Skip to main content

Reliable Aircraft electrical Insulation System sElection

Periodic Reporting for period 2 - RAISE (Reliable Aircraft electrical Insulation System sElection)

Reporting period: 2019-09-01 to 2020-12-31

In the last years, the need for high power density and efficiency has become a central concern in all industrial sectors. For this reason, the arrival on the market of new power devices based on wide bandgap (SiC and GaN) semiconductors, with extremely short switching times, has raised a relevant interest.
Aerospace industry is one of the main areas which can take advantage from these devices. According to the 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, some reliability issues may arise in this type of electric drives when higher voltage gradient are adopted, and the power cables, connecting the inverter to the motor, exceed the so called “critical length”. In this case, steep rising and falling voltage pulses make the cables act like transmission lines, with waves travelling along the cables forward and backward. In fact, due to cable-motor surge impedance mismatch, damped high frequency ringing at the motor terminals occurs leading to dangerous overvoltage (almost twice the bus voltage), which may seriously affect the motor’s windings dielectric insulation. In particular, if the voltage between the stator windings’ turns exceeds partial discharge (PD) inception voltage, PDs occur on the surface of dielectric insulation causing progressive erosion until complete breakdown. Critical length is proportional to the switching time, therefore the recent availability of SiC and GaN devices with very short switching time made troublesome to use even cables of few meters length.
RAISE project comprehends the analysis and the experimental assessment of state-of-the-art insulation materials and systems used in aircraft applications. Models will be proposed to assess the voltage gradients and the magnitudes observed for typical converter – cabling – machine systems, along with the voltage distribution within typical electrical machine windings, and its dependency on machine’s key parameters, such as the number of turns, the winding method used and the stator length.
RAISE aims to offers solutions to improve the reliability of the electrical machines and drives without compromising the overall performance and efficiency. The results will permit to drawing up design, qualification and verification guidelines supporting the development of machines operating with wide-bandgap switches in aerospace environmental conditions.
The project comprehends 4 work packages, reflecting different project stages. Each work package consists of two or more tasks and it is supervised by a Work Package Leader in charge of controlling the correctness of the work and respect of the timeline.
• WP1 - Assessment Of Voltage Gradients /Magnitudes Observed Within Insulation Systems
• WP2 - Assessment Of Partial Discharge And Breakdown Behaviour Of Components Exposed To High Voltage Gradients:
• WP3 - Lifetime Characterisation Of Components Exposed To High Voltage Gradients:
• WP4 - Management and Dissemination:

For WP1, UniMore proposed a model to assess the voltage gradients and the magnitudes observed for typical converter – cabling – machine systems, along with the voltage distribution within typical electrical machine windings, and its dependency on machine’s key parameters, such as the number of turns, the winding method used and the stator length. UniMore also conducted a survey on the best solutions to reduce voltage gradient without compromising converter performance.

For WP2, the impact of increasing slew rates on the partial discharge and breakdown behaviour of typical components used in the aerospace environment has been investigated, including the impact of varying pressure, humidity and temperature, voltage level, rise time. The behaviour of the partial discharge inception voltage (PDIV) as a function of voltage gradient, frequency and ambient pressure has been investigated. A test set-up was built for the investigation. The test set-up accounts for a custom SiC pulse generator designed by UniMore and a variety of Partial Discharge detectors, as photon-counters, photomultipliers and antennas.

For WP3 the activities quantified the impact that higher voltage gradients have on the lifetime of electrical machines and the connecting cables / connectors. Possible solutions identified in WP 1 and 2 were investigated.
This analysis led to an update of the verification / qualification methods considered in WP 2. The WP has provided recommendations for the design of insulation systems when using high voltage gradient systems and defined Type/Qualification test guidelines for the verification of complete stators. Potential limits and challenges of the current methods used in aerospace have been identified and solution proposed.

The consortium disseminated non-confidential results to the Research, Industrial and Public communities:
A web-site was realized within the first year of the project https://www.raise.unimore.it/.
A special session at international conference IEEE IECON 2019 was organized
The results of the research have been submitted to international conferences (6) and journal (3).
RAISE aims to offers solutions to improve the reliability of the electrical machines and drives without compromising the overall performance and efficiency. The results will permit to drawing up design, qualification and verification guidelines supporting the development of machines operating with wide-bandgap switches in aerospace environmental conditions.
RAISE proposes a comprehensive model able to predict the voltage gradients and magnitudes that can be observed in different part of the system composed by drive converter, cables and electrical motor. The model takes in consideration the system layout and geometrical and physical parameters, along with all the electrical parameters e.g. rise and fall times of the voltage during device commutations, switching frequency, duty cycle of the PWM, DC voltage of the converter.
Today models that analyse the voltage amplitude and gradient within drive systems are main related to study of Si-based power converter and the reflections originated in very long cables by the PWM voltage commutations. An improvement on these models is expected to be achieved for including the impact of new WBG devices with shorter rise and falling times of the converter’s synthetized voltage. Such studies, which today have never been conducted in too much detail or applied to generic electrical machine systems (applicable for a wide range of electrical machines) represent a considerable improvement in terms of progress beyond the state of the art. Designers and manufacturers of electrical machines would benefit greatly from such models. The results of the model may be used by motor and electric drive designers to test solutions and adopt precautions before realization of physical prototypes. The model will enhance the reliability of electric drive and will short the design procedures of new products, since the weaknesses of the systems will be individuated even at project early stage.

The procedures to realize verification and qualification of insulation materials and motors that have been defined during the project will be helpful to further push the limits of testing of electrical drives.
Motorette used in WP1 and WP2 for the investigation
particular of the gate driver able to change rise and fall time of the generated voltage waveform
Inverter used in WP1 and WP2 for the investigation