Rodolfo A. R. Moura

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.

Analysis of the cumulative probability distribution of the stroke angle in lightning incidence to three-phase overhead transmission lines

Lightning leaders follow a random behavior when approaching ground surface, so the usual assumption of a vertical stroke is not realistic. This paper is aimed at analysing the sensitivity of the shielding failure flashover rate (SFFOR) of three-phase overhead transmission lines to a probability distribution of the stroke angle, i.e. both vertical and non-vertical strokes. A modified Electrogeometric Model (mod-EGM), derived from a probabilistic computation of the exposure area of phase conductors is described. A computational code is developed in MATLAB, along with IEEE Flash algorithm implementation. Sensitivity analyses are carried out for a waist-type transmission tower. Results show that considering particularly vertical leaders may lead to underestimated values for flashover rates.

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.

Adjustment of current waveform parameters for first lightning strokes: Representation by Heidler functions

This paper presents a method for adjusting parameters of first stroke lightning currents, according to measured patterns of their waveforms for negative downward flashes. Waveforms are represented by a sum of Heidler functions, an approach previously adopted by other authors. As published in literature, in order to reproduce important characteristics of first stroke currents, such as the existence of a double peak, several Heidler functions are needed, and parameter fitting is a cumbersome task. The simplicity of the method presented here allows the direct adjustment of each parameter with low computational effort. Thus, this method is suitable for the generation of current waveforms to be applied in evaluating the lightning performance of overhead transmission lines according to a probabilistic-statistical approach.

Modeling of lightning strikes at overhead lines: A method to determine insulated cable joints protection

The direct impact of a lightning strike at electrical systems and living beings can lead to destruction of system elements, equipment and eventually death. The resulting magnitude of voltage and current can be attenuated or amplified, due to the reflection process of their waves in points of the system where there is discontinuity of the surge impedance value. The objective of this study is to present an incipient method for investigation of electrical transients due to lightning strikes that reaches directly on overhead lines with an underground insulated cable section. The influence of varying number and positioning of MO arresters in the system, also the variation of the underground cable length, on the maximum voltage at the insulated cable joints are evaluated. This voltage cannot exceed the safety margin considered for the Basic Insulation Level Margin (BIL) of the insulated cable. The energy absorbed by the MO arresters is also analyzed. The results are representative regarding the use of MO arresters to protect transmission lines with insulated cable sections. From the obtained total energy and safety margin, it can be determined which configuration and protection arrangement is the most technically viable.