## 1 Introduction  
The health monitoring of stator windings in electrical machines is one of the most important challenges in the automotive traction industry. Increasing supply voltages and inverter switching frequencies lead to accelerated aging of these windings [1]. Such aging is characterized by the degradation of insulation layers between conductors. Factors such as the duration and level of exposure to electrical stress influence the quality of these insulators [2]. In order to reduce the risks of premature insulation degradation and to increase winding robustness, several techniques can be considered: improving the conductor impregnation or using enamels that are particularly resistant to partial discharge phenomena [3]; promoting the design of hairpin winding machines that help reduce vibration phenomena in the winding heads [4]. However, it remains essential to monitor the health of these windings to identify their aging as early as possible. Therefore, if a given signature were to exceed a certain threshold, preventive maintenance could be considered to protect the system and reduce maintenance and repair costs and times [5,6]. There are several approaches to monitoring the health of an electrical machine winding. First, partial discharge measurement can be highlighted [7; 8; 9]. Some strategies use insulation resistance measurement and polarization index calculation to perform the diagnosis [10; 11]. Estimation of equivalent phase-to-ground or interturn capacitance [12; 13] may also be conducted. The main drawback of these methods is that they are usually performed on a machine at standstill, disconnected from the power supply. In some applications, particularly industrial applications, these test conditions cannot be met, making aging monitoring impossible. Furthermore, resistance tests can give an indication of windings state of health [14], but since these tests are destructive, they cannot be applied industrially. Statistical studies are used in [15] to estimate the remaining useful life (RUL) of a system. Finally, in recent literature, many aging monitoring methods are based on the analysis of systems frequency response to determine the winding condition. The various approaches used are usually based on the variation of the frequency signatures of the machine impedance characteristic [16; 17; 18; 19]. This characterization over a wide frequency band takes into account leakage and magnetization inductances, iron and eddy current losses, as well as interturn and phase-to-ground capacitive couplings. As a result, the different resonance frequencies reflect these different phenomena. It has been shown in [12] that the decrease of the winding breakdown voltage correlates with an increase of the machine equivalent capacitance. Furthermore, ref. [16] explains that this equivalent capacitance is particularly impacted by the evolution of the capacitance between windings. It can therefore be concluded that its monitoring will help to detect the aging of a machine inter-turn insulator at an early stage. Finally, the model developed in [20] shows that this inter-turn capacitance has a strong influence on the impedance frequency behavior of the machine. As a result, this characteristic will evolve as the machine age. Ref. [16] emphasizes that the first resonance frequency of the impedance characteristic decreases with time, indicating degradation of the winding insulation. It has also been observed in [18] when the windings are subjected to adverse environmental stresses, such as an exposure to a corrosive substance on their insulators. The resonance frequency decreases with prolonged exposure to the corrosive agent. The amplitude of this resonance also decreases as the conductors age. All of these results demonstrate the benefits of monitoring the evolution of frequency impedance over time to assess the aging of electrical machine windings.