Partial Discharge Inception Voltage in Converter-Fed Machines for the More Electric Aircraft

In this thesis the Partial Discharge Inception Voltage (PDIV) of Twisted Pairs (TP, representative models of the turn/turn insulation) is investigated, trying to unveil the effect of ambient conditions like pressure, temperature and humidity and the effect of wide bandgap (WBG) converters on it. This is the first step to find qualification methods for More Electric Aircraft actuators fed by WBG converters. A setup for PDIV tests at different pressure and temperature levels has been built. A suite of applications in MATLAB to perform acquisitions and automatic tests has been coded, it takes advantage of a photomultiplier tube for optical detection of the discharges. The optical detection in a dark environment has demonstrated to be as sensitive as a commercial PD detection instruments and free from the electrical noise of the converter. The humidity and temperature coupled effects have been investigated at ambient pressure, finding that an increase in relative humidity or temperature decreases the PDIV, but possible changes in the insulation permittivity and the confirmed presence of free charges on the surface of twisted pair renders difficult to model the phenomena. The effect of coupled temperature and pressure changes have been investigated, finding that PDIV highly decreases when the pressure is increased and that a Paschen-Like law doesn't model properly the phenomenon at very low pressures where a plateau forms instead of a minimum. Temperature increase and pressure decrease both act on the density of the air, lengthen the mean free path of the electron and reducing the electric field needed for the ionization. A SiC unipolar converter has been used to test the effect of the switching frequency on the PDIV, no impact has been found.