Ivan J. S. Lopes

An Approximate Formula for the Peak Value of Lightning-Induced Voltages in Overhead Lines

An approximate formula is proposed to estimate the peak value of lightning induced voltages in an infinite line, considering the soil resistivity. The formula is derived from results of a computer code for lightning induced voltage calculation. The code is based on Ruscks formulation for the vertical electric field, for a perfect conducting earth, and the formulation for the horizontal electric field for finitely conducting earth proposed by Barbosa and Paulino. The electromagnetic field coupling with the line is carried out using the model proposed by Agrawal. The results are compared with experimental data from Barker for validation purposes. The paper also presents an extension of the methodology proposed in IEEE Std.1410 Guide to include the effect of the soil finite conductivity in the assessment of the lightning performance of distribution lines due to nearby strikes. The annual indirect lightning induced voltage flashover rates obtained with IEEE Std. 1410 method are compared with the approximate formula results.

The Peak Value of Lightning-Induced Voltages in Overhead Lines Considering the Ground Resistivity and Typical Return Stroke Parameters

This paper presents an approximate formula for the peak value of lightning-induced voltages in an overhead line, considering the ground resistivity and a trapezoidal lightning return stroke current waveform with typical parameters 3.8-μ s front time and 120-m/μ s velocity. It presents an improvement in a formula previously proposed by the authors. The formula is used with a probabilistic approach, and the results are compared with theoretical and experimental results available in the literature. The results show that the proposed formula, although very simple, is in good agreement with results obtained using complete formulations and experimental data.

Indirect Lightning Performance of Aerial Distribution Lines Considering the Induced-Voltage Waveform

This paper shows that the wave-shape parameters of lightning-induced voltages (front-time and time-to-half value) are influenced by the ground resistivity, the line length, and the distance between the flash and the line. In general, these parameters increase with the ground resistivity, the line length, and the distance from the flash. The probabilistic distribution of the waveshape parameters are presented, considering different values of line length and ground resistivity. The effect of the induced voltage waveform on the indirect lightning performance of aerial distribution lines is evaluated using the disruptive effect (DE) method as a flashover criterion. The resulting flashover rates differ significantly from those obtained when only the peak value is used as flashover criterion. The use of a correction factor to multiply the line critical flashover when using the peak value as a flashover criterion is investigated.

Experimental investigation of corona onset in contaminated polymer surfaces

Concerns about corona and its harmful effects, particularly in polymer insulators, have increased in recent years. Research involving corona identification, quantification, and formation mechanism is important to understand the aging process and to avoid premature failures. This paper investigates the corona inception and its relation with polymer surface conditions. Experiments of visual corona identification and Radio Interference Voltage (RIV) measurements were performed. Corona onset voltage was investigated in arrangements simulating different service conditions. Wet and contaminated insulating surfaces were reproduced. An RTV coated insulating sample and a polymer insulator were used. The electric field (E-field) at corona onset was numerically evaluated through computational simulations using the Finite Element Method. Corona onset voltage, E-field and RIV were analyzed according to the contamination level. The results clearly show how certain service conditions, particularly the association water droplets and pollution, enhance the E-field and can lead to corona onset.

Electric field distribution along the surface of high voltage polymer insulators and its changes under service conditions

This paper investigates the electric field distribution along the surface of high voltage polymer insulators and its enhancement due to water droplets by means of computational simulations and laboratory experiments. Different factors affecting the electric field distribution are analyzed through finite element method simulations. Among them, the presence of a grading ring and water droplets on the insulator surface are studied. The insulator surface is simulated for dry conditions, as well as its changes under wet conditions. Additionally, radio-influence voltage (RIV) measurements are performed on a 138 kV polymer insulator. A model for the presence of humidity on the housing is presented and its effect on the field distribution is discussed. The results show that the electric field enhancement along the surface is considerable and reaches values that cause corona and early ageing of the material

Regenerative PWM source for power transformer loading tests

Most of power transformers, before being commercialized, must undertake a series of tests with the objective of analyzing their operational behavior under real operating conditions. Such tests sometimes demand a high amount of power increasing significantly their costs. The paper presents a proposed system which allows testing in closed loop, reducing the energy consumption to the natural losses of the equipment, thus constituting a regenerative system. A VSI-based test environment for power transformers where controlled active and reactive power can be supplied is designed, simulated and realized as a 50KVA- prototype. Special care is taken concerning the ac-filters in order to keep the harmonics within the limits set by standards valid for testing of power transformers. The simulation and experimental results show that the prototype fulfills the given requirements.

Influence of electrogeometric model and statistical current distribution in distribution lines indirect lightning performance estimation considering the ground resistivity

The estimation of overhead distribution lines lightning performance is very important for insulation design. Due to the random nature of lightning, different aspects of the phenomenon, such as the discharge points of incidence and the statistical distribution of the current intensity, are modeled in performance evaluation studies. This paper presents a comparative study of different models proposed in the literature to estimate the indirect lightning flashover rates of overhead distribution lines. The electrogeometric model, that defines the lightning striking point, and the statistical current distribution are evaluated using Monte Carlo simulations. The results show the importance of considering the most accurate information on these parameters for a more realistic performance estimation.

An efficient method to assess the indirect lightning performance of overhead lines

This paper presents an approximate formula for the evaluation of the peak value of lightning-induced voltages in overhead lines over lossy ground, which considers the front-time of the channel-base current. The formula is used in the assessment of indirect lightning performance of overhead power distribution lines, and its results are compared with results available in the literature. The formula is then used to analyze the influence of some parameters on the lightning performance of overhead lines. The results show that the front-time influence decreases as the soil resistivity increases, and that a higher front-time leads to a lower flashover rate. It is shown that the use of a fixed front-time T = 5.63 μs leads to results that matches well the results obtained from Cigrés front-time probabilistic distribution. Regarding the return stroke velocity, the results show that, for soils with resistivity higher than 100 Ωm, the flashover rate increases as the return stroke velocity increases. The results also show that a relative velocity vr = 0.4 leads to flashover rates that match well the results obtained considering some correlations between return stroke velocity and peak current proposed in the literature.

On the use of homogeneous ground model for lightning-induced voltage calculation

Lightning induced voltages on aerial and underground lines depend on the ground resistivity. Most calculating methodologies were developed considering the ground as a homogeneous medium. However, actual ground is better represented if stratified into horizontal layers of different resistivity values. This paper proposes the use of a simplified expression for the calculation of the equivalent resistivity of a uniform ground that can represent a two-layer stratified ground in lightning induced voltage calculations. The analysis is made by comparing the results from a homogeneous ground having the equivalent resistivity value with results from two-layer ground. It is shown that, for the range of parameters considered, the absolute average and maximum errors on the induced voltage peak values are 2.9% and 8.2%, respectively. It is also shown that, with the equivalent resistivity, a peak-value formula developed for uniform ground could be used for the calculation of lightning induced voltages on lines above two-layer ground.

Influence of the return stroke velocity and the statistical method used in distribution lines indirect lightning performance estimation considering the ground resistivity

Disturbances to power distribution lines are mainly caused by lightning. Due to the random nature of lightning, the estimation of overhead distribution lines lightning performance is obtained by statistical methods. This paper investigates the influence of the return stroke velocity and the statistical method used to estimate the indirect lightning flashover rates of overhead distribution lines.