Insulating Gases for Partial Discharge Management of Electrical Machines in Aerospace Applications

There is increasing interest in developing aircraft that are wholly electric or utilize hybrid-electric architectures to deliver a reduced carbon footprint. The higher operating voltage is necessary to deliver electrical machines with an acceptable power density, which will increase the risk of partial discharge (PD), especially in the low-pressure environment at high altitude. This paper reviews the opportunity to use high dielectric strength gases to improve the PD performance of electrical machines for aerospace applications. PD characteristics of machine winding wires are tested in air, CO2, SF6, CF3I, C3H2F4, and C3F7CN. Mixture ratios of 60% C3H2F4 / 40% CO2 and 30% C3F7CN / 70% CO2 can remain gaseous under atmospheric pressure at a temperature of -30 ºC, with the partial discharge inception voltage (PDIV) of machine winding wires approximately three times higher in these mixtures than air at 0.2 bar absolute. The test results have been applied to demonstrate that a machine using a high dielectric strength gas as part of its insulation system could tolerate a higher DC bus voltage while still being considered as Type I (PD-free). Additionally, endurance tests using insulating gases demonstrate the prospect of an improved time to failure for Type II insulation systems.