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Since the proposal of the notion of nanodielectric, the space charge characteristics of nanodielectrics have been widely investigated. With the addition of nanoparticles, some nanodielectrics exhibit smaller space charge accumulation amount during the polarisation, and also faster charge decay during the depolarisation, which is believed to be related to the interfacial layer between the nanoparticle and the base material. In this study, the space charge measurement methods, theoretical models, and experimental results are reviewed in detail. Space charge measurement methods have been greatly improved after more than 20 years development. The nature of the interfacial layer between the nanoparticle and the base material has been investigated in detail, and based on that, several models are proposed to explain some electrical experimental results of nanodielectrics. Furthermore, the parameters of nanoparticle (such as the type, size, amount, shape, and the surface modification), the experimental conditions, and the base material properties can strongly affect the space charge characteristics of nanodielectrics. This study will provide useful research results and conclusions for researchers, and may also be an overview for recent study and an outlook for future investigation on space charge characteristics of nanodielectrics.

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Electric machine for aircraft propulsion is required to have high power density and high reliability, but the harsh operating environment, such as low air pressure, will challenge the motor's insulation system. This article presents the partial discharge (PD) investigation of the form-wound winding in high-frequency electric machines for electric aircraft propulsion. Experimental results show that the PD activity is largely enhanced due to the decrease in air pressure. PD inception voltage (PDIV) decreases to ∼50%-60%, and the PD magnitude dramatically increases when the pressure drops from 1 to 0.2 atm. Several windings with introduced defects were also tested to extract features from phase-resolved PD (PRPD) analysis, which can be used for PD pattern recognition. In addition, a PD-based failure precursor, combining the apparent charge, number, and phase interval of the monitored PD profile, is proposed to monitor the insulating condition of the thermally and electrically aged windings. The proposed failure precursor generally shows a rising trend with the aging time in this article. The application of the proposed failure precursor on online condition monitoring needs more investigation through long-term online observation of the electric machine.