Voltage-time characteristics of air gaps and insulation coordination — Survey of 100 years research

With the first application of HV-power lines at the early 20th century the protection of electric components against lightning strokes became a crucial problem. Albeit the overhead insulators were designed to withstand high ac-voltages above the operating level, they failed under stress of high impulse voltages with unknown shape. The need to generate short-time HV-impulses was solved in the 1920th by the invention of the “Marx-Generator”. At least it was observed that the flashover voltage of an insulator is higher for shorter than for longer pulses. During the following decades HV-power networks expanded worldwide rapidly and with it grew the need of HV-impulse tests in laboratories. Progress in short time measurement techniques allowed to reproduce impulses of defined shape and amplitude. In order to establish compatibility of discharge tests in the Megavolt-range, standard pulse shapes were internationally agreed upon. Of special interest for an effective coordination of various insulation structures became data of spark-over values of air gaps with very short time lags. However, a great number of measurements did not lead to satisfying results for non-standard test-voltages. Attempts to derive formulae for good results were neither easy to use nor reliable. Finally, an approach starting from the first principle of discharge in an air gap assuming the speed of leader growth being proportional to the instant voltage above the withstand value led to a simple and generally applicable criterion. Further development of physical models for the leader propagation process leads to self-reliant calculation methods, which are simply coupled to average field calculations.