Showing papers on "Power system simulation published in 1989"

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. >

Implementation of a real-time expert system for a restoration guide in a dispatching center

Abstract: The authors describe a real-time expert system for a restoration guide which is intended for use in a dispatching center to evaluate the performance of a knowledge-based system. The system configuration is decentralized by workstations, one of which is attached to a SCADA (supervisory control and data acquisition) computer to obtain on and offline information on the current state of the power system. The restoration guide of this system has two operation modes. One is the online guide mode, which gives the operator an appropriate restoration plan and restoration procedures by using the knowledge base of expertise and heuristics on power system restoration. The other is the offline simulation mode which can be used by the knowledge engineer to verify the validity of the acquired knowledge. Implementation issues of this system, such as system function, knowledge acquisition, knowledge representations, the interface of the SCADA system, and the man-machine interface, are discussed. Some execution results from the online guide mode are provided to demonstrate the capability and efficiency of this system. >

Dynamic simulation of voltage collapse

Abstract: The problem of voltage collapse is approached by simulation of a power system using a slightly modified transient stability program. A sufficiently complex system with 39 buses and ten generators is used in the simulations. The system is stressed by progressively increasing the system load through a multiplier k. A very small change of k (order of 1%) is used as the collapse-inducing disturbance. Total system voltage collapse was observed after the disturbance. The significance of these findings and directions for future research are considered, and the applicability of real-time control is discussed. >

Combination of Lagrangian-relaxation and linear-programming approaches for fuel-constrained unit-commitment problems

Abstract: The authors present a new method for determining the unit-commitment schedule of a power system which operates under fuel utilisation constraints. The proposed method employs the Lagrangian relaxation approach to determine a feasible suboptimal schedule. Then, linear programming is applied to improve the feasible solution, as well as seeking the optimal economic dispatch of the committed generators. The algorithm is implemented on a VAX11/780 machine to solve the unit-commitment problem of a system which consists of 26 thermal units and 6 fuel-constrained units. Numerical results presented in this paper describe the usefulness and practicality of the proposed method.

A self-tuning power system stabilizer for wide-range synchronous generator operation

Abstract: A generalized minimum-variance self-tuning power system stabilizer (PSS) is suggested. Computer simulations of a lightly damped single turbine-generator supplying power to an infinite bus system show that the PSS can effectively reduce oscillations over a large operating range and significantly improve rotor damping. The algorithm not only identifies the controller parameters directly from identification (such an algorithm is called an implicit algorithm), but also gives more freedom in engineering applications when choosing parameters. It also can handle the nonminimum-phase plant by choosing different weighting polynomials. >

Evaluation of power flow techniques for personal computers

Abstract: An evaluation study is presented for running power flow programs on an IBM/AT personal computer (PC). Different issues that have been the subject of discussion for the mainframe computer implementation are addressed regarding the PC applications. These include convergence rates, iteration counts, memory requirements, effects of low precision arithmetic, ill conditioning, and the CPU times. A number of formulations of the power flow problem are considered and compared with regard to the mentioned issues. The investigated formulations are: the full Newton-Raphson power flow, the fast decoupled power flow, the Gauss-Seidel flow, and the second-order power flow using cartesian coordinates. The numerical results of running power flow studies for the IEEE 5, 14, 30, 57 and 118 bus test systems are presented and discussed. >

A supplementary adaptive VAr unit controller for power system damping

Abstract: A multivariable adaptive LQ control strategy for static VAr compensators is described which can be added to an existing fixed controller to enhance the robustness and effectiveness of the VAr unit in damping power system oscillations. The control strategy uses only local network information to damp oscillations between machines in the network. Simulation results are presented for a nine-bus network. The controller is effective in reducing local machine oscillations even in cases where the oscillatory disturbance is not controllable. >

Simulation of thermal effects in electrical systems

Abstract: Simulators based on the SPI CE program have one major limitation in that every device in a system must operate at the same temperature. A discussion is presented of modifications implemented in SPICE such that each device's operating temperature can be set independently by operating-point power dissipation and/or user input, and can vary during a transient analysis controlled by a user-defined parallel thermal network. The method used to describe the thermal system is an electrical analog. The temperature is characterized as voltage and the power as current. The thermal resistances and capacitances are represented with resistors and capacitors. The thermal network is then solved simultaneously with the electrical network using the same algorithms. >

Power plant models for operator training simulators

Abstract: Models for power plants have been developed over several decades for use in long term dynamic simulation of large power systems. These models are derived from, but are significantly simpler than, models used for studying power plants. This is because in system studies, as opposed to power plant studies, only the trajectories of the output variables of many plants are of interest as opposed to the many variables internal to the plant. The first use of these models was in the simulation and study of automatic generation control. This was done by interconnecting the power plant models through a uniform frequency dynamic model of the power system. Later, the addition of the network model and models of other system components provided the capability to simulate all aspects of power system long term dynamics. The power system simulation required in an operator training simulator is very similar to that needed for long term dynamic studies with the added requirement that it must be run in real time. These available power plant models have been used in operator training simulators and the suitability for such application of these models has also been shown. The fast solution requirement of the operator training simulator, however, has often led to the use of simple low order models whose accuracy, though suspect, has never been reported. In this paper, an attempt is made to provide models of varying complexity.

A real-time calculation method for optimal reactive power compensator

Abstract: A real-time calculation method is presented to determine the optimal reactive power compensator for power systems. The reactive power compensators are expressed as both the continuous control scheme and the discrete control scheme. The optimization problem is written in terms of the minimization of power line loss by minimizing the line currents. Three-phase current balancing is also considered in this problem. By an integerization method, suboptimal solutions are approached for utilizing the discrete-tap compensators. Theoretical derivations and computation procedures are formulated by including digital techniques for real-time data acquisition. Applications to radial-type systems are described and discussed. Examples of real-time simulations and experiments are discussed to show the performance of the method and to support the application capability. >

Reliability assessment of generation systems containing multiple hydro plant using simulation techniques

Abstract: Models and evaluation techniques based on Monte Carlo simulation are described for the reliability assessment of mixed hydro-thermal generation systems. The effects that system operating and water management policies have on the reliability indices are considered. These effects are described and discussed using the basic IEEE Reliability Test System (RTS) extended by additional hydro plant data. The outcome of these studies is an improved understanding of the effects that operational policies have on the behavior of mixed hydro-thermal systems and, in particular, an improved knowledge of the response of the RTS. >

Operator Training Simulator: Algorithms and Test Results

Abstract: The power system simulation in an operator training simulator (OTS) is made up of a periodic solution of the steady state power network synchronized with a solution of the slow dynamic behavior due to the power balance of the system. The periodicity of the static solution (the OTS cycle time) has to coincide with the data scan rate of a control center so that changes in the variables of the simulated system accurately mimics the actual measurement data acquisition system. The design of such a computationally demanding application requires many considerations almost all of which distill down to a compromise between accuracy and computation. A companion paper describes the models developed to obtain the required accuracy for such a simulation. This paper describes the algorithms developed to meet the computation times required for real time simulation. In a control center, the data acquisition system is usually designed to scan different classes of data at different scan rates. Although every system is different in detail, the AGC data (generator powers and tie-line flows) scan is the most frequent, typically around 4 seconds. This implies that the OTS cycle should be at least as fast as the AGC scan cycle. To achieve this on the size of computers normally used in control centers (it can obviously be done on faster processors) has been difficult for system sizes of say, 1000 buses. The main hurdle has been the simulation just following a large disturbance.

A self-tuning stabiliser for excitation or governor control of power systems

Abstract: Recently there has been increasing interest shown in utilising self-tuning control for stability enhancement of power systems. This paper proposes a new method of designing a self-tuning stabiliser for excitation or governor control for stability enhancement of power systems. In this method, a linear discrete-time model of preassumed order is used for the stabiliser design. Based on this model, a modified version of a conventional quadratic performance index is so defined that upon minimization, the resulting optimal stabiliser possesses an additional derivative term not found in the conventional stabiliser. The method also employs the recursive least squares algorithm for the purpose of estimating the model parameters every sampling interval. Based on the latest parameter estimate, the optimal stabilising signal is computed and applied to the power system under control. The proposed strategy is, therefore, self-tuning. Fig. 1 shows the block diagram of the proposed strategy for self-tuning excitation or governor control of a power system. An advantage of the derivative control term is that it possesses an anticipatory characteristic and initiates an early stabilising action. Consequently, the proposed self-tuning stabiliser is suitable for improving the damping characteristics of power systems. In fact, simulation results show that the damping characteristics of a power system, under different disturbances and over a wide range of operating conditions, can be enhanced when the proposed stabiliser is applied to the excitation or governor loop. Fig. 2 shows a set of typical nonlinear test results.

Unit commitment in thermal power systems with long-term energy constraints

Abstract: The authors describe a method for unit commitment, considering energy constraints obtained from a long-term optimization. In many electrical utilities, there exists the problem of fulfilling energy supply contracts with other utilities or with fuel suppliers. Furthermore, energy prices may change during the year or depend on the consumed electrical energy. The proposed method solves this complex task in two steps. The first step calculates a long-term, e.g., yearly, operation schedule with daily energies allocated to each unit. These daily energy constraints of the long-term optimization have to be incorporated in the second step, the daily unit commitment. The long-term optimization is solved by mixed integer programming, and the short-term is solved by a Lagrangian relaxation. Special attention is given to the coordination between long-term and short-term optimization. >

HVDC models used in stability studies

Abstract: The author discusses the new generation of detailed models for HVDC systems that were recently introduced in power system stability programs. These models represent the high-speed dynamics of the converter controllers as well as the L/R dynamics of the DC transmission. Older DC models based on pseudo-steady-state relationships are still in general use. The author addresses the following questions concerning the two types of models: (1) To what extent is simulation accuracy impacted by using the older HVDC model? (2) Is the difference in precision significant compared to other uncertainties which are inherent in stability calculations? He also considers a third type of HVDC model, which relieves some of the assumptions associated with the pseudo-steady-state models. The third type of model does not represent high-frequency effects but can represent the temporary dynamic condition when neither the rectifier nor the inverter are at control angle limits. It is concluded that this model is more accurate than the pseudo-steady-state model during the recovery period and is just as easy on the computer budget. >

Operator training simulator: component models

Abstract: The power system simulation in an operator training simulator (OTS) is made up of a periodic solution of the steady state power network synchronized with a solution of the slow dynamic behavior due to the power balance of the system. The periodicity of the static solution (henceforth, called the OTS cycle time) has to coincide with the data scan rate of a control center so that changes in the variables of the simulated system accurately mimics the actual measurement data acquisition system. Thus, if the computation time for one static solution and the dynamic solution for the cycle time, can be kept to less than or equal to the cycle time itself, then effective real time simulation is achieved. Obviously, a matching of the computer size to power system size is needed to ensure this real time simulation, assuming, of course, that the fastest available solution algorithms are used. The design of such a computationally demanding application as the OTS requires many considerations almost all of which distill down to a compromise between accuracy and computation. Thus, the models used to represent the various components of the power system have to be carefully chosen to provide adequate accuracy with the highest computational efficiency. Such models have been developed and refined over the years for such applications as power flow and transient stability. However, the requirements of an OTS are sufficiently different from these other applications that many of these existing models were not found to be directly usable without change.

A Model for Economy Energy Enchanges in Interconnected Power Systems

Abstract: A model for the study of improvements in production efficiency in interconnected systems that takes into account the random outages of machines, transmission loadings, and wheeling penalties is presented. The model simulates up to five interconnected systems in any manner. The simulation procedure uses a Monte Carlo technique to model machine outages. Input data matrices account for the topology of interconnections, transmission capacities, loop flows, and wheeling penalties. Economic benefits of using the model are discussed, and some results obtained with it are presented. >

Effect of load models on AC/DC system stability and modulation control design

Abstract: This paper investigates important aspects related to the effect of load models on the modulation control design and stability of a modulated ac/dc system. Static load is modeled as a nonlinear function of load bus voltage and dynamic load is modeled by an equivalent induction motor. DC power and reactive power modulations are considered for the modulation controllers. A method for eigenvalue sensitivity calculation is developed to predict the effect of load characteristics on system stability. Eigenvalue sensitivity and simulation results show that static and dynamic load characteristics may have a considerable effect on the system stability. Figure 1 shows an ac/dc power system model used for studying the effect of nonlinear load on system stability. Reactive power modulation gain is obtained via optimal control theory. Figure 2 shows speed response of synchronous generator for a 5% change in reference current (Iref) of the rectifier terminal. Reactive power modulation by static var compensator improves system stability with constant impedance load model. However, reactive power modulation makes the system unstable when the modulation gain is based on constant impedance load model and the actual load is represented by induction motor. Important conclusions resulting from the computations and simulations performed for an integrated ac/dc system are listed below. 1. The dynamic behavior of induction motor load has a significant effect on the system stability. Induction motor in most cases reduces the overall system damping.

Large-signal analysis of spacecraft power systems

Abstract: A large-signal analysis of the direct energy transfer power system is performed to predict the main bus dynamics in various modes of operation. Stability criteria for the operating points on the solar-array I-V curve are identified in each mode. The large-signal trajectories of the system's operating point for a complete orbit cycle are analyzed, employing a qualitative graphical representation. The analyses are verified through computer simulation. The results of the analysis are presented and discussed in great detail. >

Device level simulation for power converters

Abstract: The objective of device-level simulation of power converters is to obtain detailed performance information such as device voltage and current waveforms and power dissipations. The resulting model can also be used in the course of design to help optimize component choices. Issues involved in device-level simulation such as applications, problems, limitations and advantages are discussed. An example full-bridge converter is simulated and used for illustration. >

Control for stability in interconnected power systems

Abstract: The development of a number of stabilizing controls is described. The robustness of the overall power system is emphasized along with modifications to the basic controls necessary to achieve this robustness. Analytical tools used in the design of practical power system controls are described. The importance of validation of modeling and simulation methods by planned system tests and by the analysis of naturally occurring faults is stressed. Likely future system developments are reviewed, including implications for the types of controls that may be necessary. >

Scenario building for operator training simulators using a transient stability program

Abstract: The operation of a modern power system has become more complicated and as a result the need for advanced and extensive operator training is becoming more critically important. An Operator Training Simulator (OTS) can significantly enhance the efficiency of power system operator training. An OTS can provide a realistic environment for operators to practice operating procedures, and understand and learn how to cope with emergency and restorative situations. To achieve efficient training through an operator training simulator, suitable training scenarios have to be developed to familiarize the trainee with the characteristics of the system under different (and stressed) conditions. Each training scenario consists of a base (starting) operating case, a system load profile to drive the simulation and a sequence of events including generator tripping, network switching, etc. The scenario building is usually a very complicated and very difficult task, imposing a heavy burden on the OTS scenario builder. One of the drawbacks of the existing power system operator training simulators is the lack of the efficient scenario building aids. As recommended in an EPRI final report, various scenario building aids should be provided for the scenario builder to efficiently develop training scenarios. One such scenario building aid is a suitable transient stability program. Due to the OTS modeling limitations, it is not possible to simulate in real time the power system transient response, such as protective relaying operations, to system faults.

A fast accurate method for midterm transient stability simulation of voltage collapse in power systems

Abstract: The effects that are modeled in recently developed midterm stability programs are described. A tool that can screen large numbers of contingencies and operative condition changes to determine which need more careful study by full midterm stability simulation has been developed. The screening tool, which covers all of the effects modeled by the program, provides a series of snapshots of the effects of various controls over time. The snapshots are obtained by appropriately modifying the input data to a conventional Newton-Raphson load flow. The modeling performed and the modifications of a conventional load flow to provide each snapshot are discussed. >

Adequacy evaluation of a small area in a large composite power network

Abstract: The adequacy evaluation of a composite power system involves the examination of both generation and transmission facilities in a network. The major developments in the area of adequacy evaluation of composite power networks to obtain appropriate indices at the major load points have occurred over the last twenty five years. During this time, there has been considerable interest amongst power utilities, research organizations, and universities in developing suitable models, solution techniques, adequacy indices and in collecting relevant data to conduct studies on bulk power systems. Most power utilities in North America are, however, still using deterministic criteria as opposed to probabilistic criteria in adequacy assessment of their bulk power systems. There are many reasons for this but one of the important ones is the lack of a suitable methodology that provides the required information regarding adequacy performance. An actual power system tends to be quite large and a thorough examination of all credible outages involving system components is not practical due to the large computation time required to solve the network under these outage situations. Attempts have been made to calculate the adequacy indices of large power systems using approximate techniques which either make use of simplifying assumptions or utilize approximate solution techniques. They also do not consider all the credible outages that might significantly affect the performance of a power system. It should be noted that there is no simple or magic solution for overcoming these difficulties.

Interactive simulation of dynamic systems on a personal computer to support teaching

Abstract: A system is described that can be used to simulate dynamic systems described by differential equations. The results are displayed, using graphs, on the computer screen. The user can halt the simulation at any time in order to change parameters. It is also possible to change parameters during the simulation and to observe immediately the impact of the changes on the simulated system. The system operates on a standard personal computer. The concept of the simulation system is described. In addition the specification and implementation of the simulation system are discussed. Examples of the performance of the system are provided. >

The Inclusion of Dynamic Factors in Statistical Power System Cost Models Part I: Assessment of Startup and Banking Costs

Abstract: Methods are developed for approximating dynamic aspects of power system costs within the load duration and probabilistic framework. The approach uses a transition-frequency function, as applied in frequency and duration methods of system reliability assessment, together with a function expressing the RMS prediction error in load forecasting. The author shows how the transition-frequency function can be used to estimate plant startup and banking costs under various conditions. He demonstrates how the calculations can be performed in terms of cumulants of the functions involved, making possible a very efficient formulation. >

Modeling and analysis of computer power systems

Abstract: Two techniques for time-domain analysis, cycle-by-cycle simulation of switched linear circuits and state-space averaging, are discussed in the context of modeling computer power systems. These techniques are compared by studying the output voltage response of a single DC-to-DC power converter to a transient load. Measured data are used to validate the simulated results. Simulation CPU times are also presented along with a discussion on the pros and cons of each approach. Predicting the response to line and load transients of a much larger system composed of three paralleled DC-to-DC converters is studied using state-space averaged models implemented in SPICE. >

Simulation of induction machinery for power system studies

Abstract: A simple, fast, and accurate algorithm for determining induction-machine transient behavior is presented. The differential equations for the full-order model of induction machines are simplified and solved using linear system techniques. Examples show that the algorithm is accurate for large disturbances and requires as little as 29% of the CPU time required by the full-order model. The algorithm may offer computational advantages in some studies of large-scale power systems where the detailed induction machine models can not be used due to the complexity of the overall system and where the overall CPU time may be costly. >

Convergence properties of the waveform relaxation method as applied to electric power systems

Abstract: Several theoretical results are presented and simple examples examined in order to determine the suitability of waveform relaxation for transient power system simulation. This examination leads to two practical suggestions that can easily be satisfied for power systems. The first is to break the simulation interval into sections, the first of which should be narrow and be used to kill off errors in the initial guess that activate the stiff modes. The second is to partition the system into subsystems that are stable, as this not only aids convergence but ensures multirate numerical stability. >

Operations research in forestry.

Abstract: The applications of Operations Research techniques in forestry and forest industries are numerous. The most widely used mathematical models are: linear programming, integer programming. goal programming, dynamic programming, network analysis, and computer simulation. The demand for improved efficiency, combined with multiple-use requirements, and the availability of computers, will result in a continuing increase of the use of Operations Research in natural resource managerial decision-making.