Showing papers in "IEEE Transactions on Power Systems in 1989"

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].

A regression-based approach to short-term system load forecasting

Abstract: The authors describe a novel linear regression-based model for the calculation of short-term system load forecasts. The model's most significant new aspects fall into the following areas: innovative model building, including accurate holiday modeling by using binary variables; temperature modeling by using heating and cooling degree functions; robust parameter estimation and parameter estimation under heteroskedasticity by using weighted least-squares linear regression techniques; the use of 'reverse errors-in-variables' techniques to mitigate the effects on load forecasts of potential errors in the explanatory variables; and distinction between time-independent daily peak load forecasts and the maximum of the hourly load forecasts in order to prevent peak forecasts from being negatively biased. The significant impact of these issues on the accuracy of a model's results was established through testing of an existing load forecasting algorithm. The model has been tested under a variety of conditions and it was shown to produce excellent results. It is also sufficiently general to be used by other electric power utilities. >

A reliability test system for educational purposes-basic data

Abstract: The IEEE Subcommittee on the Application of Probability Methods (APM) published the IEEE Reliability Test System (RTS) [1] in 1979. This system provides a consistent and generally acceptable set of data that can be used both in generation capacity and in composite system reliability evaluation [2,3]. The test system provides a basis for the comparison of results obtained by different people using different methods. Prior to its publication, there was no general agreement on either the system or the data that should be used to demonstrate or test various techniques developed to conduct reliability studies. Development of reliability assessment techniques and programs are very dependent on the intent behind the development as the experience of one power utility with their system may be quite different from that of another utility. The development and the utilization of a reliability program are, therefore, greatly influenced by the experience of a utlity and the intent of the system manager, planner and designer conducting the reliability studies. The IEEE-RTS has proved to be extremely valuable in highlighting and comparing the capabilities (or incapabilities) of programs used in reliability studies, the differences in the perception of various power utilities and the differences in the solution techniques. The IEEE-RTS contains a reasonably large power network which can be difficult to use for initial studies in an educational environment.

Analysis and evaluation of five short-term load forecasting techniques

Abstract: Load forecast has been a central and an integral process in the planning and operation of electric utilities. Many techniques and approaches have been investigated to tackle this problem in the last two decades. These are often different in nature and apply different engineering considerations and economic analyses. In this paper a comparative evaluation of five short-term load forecasting techniques is presented. These techniques are: 1. Multiple Linear Regression; 2. Stochastic Time Series; 3. General Exponential Smoothing; 4. State Space Method; and 5. Knowledge-Based Approach. The authors have applied these algorithms to obtain hourly load forecasts (for up to 24 hours) during the winter and summer peaking seasons. Thus the five forecasting methodologies have been applied to the same database and their performances are directly compared. The forecast error analyses are provided in Tables 1 and 2 for the winter and summer days respectively. As these results are based on forecasts of two single days, these should be used for comparative purposes only. Some interesting observations are made about the results presented in Tables 1 and 2. For example, for the peak summer day the transfer function (TF) approach gave the best result, whereas for the peak winter day the TF approach resulted in the next to the worst accuracy. During the peak summer day the temperature profile was typical whereas during the peak winter day the profile was unseasonal.

Artificial neural-net based dynamic security assessment for electric power systems

Abstract: In the post-fault dynamic analysis of interconnected power systems, the critical fault clearing time (CCT) is one of the parameters of paramount importance. Critical clearing time is a complex function of pre-fault system conditions (operating point, topology, system parameters), fault structure (type and location) and post-fault conditions that are in part dependent on the protective relaying policy. To define analytically such a relationship would be highly desirable but diversity of variable involved makes this task extremely complicated. Our efforts focus on examination of that complex mapping and investigation of the influence of the various parameters on CCT. The evaluation of CCT involves elaborate computations that often include time-consuming solutions of nonlinear on-fault system equations. Existing conventional pattern recognition techniques are incapable of synthesizing such complex and transparent mappings. Thus, when a human operator tells the machine learning unit (that is the pattern recognizer) that system state belongs to a certain class, say "emergency", the pattern recognizer merely records that classification mindlessly and is not able to look at the pattern with insight and discover what underlies the "emergency" nature of pattern. It is, therefore, highly desirable to have a mechanism which when presented with a sequence of class labeled patterns not only learns an internal structure which allows it to generalize and to classify other and to classify other patterns correctly, but also is able to shed some light on what combination of features give rise to the particular class membership.

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. >

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. >

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. >

Implementation of a Lagrangian relaxation based unit commitment problem

Abstract: The unit commitment problem in a power system involves determining a start-up and shut-down schedule of units to be used to meet the forecasted demand, over a future short term (24-168 hour) period. In solving the unit commitment problem, generally two basic decisions are involved. The "unit commitment" decision involves determining which generating units are to be running during each hour of the planning horizon, considering system capacity requirements including reserve, and the constraints on the start up and shut down of units. The related "economic dispatch" decision involves the allocation of system demand and spinning reserve capacity among the operating units during each specific hour of operation. As these two decisions are interrelated, the unit commitment problem generally embraces both these decisions, and the objective is to obtain an overall least cost solution for operating the power system over the scheduling horizon. The unit commitment problem belongs to the class of complex combinatorial optimization problems. During the past decade a new approach named "Lagrangian Relaxation" has been evolving for generating efficient solutions for this class of problems. It derives its name from the well-known mathematical technique of using Lagrange multipliers for solving constrained optimization problems, but is really a decomposition technique for the solution of large scale mathematical programming problems. The Lagrangian relaxation methodology generates easy subproblems for deciding commitment and generation schedules for single units over the planning horizon, independent of the commitment of other units.

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 >

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.

Evaluation of transmission network capacity use for wheeling transactions

Abstract: The authors describe the principles and the implementation of a methodology to evaluate the transmission network capacity use for firm transmission services including wheeling transactions. In this methodology, transmission network capacity use of a transaction is a function of the magnitude of electric power, the length of the transmission lines, and the type of facility involved in the transaction. This capacity value provides an equitable means of allocating the cost of transmission facilities among users of the firm transmission service. Through examples, it is shown that this methodology, called the MW-mile methodology, is more reflective of the actual usage of the transmission network in allocating the transmission network capacity cost than the now widely used postage stamp rate procedure. Also indicated is the potential for realizing greater economic efficiencies through the use of this methodology. >

A general-purpose version of the fast decoupled load flow

Abstract: A new version of the fast decoupled load flow, in which a more broad range of power systems can be solved, is presented. The key lies in the different way in which the resistances are ignored and in a different iteration scheme. In the standard algorithm the resistances are ignored while building the B' load flow matrix: it is shown that it is preferable that the resistances are ignored in the B" matrix instead of the B' matrix. For normal test systems there is hardly any difference in the number of iterations. However, the new algorithm iterates faster if one or more problematic R/X ratios are present. An iteration scheme with strict successive P and Q iterations prevents cycling convergence behavior which can be found in some low voltage systems. The advantages of the new version are demonstrated with runs on IEEE test systems, with both uniformly and nonuniformly scaled reactances. R-scaling up to 3 is always possible, and sometimes values up to 5 can be used. X-scaling of at least 0.1 is possible without losing convergence and with iteration counts which are significantly lower than with the standard scheme. >

Optimal demand-side response to electricity spot prices for storage-type customers

Abstract: The customer response to spot prices is discussed. The factors that allow flexible customer response without service curtailments are identified. An overview of a fast, optimal, nonsimplex algorithm applicable to single storage electricity consuming processes is presented. A case study involving an air compression company demonstrates the application of the algorithm and shows the economic effects of industrial customer response to the spot pricing of electricity. >

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. >

An artificial intelligence framework for online transient stability assessment of power systems

Abstract: A framework is proposed to tackle the online transient stability problem of power systems. Based on artificial intelligence, it successively makes use of an inductive inference method to build decisions automatically and a deductive inference method to apply them online. The authors lay the foundations of an inductive inference method, where the rules explicitly relate a system's stability with relevant parameters of it. A simple but realistic power system is treated to illustrate important features of the method and to suggest how the derived decision rules could be used online. >

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. >

Suppression of numerical oscillations in the EMTP power systems

Abstract: The integration scheme in the electromagnetic transients program EMTP has been modified to solve the problem of sustained numerical oscillations that occur when the trapezoidal rule has to act as a differentiator. These oscillations appear, for instance, on the voltage across an inductance after current interruption. The technique presented prevents these oscillations by providing critical damping of the discontinuity within one Delta t of the simulation. The critical damping adjustment (CDA) is achieved by means of two Delta t/2 integration steps using the backward Euler rule. With the CDA scheme the trapezoidal rule can still be used throughout the entire simulation without the problem at discontinuities. The effectiveness of the scheme is illustrated with simulation results. >

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. >

Application of sparse eigenvalue techniques to the small signal stability analysis of large power systems

Abstract: Two sparsity-based eigenvalue simultaneous iterations and the modified Arnoldi method are presented and their application to the small signal stability analysis of large power systems is discussed. An algorithm utilizing these two methods is proposed for calculating the eigenvalues around a fixed point which can be placed at will in various parts of the complex plane. The sparsity is fully preserved in the algorithm by using the augmented system state equations as the linearized power system small signal model and performing the corresponding sparsity-oriented calculations. Several applications of the algorithm are discussed and illustrated by numerical examples. Comparisons are made for the two eigenvalue methods with other techniques. >

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. >

Short term hydrothermal coordination using multi-pass dynamic programming

Abstract: A technique that combines multipass dynamic programming (MPDP) technique with successive approximations to solve the daily hydrothermal coordination problem is presented. The long computation time and the large storage memory requirement are reduced. Due to the characteristics of hydrothermal coordination, the resulting load-following capability is much better than that achieved by applying MPDP to area automatic generation control. The generation can follow any shape load curve instead of only constant loads and ramps as in previous applications. >

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.

Discrete-mode automatic generation control of a two-area reheat thermal system with new area control error

Abstract: Most of the work reported in the literature pertaining to Automatic Generation Control (AGC) of interconnected power systems is centered around tie-line frequency bias control strategy. Supplementary controllers are designed to regulate the area control errors to zero effectively. Several modern design techniques have been used to optimize the parameters of the supplementary controllers. Supplementary controllers regulate the generation to match the load variation. As the generation change chases the load variation, the frequency and tie-power deviate from the scheduled values. This would result in accumulations of time error and inadvertent interchange. They would also occur due to various measurement errors or intentional offsets in scheduled settings. It is expected that individual areas will make all reasonable efforts to minimize time error and inadvertent interchange accumulations by minimizing or eliminating source causes. There is a need for correcting these accumulations. Such corrections are achieved by making appropriate offsets in system frequency schedules to compensate for time error accumulations and offsets in area net interchange schedules to compensate for inadvertent interchange accumulations. Detailed literature survey clearly shows that the two-step correction scheme, mentioned above, has been used by utilities inspite of some practical difficulties. To avoid those difficulties they are looking forward for a suitable control strategy that not only maintains constancy of system frequency and desired tie-power flow but also achieves zero steady state time error and inadvertent interchange. It is essentially in this area, the investigations are carried out in this paper.

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

Hydrothermal optimal power flow based on a combined linear and nonlinear programming methodology

Abstract: The author's consider a new formulation of optimal power flow (OPF) that makes it eminently suitable for accurate incremental modeling. G. Zoutendijk's method of feasible directions (1960) for solving the nonlinear programming problems is adapted for the solution of the OPF. Hydraulic modeling of systems with a considerable share of hydraulic generation is considered. The method is very efficient as it is designed to exploit the special structure of the problem. >

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. >