How does PDIV change during isothermal aging of magnet wire

Abstract

In the context of the energy transition in transportation systems, electric machine insulation systems will endure severe environmental and electrical constraints. To keep high level of performances and reliability for such Electric Insulation Systems (EIS), it is necessary to master several aspects linked to their aging. For type I machines (so-called Partial Discharge (PD) free) the thermal constraint is often the most important aging factor. However, the knowledge of how the Partial Discharge Inception Voltage (PDIV) evolves is crucial to ensure that the machine is exempt from PD activity throughout its lifespan.In this article, the thermal ageing of twisted pairs of magnet wire is reported. The aim of this study is to highlight the importance of electrical markers such as the Partial Discharge Inception Voltage (PDIV) and the dielectric strength as relevant makers to monitor the state of health of machine EIS and extract ageing laws.The samples studied are twisted pairs of 0.6 mm copper coated with PEI/PAI enamel grade 2. They were subjected to three different ageing temperature (180°C, 200°C and 220°C) in the oven. Periodically samples were taken out to measure their PDIV at ambient temperature. In order not to influence the purely thermal aging with electric measurements, the samples were not replaced in the oven.To understand the reason for the change in PDIV we also performed measurements of enamel thickness at different durations. This allowed to predict PDIV using simulation software, called AIRLIFT, developed during HIGHVOLT project.A clear decrease of PDIV was observed for each of the three aging temperatures. Microscopic observations allowed us to show that the thickness of the enamel decreases during aging. The simulations using AIRLIFT confirmed our observations.The decrease of the twisted pairs PDIV during thermal aging was mainly due to decrease of the enamel thickness. The empiric formula, based on Arrhenius law, is given to assess the decrease in PDIV during aging at different temperatures. This change need to be taken into consideration during the design of the type I insulation system in order to prevent the machines from PD activity they are not designed to withstand.