Determining Ionization Threshold in Layer Insulation

Abstract

This research relates to the dielectric strength of layer insulation of HV electronic transformers and similar structures. A simple method of evaluating layer insulation has been devised and validated. It requires measurement of the insulation sample voltage and current, and digitally manipulating these variables. Thus, active energy absorbed by the sample insulation, consisting of pure dielectric and ionization losses, can be derived. The first depend quadratically on the applied voltage, whereas the ionization losses are zero below a certain level [ionization threshold (IT)] but rise sharply above it. By studying the behavior of the loss curve we can determine IT. Several samples imitating layer insulation were manufactured and tested. The main construction variables were wire diameter and layer insulation parameters. Samples were tested at high frequency, 20–120 kHz, HV at different voltage shapes. Testing of unpotted samples revealed that ITs had small spread between the samples and depended only on the voltage amplitude. Above IT, ozone was copiously generated, and the sample temperature rose rapidly. An important finding was that ITs of potted samples not necessarily were higher than ITs of their unpotted counterparts. FEA of the electrostatic field in the winding has been performed. The windings’ capacitance was calculated. It became evident that a simplistic calculation using a flat electrodes’ approximation was too crude to be useful. Calculation in a “real” geometry yielded close match to the experimental results. Comparing the voltage between the wire and the insulation layer with the Paschen’s curve for air, one notes good agreement between the two sets at voltages close to the experimentally found ITs. Thus, probably the most important information that can be drawn from a rigorous field analysis is the IT value.