On the influence of humidity on the breakdown strength of air—with a case study on the PDIV of contacting enameled wire pairs

Abstract

The partial discharge inception voltage (PDIV) in contacting enameled wire pairs exhibits a marked decrease with increased air humidity. While existing literature mentions several potential mechanisms for this reduction, a comprehensive quantitative assessment of the associated effects is lacking. This research paper addresses this knowledge gap by providing a quantitative estimation of the combined impact of water on the gas’s ionization yield (effective ionization coefficient) and the modification of the gap electric field caused by water absorption into the bulk of the insulating coating and the associated microscopic and macroscopic polarization processes (dielectric permittivity). However, a comparison of the theoretical predictions with experimental data reveals that these factors alone cannot fully account for the observed reduction in PDIV. Therefore, the study explores additional mechanisms mentioned in the literature, with particular focus on the development of a semi-conductive layer on the insulation coating in humid atmospheres. The numerical simulations of the surface charge dynamics within this layer suggests that the frequency-dependent decrease in PDIV under high-humidity atmospheres can indeed be attributed to the modification of the gap electric field due to the accumulation of surface charge in the semi-conductive layer.

On the frequency dependence of the PDIV in twisted pair magnet wire analogy in humid air

“…Regarding the other two points, in [FSF23], we have shown that the role of the second mechanism-effective attachment of free electrons-should play only a negligible role in affecting the PDIV due to changing humidity. This we also confirm indirectly later in this paper.…”

“…where {d gaps } is the set of the discharge cells' air gap lengths. In previous work [SFF23] it was shown that equation predicts the PDIV under dry air conditions satisfyingly In [FSF23] it was demonstrated that the RH has only a negligible effect on the gas parameters and thus affects the breakdown voltage only minimally. Therefore, the original, unaltered model is employed for the PDIV predictions in the result sections.…”

“…This is respected in the model by setting the two electrodes slightly apart to prevent such a transfer of charge. As a result, the surface charge movement leads to a concentration of the charge, which increases the local field and thus reduced the PDIV [FSF23]. Should the charge transfer be possible, the migrating charge would create an opposing field, which would then increase the PDIV.…”

“…The amplitude of the periodic movement then depends only on this movement speed, which is inversely proportional to surface resistance, and the duration of the applied single-polarity field, which is inversely proportional to the test voltage frequency. For this purpose, the onset frequency of the transition can be treated as indirectly proportional to the surface resistance, with the proportionality (empirically) quantifiable as [FSF23]:…”

“…According to the IEC 60052 standard, rising absolute humidity of air increases the breakdown voltage in uniform (sphere-gap) field by about 0.2% g −1 m 3 . While the field in between the magnet wires is quasi-uniform [Fär+23a], the air gaps are much shorter (µm-range) than those tackled by the standard, making it not directly applicable [FSF23]. Furthermore, the PDIV is also affected by the possible change in the relative permittivity of the magnet wire coating due to ingress of water molecules from the humid air.…”