Análise de sistemas de aterramento submetidos a correntes impulsivas.

Abstract

A grounding system consists of a purposeful electrical connection of a physical system to the ground. Its main characteristic is to make the ground a path of low impedance, preferential to currents coming from short circuit, phase unbalance, switching surge or lightning surge. Therefore, grounding systems can act for currents at different levels of intensity and frequency. The behavior of ground systems against low frequency current is widely known. However, the high current and frequency current behavior is influenced by ionizing, inductive or capacitive effects, individually or simultaneously. As an example of a current of high intensity and frequency, one has the impulsive current coming from an atmospheric discharge (lightning surge). In this way, the present work has proposed to analyze the behavior of grounding systems against impulsive currents. For this, field experiments (with impulsive current injection) were carried out in four different grounding systems, installed in different resistivity soils. From the field trials, the parameters, or variables were obtained: soil stratification; ground resistance; impulse current curve; voltage response curve; characteristic curve of the relation between measured voltage and applied current (curve V x I); transient impedance; impulsive impedance; impulse coefficient; forward and half-stroke times of the current and voltage signals. From then on, the transient behavior of the grounding systems was analyzed, and the transient effects (ionization, inductive or capacitive) present were identified. It was verified that the inductive effect is influenced by the number of ground rods, independent of soil resistivity, however, the higher the resistivity, the more the capacitive effect exerts influence. The opposite occurs with the inductive effect that increases with decreasing soil resistivity. Finally, a generic equation of undefined coefficients was identified that represents the contribution of the frequency dependent transient effects to the voltage response of the analyzed systems.