Modeling Humidity Impact on PDIV for Turn-to-Turn Insulation of Inverter-Fed Motors at Different Temperatures

Abstract

This paper models the partial discharge inception voltage (PDIV) as a function of ambient humidity (H) at various temperatures (Ts) using Schumann's streamer inception criterion (SCSIC) for turn-to-turn insulation, which is the most vulnerable part in inverter-fed motors' insulation system. The Schumann constant (i.e., the natural logarithm of the critical electron number defining the Townsend-tostreamer discharge transition, K) varies with H, showing distinct patterns at low and high Ts. The K equations are derived across a wide range of relative humidity (RH) levels (20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%) at four Ts (25°C, 40°C, 60°C, and 90°C). These equations can be used in finite element analysis software to predict PDIV under varying H with outstanding accuracy. Additionally, a novel approach is presented for partial discharge (PD) phenomenology under H variations at different Ts using SCSIC-derived streamer inception parameters (SIPs): critical field line length (CFLL), air effective ionization coefficient (𝜶 𝐞𝐟𝐟 ), partial discharge (PD) inception field (Einc) and firing voltage (Vfiring). Notably, at high Ts (e.g., 90°C), a transition phase emerges concerning RH, leading to significant SIP changes due to the disappearance of a critical region (CritR) in 𝜶 𝐞𝐟𝐟 at specific electric field intensities. The developed humidity-dependent PDIV model supports insulation designers in achieving PD-free designs that account for H variations and sheds light on SIPs variations concerning H changes.

Pressure scaling laws for partial discharges in wedge-shaped, dielectric-bounded gas gaps

“…This has been observed to lower the PDIV below a certain threshold frequency [19]. In sufficiently humid atmospheres (≳100 gm −3 in [21,22]), the PDIV has been observed to increase again, possibly due to the formation of a connected conductive film which allows charge transport not only on the dielectric layers but also in between them, thus enabling a screening of the electric field in the air gap [23]. Such effects are not included in the present model.…”