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High Voltage Aerospace Cable System

Periodic Reporting for period 2 - HIVACS (High Voltage Aerospace Cable System)

Reporting period: 2020-04-01 to 2021-07-31

Current research and development is focusing on propulsive energy components for hybrid aircraft, opening the path to an all-electrical aircraft. Power levels are predicted to be between 2 and 4MVA for hybrid systems and up to 40MVA for all electrical systems. This will require the transmission of electrical power across the airframe at previously unseen scales. This will not be possible without the development of power dense and safe cabling systems operating at higher voltage and current levels.
To this end, the objectives of HIVACS were to:
- Bring together a coherent suite of experimentally validated simulation models to permit the design exploration and optimisation of future aerospace cable systems to allow the aeronautical industry to meet high-power design requirements of future aircraft programs.
- Share gained knowledge for future standardisation to relevant standard committees and identify key axis for further development.
- Establish for future developments beyond HIVACS, a validated HVAC aerospace cable design methodology supported by simulation models and experimental means: performing a state-of-the art review and a requirements and Failure Mode Effect and Analysis (FMEA) on the design and manufacturing processes on a selection of designs; assessing the performance of these designs in the aerospace environment with a range of existing models; validating models by comparison to experimental test bench activities undertaken on existing cables.
- Develop an AC cabling system up to TRL5 that has optimal performance through the use of innovative conductor design and insulation configurations for aerospace applications with the following characteristics: qualified and accepted by NEXANS for adoption in an industrial setting; used in a design optimisation process to determine optimal geometry and sizing of candidate cables and predict their expected performance. With an optimum cable design, two cable types will be produced using different manufacturing techniques. Both will be tested and qualified.
The project will draw upon existing test bench facilities and develop a specific thermal test bench for thermal cycling ageing.
Review of Failure modes: The state of the art in cable design & manufacturing for aerospace applications and failure mechanisms was reviewed. The potential impact and risk of increased power levels of 40MVA, related voltage (3kV), current (600A) levels and increased frequency related to PWM systems (>20kHz switching frequency; 1-3kHz fundamental frequency) were considered. A formal FMEA was conducted: allowing to identify and investigate risks related to the interface between the cable & connector. For screened cables, the field distortion at the triple point (between insulation, screen & air) may cause partial (surface) discharges. This may require field grading techniques specific to the aerospace environment. A UNIMAN PhD will investigate the subject further.

Development of electrostatic, electro-thermal and ageing models: Models using the streamer inception criteria to estimate the partial discharge (PD) inception voltage (PDIV) of unscreened cables were established & confirmed. The PDIV does not increase proportionally with the insulation thickness. Electro-thermal models were developed & confirmed in the test bench set up in the project, taking into account the cable design (dimensions & material properties), current waveform (amplitude, fundamental frequency & harmonics), shield (EMC braid) configuration (unshielded, individual shield on each phase, common shield around the 3 phases) and installation configuration (trefoil, flat). The current carrying capacity of screened cables increase slightly with increasing insulation thickness as the heat dissipation is favored by the increased cable surface. For unscreened cables the highest power density (VA/kg) is obtained with an optimum insulation thickness of 2mm (for PFA insulation). Cable ageing and long term risk of failure were analysed. The isothermal degradation kinetics of aerospace insulation materials polyimide, PTFE and PFA were compared. At temperatures below 250°C PFA has a higher lifetime compared to polyimide and PTFE. At higher temperatures polyimide behaves better than PTFE and PFA.

Development of HVAC cables: Several small (AWG12) and big size (AWG0) model cables were manufactured and analysed. Different manufacturing techniques to apply the insulation on the conductor were investigated. Advantages & disadvantages were highlighted, the most appropriate technique selected. Different design concepts were also evaluated. Based on this and simulation results, 3 HIVACS cables with the same conductor AWG00 (aluminium) but different electrical insulation systems (screened, unscreened) and shielding (metallic braid, unshielded) were designed, manufactured & tested for a PDIV above 9000V peak to peak PWM phase to phase. The screened cable has a PD inception and extinction voltage that do not change with pressure (semiconductive layers contain the electrical field inside the insulation). The advantage of a EMC braid around each phase is that phase-to-phase voltage is reverted to phase-to-ground voltage requiring less insulation thickness. For example the unscreened but shielded cable required a 2mm insulation thickness whereas the unscreened and unshielded cable required a thickness of 3.2mm (i.e. 6.4mm between 2 phases). On the other hand the unscreened & unshielded cable has a current carrying capacity 20% higher than shielded cables.
The HIVACS project went beyond the state-of-the-art in many aspects of cable design & manufacturing:
Insulation performance of new high voltage AC cable systems:
- Developed clear guidance on the preferred electrical insulation system within high voltage aerospace applications,
- Identified the critical electric fields that determine the threshold for partial discharge (PD) inception and methods to manage them,
- Identified the limitations of the insulation manufacturing techniques,
- Demonstrated which manufacturing process will lead to improved HVAC cable performance.
Modelling activities to support cable design:
- Introduced a thermal model that considers effects related to the frequency, the insulation material proprieties and thickness, and the aircraft installation conditions.
- Developed electrostatic models of HVAC cable systems that can be used to support the development of optimal cable designs and inform the production of suitable guidance in standards such as AS50881.
- Introduced a model predicting the behaviour of partial discharge inception voltage (PDIV) and of thermal ageing for future HVAC cables.
Experimental test means and optimised methodologies:
- Delivered a validated practical test system that can be used to confirm the thermal performance of HVAC aerospace cabling systems and be used in conjunction with ageing assessment.
- Developed a process that allows the design of a HVAC aerospace cable to be optimised using a combination of experimentally validated electrostatic, thermal and ageing models.
The modelling and experimental test means will support the future development of HVAC cables that are expected to become far higher in power rating, voltage & current. The knowledge gained & shared with standardization bodies will also assist the aeronautical community with value guidance for development activities across all types of future aircraft programs.
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