Electric Insulation of a High Voltage Rotating Machine for Aircraft Propulsion

Abstract

One fifth of the global carbon dioxide (CO2) emissions can be attributed to the transportation sector. Consequently, the growing desire to participate in the energy transition begs for the exploration of alternative modes of transportation. This endeavor is particularly challenging in the field of aviation and aircraft propulsion, where the injection of electrification and electric motors (EMs) pose many hardships such as the need for high autonomy, light weight, high power density and reliability. This work will mainly focus on the latter.

Unlike the low voltage machines used in road mobility, the motors necessary to propel an airplane demand substantially higher voltages due to their power requirements. Despite the availability of dielectric materials and insulation techniques, an improper design of the insulation system may make it susceptible to electrical stress, possibly reaching its breakdown voltage, generating partial discharges (PDs) and degrading the insulating material overtime. This will eventually lead to the electrical failure of the machine.

This danger is further increased by the recent rise in popularity of new wide-band gap power electronic devices. These devices offer many advantages: reduced size and weight, the ability to operate in higher temperatures, and the improved efficiency due to the reduction in power losses, caused by their steep switching speed in the order of 10−100 ns. However, the steep voltage transients (dv/dt) they produce create harmful side-effects to the machine. Firstly, the feeder cable connecting the voltage inverter to the machine terminals suffers Reflected Wave Phenomenon (RWP), where the voltage pulses are reflected at both ends of the line generating intereferences, which in turn generate overvoltages that can rise to up to twice the original voltage pulse. Secondly, the inherent leakage capacitance between the cable turns and the core of the stator slots produce an uneven voltage distribution along the wiring turns. These effects could increase the voltage stress in the insulator and momentarily reach the breakdown voltage in certain points of the geometry, producing Partial Discharges (PDs) that degrade the insulation over time.

To avoid this degradation, the aim of this thesis is to study these harmful effects in depth, back-up the literature review with accurate simulations and experimentation and realize the worst case scenario considering the geometry of a given motor that is currently under design. Once the phenomena is understood, some mitigation techniques are proposed to lower the chance of any discharge occurring.