Elham B. Makram

A new time domain voltage source model for an arc furnace using EMTP

This paper presents a new time domain controlled voltage source (CVS) model for an arc furnace using the Electro Magnetic Transients Program (EMTP). The developed model is based on a piecewise linear approximation of the V-I characteristic of the arc furnace load. In addition to this, the novelty of this simulation technique lies in the fact that the active power consumed by the load is also considered in the proposed load model, thus making the proposed load model dependent on the operating conditions of the load. Dynamic variation of the arc is incorporated through sinusoidal and band limited white noise variation of the arc resistance in the V-I characteristic of the arc furnace load. Depending on the variation of the arc resistance, the developed load model accurately represents both static (deterministic) and dynamic (random) arc characteristics. While static arc models are useful for harmonic studies, dynamic models are useful for power quality studies and in particular, the study of voltage flicker. The results are compared with actual voltage measurements taken from a local utility to prove the validity of the proposed load model.

Impact study of energy storage for optimal energy scheduling in microgrid

Microgrid is one essential part of study in smart grid. In this paper, an optimal energy management in microgrid considering the impact of energy storage system is studied. The purpose is to minimize the total cost, improve the renewable energy utilization efficiency and maximize the profits gained by a microgrid. The model for this problem is found as a multi-parametric programming constrained by dynamic programming. In this study, an advanced dynamic programming is proposed in solving this model. This approach is tested on a typical microgrid case in different operation conditions. At last, the results are presented and conclusions are drawn.

A hybrid wavelet-Kalman filter method for load forecasting

Abstract This paper presents a wavelet transform method for load forecasting. The stochastic nature of the wavelet coefficients for the daily load variation is studied by the decomposition scheme of multiresolution analysis (MRA). The study indicates that the stochastic process of the wavelet coefficients can be modeled as a random walk process. Therefore, the wavelet coefficients are modeled as the state variables of Kalman filters. The best estimation of the wavelet coefficients is obtained by the recursive Kalman filter algorithm. The predicted daily load is the inverse of the predicted wavelet coefficients. Based on the above procedure, two models (weather insensitive and sensitive models) are presented in this paper. Results from an actual system are also presented.

Effect of harmonic distortion in reactive power measurement

The authors consider the effect of harmonics on reactive power measurements, and present a novel method for calculating active, reactive apparent, and residual power in nonsinusoidal conditions. The method considers common and uncommon harmonic components of voltage and current waveforms. It properly defines all power components at different harmonic frequencies, such as rotating real power, quadrature power, and the residual power (distortion). Each component is calculated correctly at different frequencies to define its physical meaning. A computer algorithm is developed to calculate all the power components for any distorted voltage and current waveforms. A numerical example is discussed. >

Application of adaptive Kalman filtering in fault classification, distance protection, and fault location using microprocessors

An adaptive Kalman filtering scheme is presented for estimation of the 60 Hz phasor quantities, fault type identification, distance protection, and fault location. The current and voltage data of each phase are simultaneously processed in two Kalman filter models. One model assumes that the phase is unfaulted, while the other model assumes the features of a faulted phase. The condition of the phase is then decided from the computed a posteriori probabilities. Upon the secure identification of the condition of the phase, the corresponding Kalman filtering model continues to obtain the best estimates of the current or voltage state variables. Upon convergence to highly accurate values, the appropriate current and voltage pairs are selected to decide the zone of the fault and the fault location. The scheme was tested on digitally simulated data. The fault classification was doubly secure using both voltage and current data. The convergence of estimates reached exact values within half a cycle. >

Wavelet representation of voltage flicker

Voltage flicker can be considered as a voltage magnitude modulated signal with a frequency ranging from 0.5 to 30 Hz. Wavelet transform is a powerful tool to analyze this kind of nonstationary, wide-range frequency signal. The wavelet representation of voltage flicker signal is presented in this paper. The proposed scheme is implemented in three steps. Firstly, a low-pass demodulation filter is designed to find the magnitude of 60 Hz component. Accordingly, a high degree of accuracy may be achieved and the effect of the transient, harmonics and white noise may be eliminated. Secondly, a multiresolution analysis (MRA) scheme that is an orthonormal wavelet transform, can then be applied to decompose the demodulated signal into several components according to the scales. Components at high scale can be considered as white noise, while components at the low scale represent the voltage fluctuation. The smooth version left by the MRA scheme represents the DC component. Finally, the voltage flicker level can be estimated by the wavelet coefficients at different scales, which give the time and frequency information. Numerical examples are also presented in this paper to show the efficiency of the proposed scheme.

Selection of lines to be switched to eliminate overloaded lines using a Z-matrix method

There are many ways in which a system can be adjusted to compensate for an overloaded line. Earlier studies have shown that shifts in generation or phase shifter adjustments can be calculated to correct an overload. These corrections are based on an analytical solution of the system. Many algorithms have been developed to use these techniques in optimal power flow solutions. Generation schedules are based on economic dispatch and any deviation from the proper schedule to eliminate overloads will result in an increase in production costs. There have been a considerable number of papers in the literature that have presented methods of solving large problems, such as the power system load flow problem, using different approaches with a large amount of numbers in storage and more computer time. Methods have been developed to identify overloaded lines and to select one or more lines to be removed in order to reduce the overload in a power system. Recent research in reference [1] indicated that a simple method of redirecting power flow to reduce the overload in a line is to selectively switch the network. A line can be added to or removed from the system in order to cause a shift in the power flow and to eliminate the overload. The selection of a line to be switched is a complicated process. The addition or removal of a line must not cause any other overloads on the system as it eliminates the original overload.

Evaluating power system unbalance in the presence of harmonic distortion

This paper presents a new method to evaluate the unbalance in the presence of harmonic distortion. The balanced, the first unbalanced, and the second unbalanced components are obtained from the original three-phase current and voltage phasors of each harmonic component. The equivalent rms value of three-phase voltage and current is then decomposed into the balanced fundamental, the unbalanced fundamental, the balanced harmonic, and the unbalanced harmonic components. The same decomposition is applied to the apparent power, and the system unbalance can then be evaluated through the proposed system total unbalance distortion factor. The unbalanced fundamental, the balanced harmonic, and the unbalanced harmonic distortion factors are defined. These distortion factors clarify the interleaving between the unbalance and harmonic distortion and are suitable for unbalance evaluation in the presence of harmonics. Numerical examples are also presented to illustrate the application of the proposed method on the study and characterization of unbalance in power systems.

Effect of different arc furnace models on voltage distortion

In this paper, different arc furnace methods are reviewed for the purpose of harmonic analysis. In general, these models may be classified into time domain method and frequency domain method. The difference between these two methods is discussed and evaluated. In this paper, six typical arc furnace models from the time domain and frequency domain methods are selected to study their effects on harmonic analysis. Simulation results are also provided. Comparisons between these results show the effects of different arc furnace models on voltage waveform and percentage of harmonic components distribution, which provides a discussion of the differences between time domain and frequency domain methods. Finally, recommendations are made for the application of some of these models.

Harmonic filter design using actual recorded data

The authors present a study of harmonic filter design to minimize harmonic distortion caused by a harmonic source such as drives. Several types of shunt harmonic filters are presented. The analysis includes the basic principles, the application of the Z-bus method, and the economic aspects for harmonic filter design. Offline steady-state simulation programs were used to model loads, to study variation of the harmonics, and to evaluate the effect of harmonic filters at various buses in the system. The performance of each filter design was tested using data recorded in local utility systems. A method of reducing resonance that may occur due to the presence of the filter is also suggested. The transient analysis of harmonic filters using the electromagnetic transients program (EMTP) is reported.<<ETX>>