Insulation in high temperature superconducting (HTS) equipment suffers severe thermal shock during quench events from short-circuit faults, causing performance degradation. This paper reveals the mechanism behind the nonlinear variation of the insulating properties of polypropylene laminated paper (PPLP) under cryogenic thermal shocks from interfacial and bulk perspectives. To simulate quench events in HTS cables, PPLP samples were prepared under different temperature ranges and numbers of thermal cycles. A cohesive zone model of PPLP interlaminar damage was established to investigate crack evolution between polypropylene (PP) and kraft paper layers. Single thermal cycles from liquid nitrogen to room temperature caused complete interface damage with a damage factor D = 1corresponding to microcracks less than 30 μm in width observed by 3D-CT and SEM, adversely affecting electrical insulating behavior. As cycles increase, cracks reduce interfacial stress, slowing crack expansion. Macrocracks larger than 30 μm are constrained by winding stress, limiting adverse effects on insulation performance. After multiple thermal cycles, changes in PPLP electrical properties mainly depend on its bulk structure due to non-monotonic changes in kraft paper compactness. Cellulose density first rises then falls as aging cycles increase, whereas PP bulk structure changes slightly under these conditions. The development of a model for the non-monotonic degradation of insulation, dominated by interfacial cracking induced by mismatched thermal stresses and supplemented by bulk structural changes, contributes to a deeper understanding of the aging mechanism of thermal cycling in multilayer thin-film during quenching failure of HTS equipment. • Thermal cycling initiates interfacial cracks; thermal expansion causes severe damage. • Kraft paper properties evolve non-monotonically; PP properties remain unchanged. • Aging stage 1: microcracks (<30 μm) cause sharp drop in DC breakdown strength. • Aging stage 2: bulk structural changes lead to non-monotonic performance.
Tan et al. (Fri,) studied this question.