On the Frequency Dependence of the PDIV in Twisted Pair Magnet Wire Analogy in Dry Air

Abstract

This paper presents an experimental verification of a PDIV model for twisted-pair magnet wires (TPMW). The model is physically rooted in an inception criterion based on secondary electron feedback and parametrizes the PDIV of wedge-shaped gas gaps (such as TPMW) solely in terms of its reduced coating thickness—the ratio of coating thickness to its relative permittivity. According to the literature, the coating’s permittivity and thickness should play a significant role in determining the PDIV. Still, no explicit quantification of their concurrent effects is often given, and they are frequently treated as decoupled parameters. Therefore, extensive measurements of PDIV of samples analogous to TPMW coated with dielectric materials of different permittivity (sealed and unsealed alumina, silicone polyester, and perfluoroalkoxy alkane) and thickness (≈45 to 80 μm) are performed. The PDIV of the test samples is determined under sinusoidal test voltage in the frequency between 10 to 60 000 Hz. It is shown that the PDIV can be predicted based on the relative permittivity and thickness of the coating, and the frequency dependence is due only to the change of permittivity with frequency. Temperature and pressure are kept approximately constant at their ambient values (ca. 23 °C & 970 mbar), and only dry air is used to exclude the effect of humidity on the insulation’s permittivity. Ultimately, the role of surface charge is discussed qualitatively by comparing the PDIV with the partial discharge extinction voltage (PDEV) and their repetitive counterparts (RPDIV, RPDEV).