Jorge M. Janiszewski

A Scale Model for the Study of the LEMP Response of Complex Power Distribution Networks

This paper deals with scale models of power distribution systems for the study of lightning induced voltages on overhead lines. The scale model technique is useful for the investigation of situations which are prohibitively complex to be treated theoretically. For instance, urban distribution networks are usually characterized not only by complex topologies but also by the presence of nearby buildings, whose influence on the lightning induced effects can be successfully evaluated by means of reduced models. The paper first describes the scale model implemented for such a purpose at the University of Sao Paulo, Sao Paulo, Brazil. It then presents a comparison between the experimental data obtained with the scale model and the computer simulations obtained by using the LIOV-EMTP code, a software tool able of calculating lightning-induced electromagnetic transients in distribution systems having complex configurations. Finally, the paper shows an application of the scale model in the evaluation of lightning induced voltages on distribution networks considering the presence of nearby buildings

Lightning-Induced Voltages on Overhead Lines—Application of the Extended Rusck Model

Lightning-induced overvoltages have a considerable impact on the power quality of overhead distribution and telecommunications systems, and various models have been developed for the computation of the electromagnetic transients caused by indirect strokes. The most adequate has been shown to be the one proposed by Agrawal ; the Rusck model can be visualized as a particular case, as both models are equivalent when the lightning channel is perpendicular to the ground plane. In this paper, an extension of the Rusck model that enables the calculation of lightning-induced transients considering flashes to nearby elevated structures and realistic line configurations is tested against data obtained from both natural lightning and scale model experiments. The latter, performed under controlled conditions, can be used also to verify the validity of other coupling models and relevant codes. The so-called Extended Rusck Model, which is shown to be sufficiently accurate, is applied to the analysis of lightning-induced voltages on lines with a shield wire and/or surge arresters. The investigation conducted indicates that the ratio between the peak values of the voltages induced by typical first and subsequent strokes can be either greater or smaller than the unity, depending on the line configuration.

Induced voltages on distribution lines due to lightning discharges on nearby metallic structures

This paper presents a numerical method for the calculation of voltages induced on overhead lines due to lightning discharges striking a metallic structure in its vicinity. The calculation is done from the determination of the electric and magnetic potentials associated with the charges in the return stroke channel and with the currents that propagate in the channel and in the structure. It is shown that in this situation the induced voltages may differ significantly from those originated by lightning discharges direct to the ground. Comparisons between measured and calculated voltage waveforms confirm the validity of the procedure.

An improved model for lightning induced voltages calculations

Overvoltages induced on overhead distribution lines by nearby lightning are responsible for a significant number of supply interruptions, due to their high frequency of occurrence. The importance of the phenomenon has motivated several researches, aiming at a simple and reliable model that can be used for the analysis of the lines performances concerning indirect strokes. This paper presents initially some comparisons between lightning induced voltages recorded in Japan-simultaneously with the associated stroke currents-and those calculated according to some of the existing theories. The results show the consistency of the extended Rusck model (ERM), a method derived from Ruscks theory but with the ability to take into account the finite lengths of line and stroke channel, the occurrence of upward leaders and the effects of lightning incidence to tall structures. Then the effects of these parameters are discussed in order to illustrate the application of the ERM on the analysis of lightning induced voltages.

The effectiveness of shield wires in reducing induced voltages from lightning electromagnetic fields

The mechanisms from which lightning overvoltages can be produced on a power line depend on the system voltage. In medium voltage (MV) overhead distribution systems, lightning transients can be originated by either direct or indirect strokes. The main methods that can be adopted to improve the line lightning performance concern the increase of the critical flashover voltage (CFO) of the line structures, the installation of surge arresters, and the use of one or more periodically grounded shield wires. This paper deals with the evaluation of the effectiveness of shield wires in reducing the magnitudes of the surges induced by nearby strokes on MV distribution lines. Such effectiveness depends on the combination of several parameters such as the relative position of the shield wire with respect to the phase conductors, the grounding interval, the ground resistance, the stroke current steepness, and the relative position of the stroke channel with respect to the grounding points. Realistic situations corresponding to typical configurations of a rural distribution line with either an overhead ground wire or a neutral conductor are considered. The analysis is carried out based on both computer simulations and test results obtained from scale model experiments performed under controlled conditions.

Utilization of reduced models for the analysis of lightning induced overvoltages on overhead lines

This paper deals with the application of the scale model technique to the analysis of lightning induced voltages on overhead power distribution lines. The paper first describes, briefly, the main methods for evaluating lightning transients on power lines. It then presents the basis for the utilization of reduced models and the scale factors for the most important quantities. Finally, examples are shown that illustrate the usefulness of the technique for the validation of theoretical models and relevant computer codes, as well as for investigations involving complex situations.

Analysis of lightning-caused distribution transformer failures

This paper investigates the effects of direct strokes to medium-voltage (MV) lines by analyzing the surges at the primary and secondary sides of a single-phase distribution transformer installed in a typical rural network of the state of Rio Grande do Sul, located in the South of Brazil. The distribution transformers of AES Sul, the electric utility, present a high failure rate and a significant number of the failures are attributed to lightning. The transformers are in general protected by surge arresters at the MV terminals and in a few cases also at the low-voltage (LV) side. Different distances between the MV arrester and the transformer as well as various values of ground resistance are considered in the analysis.

Lightning-caused transformer failures in distribution systems

This paper presents the main results of an investigation conducted with the aim of reducing to an acceptable level the lightning-caused distribution transformer failure rate in a region in the South of Brazil. The region, in the border of the State of Rio Grande do Sul and Argentina, is characterized by a high lightning activity. The analysis of failed transformers and field results shows that the installation of surge arresters at the transformer LV side does not drastically change the failure rate. Computer simulations corroborate this result and confirm that failures are mostly associated with surges coming from the MV side. Therefore, the recommendations are mainly related to the surge arrester installation procedures at the primary side. A discussion is provided on the reasons for the apparent discrepancy with regard to the conclusions of similar investigations carried out in Australia, Norway, and the USA, according to which the installation of secondary arresters leads to substantial reduction in transformer failure rates.

The effect of the distance between transformer and MV arresters on the surges transferred to the LV side

The western part of the state of Rio Grande do Sul, in South Brazil, is characterized by high ground flash densities. In the period 2003-2011, lightning overvoltages accounted for about 47 % of the total number of distribution transformer failures observed in the service area of the power company AES Sul. This paper presents the results of an investigation on the influence of the distance between transformer and medium voltage arresters on the surges transferred to the secondary side. The analysis, performed through simulations using the Alternative Transients Program (ATP), shows that in general higher voltages are produced by subsequent strokes. Although the amplitudes of the voltages transferred to the secondary side tend to increase as the surge arrester approaches the transformer, the distance between these equipment should be as short as possible.

Lightning induced voltages on low-voltage lines with different configurations

Lightning overvoltages originated both on the primary and secondary circuits can lead to distribution transformer failures and to damages to the customers sensitive electronic equipment as well. In view of the possibility that an important part of such problems may be associated with the so-called low-side surge phenomena, this paper analyses the characteristics of lightning induced overvoltages on secondary networks. The calculations are performed through the use of the “Extended Rusck Model” (ERM) and the surges are evaluated considering different topologies for the low-voltage network. The high frequency behaviors of loads and distribution transformer are taken into account and the influences of parameters such as the stroke current front time and the ground resistance are investigated, as well as the effect of the presence of surge protective devices at different points of the low-voltage line.