based on a Phase I-Phase II feasible directions approach. Novel power flow equations and a solution method, called DistFlow, for radial distribution systems are introduced. The method is computationally efficient and numerically robust, especially for distribution systems with large r/x ratio branches. DistFlow is used repeatedly as a subroutine in the optimization algorithm for the capacitor sizing problem. The test results for the algorithm indicate that the method is computationally efficient and has good convergence characteristics. >

Application of power system stabilizers for enhancement of overall system stability

Abstract: This paper provides a detailed account of analytical work carried out to determine the parameters of power system stabilizers (PSS) for the Darlington nuclear generating station presently under construction in eastern Ontario. The results presented are, however, of general interest and provide a comprehensive analysis of the effects of the different stabilizer parameters on the overall dynamic performance of the power system. They show how stabilizer settings may be selected so as to enhance the steady-state and transient stability of local plant modes as well as inter-area modes in large interconnected systems. In addition, it is shown that the selected parameters result in satisfactory performance during system islanding conditions, when large frequency excursions are experienced. Darlington GS, when completed by 1992, will comprise four 1100 MVA, 0.85 p.f., 1800 RPM turbine generators with "CANDU-PHW" reactors, moderated and cooled by heavy water. The station will be incorporated into the 500 kV network through three double-circuit lines. The units will be equipped with transformer-fed thyristor excitation systems and Delta-P-Omega type PSS [1, 2].

Determination of suitable locations for power system stabilizers and static VAr compensators for damping electromechanical oscillations in large scale power systems

Abstract: Efficient algorithms are presented for the solution of two important problems in the area of damping control of electromechanical oscillations in large-scale systems. The proposed algorithms allow the determination of: the most suitable generators for installing power system stabilizers; the most suitable buses in the system for placing static VAr compensators in order to damp the critical modes of oscillation. These algorithms involve the calculation of transfer function residues and represent an important extension of the powerful methodology described by V. Arcidiaconos et al. (see IEEE Trans. Power Apparatus and Systems, vol.PAS-99, p.769-78, 1980), whose use was up to now restricted to power systems of limited size. A major advantage of this methodology is that there is no limitation on the degree of modeling of the power system being studied. >

Update of harmonic standard IEEE-519: IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems

Abstract: The authors present the changes in IEEE STD 519-1981, titled IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, that are to be made in 1988 as the result of a two-year review. The voltage distortion criteria in the 1981 edition could not be used to distribute among users, the ability of the utility system to absorb harmonic currents. This problem is addressed in the revision. There are now two criteria that are used to evaluate harmonic distortion. The first is a limitation in the harmonic current that a user can transmit into the utility system. The second criterion is the quality of the voltage that the utility must furnish the user. Examples of the application of the revised standard, which has been upgraded to a Recommended Practice, are given. >

An adaptive controller for power system load-frequency control

Abstract: An adaptive controller is presented for load-frequency control of power systems. It uses a PI (proportional-integral) adaptation to satisfy the hyperstability condition for taking care of the parameter changes of the system. Only the available information on the states and output are required for the control. No explicit parameter identification is needed. The controller can be designed by using a reduced plant model to simplify the design without degrading the performance much, so it is very easy to implement practically. The simulation results indicate that good control performance can be obtained and that the performance is sensitive to the plant parameter changes. The control remains effective in the presence of generation rate constraints. >

Subsynchronous resonance in power systems

Abstract: This book provides an introduction to subsynchronous resonance in power systems and includes material on system modeling, system parameters, and system analysis.

Extended equal area criterion justifications, generalizations, applications

Abstract: The extended equal area criterion (EEAC) for online transient stability analysis is considered with the following objectives. The first is to state systematically its main hypotheses and key conditions, justify the former, and suggest means to guarantee the latter. The identification and error analysis of critical machines are among the investigated issues. The second is to scan all possible types of instabilities likely to arise in practice and devise means to treat them. The extension of the EEAC to cases beyond the so-called first-swing stability makes it more robust than all direct methods developed up to now. The third objective is to extract essential information out of a large body of simulations and show that the above improvements and extensions enhance the EEAC accuracy and its capability to work properly even under stringent conditions. Possible EEAC applications are also discussed, and uses of the method as such or as an auxiliary technique for more sophisticated approaches are suggested. >

Structural analysis of the electrical system: application to secondary voltage control in France

Abstract: The voltage control of the French EHV power system is organized in three levels (primary, secondary, and tertiary) which concern distinct geographical areas and time constants. The authors present studies conducted to reach an automatic determination of zones and pilot nodes based on the examination of the structure of the network. The concept of electrical distance has been developed, and its close relationship with the theory of information is underlined. This concept allows the concept of structural controllability and observability of proximity to be extended to the power system. The combined use of this electrical distance and typological analysis algorithms has proven an effective method for the identification of the secondary voltage control zones of the French grid. >

On voltage collapse in electric power systems

Abstract: Several voltage collapses have had a period of slowly decreasing voltage followed by an accelerating collapse in voltage. The authors clarify the use of static and dynamic models to explain this type of voltage collapse, where the static model is used before a saddle-node bifurcation and the dynamic model is used after the bifurcation. Before the bifurcation, a static model can be used to explain the slow voltage decrease. The closeness of the system to bifurcation can be interpreted physically in terms of the ability of transmission systems to transmit reactive power to load buses. Simulation results show how this ability varies with system parameters. It is suggested that voltage collapse could be avoided by manipulating system parameters so that the bifurcation point is outside the normal operating region. After the bifurcation, the system dynamics are modeled by the center manifold voltage collapse model. The essence of this model is that the system dynamics after bifurcation are captured by the center manifold trajectory. The behavior predicted by the model is found simply by numerically integrating the system differential equations to obtain this trajectory. >

On sensitivities, residues and participations: applications to oscillatory stability analysis and control

Abstract: Techniques are presented for the evaluation and interpretation of eigenvalue sensitivities, in the context of the analysis and control of oscillatory stability in multimachine power systems These techniques combine the numeric power of model analysis of state-space modals with the insight that can be obtained from transfer-function descriptions Relationships with tools from selective model analysis (namely, participations) are stressed Examples of applications to a detailed multimachine power system model are given >

A survey of neutral currents in three-phase computer power systems

Abstract: Neutral current in three-phase power systems is often thought to be only the result of the imbalance of the phase currents. With computer systems, very high neutral currents have been observed even when the phase currents are balanced. Measurements from a sample of computer power systems in the United States are presented to determine the extent of the neutral current problem. The author explores the cause of high neutral currents in three-phase computer power systems, potential problems, and recommended remedies. It is found that, while very high neutral currents are possible due to the additive nature of triple harmonic currents, a low percentage of data-processing sites in the US are actually experiencing neutral currents in excess of the rated phase current. However, recent trends in computer systems make high harmonic neutral currents more likely. Power system problems associated with high harmonic neutral currents include overloaded transformers, voltage distortion, and common-mode noise. It is suggested that whenever three-phase, wye power systems are used to supply power to computer systems or other similar electronic loads, the power system design should allow for the possibility of high harmonic neutral current to avoid potential problems. >

Nonlinear stabilizing control of multimachine systems

Abstract: It is of great importance to improve stability as well as dynamic performances of power systems for both small and large disturbance. Such issues have received a great deal of attention and many contributions have been made to this objective. However, some of the design methods developed previously are based on second-order synchronous generator model, so excitation systems or/and governor systems could hardly be considered by them, the others based on linear models, which are set up by approximate linearizing at an equilibrium point of the system. Since, in fact, a power system is a nonlinear dynamic one, the controller designed by using the approximate linearized model may cause untolerable errors, while the state point of the system is changed away from the equilibrium point at which the linearization is realized. A new approach to nonlinear decoupled optimal control based on the differential geometric control theory for interconnected power systems is proposed. And it is successfully applied to nonlinear steam valving control of multimachine systems. For an m-machine system the optimal steam valving control law of the ith-machine can be expressed as ui= TS; ii(K1-M& 1+ K2 cw-+ K3; | wdt + TsiPei+ (Pmj-Pmjo) (18) where Mi inertia coefficient (in sec.) Tsi, time constant of servomotor and steam (in sec.) wi, speed (in per unit) Di damping coefficient Pei electrical power (in per unit) Pmi mechanical power (in per unit) and K1j, K2j, and K3j are optimal feedback gain coefficients.

Least-Cost Electric Utility Planning

Abstract: The Utility Perspective. Introduction to Utility Financial Accounting. Time Value of Money. Economic Evaluation. Financial and Regulatory Analysis. Industrial Power Generation Economics. Electricity Load--Demand Forecasting. Load Forecasting II. Power Plant Reliability Characteristics. Generation System Reliability. Generation System Reliability II. Production Simulation. Production Simulation II. Generation Planning. Capacity Resource Planning. Bulk Power Transmission Planning. Power System Stability. Preparing for the Next Century. Index.

Structural analysis of the electrical system

Abstract: The voltage control of the French EHV power system is organised in three levels (primary, secondary and tertiary) which concern distinct geographical areas and time constants. The secondary voltage control co-ordinates the action of the different generating sets at regional level. Proper operation of this control pre-supposes the possibility of dividing the EHV network into uncoupled zones and of choosing a descriptive node (pilot node) for each zone. This article presents the studies conducted to reach an automatic determination of zones and pilot nodes based on the examination of the structure of the network. The concept of electrical distance has been developed and its close relationship with the theory of information underlined. In addition, this concept allows the concepts of structural controlability and observability of proximity to be extended to the power system. Combined use of this electrical distance and typological analysis algorithms has proven an effective method for the identification of the secondary voltage control zones of the French grid.

Identification of harmonic sources by a state estimation technique

Abstract: A reverse power flow procedure is described to identify the sources of harmonic signals in electric power systems. Methods have been reported elsewhere to analyze the propagation of these harmonics signals throughout the network. Here the reverse problem of how to identify the source (and potentially the type) of harmonic signals is considered. Line and bus data at several points in the network are used with a least-squares estimator to calculate the injection spectrum at buses suspected of being harmonic sources. When energy at harmonic frequencies is found to be injected into the network at a bus, that bus is identified as a harmonic source. >

Combined low-cost, high-efficient inverter, peak power tracker and regulator for PV applications

Abstract: A novel compound power converter that serves as a DC-to-AC inverter, maximum power point tracker (MPPT), and battery charger for stand-alone photovoltaic (PV) power systems is introduced. A theoretical analysis of the proposed converter is performed, and the results are compared with experimental results obtained from a 1.5 kW prototype. The overall cost of PV systems can thus be reduced by using load management control and efficiency-optimization techniques. Power flow through the converter is controlled by means of a combination of duty cycle and output frequency control. With load management, large domestic loads, such as single phase induction motors for water pumping, hold-over refrigerators, and freezers, can be driven by day at a much higher energy efficiency. This is due to the high efficiency of the inverter with high insolation, and because the inverter uses the energy directly from the solar array. The battery loss component is thus reduced. >

Energy functions, transient stability and voltage behaviour in power systems with nonlinear loads

Abstract: The authors present preliminary results in a computer program aimed at energy function analysis of transient behavior of power systems with nonlinear loads. The model, which preserves the network structure, is of a differential-algebraic type. This introduces some new analytical issues, but the concepts permit the establishment of a connection between transient (angle) stability, multiple stable equilibria, and voltage behavior. A practical method for determining and classifying the equilibrium points of the model is developed. >

Detection of topology errors by state estimation (power systems)

Abstract: Errors in the telemetered data of breaker and switch status, through the network topology processor in the EMS (energy management system) computer, may result in errors in the determination of the current network topology of the system. The use of normalized residuals that result from state estimation is proposed for the detection of topology errors. Three types of topology errors are considered: line or transformer outage, bus split, and shunt capacitor/reactor switching. Conditions for detectability of topology errors are presented. The conditions are tested on the IEEE 30 bus system, and the results confirm the theoretical predictions. The problem of topology error identification is also discussed. >

Application of Newton's optimal power flow in voltage/reactive power control

Abstract: The authors consider an application of Newton's optimal power flow to the solution of the secondary voltage/reactive power control in transmission networks. An efficient computer program based on the latest achievements in sparse matrix/vector techniques has been developed for this purpose. It is characterized by good robustness, accuracy and speed. A combined objective function appropriate for various system load levels with suitable constraints for the treatment of power system security and economy is also proposed. For the real-time voltage/receiver power control, a suboptimal power flow procedure has been derived by using the reduced set of control variables. This procedure is based on sensitivity theory applied to the determination of zones for the secondary voltage/reactive power control and a corresponding reduced set of regulating sources, whose reactive outputs represent control variables in the optimal power flow program. As a result, the optimal power flow program output becomes a schedule to be used by operators in the process of the real-time voltage/reactive power control in both normal and emergency operating states. >

Security enhancement using an optimal switching power flow

Abstract: The method presented allows the determination of the optimal and n-1 secure power system state by switching operations. A linear switching model is applied in an active way to model the control actions and in a more passive way to set up contingency constraints. Using the switching model in a dual role permits the integration of contingencies and control actions into one optimization algorithm. Thus, with exactly one optimization step the power system can be improved considering various security levels. A comparison is made between preventive conservative and postcontingency rescheduling. The main difference lies in the time-dependent performance of the topology changes in establishing a normal undisturbed system state if any contingency occurs. The preventive way of rescheduling has the advantage that there are no limit violations produced by a contingency. In postcontingency rescheduling short-term overloads are tolerated. This system state has the advantage of lower production costs. The performance of the algorithm is shown in a test case. >

Large-scale optimal power flow: effects of initialization, decoupling and discretization

Abstract: This paper presents the results of extensive numerical testing of a second-order OPF solution method. The testing was conducted using a 1500 bus network under various loading conditions. Three issues were studied: ? Sensitivity of the solution with respect to the initial conditions ? Accuracy of decoupling the original OPF problem into separate active power and the reactive power OPF problems ? Effects of the discretization of transformer taps on solutions. The first issue is particularly important in an EMS environment. An OPF must produce consistent solutions if it is to be used to guide the decision-making of power system operators. This requires that the OPF solution not be sensitive to the (arbitrarily or randomly) selected starting point used by the OPF program, and that changes in the OPF solution point be consistent with the changes in the power system operating constraints. Such changes include the evolution of bus loads over time, control variables which move to their limits over time, and changes in topology due to disturbances. Hence the first objective of the OPF testing was to study the sensitivity of OPF solutions to the choice of initial points. The second issue is of importance because most of the OPF approaches proposed by EMS vendors rely on decoupling of the active and reactive OPF problems in order to achieve acceptable computational performance.

Off-nominal frequency measurements in electric power systems

Abstract: Some power system protection applications require accurate estimates of voltage magnitude and frequency over a wide measurement range. Most digital techniques for measuring frequency have acceptable accuracy over a small range in the neighborhood of the nominal value. This paper presents a method for extending the measurement range of the Least Error Squares (LES) technique. The paper also examines the effectiveness of the technique and presents some results from simulated studies and laboratory experiments. It is essential to maintain the frequency of a power system close to its nominal value. Usually, frequency deviations in the range of two to three percent only are allowed for short durations of time. In this range, the Least Error Squares algorithm (LES) provides accurate estimates and is, therefore, suitable for implementing in most frequency relays. However, the measurement errors increase as the frequency deviation from the nominal value increases. During start-up and shut-down of a generator, its frequency varies over a wide range. If the generator is connected to a transformer, both the generator and the transformer can experience overexcitation. Excessive volts-perhertz (V/Hz) indicates that overexcitation has occurred. One method of estimating this parameter is to measure the voltage magnitude and frequency and then calculate volts-per-hertz. Voltsper-hertz relays that provide accurate measurements over 10 to 70 Hz range would be desirable for use on steam turbine driven generators. The frequency of the voltage of a thermal generator is low during start-up and shut-down procedures.

Complete bounding method for AC contingency screening

Abstract: The analysis of the effects of hundreds of outages on line flows and bus voltages is required for the real-time security analysis and contingency enumeration. This increases the demands on the speed, accuracy and adaptability of the solution methods. Ideally, an ac power flow should be solved for each contingency, followed by a check for limit violations and major shifts from the initial system conditions. Such an approach is not feasible for practical systems consisting of hundreds of buses. To cope with this computational barrier, various approximate methods have been developed based on the idea that the vast majority of outages does not cause major shifts/violations. There are two classes of such methods, explicit and implicit techniques, which ease the computational burden by identifying cases with severe system limit violations. The explicit methods do not identify or solve for specific violations. Rather, they quantify the severity of each outage by a scalar index by which all the contingencies can be ranked. The explicit methods are not completely reliable since they are prone to masking errors. Specifically, a contingency with a few severe violations can be ranked equally with one with many minor violations or even worse, with one without violations. The implicit methods, though more demanding in CPU resources, permit the identification of actual violations/ major shifts and, therefore avoid masking errors. This paper describes a new contingency analysis technique which overcomes the deficiencies of existing methods. This technique has been implemented in a production-grade program designed for real-time applications.

Real time pricing to assist in load frequency control

Abstract: We study the use of real time prices to assist in the control of frequency and tie line deviations in electric power systems The role of such prices, if any, would yield the practical limit to the trend in electric power systems of varying prices on ever faster time scales The application of prices in electric power systems to increase the efficient use of resources is an established technique The pricing schemes can be classified by time scales Energy adjustment charges vary seasonally or monthly, while time of day rates vary two or three times per day The power brokering system of 18 Florida Utilities operates on an hourly time scale In a spot price market of buyers and sellers of electric power, prices adapt to system operation conditions such as changes in system lambda, the effect of generation shortages, and the effect of line overloads The fastest spot price that has been implemented to date is 30 minutes (most implementations involve 1 hour time steps, which may be prespecified 24 hours in advance) On a five minute time scale is system lambda, a shadow price, used internally by electric utilities for economic dispatch A key assumption of spot pricing and economic dispatch is that the power system is in quasi-steady state; ie power system dynamics involving frequency, voltage, etc are ignored, and only Kirchoff's laws for network are considered The paper explores pricing at time scales where the quasi-steady state assumption is no longer valid

A digital signal processing algorithm for detecting arcing faults on power distribution feeders

Abstract: Signal processing hardware and software that can be used to improve the detection of certain power system faults using computer relays are discussed. Integrated systems and architectures for monitoring several fault-sensitive parameters have been investigated. A suggested architecture utilizing several processors is presented. Several fault-sensitive parameters for the detection of arcing faults are presented. A detection methodology based on these parameters is described, and a partial solution to the problem of directionality is discussed. The use of a knowledge-based environment to modify protection criteria is suggested. >

Low-order harmonic interactions on AC/DC systems

Abstract: A technique is described to evaluate harmonic interaction. It is based on linearizing the harmonic interaction between AC and DC sides of the converter, including the effect of control action. The technique provides an efficient tool for the evaluation of possible adverse amplification of low-order harmonics as well as the design of mitigating measures. It is especially valuable for complex HVDC systems where performance must be evaluated for many contingencies. >

STAG-a new unified software program for the study of the dynamic behaviour of electrical power systems

Abstract: A program is developed for the numerical simulation of dynamic phenomena in power systems. This program unifies the conventional fields of transient, mid-term and long-term stabilities. The integration algorithm ensures a constant level of accuracy in the calculation of fast or slow transients through a continually variable step and through suitable numerical stability properties. The performance levels reached, both as regards ease of utilization and the size and complexity of the simulated systems, allow the majority of cases to be covered so far as the dynamic system behavior study requirements of the Belgian power companies are concerned. >

Expert systems in electric power systems-a bibliographical survey

Abstract: The authors give a bibliographic survey of research, development, and application of expert systems in electric power systems based on over 80 published articles from 1982 to 1988. The general background of the research, development, and applications is described. Requirements for expert systems technology in power system applications are clarified. A historical perspective of expert systems, an overview of their applications, and potential future applications in power systems are presented. >

Analysis of tap-changer dynamics and construction of voltage stability regions

Abstract: The destabilizing behavior of onload tap changers (OLTC) is an important mechanism responsible for the voltage collapse of interconnected power systems. A nonlinear dynamic model of the OLTC, impedance loads and decoupled reactive power-voltage relations is used to reconstruct the voltage-collapse phenomenon. Trajectories leading to a monotonic fall of bus voltages are obtained from initial conditions outside the stability region of a simple power network. The construction of voltage stability regions is desirable for the prevention of voltage collapse. Based on the proposed M-bus power network model, this research results in (1) a simple criterion for stability of an equilibrium, and (2) a method to obtain a stability region by forming the union of hyperbox subsets of the true region. The theoretical foundations of the proposed method, i.e., characteristics of the equilibria and monotonic behaviour of system trajectories, are thoroughly studied. >