Antonio C. S. Lima

Rational approximation of frequency domain responses in the S and Z planes

In time domain analysis, approximated rational functions are used to model components or parts of a power system whose characteristic is known in the frequency domain. Over the years, many techniques have been proposed to fit the frequency response to rational functions to ensure accurate computational models for transient studies. In this work, a comparison of different approaches is made. Linear and nonlinear fitting routines are applied to the synthesis of lumped and distributed parameter circuits. The fittings are performed in S and Z-domains. The following methods have been considered: Golubs algorithm, singular value decomposition (SVD), vector fitting, Levenberg-Marquardts method and a full-Newton implementation developed by the authors. The presented techniques may be applied on modelling transmission lines and transformers, and to the development of frequency-dependent network equivalents.

Preventive Control Stability Via Neural Network Sensitivity

This paper discusses the power systems stability margin improvement by means of preventive control based on generation re-dispatch using a neural sensitivity model. This model uses multilayer perceptron networks with memory structure in the input layer. The training of this model is made with temporal data samples from time domain simulations, incorporating information about the dynamic behavior of the system, unlike the methods proposed in the literature in which the pre-fault system data are used instead. The sensitivity is used as a guideline in selecting the most effective set of generators in the reallocation of the amount of active power capable of increasing system security. The effectiveness of the proposed methodology has been demonstrated through the application to a large system.

Numerical Issues in Line Models Based on a Thin Wire Above a Lossy Ground

This paper investigates the causes of numerical instabilities that might occur in transmission line models if the so-called image approximations are used for the evaluation of the per-unit-length parameters when the ground is assumed lossy and a wide frequency range is considered. To assess whether these instabilities are inherent to the formulation of the line parameters, results were compared with two well-established models: a full-wave model and a quasi-transverse electromagnetic approximation. Both approaches provided stable models throughout the whole frequency range. Two different methodologies are used for the determination of the modal propagation constant in a full-wave model leading to similar results. Time-domain responses based on the numerical Laplace transform and the method of characteristics were used to verify the accuracy and stability of the tested models. The input voltage considered for the time-domain responses included a small perturbation to excite the passivity violations found in the modeling using image approximations. Some aspects related to a full-wave model based on the method of characteristics using rational fitting are also presented.

Noninvasive monitoring of residential loads

This paper presents a new methodology for the noninvasive identification of residential loads. The main goal is to develop a prototype capable to determine in real-time consumers loads based upon their energy signatures. The mathematical modeling is based on identification using signal processing techniques. Both simulation and measurements were considered in the analysis.

Validation Limits of Quasi-TEM Approximation for Buried Bare and Insulated Cables

For the analysis of power system transients in overhead transmission lines and underground cables, a common assumption is related to the quasi-TEM (transverse electromagnetic) approximation. In this paper, we investigate the applicability of this approximation throughout a wide frequency band when both conduction and displacement currents in ground are considered, typically only the former is considered. A full-wave model is established to assess the behavior of the propagation constant for bare and insulated underground cables in a wide frequency band and verify the adequacy of the quasi-TEM approximation. The results demonstrate that it is possible to retain quasi-TEM approximations in the high-frequency range as well. One has only to consider different assumptions for the propagation constant. For the evaluation the adequacy of the quasi-TEM approximation, a simple, single-phase buried cable is considered. The effect of the cable coating is also taken into account and the results indicate that the quasi-TEM approximation is rather distinct for bare or insulated cables. The accuracy of the approximation is evaluated from comparison of step and impulse voltage tests considering different soil models.

An improved scale dependent wavelet selection for data denoising of partial discharge measurement

The detection of partial discharge (PD) signals has proven extremely important to diagnose the integrity of the insulation in high voltage equipments. The measurements of such signals are often accompanied by noise from different sources, which can compromise the data analysis. Numerous wavelet shrinkage denoising techniques have been discussed recently in the literature. This article proposes an alternative criterion for selection of the mother wavelet, named signal to noise ratio based wavelet selection (SNRBWS), where the wavelet selection is done, for each scale, based on the maximization of the signal to noise ratio (SNR) of the estimated signal. The detail or approximation coefficients corresponding to the largest peak amplitude value are seen as from the PD signal, while the ones corresponding to the lowest peak amplitude value are seen as from noise. Such coefficients are conceived through the decomposition of the PD signal previously obtained for a given equipment. The proposed method is compared to the energy based wavelet selection (EBWS) method and to the correlation based wavelet selection (CBWS) method for signals measured from current transformers, circuit breakers, generators, gas-insulated switchgears (GIS), surge arresters and transformers, and for simulated PD signals embedded in noise. The proposed method presented better denoising results when compared to the previously proposed methods for most of the tested signals, where among various assessment parameters analyzed were the correlation coefficient, the mean square error (MSE), and the SNR. The algorithm also showed superior performance compared to the others with respect to the processing time.

Assessment of a frequency dependent soil model impact on lightning overvoltages

A key aspect in the evaluation of the lightning performance of an overhead line is the tower grounding modeling approach. It affects the insulator string overvoltages and the ground potential rise (GPR). This work aims to assess the impact of the soil model i.e. using frequency independent and frequency dependent parameters. The developed grounding system model was implemented in the EMTP-ATP program. A RLC synthetized network is used instead of a constant lumped resistance. The RLC elements are derived from the rational approximation of the grounding system response obtained using a rigorous model based on the direct solution of Maxwells equations in the frequency domain. An application example is presented, showing that the computed overvoltage is sensitive to the frequency dependence of the soil parameters. This effect is more pronounced for high resistivity soil.

Approximation of Lightning Current Waveforms Using Complex Exponential Functions

A key aspect to evaluate the lightning performance of transmission lines is the correct representation of the lightning current waveform considered. Traditionally, the analytical modeling of such data might involve the use of double-exponential functions or even Heidler functions series, i.e., partial fractions with polynomial functions of variable degree. Herein, we propose the fitting of lightning current waveforms to obtain a pole-residue model as an alternative procedure. Purposely, we investigate the performance of the vector fitting (VF) and time-domain vector fitting (TD-VF) methods to represent continuous lightning current waveshapes originally modeled using Heidler function series. Four nontrivial test cases of single-crest and double-crest peaked lightning waveforms are analyzed. Albeit comparatively accurate results are achieved, the TD-VF provides models with lower pole orders than the VF approach for equivalent window observation times and time steps. Incorporation of pole-residue models in transient simulation programs can be straightforward as compared to a Heidler function series heuristic approach.

Synthesis of controlled reactances using VSC converters

This work presents a review of the main techniques for synthesizing positive and negative inductance using power electronics: VAPAR (variable active-passive reactance), BVI (bootstrap variable inductance), AVI (active variable inductance) and DRS (direct reactance synthesis). Based on this review a new topology for synthesizing a fundamental frequency variable reactances based on synchronism circuits (PLL - phase-locked loop) is presented. The main advantage of this new technique is that the control signals are immune to the noise from the grid. Some simulation results, using PSCAD/EMTDC, are presented for the proposed circuit.

Rational modeling of overhead transmission lines considering finite length impedance and admittance expressions

An upcoming challenge in overhead lines is wide river crossing as the span between towers may be over 2 km. One challenge in this configuration is to accurately represent the line behavior. The expressions for the evaluation of impedance and admittance matrices in overhead lines are based on the assumption that the total line length must be at least a few order of magnitude higher that the highest conductor average height. However, in case of a wide line span such hypothesis is not strictly valid and the per-unit of length impedance and admittance must consider the finite length of the circuit. The cascading of short line in time-domain is inefficient, however, if a rational approximation of the equivalent admittance is used a more efficient line model is obtained. A test case based on actual line configuration used to cross the Amazon River is considered and the results indicate that a more accurate frequency domain behavior is obtained leading to a more suitable rational approximation, which is not only more accurate but also is less prone to the residue perturbation in the case of passivity enforcement.