Characterisation and Generation of High Impulse Voltages and Currents

Abstract

Transmission and distribution of electrical energy involves the application of high-voltage apparatuses like power transformers, switchgears, surge arrestors, insulators, power cables, transformers, etc. They are exposed to high transient voltages and currents due to internal and external overvoltages. Before commissioning, they are therefore tested for reliability with standard impulse voltages or currents. Depending on the apparatus and its proposed application, the specifications prescribe different types of test impulses, e.g., lightning, switching and chopped impulse voltages as well as exponential, rectangular and short-time alternating currents. For on-site voltage tests, oscillating lightning and switching impulse voltages are specified in addition. The standard impulses are defined by their test voltage value (or test current value) and at least two time parameters, with tolerances during generation and uncertainties during measurement. The background and specification of the new evaluation procedures in IEC 60060 concerning overshoots and oscillations superposed on lightning impulse voltages are treated in detail. This includes the presentation of the frequency-dependent test voltage function k(f) and the filtering method, obtained both as the result of world-wide round-robin tests. In the latter part of this chapter, fundamental circuits for generating high-voltage and high-current impulses are given, e.g., the multi-stage Marx generator for generating impulse voltages of up to several megavolts and the impulse current generator with crowbar gap arrangement for preventing undershoots of impulse currents.