On the differences between partial discharge inception voltages of a turn insulation analogy under unipolar and bipolar excitation

Abstract There is a disagreement in the scientific community regarding the impact of uni- and bipolar voltage excitation on the inception of partial discharge in electric motor insulation. In this paper, we conduct repetitive and single partial discharge inception voltage (RPDIV and PDIV) measurements on a well-defined turn insulation analogy using both unipolar and bipolar voltages. The experiments are specifically designed to suppress the commonly identified sources of scatter, namely lack of starting electrons, presence of surface charge, insufficient measurement sensitivity and voltage overshoots. It is confirmed that the PDIV under either excitation type depends on the peak-to-ground voltage of the waveform and does now show any polarity dependence. Conversely, the RPDIV is determined by the peak-peak voltage and is independent of offset for samples with very low surface conductivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>&lt;</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>15</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">S</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ). For unipolar waveforms, the RPDIV is first reached at approximately twice the PDIV, whereas for bipolar, the two inception voltages are practically identical. A previously-developed analytical breakdown model is shown to deliver PDIV predictions perfectly matching the experimental results.