Showing papers in "IEEE Transactions on Power Systems in 1988"

A compensation-based power flow method for weakly meshed distribution and transmission networks

Abstract: A power flow method is described for solving weakly meshed distribution and transmission networks, using a multiport compensation technique and basic formations of Kirchoff's laws. This method has excellent convergence characteristics and is robust. A computer program implementing this scheme was developed and successfully applied to several practical distribution and transmission networks with radial and weakly meshed structures. The method can be applied to the solution of both the three-phase (unbalanced) and single-phase (balanced) representation of the network, however, only the single-phase representation is treated in detail. >

Towards a more rigorous and practical unit commitment by Lagrangian relaxation

Abstract: A Lagrangian relaxation algorithm for power system generator unit commitment is proposed. The algorithm proceeds in three phases. In the first phase, the Lagrangian dual of the unit commitment is maximized by standard subgradient techniques. The second phase finds a reserve-feasible dual solution, followed by a third phase of economic dispatch. A mathematically based, systematic and generally applicable procedure to search for a reserve-feasible dual solution is presented. The algorithm has been tested on systems of up to 100 units to be scheduled over 168 hours, giving reliable performance and low execution times. Both spinning and time-limited reserve constraints are treated. >

An expert system based algorithm for short term load forecast

Abstract: Existing studies on 1-24 hr load forecasting algorithms are reviewed, and an expert-system-based algorithm is presented as an alternative. The logical and syntactical relationships between weather and load as well as the prevailing daily load shapes have been examined to develop the rules for this approach. Two separate, but similar, algorithms have been developed to provide 1-6 hr and 24 hr forecasts. These forecasts have been compared with observed hourly load data for a Virginia electric utility for all seasons of the year. The 1 hr and 6 hr forecast errors (absolute average) ranged from 0.869% to 1.218% and from 2.437% to 3.48% respectively. The 24 hour forecast errors (absolute average) ranged from 2.429% to 3.300%. >

An expert system operational aid for restoration and loss reduction of distribution systems

Abstract: An application of expert system techniques to the restoration of distribution systems is reported. Primary feeders are typically radial in structure. To increase system reliability, neighboring feeders are connected through a normally open tie switch. When load zones on a feeder interrupted due to a fault, system operators need to identify neighboring feeders and try to restore customers through the tie switches. To restore the maximum number of zones, several steps are followed: group restoration, zone restoration and, if necessary, load transfer. Based on the methodology, an expert system is developed which can be utilized as an online aid to system operators in a distribution SCADA (supervisory control and data acquisition) environmental. The proposed expert system is implemented in Prolog. Knowledge representation, portability of the system, and computational efficiency are discussed. Several examples illustrate the capabilities of the system. >

A posturing strategy against voltage instabilities in electric power systems

Abstract: The problem of system collapse or blackout characterized by a local severe voltage depression is generally believed to be associated with inadequate VAR support at key busses. The authors discuss techniques for improving the system security with respect to this index. The effect of adding capacitors are examined. They also present a continuation technique that redistributes the system generation to the optimal operating condition with respect to the minimum singular value index. >

Multiobjective optimal generation dispatch based on probability security criteria

Abstract: An efficient algorithm is proposed to obtain the optimal power flow in power system operation and planning phases by multiobjective optimization. The algorithm makes it possible to treat the optimal dispatch problems with multiple performance indices and to include tradeoff relations between selected indices. The effect of uncertain factors pertaining to power systems can also be taken into account. The effectiveness of the approach is illustrated with numerical examples of the IEEE 30-node, 6-generator system. >

A simple direct method for fast transient stability assessment of large power systems

Abstract: A method for online transient stability assessment of large power systems is proposed. It consists of: replacing the multimachine system by a two-machine dynamic equivalent, further amenable to a one-machine-infinite-bus system; reducing the stability problem to a sole algebraic equation, devised from the equal area criterion, or equivalently from the Lyapunov direct criterion; and using this equation to derive one-shot stability analysis strategies. A technique for system admittance matrix reduction is developed that proves efficient, especially for large systems and multiple-contingency evaluation. The method's main appeal is rapidity: it is about one order of magnitude faster than the most efficient direct criterion. Other attractive features are flexibility and ability to encompass various simulation conditions. Extensions to online sensitivity analysis and control are suggested. >

Load modeling for power flow and transient stability computer studies

Abstract: A novel method is presented for preparing load models for power flow and stability studies. The LOADSYN load model synthesis software package transforms data on load class mix, composition, and characteristics into the form required for commonly used flow and transient stability simulation programs. Typical default data have been developed for load composition and characteristics. The load-modeling techniques used in this software and results of initial testing are described. Significant improvements in simulation accuracy are demonstrated. >

An enhanced dynamic programming approach for unit commitment

Abstract: To investigate modeling problems with predefined unit hourly start-up ramps, the authors look at the method by which dynamic programming (DP) considers and reject combinations. An enhanced DP approach which saves predecessor options was developed and implemented in an online energy-management system. The method not only supports realistic modelling of unit start-up ramps, but it also involves the analysis of solution paths that may have been eliminated under traditional methods, thereby making better solutions possible. Sample results are given to demonstrate the benefits of the proposed algorithm. >

Comparison of different methods for state estimation

Abstract: Ill-conditioning in the gain matrix of the classical normal-equations-approach for state estimation has created a numerical stability problem for large power systems. Several methods have been proposed to circumvent the problem. A comparative study of five methods, namely, the normal equations method, the orthogonal transformation method, the hybrid method, normal equations with constraints, and Hachtel's augmented matrix method for state estimation has been conducted. The comparison is made in terms of their (i) numerical stability, (ii) computational efficiency, and (iii) implementation complexity. A theoretical analysis indicates that the orthogonal transformation method is numerically most stable. But the orthogonal transformation method cannot be implemented in the efficient fast decoupled version. It is shown that the hybrid method and Hachtel's method are both good compromises between numerical stability and computational efficiency. >

Application of superconducting magnet energy storage to improve power system dynamic performance

Abstract: The application of superconducting magnet energy storage (SMES) to the stabilization of a power system with long-distance bulk power transmission lines which has the problem of poorly damped power oscillations is presented. Control schemes for stabilization using SMES capable of controlling active and reactive power simultaneously in four quadrant ranges are proposed. The effective locations and the necessary capacities of SMES for power-system-stabilizing control are discussed in detail. Results of numerical analysis and experiments in an artificial power-transmission system demonstrate the significant effect of the control by SMES on the improvement of power-system oscillatory performance. >

Optimal VAr planning by approximation method for recursive mixed-integer linear programming

Abstract: The authors propose an algorithm for solving reactive power planning problems. The optimization approach is based on a recursive mixed-integer programming technique using an approximation method. A fundamental feature of this algorithm is that the number of capacitor or reactor units can be treated as a discrete variable in solving large-scale VAr (volt-ampere reactive) planning problems. Numerical results have verified the validity and efficiency of the algorithm. >

A stochastic computer model for heating and cooling loads

Abstract: The authors review the concept of demand-side management, particularly in regard to utility intervention into the setpoint adjustment of customers' thermostats for heating and/or cooling loads. As part of a larger effort to develop a large simulation to evaluate performance of various management strategies, they have created a simple, realistic, easily implementable model for the temperature of a house having a heater/air-conditioner which is regulated by a thermostat and operates in an environment that contains random elements. Two models previously given in the literature are reviewed before giving the details of the model, the feature of which is that it is a stochastic model operating in discrete time. Computer results show good agreement with basic physical principles, and suggest that the probability distributions of on-time and off-time are negative binomial. It is concluded that this model will provide a useful tool in the evaluation of load-management strategies. >

An optimization method for load management scheduling

Abstract: Many utilities have load management programs whereby they directly control residential appliances in their service area. An algorithm for scheduling the load control using dynamic programming is presented. This method is based on an analytic dynamic model of the load under control. The method can be used for different utility objectives, including minimizing production cost and minimizing peak load over a period of time. >

An efficient technique for reliability analysis of power systems including time dependent sources

Abstract: A method of reliability analysis of electric power systems with time-dependent sources, such as photovoltaic and wind generation is introduced. The fluctuating characteristic of unconventional generation units affects the reliability of the generation system in a different manner than the conventional units. The method proposed by the authors groups the units into several subsystems. One subsystem contains the conventional units and the remaining subsystem consist of unconventional units. A generation system model is built for each subsystem. The outputs for the unconventional units and the load are treated as correlated random variables. Using a clustering procedure states are identified wherein for a given value of load there are specific mean values of the outputs of the unconventional units. Reliability analysis is performed by combining the conventional subsystem with the unconventional subsystems in each state. The output from all the states is combined to compute the loss of load expectation and expected unserved energy. >

Detection and identification of topology errors in electric power systems

Abstract: A method for detecting topology errors in electric power networks is developed by providing a geometric interpretation of the measurement residuals caused by such errors. A test for single topology errors is presented that is similar to the normalized residuals test for measurement errors. This test is generalized to multiple topology errors. The concept of critical network branches (where their removal renders the network unobservable) is introduced and extended to critical-branch k-tuples. It is shown that topology errors on critical branches cannot be detected from measurement residuals. An equation is developed for a matrix whose column linear dependencies determine topology error detectability and identifiability. An example for an IEEE 14 bus network is provided. >

MW/voltage control in a linear programming based optimal power flow

Abstract: A functional improvement to the linear-programming-based optimal power flow technique is reported. The new feature allows the rescheduling of the active power controls to correct voltage-magnitude-constraint violations. It preserves the reliability and speed characteristics of the traditional linear programming approach. Results demonstrating the effectiveness of the method on a small and a large power system are presented. >

A modification to the fast decoupled power flow for networks with high R/X ratios

Abstract: The reliability of the fast decoupled load-flow method for most power systems is high, but it presents difficulties in convergence for systems with high ratios of branch resistance to reactance. Modifications that retain the advantages of this method but can handle high r/x ratios are of interest, and compensation techniques have been used for this purpose. Both the series and parallel compensation techniques, however, give mixed results. A modification is presented that performed better on several test systems. The test results show that this method not only converges well for systems with high r/x ratios but also is only slightly less efficient than the fast decoupled method for systems with normal ratios. >

Short-term thermal unit commitment-a new method

Abstract: A method and an algorithm based on this new method are presented. The effectiveness of the algorithm is illustrated by studying a 20-unit midwestern utility system, the EPRI 174-unit synthetic utility system D, and the EPRI 155-unit synthetic utility system E. The algorithm produces the same unit commitment schedule for the 20-unit system as a frequently used DP-STC algorithm in 15 s of computation time versus 524 s, respectively. The computation time is approximately linear with the number of hours in the unit commitment horizon. For the EPRI 174-unit system the algorithm requires only 205 s of computation time on a VAX 11/780 for a 48-hour horizon. >

Practical reactive power allocation/operation planning using successive linear programming

Abstract: A method for reactive power planning is presented that it finds an optimal solution for both allocation and operation planning in large systems using linear programming (LP). The method utilizes calculated linear sensitivities including active power and voltage phase angle in the formulation. Although the overall method includes these relations, the number of constraints and variables are not augmented in its first procedure, APPROACH-1. Its second procedure, APPROACH-2, overcomes numerical problems caused by a dense constraint matrix. This is achieved by retaining untouched sparse sensitivities in the constraint matrix and by eliminating any calculations related to the inverse matrix. The results of applying this method to a practical 224-bus system and the IEEE-30 bus system verify its robustness and fast convergence. >

Efficient bounding method for linear contingency analysis

Abstract: A very efficient contingency analysis method for detecting branch megawatt flow violation is presented. The efficiency stems from the use of a bounding criterion that drastically reduces the number of branch-flow computations and limits checking, and the use of state-of-the-art compensation and sparse matrix/vector methods. The method requires no offline setup, is highly efficient, and can handle contingencies with any time of network topology and load/generation changes. The method is based on the linear incremental-power-flow model and consequently does not consider reactive power. >

Fast contingency screening and evaluation for voltage security analysis

Abstract: The authors describe a fast security-analysis technique for voltage security assessment in an energy-management system. The proposed method identifies the location of buses with potential voltage problems and thereby defines a voltage-sensitive subnetwork for contingency screening. This allows the evaluation of a large number of contingencies. The efficiency of this method is derived from the use of a voltage subnetwork to drastically reduce the number of bus voltages to be solved; and subsequently from the use of compensation techniques and sparse-vector methods (including adaptive reduction) for screening and final solution of contingencies. Results demonstrating the effectiveness of the method on a small and a large power system are presented. >

Some deficiencies in optimal power flow

Abstract: Optimal power flow programs have deficiencies that limit their practical value and scope of application. Three of these deficiencies are: (1) the use of equivalents causes errors, (2) the methods for adjusting discrete variables are suboptimal; and (3) the number of control actions is too large to be executed. These deficiencies are explained, and approaches for correcting them are discussed. >

Secondary voltage control using pilot point information

Abstract: The authors study the minimal information structure needed for monitoring and control of the voltage profile and the reactive power flow of a power system. Emphasis is on the minimum amount of information (data) required for reliable decision-making. An effort is made to work with fewer voltage data and therefore make the real-time monitoring and control more manageable. An annealing algorithm for selecting pilot points is presented. These are the load buses at which the voltage is to be measured in real time. Simulation results obtained with the algorithm for the Central Illinois Light Company system are presented. >

Integrated security control using an optimal power flow and switching concepts

Abstract: A synthesis of methods is presented which combines security analysis and optimal power flow in order to achieve n-1 security. The concepts of corrective switching are used to model outages, making it possible to generate constraints to be attached to the optimal power flow. A distinction is made between the secure state (conservative) and a state which is achieved by postcontingency rescheduling. Results for an 80-node system show the dispatch of control variables for the source states, the losses and the CPU-times. >

Some applications of phasor measurements to adaptive protection

Abstract: The application of microprocessor-based phasor measurements to novel adaptive protection schemes is presented. The concepts of digital adaptive protection in which relay characteristics are modified in response to external signals and conditions in the system, are examined. The importance of real-time phasor measurements in adaptive protection systems is illustrated by two examples. The first involves obtaining synchronizing information about phasor measurements to be used for current differential protection of a multithermal line. By estimating the reference angles for the phasor measurements the effect of synchronized sampling can be obtained without the expense of synchronizing equipment. The second use is the adaptive setting of out-of-step blocking and tripping. Traditional approaches experience problems because the settings are, of necessity, compromises based on offline studies. By reacting to real-time phasor measurements from selected buses in interconnection, it is shown that improved out-of-step blocking and tripping is possible. >

Observability analysis and bad data processing for state estimation with equality constraints

Abstract: A factorization-based observability analysis and the normalized residual-based bad-data processing have been carried out for state estimation using the normal equation approach. The observability analysis is conducted during the process of triangular factorization of the gain matrix. The normalized residuals are calculated using the sparse inverse of the gain matrix. The method of Lagrange multipliers is applied to handle state estimation with equality constraints arising from zero injections, because of its better numerical robustness. The method uses a different coefficient matrix in place of the gain matrix at each iteration. The factorization-based observability analysis and normalized residual-based bad-data processing are extended to state estimation with equality constraints. It is shown that the observability analysis can be carried out in the triangular factorization of the coefficient matrix, and the normalized residuals can be calculated using the sparse inverse of this matrix. Test results are presented. >

Determination of voltage stability limit in multimachine power systems

Abstract: A method of determining the load limit and the critical state of a general multimachine power system is presented. In the method, the search for the load limit is formulated as an optimization problem. With this formulation, difficulties related to singularity of the load-flow-equations Jacobian matrix, and convergence of the load-flow solution around the voltage stability limit, are avoided. Modification to the basic formulation to allow consideration of load-voltage characteristics is also shown. A voltage stability margin is defined which may serve as a measure of the security of a given operating condition from voltage instability or collapse. Results of application of the proposed method to the AEP 14-bus network are presented. >

Loss reduction by network switching

Abstract: Systematic and fast switching for the purposes of reducing losses in power transmission networks is treated as an optimization problem whereby switching is to be understood in a general and comprehensive way. Injected currents applied to a base network are used to model the switching operation. These currents are used as variables in a linear programming (LP) problem formulation. The objective function, i.e. the change in losses, can be expressed by the injected currents, taking into account that all nodes are constrained by constant active powers except for the slack node. The change of power of the slack node is the change in losses, which is obtained by a two-step approximation. Each single optimal switching operation is obtained by an LP-like operation followed by a load-flow update. The interaction between LP and AC load-flow leads to a sequence of optimal switching operations whereby losses are reduced to a minimum subject to the given constraints. >

A new eigen-analysis method of steady-state stability studies for large power systems: S matrix method

Abstract: The authors discuss an advanced version of the S matrix method, an eigenvalue technique for the analysis of the steady-state stability (or the stability against small signals) of large power systems. The dynamic characteristics of power systems can be linearly approximated with a set of differential equations. The technique transforms the matrix A into the matrix S and then determines several eigenvalues with the largest absolute values from matrix S that correspond to the dominant eigenvalues of matrix A. In the process of identifying the appropriate eigenvalues, the method uses the refined Lanczos process, which makes high-speed calculation possible through the use of the sparsity and the structural uniformity of matrices. >