In recent decades, extensive research has been conducted on the electrification of commercial aircraft to reduce the dependence on mechanical, hydraulic, and pneumatic systems and replace them with electrical systems. A primary goal of this path is to make the power density of the more-/all-electric aircraft (MEA/AEA) closer to that of conventional aircraft. While current commercial aircraft operate at voltages below 1 kV, it is widely accepted that higher operating voltage is necessary for MEA/AEA. NASA has envisaged a voltage level of at least 6 kV for MEA. In the language of electrical insulation technology, higher voltage levels translate into higher electric tension on the insulation system. A serious challenge stems from the fact that, at high altitudes, high-voltage insulation design strategies are not necessarily as efficient as at sea level due to differences in environmental conditions, including lower pressure, higher moisture level, microgravity, and plasma radiation. In this article, challenges associated with the electrical insulation design of future aircraft are reviewed. These challenges extend to almost any part of the electric power system in an aircraft, such as electric machines, power converters, cables, and printed circuit boards (PCBs). An overview of the aging factors, such as internal discharges, arc tracking, and thermal degradation, is accompanied by a discussion of the potential of novel insulating material and the ways to reinforce the current commercial dielectric materials. Finally, considerations for testing at simulated high-altitude conditions and the existing standards and their deficits are investigated.

Insulation Materials and Systems for More- and All-Electric Aircraft: A Review Identifying Challenges and Future Research Needs

Next generation aircrafts will use more electrical power to reduce weight, fuel consumption, system complexity and greenhouse gas emissions. However, new failure modes and challenges arise related to the required voltage increase and consequent rise of electrical stress on wiring insulation materials, thus increasing the risk of electrical arc appearance. This work performs a critical and comprehensive review concerning arc tracking effects in wiring insulation systems, underlying mechanisms, role of materials and possible mitigation strategies, with a special focus on aircraft applications. To this end an evaluation of the scientific and technological state of the art is carried out from the analysis of theses, research articles, technical reports, international standards and white papers. This review paper also reports the limitations of existing insulation materials, standard test methods and mitigation approaches, while identifying the research needs to comply with the future demands of the aircraft industry.

https://www.mdpi.com/1424-8220/20/6/1654/pdf

Arc Tracking Control in Insulation Systems for Aeronautic Applications: Challenges, Opportunities, and Research Needs

Microstructures are critical for designing composite materials with admirable properties required by various applications. However, desired inclusion arrangements are pre-required and have to be reached by appropriate processing as well as quantitative evaluation. Up to date, visual characterization like transmission electron microscope is still the “golden rule” for measuring inclusion arrangement, together with qualitative descriptions in most studies. In order to establish structure-property relationship, optimize processing parameters and even predict composite performances, quantitative evaluation of inclusion arrangement becomes a must. This review focuses on the numerous developed mainstream quantitative approaches and their recent advances. Methods, including ASTM index (American Society of Testing Materials), Morishita’s index, quadrat method, linear intercept distance, free-path spacing, nearest neighbor distance, inter-particle distance, free-space length, functional analysis, Delaunay triangulation and Dirichlet tessellation, are classified into three categories: methods based on inclusion features, methods based on inclusion position and methods based on inclusion position and inter-distance, which are introduced in the manuscript. Definition, advantages, updated progress and recent application examples of specific method are included. The differences between each methods and their limitations are summarized.

Quantitative evaluation of inclusion homogeneity in composites and the applications

Polymeric composite insulators are subjected to varying work conditions like rain and heat, which create an impact on degradation during their long service period. Electrical tracking under the Alternating Current (AC) field plays a predominant role in surface degradation, which can be different for fresh and aged insulations. The tracking studies on the fresh and aged polymeric insulation therefore become significant. Motivated by this, an indigenous low-cost electrical tracking setup was developed, and the tracking studies were carried out as per International Electro technical Commission standard (IEC) 60587 on fresh, thermal-aged and water-aged silicone rubber samples. Contact angles of samples were measured to analyse the effect of ageing on hydrophobicity. Further, to analyse the influence of ageing on insulation integrity, tracking tests were conducted and parameters like leakage current pattern and magnitudes, tracking length and loss of weight in the material due to tracking were examined. The physicochemical impacts of ageing on the surface degradation of the samples were also analysed using X-ray diffraction analysis and Fourier Transform Infrared Spectroscopy analysis. The investigations added insight into the degradation mechanism of polymeric insulators in terms of their electrical performance and physicochemical changes in the material. Comparison of these changes showed that ageing could influence surface degradation of samples.

https://www.mdpi.com/1996-1073/12/19/3769/pdf

Investigation of Surface Degradation of Aged High Temperature Vulcanized (HTV) Silicone Rubber Insulators

Glass-fiber reinforced epoxy nanocomposites are widely used in high voltage outdoor applications. One of the serious problems of outdoor insulation is the surface tracking and erosion that leads to premature failure of insulation. In this paper free-space length, surface energy and surface tracking of epoxy composites with nano alumina, silica and micro alumina trihydrate fillers and unidirectional glass fiber reinforcement are discussed. Quantitative interfacial analysis has been used by determining the freespace length of the composites. The results of tracking are explained on the basis of contact angle measurements and the concept of surface energy is used to substantiate the results. The time-to-track and surface erosion of dry and water treated nanocomposites are determined by the inclined plane tracking and erosion method. The experimental results have revealed that the hydrophobic nature of glass epoxy composites improves with addition of fillers. The surface energy, wettability and interaction parameters show reduction with the addition of fillers. It is observed that the Glass Epoxy composite with combination of nano alumina, silica and micro ATH fillers exhibits good thermal, electrical and mechanical properties with improved tracking and erosion resistance.

Correlation of free space length and surface energy of epoxy nanocomposites to surface tracking

The paper reports the results of moisture and filler induced effects on the dynamic mechanical properties of glass fiber reinforced epoxy hybrid composites. The rate of water diffusion into the composites has been determined to understand the effect of moisture on the dynamic mechanical properties. Significant differences in the moisture uptake properties of the composites are observed which induce changes in storage modulus (E′), loss modulus (E″), and damping factor (tan δ). It is also observed that water diffusion depends on the free volume content of the hybrid composites. Results of this study indicate that hybrid composites with combination of micro and nanofillers possess good dynamic mechanical properties in presence of moisture at higher temperatures.

C. Ranganathaiah

Papers

Correlation of free space length and surface energy of epoxy nanocomposites to surface tracking

Moisture and filler induced effects on the dynamic mechanical properties of glass fiber reinforced epoxy hybrid composites