Going electric with the RAISE project
In the last years, the need for high power density and efficiency has become a central concern in the green transportation sector. The aerospace industry is one of most demanding areas in this sense, and according to the More Electric Aircraft (MEA) concept, replacing the aircraft’s hydraulic/mechanical actuators with electrical drives yields an improvement in efficiency and power density, which means a reduction in weight, fuel consumption and noise/pollutants emission.
However, to meet the challenging requirements of modern aerospace applications, power electronics need to be not only efficient but also very reliable. New power devices based on wide bandgap semiconductors (SiC and GaN), with extremely short switching times and the ability to operate efficiently at high temperature, could provide a solution.
Adopting these technologies will allow for decreases in total power losses and reductions in the cooling system requirements (and thus heatsink size and volume), as well as increasing the fundamental operating frequencies of electrical machines, leading to smaller and lighter motors and generators.
Nevertheless, some reliability issues may arise in this type of electric drives, as faster devices’ commutations are known to trigger faster degradation of coil insulation and cause greater susceptibility to Electromagnetic Interference EMI. Thus, there is a stringent urgency to understand the causes of these issues, model the phenomena and evaluate the impact on the relatability side of these devices against their superior efficiency, in order to propose solutions so as to safely deploy this technology in the aerospace sector.
That was the goal of the RAISE project – to investigate these issue surrounding wide-bandgap devices (WBG), which are seen as a key technology for future aircraft electrification. In the end, the results of this project will help to safely deploy WBG devices in aerospace applications, to pursue the More Electric Aircraft (MEA) concept, and replace the aircraft’s hydraulic/mechanical actuators with electrical drives. This will lead to an improvement in efficiency and power density, which means a reduction in weight, fuel consumption and noise/pollutants emission.
To carry out the research, the project undertook a twofold approach, considering both simulations and experimental validations.
On one hand, a propagation model was realised to evaluate the electrical stress. On the other hand, a thorough investigation was carried out to determine the stress levels required to incept partial discharges as a function of several parameters (pressure, temperature, humidity, materials, and impulse voltage features). The stress levels due to the inverter were thus matched to the threshold for partial discharge inception, considering the worst case (cruising altitude).
To match the drive characteristics with the insulation, the project also aimed at verifying the inception of partial discharge (PD) on new and aged insulation systems.
The capability of designing an insulation system that is PD free by design throughout its lifetime will deliver reliable components.
Solutions have been identified to reduce the voltage gradient at the electrical machine, and so the electrical stress for the winding insulation, without compromising the performance of the electric drive. These include use of passive filters at the inverter output, new material or thicker dielectric in the design of the windings’ insulation system, form-wound windings and an innovative design for the inverter.
The RAISE project published 6 conference papers and 3 journal articles. The data collected during the project have huge potential and they continue to be probed for possible future publications.
In particular, based on the results of RAISE, the research group is investigating the ability to contain voltage overshoot at the machine terminals. This would minimise the detrimental effects of increased dv/dt, while maintaining the benefits derived from the fast commutations of the devices.
The consortium is made up of the University of Modena and Reggio – UniMore and Alma Master Studiorum – University of Bologna – UniBo. UniMore leads the consortium as project Coordinator, whereas the company Liebherr-Aerospace, a multinational operating in the aerospace sector and a Clean Sky 2 Leader, acts as Topic Manager for the project. The project lasted for 34 months and the overall budget was € 396 401.25.