Application of Dynamic Models to predict Switching Impulse Withstand Voltages of Long Air Gaps

The purpose of this paper is to study the critical breakdown voltage or U-50 - corresponding to the 50% probability of breakdown for a single impulse application - of long air-gaps under positive and negative switching impulse voltages, using predictive dynamic models. The knowledge of this important parameter is of practical interest to the design engineer. The models previously developed by the authors are based on equivalent electrical network, gas discharge theories and physical laws. Various gap geometry and atmospheric conditions were considered. The U-50, determined using the up and down method along with other important parameters, were compared to experimental data. Good agreements were found between simulated and experimental results. The results obtained indicate that U-50 prediction is possible, using the proposed models. Knowledge of the gap factor and weather conditions might help applying corrections to determine the breakdown strength under various conditions. These dynamic models might therefore be useful to estimate the required clearances of air gaps in the preliminary design phase over a wide range of geometry and some atmospheric factors; therefore reducing the required full-scale testing.