INSUCON 2017 13 th International Electrical Insulation Conference , Birmingham , UK SURFACE CORONA INCEPTION ON STRESS GRADING SYSTEM IN END-TURN REGIONS OF ROTATING MACHINES

The transients generated by drives based on pulse width modulation (PWM) have adverse effects on the insulation system of rotating machines, especially the stress grading (SG) system used in the endturn region. The surface corona inception voltage on SG system under repetitive impulse, 1kHz sinusoidal, and 1kHz square voltages was measured using a photomultiplier connected to optical fiber bundle, and the evolution of corona was observed using a ICCD camera. Four samples were evaluated to assess process-induced variations in performance. The macroscopic repetitive corona inception field was computed for each waveform from the measured surface corona initiation voltage on the SG system and was 25% to 50% of critical field for gas ionization. Pulse width modulated drives have been used widely to control medium voltage motors based on improved efficiency and control of rotational speed. High frequency components in the PWM voltage waveform cause increased electric field along the end-turn stress grading (SG) systems of form-wound coils as a result of increased capacitive current from the conductor. The greater capacitive current creates local heating of the SG, while the increased field along the SG surface can cause partial discharge (PD). Unlike PDs generated in voids enclosed by mica-main insulation, PDs along SG surface release chemically active species to the surroundings. Both UV emission from PD and such active species accelerate degradation of the SG in combination with thermal stress caused by the local heating in SG [1-3]. The IEC 60034-18-42 standard, now under discussion, is intended to define technical requirements necessary to assure that form-wound motors operate reliably when driven by PWM waveforms. The authors have investigated the electric field distribution of SG system, which consists of corona armour tape (CAT) and stress grading tape (SGT), under PWM waveforms with computational and experimental approaches [4-6]. The transient potential distributions along SG system under various voltage waveforms were computed with FEM software (COMSOL) and were compared with the measured data from a Pockels sensor [4,6]. The potential drop in the CAT is large under the application of a voltage at frequencies higher than a critical value, which is several MHz for the model bar-coil used in this research. A high potential gradient (electric field) is present at the CAT/SGT interface under the application of a voltage whose frequencies are lower than the critical frequency. In other words, the electric field becomes high on the CAT, especially on the edge of stator core, during the wavefront of an impulse voltage whose slew rate is large, and becomes high on the CAT/SGT interface during the wavetail of the impulse voltage [6]. Little information is available related to the surface corona inception field along SG system under transient conditions, for which the surface condition can have a large effect on the local field distribution. The PD inception field along SG system of a model bar-coil is investigated in this paper by observing light emission on SG with a photomultiplier tube (PMT) and an ICCD camera.