Performance of Large Air Gaps under Lightning Overvoltages: Experimental Study and Analysis of Accuracy of Predetermination Methods

The paper summarizes the results of the research recently carried out at CESI, with lightning overvoltages. Tests were performed on various gap configurations and with several impulse shapes of both polarities to get information about macroscopic parameters and basic processes associated with discharge development. The basic aim was to have a consistent amount of data to carry out a validation and comparison of the existing models, while providing the required information to support the refinement of those based on physical approach. Breakdown voltages versus time to breakdown characteristics, (Volt-time curves), were determined. The curves give an indication of the importance of the impulse-shape and configuration, thus underlying the necessity of calculations methods. Discharge parameters have been presented and discussed to put into evidence the relative importance of the impulseshape characteristics. Data about the discharge process have been compared with other available data and with evaluations according to methods suggested in the literature. A refinement of the available formulas with reference to leader velocity has also been proposed, as in the following: v1 = 170 * d * e(1.5* 10-3* U/d) * (U/x-Eo) where ¿1 is the velocity (m/s), U is the applied voltage (kV), d the gap clearance (m), x the part of the gap unbridged by the leader (m) and Eo the average gradient in the gap of the configuration examined at 50% breakdown voltage with standard lightning impulse (kV/m). The equation is of general type, applicable for all the configurations and for impulses of different shape and polarity.