The Measurement and Characterization of Charge Transport and Growth in Decomposition Gases from Electrical Insulation Used in Low Pressure Environments.
Several practical issues must be considered when solid electrical insulation is used in a low pressure environment. An important one from a reliability viewpoint is the physical basis for the electrical breakdown mechanisms in the gaseous medium, which inevitably accompanies the solid insulation. Whether the volume of the breakdown site is macroscopic or microscopic, the Townsend mechanism can be dominant if the geometry and surface conditions are appropriate. This Townsend mechanism theory provided the foundation for the parameter measurements conducted in this investigation. The gases investigated covered a range of typical polymers and were not limited to any specific insulating material. This was achieved by selecting gaseous decomposition products reported to evolve from a number of polymers in published pyrolysis studies. The relevant gas mixtures were then synthesized and characterized in a series of experiments. To further the objective of understanding the electrical failure modes of gases which evolve from insulation at low pressures, a methodology was established to enable the measurement of drift velocities and apparent ionization coefficients (and the statistical variations of the these coefficients) within a single experimental apparatus. An original version of a pulsed-Townsend drift tube was designed, constructed and instrumented to this end. For the first time, these parameters were measured in a broad range of synthesized binary mixtures chosen from the three molecular gases: CO_2, CO, N_2 O. In general, all measurements of drift velocity and apparent ionization coefficient were made within the E/n range from 0.5 to 500 townsends. Additionally, the statistical fluctuations of the apparent ionization coefficient for these gases, as well as N_2, Ar, and one CO/N_2O mixture were initially documented and quantified. The relevancy of these measurements to specific breakdown calculations and other theoretical work is discussed. Recommendations for future experimental efforts based on this work, are presented, such as: the investigation of decomposition gas mixtures relevant to specific polymers and further study of the statistical variations of the apparent ionization coefficient.
- Pub Date:
- Engineering: Electronics and Electrical; Physics: Electricity and Magnetism; Physics: Fluid and Plasma