Gustavo P. Lopes

Estimation of the influence of direct strokes on the lightning performance of overhead distribution lines

When assessing the lightning performance of distribution lines, major attention is in general devoted to the induced voltages from nearby strikes rather than to direct events hitting the line, as the latter are always expected to cause a fault in the line. This paper proposes a Monte Carlo procedure capable to make it possible the evaluation of the overvoltage probability due to both indirect and direct strokes. The results are presented making reference to specific nodes of medium voltage overhead distribution networks, namely the locations of MV/LV transformers connections. The problem is of interest when a major upgrade of surge arresters is required, for instance when the neutral grounding practice needs to be changed (e.g. from grounded to resonant).

Protection systems against lightning-originated overvoltages in resonant grounded power distribution systems

A joint research project between AES Sul, the Federal University of Itajubá and the University of Bologna aims at defining the required protections against lightning-originated overvoltages in a resonant grounded distribution system in order to achieve the adequate lightning performance level with minimum cost. The paper aims at illustrating the effectiveness of the presence of a surge arrester or an arcing-horn gap and its protective zone by means of simulations obtained with a computer code for the calculation of lightning induced voltages that takes also into account the steady-state voltage. The results of a statistical analysis for a single multiconductor line are eventually presented and discussed.

Selection of MV/LV transformers to be protected by surge arresters against indirect lightning overvoltages

The papers addresses the problem of locating surge protective devices in a medium voltage distribution network by means of an accurate computer model for the calculation of lightning induced overvoltages and a Monte Carlo statistical procedure. The indirect lightning performance of small network cells of different configuration are evaluated and used as a benchmark for the validation of the surge arrester locations provided by a genetic algorithm procedure. The obtained results appear useful for the lightning protection design of networks with complex topology.

Assessment of lightning overvoltages on lines with different voltages sharing the same structures

The objective of this paper is to evaluate the lightning overvoltages that appear on a conventional 69 kV line, a compact overhead distribution line (spacer cable system) rated 11.4 kV and a low voltage line rated 220 V sharing the same tower. The simulations were performed using the software ATP (Alternative Transient Program) considering lightning strikes at the top of the tower, the mid span of the ground wire and one of the phases of the high voltage line.

Lightning withstand of medium voltage cut-out fuses stressed by nonstandard impulse shapes experimental results

This paper aims at evaluating the lightning withstand voltage of cut-out fuses designed for overhead distribution lines of 15 kV and 25 kV. For this purpose, it is used the critical flashover voltage (CFO) and the V-t curves parameters. These two parameters are usually derived for 1.2 × 50 μs impulse shape, which represents overvoltages originated by direct lightning strikes on phase conductors or shield wires of transmission and distribution lines. In order to assess the lightning withstand levels with respect to induced overvoltages, it has been chosen five nonstandard impulse shapes with front and tail times of 0.5 × 5 μs, 0.5 × 20 μs, 1.2 × 10 μs, 3 × 16 μs and 5 × 50 μs. The standard impulse shape 1.2 × 50 μs results are also provided for the purpose of comparison with nonstandard ones. The tests for determining the CFO and V-t curves were performed on dry condition, for positive and negative polarities. The CFO and V-t curves results show an important issue considering lightning withstand of cut-out fuses. For both test methods and impulse shapes, the lowest withstand values are observed on negative polarity. Considering porcelain insulators and manual disconnectors for distribution systems, it means a change on the critical polarity with impacts on insulation coordination procedures.

Chopped Impulses for Distribution Transformers Their Importance for Dimensioning and Performance Evaluation

Impulse tests on distribution transformers are requested by all main international standards as IEC, IEEE and ANSI. They are also adopted by most national standards and normative. These tests consists in the application of a sequence of impulses, including chopped ones, to evaluate the insulation withstanding of the equipment, mainly failures on paper and windings. Although the chopped impulses are of main importance in this evaluation process, there is a discussion in some countries considering removing them from the test sequence. This paper discusses the importance of the chopped impulses and how they aid in the evaluation of the transformer failure probability.