Development of a generic physical model for breakdown and withstand voltages of air-gap insulated configurations in dependence of the atmospheric conditions

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In power supply systems, the insulation of the equipment is stressed by impacts of lightning and switching. An adequate insulation co-ordination and dimensioning requires the knowledge of the correct breakdown voltage in order to avoid damage as a result of dielectric breakdown.Especially for air-gap insulated equipment, the dielectric strength depends heavily on the atmospheric ambient conditions. In turn, the main ambient parameters temperature, air humidity, air pressure and air conductivity – and thereby the breakdown voltage – are dependent on the geographical location of the installation.Current atmospheric correction methods for breakdown and withstand voltages in long air-gap configurations show deviations in comparison to measurement data when applied to correct for unusual or atypical atmospheric conditions (i.e. altitudes above 2000 m a.s.l.). The corresponding standards IEC 60060 and IEC 60071 show inconsistencies, although both are being based upon the same measurements and empirical models. A world-wide discussion of this inadequacy caused the establishment of several working groups, yet so far, a solution could not have been found (IEC JWG 42/22; CIGRE WG D1.50).Therefore, and with the support of the working groups mentioned above, the purpose of this research project is to lay the necessary groundwork for a profound solution. The main goal is to develop a generally valid, physical model of the breakdown process in dependence of the atmospheric parameters and in relation to distinct configurations and voltage forms.The proposed research is based upon the recent knowledge, that any breakdown process in air-gap insulation consists of the sub-processes streamer and leader. While their physical characteristics seem to be independent of configuration and voltage form, their ratio and proportion varies, which in turn defines the value of breakdown voltage. As a unified extension, a sub-process-differentiated, measurement-based evaluation of the entire breakdown process in dependence of the atmospheric conditions will be conducted. Based on the electrical and optical measurement results of this parameter study, the atmospheric influences on the physical characteristics of the different breakdown sub-processes will be determined.Upon the findings of the investigation, a physical model of the breakdown process with respect to its sub-processes streamer and leader and in dependence of the atmospheric conditions will be developed. In relation to any of the examined atmospheric parameters, the model shall provide a reliable and consistent atmospheric correction method for any breakdown and withstand voltage. Regarding the atmospheric correction of withstand voltages in accordance with insulation co-ordination, the model shall be used to derive a correction method in dependence of altitude.