Innovative Numerical Method for Determining the Critical Space Charge Characteristic and the Ion Current due to Superimposed Voltage Stresses

Abstract:
Under high DC voltages and/or overvoltages, the insulating medium air loses its dielectric strength, which manifests itself in so-called corona discharges and leader discharges in highly inhomogeneous arrangements. As a result of corona discharge, a space charge density occurs in converter halls, highvoltage laboratories and high voltage direct current (HVDC) overhead line arrangements. New types of overvoltages with particularly long time-to-half occur when modular multilevel converter (MMC) technology is used to generate DC voltage. Effects on the gaseous insulation medium and the corresponding distance dimensioning with the newly types of overvoltages are also the subject of current research. The aim of this work is to develop a numerical simulation to describe the development of the space charge density and ion current on full-scale arrangements under superimposed voltage stresses in order to determine critical charge quantities for the use of leader discharges and complete electrical breakdown. In addition, the required ignition field strengths on arbitrarily complex electrode arrangements and ion mobilities for arbitrarily complex insulating gases are also calculated in advance using a developed numerical method. In particular, the effects of the newly occurring overvoltages compared to the standardised switching impulses (SI) on the insulating gas air are discussed and analysed. It is shown that the charge converted by the ion current is 32 times greater with a very slow front impulse (VSFI) than with an SI.