M. E. El-Hawary

A summary of environmental/economic dispatch algorithms

Traditionally electric power systems are operated in such a way that the total fuel cost is minimized regardless of emissions produced. With increased requirements for environmental protection, alternative strategies are required. This paper presents a summary of algorithms of environmental-economic dispatch in electric power systems since 1970. The algorithms attempt to reduce the production of atmospheric emissions such as NO/sub x/ and SO/sub x/ caused by the operation of fossil-fueled thermal generation. Such reduction is achieved by including emissions either as a constraint or as a weighted function the objective of the overall dispatching problem. >

Application of least absolute value parameter estimation based on linear programming to short-term load forecasting

Short term load forecasting employs load models that express the effects of influential variables on system load. The model coefficients are found by fitting the load model to a data base of previous loads and observations of the variables, and then solving the resulting overdetermined system of equations. The coefficients thus obtained are critical to the forecasting process, as they directly affect its final predictive accuracy. This study compares two linear static parameter estimation techniques as they apply to the twenty-four hour off-line forecasting problem. Here a least squares and a least absolute value based linear programming algorithm will be used to simulate the forecast response of three twenty-four hour off-line load models. The three load models are (1) a multiple linear regression model, (2) a harmonic decomposition model and (3) a hybrid multiple linear regression/harmonic decomposition model. These models are simplistic in nature, and their primary purpose is to provide a basis for comparing the two parameter estimation techniques. The results obtained for each estimation algorithm via each load model, using the same data bases and forecasting periods, are presented and form the basis for comparisons presented in the paper.

A computational comparison of steady state load shedding approaches in electric power systems

This paper presents a formulation of the optimal steady state load shedding problem that uses the sum of the squares of the difference between the connected active and the reactive load and the supplied active and reactive power. The latter are treated as dependent variables modelled as functions of bus voltages only. An investigation of the performance of the proposed algorithm over a range of generation deficits as well as overload conditions is presented. Testing is done using IEEE 14, 30, 57, and 118 bus power systems, representing small and medium power systems. The optimal results are compared with results obtained using two earlier approaches. The results obtained using the proposed approach appear to give a better optimal state of the power system.

Application of Kalman filtering for online estimation of symmetrical components for power system protection

Abstract This paper presents a new model that enables linear Kalman filtering (KF) to be applied for the direct estimation of symmetrical components in unbalanced three-phase systems. Computational experiments implementing KF using both synthetic and actual data sets are reported. It is shown that Kalman filtering models are well suited for symmetrical component estimation online. The effects of sampling rate, data window size and frequency drift on the optimal estimates are also studied.

The Hydrothermal Optimal Load Flow, A Practical Formulation and Solution Techniques Using Newton's Approach

This paper is concerned with the problem of optimal power flow in a hydro-thermal electric power system (HTOPF). The formulation and optimality conditions for systems with fixed head hydro-plants subject to an energy limited constraint are presented. The solution to obtain the optimal operational strategy is implemented using Newtons method. Due to the large scale nature of the problem, special sparsity oriented and structural property enhancements are explored and four versions for algorithmic implementation are detailed in the paper. Special attention is paid to finding realistic initial guess estimates for the algorithms reported. Results of computational experience with six standard test systems are reported in the paper.

A new approach to AC-DC load flow analysis

This paper presents an alternative formulation of the AC-DC load flow problem and shows that neglecting the converter transformer reactance results not only in reduced accuracy but in less confidence in the validity of the solution. The formulation extends the Ong-Fudeh model by decoupling the AC system variables from the DC system of equations to provide an improved load flow solution algorithm. The proposed method is tested and compared with other available techniques.

Effects of harmonic distortion on the active and reactive power measurements in the time domain: a single phase system

A new application of the least error squares (LES) estimation algorithm is given for identifying, in the time domain, the active and reactive power from available samples of voltage and current waveforms. The digital available instruments used to monitor active, reactive and apparent power use such samples. A new definition for the active reactive and apparent power for sinusoidal and nonsinusoidal waveforms is discussed in this paper. It is shown that misleading reactive power compensation could be obtained if the circuit power factor is not well defined. Simulated results are presented for different waveforms, where the authors compare the results obtained with other available techniques.

A comparison of load models and their effect on the convergence of Newton's power flows

Abstract We consider the solution of the load flow problem that incorporates the variation of active and reactive power with bus voltages using two load model forms. An exponential model with a bias term and a double-exponential model are adopted. The load flow is solved using Newtons method. Seven test systems ranging in size from a 5-bus to a 37-bus system are used to compare the convergence patterns of Newtons method for the conventional formulation with that incorporating load models. We also compare six possible models in terms of the goodness-of-fit criterion and their effect on the end use in load flow solutions. The evaluation is based on changes in convergence characteristics and deviations in results from the values obtained from a conventional load flow assuming that the active and reactive power demends are independent of the bus voltage magnitude. The results show that the two load models give almost identical convergence patterns. The conventional load flow convergence patterns were identical to those incorporating load models in some cases, and in most cases similar. The total number of iterations required for convergence in the conventional and the load flow incorporating load models differs mainly owing to the manner in which the reactive power generation limits are reached. The numbers of iterations required for convergence between checks for reactive power generation limits are more or less the same for the conventional and the load flow incorporating load models. Our studies show that the six models considered are almost identical from a goodness-of-fit point of view. The results of incorporating each of the models in the load flow problem indicate that a straight line fit is advantageous in many cases. Minor differences in results and computational times are encountered, except for cases when the straight line fit has a goodness-of-fit value of less than 90%. In these situations the use of an exponential type of model is recommended.

Stochastic optimal load flow using Newton-Raphson iterative technique

ABSTRACT In this paper we consider the problem of stochastic optimal power flow for an all thermal electric power system. The formulation includes the effects of variables uncertainty modeled as stochastic and normally distributed. The system performance is represented by the load flow equations in polar form, reformulated to include the uncertainty aspect. Inequality constraints are handled using Powells penalty function method. We employ a variational approach to derive the optimality conditions resulting in a set of nonlinear equations. Newtons method is employed, to actually implement the optimum strategy by solving the nonlinear set of optimality conditions iteratively. An application example involving the IEEE standard 14 bus system is presented and the stochastic solution is compared with a deterministic solution for the same system in order to demonstrate the applicability of the proposed method.

SELECTION OF BUSES FOR DETAILED MODELING IN LOAD FLOW STUDIES

ABSTRACT Conventional load flow solutions are carried out assuming that the active and reactive power requirements at all load buses are independent of the voltage magnitude at the corresponding bus. While this approach yields satisfactory results in most practical cases, it has been shown that detailed modeling of the load variation with voltage may be necessary to obtain realistic load flow results. From a theoretical point of view, it appears that detailed modelling at all load buses in an electric network may be the answer. Realizing the impracticality of this proposition, the paper presents a summary, of research work carried out to examine the improvements obtained via detailed modeling of loads in a number of test systems. From our experimentation we have found that it is only necessary to deal with only a few key buses in terms of detailed modeling. The paper deals with the problem of finding criteria for selecting such buses. The concept of significance factors is introduced and provides such a c...