Influência da corrente de descarga atmosférica e dos modelos para caracterização da suportabilidade de isoladores no fenômeno de backflashover

Abstract

The more severe quality and continuity standards related to electric energy supply have motivated the search of realistic and reliable methodologies to assess the lightning performance of transmission lines in order to reduce the outage rates. With the advent of modern lightning detection and measurement systems, it became possible to apply the characteristic parameters of the real return-stroke current waveforms in the assessment of the lightning performance of transmission lines based upon real data. Moreover, it is important to highlight the development of analytical methods capable to accurately represent and characterize the insulation strength. These methods consider the physical process of breakdown established along the insulation strings during a fault, described by means of electromagnetic concepts. This panorama motivated the development of the present work whose aim is to perform investigative studies concerning the transmission lines outages caused by backflashover, considering the characteristic parameters of the real first-stroke current waveforms based on measurement data. Besides the real return-stroke current waveforms and the slope ramp current waveform recommended by IEEE and IEC, two new current source computer models were developed in order to represent the impulsive current waveform recommended by CIGRÈ as well as the double-exponential current waveform traditionally used to perform these kind of studies. The obtained results permit to identify the combination of the current waveform with the insulation strength representation method, which conduct to the most accurate results in lightning performance studies when compared to those that come from real lightning current waveform. The computer modeling presented also includes the mainly electromagnetic models presently used to predict the electrical insulation strength, namely the volt-time curve, the leader progression model (LPM) and the disruptive effect (DE) method. The volt-time curves predicted by means of these methods were compared to curves available in classical references as well as the experimental volt-time curves obtained from tests in order to validate the calculation procedures of these methods. The implemented computer modeling strategy permit to determine the magnitude of critical discharge current, the magnitude of disruptive atmospheric overvoltage and the instant of breakdown. It is also possible to identify the tower and the insulation string where the insulation rupture occurs. In this sense, the developed modeling provides an integrated analysis of the main factors that cause the backflashover outages. The results obtained may contribute to improve the insulation coordination projects of transmission lines.