Metallised polypropylene film capacitors (MPPFCs) are widely used in power electronics and are generally degraded by elevated temperatures. This work aims to determine the relationships between the structural changes of MPPFC and the microstructural variations of the PP film during the thermal ageing of MPPFC at 100°C for 38 days. The capacitance of MPPFC has a slight decrease during thermal ageing. However, the breakdown voltage of the MPPFC decreases by 39.4% by the ageing. The partial discharge (PD) number of MPPFC increases linearly with ageing time. The tear-down analysis of the MPPFC reveals that the molecular structure of the PP film has not been altered but has led to molecular chain scission and the generation of some polar fragments/groups. Meanwhile, the relative permittivity of the PP films rises as the ageing time increases. Moreover, thermal ageing causes the conversion of aluminum to alumina in the metallised electrode, which is hydrophilic for polar groups and leads to an adhesion effect between the metallised electrodes and the PP film. Contact angle measurements prove that the surface hydrophilicity of the PP sample increased after thermal ageing. Therefore, the PD/breakdown voltage in the MPPFC increases/decreases due to the uneven adhesion of the metallised PP film. | INTRODUCTIONMetallised polypropylene film capacitors (MPPFCs) are ubiquitous in power electronics, such as static synchronous compensators (STATCOM), motor drives, and modular multilevel converters, due to their high breakdown strengths, good dielectric stability, and low dissipation factors [1][2][3]. In power electronics, the PP film in the capacitor must endure electrical and thermal stresses that can rise above 200 V/μm and around 150°C [4]. In addition, capacitors with a proper performance at elevated temperatures facilitate practicable thermal management in application scenarios where the volume and weight are limited, typically in electric vehicles [5,6]. However, elevated temperatures may significantly reduce the life expectancy of the capacitors [7].Thermal ageing is considered as one of the most critical factors, which leads to severe degradation of the PP film and consequent capacitor failure [8]. It has been recognised that the PP films have a very low dielectric loss (~10 −4 ) at temperatures up to 85°C. Once the temperature exceeds 85°C, electron conduction starts to contribute to a significant increase in dielectric loss [9,10]. Qi et al. [11] discovered that the hotspot This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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