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

This article 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-to-streamer 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 (
$\alpha _{\text {eff}}{)}$
, PD inception field (
${E}_{\text {inc}}{)}$
and firing voltage (
${V}_{\text {firing}}{)}$
. 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
$\alpha _{\text {eff}}$
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 SIP variations concerning
${H}$
changes.