Infrared Thermography (IRT) is being used in an ever more broad number of application fields and for many different purposes; indeed, any process, which is temperature dependent, may benefit from the use of an infrared device.

Infrared Thermal Imaging

The main purpose of this article is to provide an instructive review of the technological challenges hindering the road toward more electric powertrains in aircraft. Hybrid, all-electric, and turboelectric powertrain architectures are discussed as possible fuel consumption and weight reduction solutions. Among these architectures, the short-term implementation of hybrid and all-electric architectures is limited, particularly for large-capacity aircraft due to the low energy/power density levels achievable by state-of-the-art electrical energy storage systems. Conversely, turboelectric architectures with advanced distributed propulsion and boundary layer ingestion are set to lead the efforts toward more electric powertrains. At the center of this transition, power converters and high-power density electric machines, i.e., electric motors and generators, and their corresponding thermal management systems are analyzed as the key devices enabling the more electric powertrain. Moreover, to further increase the fuel efficiency and power density of the aircraft, the benefits and challenges of implementing higher voltage powertrains are described. Lastly, based on the findings collected in this article, the projected roadmap toward more electric aircraft powertrains is presented. Herein, the individual targets for each technology, i.e., batteries, electric machines, and power converters, and how they translate to future aircraft prototypes are illustrated.

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Toward more electric powertrains in aircraft: Technical challenges and advancements

The idea of electric propulsion for transportation is not new; indeed, the first cars, nearly 200 years ago, were electric. However, our dependence on fossil fuels over the last 100 years is now being questioned, and as a global society, we are moving toward more-electric transportation solutions. Electric propulsion of aircraft is part of this trend, either all-electric or through a large variety of the proposed hybrid propulsion systems. This article considers some of these systems, their technological requirements, and the ongoing research and development in motors and drives necessary to make this technological change a feasible option for the future of passenger flight.

Electric/Hybrid-Electric Aircraft Propulsion Systems

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

Electrical and electronic equipment used in aerospace applications must be designed to operate over a wide range of environmental conditions that include variations in pressure, temperature, and humidity. Electrical power systems for advanced aircraft utilize voltages well above the traditional levels of 12 to 42 VDC and 115/200 VAC, 400 Hz. Current airborne systems can contain 270 VDC, and bipolar systems with a 540 V differential are appearing in certain applications. Higher potentials create increased probability of arcing and flashover compared to the risks associated with traditional ac or low-voltage dc. The low pressures of high altitude environments only serve only to worsen such concerns. This paper summarizes the development of a guideline document containing methods of managing higher voltages in aerospace vehicles. Based upon both current and archival work, the design guidance (1) provides a basis for identifying high voltage design risks, (2) defines areas of concern as a function of environment, and (3) illustrates potential risk mitigation methods and test and evaluation techniques. The document is focused on electrical discharge mechanisms including partial discharge and does not address personnel safety. Some of the key areas of concern are power conversion devices, electrical machines, connectors and cabling/wiring, as well as interactions between components and subsystems. The document is intended for application to high voltage systems used in aerospace vehicles operating to a maximum altitude of 30,000 m. (approximately 100,000 ft.), and maximum operating voltages of below 1500 Vrms. Fundamental issues addressing some of the key areas will be presented and discussed.

Design considerations for higher electrical power system voltages in aerospace vehicles

This paper reports on a method combining the use of finite element simulations and external measurements to provide preliminary condition diagnosis of oil-filled cable sealing ends (CSEs) without requiring downtime. Good agreement has been obtained between the predictions from the electric field and thermal simulations and the measurements on a 132 kV oil-filled CSE. The electrostatic computation combined with electric field measurements can provide information regarding the electric field distribution inside the CSE and help in identifying potential issues with the CSE design, the materials or the cable termination process. The thermal computation combined with thermal imaging can reveal potential problems, such as high resistance connection to the busbar, and provide information regarding the cooling efficiency of the liquid dielectric. The method presented can provide the starting point for prioritizing maintenance operations on CSEs.

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Electric Field and Thermal Analysis of 132 kV Ceramic Oil-Filled Cable Sealing Ends

With the introduction of higher voltages in ‘more electric’ aircraft, the understanding of Partial Discharge (PD) performance has never been more critical. Power frequency PD testing of insulation systems according to IEC 60270 has been well established for some time. Less well established is the measurement of PD caused by voltage impulses with fast rise times and significant peak overshoots, as described in IEC 61934. This type of test is particularly applicable to machines fed by Pulse Width Modulation (PWM) voltages. An array of tests has been performed at The University of Manchester's high voltage laboratory on typical aerospace components using both power frequency and impulse waveforms. The purpose of the tests has been to assess the performance of insulation systems under both atmospheric and simulated altitude (50,000 feet) conditions. Testing the samples using both sinusoidal and impulse waveforms provides a good opportunity to compare the practices described in the IEC standards, as well as the effect the type of waveform has on the PD performance of the test samples. Tests performed according to IEC 60270 have returned reliable results across a variety of test samples and atmospheric conditions. However, tests performed according to IEC 61934 have provided inconsistent results, and some unexpected behaviour has been observed. In particular, some test samples have shown an increase in the partial discharge inception voltage as the ambient pressure is reduced. As a result of these unreliable test results, a set of round robin tests have been proposed. Samples of identical specification have been dispatched for testing at a number of laboratories around the world. The results of these tests will be analysed and disseminated with the objective of improving PD qualification testing of aerospace components.

Comparison of power frequency and impulse based Partial Discharge measurements on a variety of aerospace components at 1000 and 116 mbar

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Operation of aerial inspections vehicles in HVDC environments Part A: Evaluation and mitigation of high electrostatic field impact

Premature failures of stator insulation account for a large percentage of repairs of marine generator systems. The failure mechanisms of such faults have been presented in many parts of the literature. Partial discharge activity, thermal degradation, thermal cycling, harmonics and transients are some examples of such failure mechanisms. Whilst there has been an insight into the failure mechanisms, there is still no definite answer to how these defects manifest in the first place. Most of the failures that have been identified within literature are on end windings, especially slot ends. Some failure mechanisms have also been linked with thermal cycling. Frequent and rigorous stop/start cycles stress coils by inducing mechanical forces between elements of the coil and housing owing to differential thermal expansion. This differential expansion is dependent on the rate of rise of temperature and also the different coefficients of thermal expansion of the materials. The present paper will evaluate the thermal degradation of insulation systems used on marine generators using Finite Element Analysis (FEA) methods. On board temperature measurements of stator coils during a high speed run are used as one of the parameters within the FEA simulations, to investigate if there is any risk of differential thermal expansions during such an operational cycle. Different ramp rates are also analyzed within the FEA simulations to understand the effect of uneven thermal expansions and the risk of material degradation of the insulation in coils on marine systems. A brief review of the standards available for thermal cycling and testing are also presented within the paper.

Richard Gardner

Papers

Design considerations for higher electrical power system voltages in aerospace vehicles

Electric Field and Thermal Analysis of 132 kV Ceramic Oil-Filled Cable Sealing Ends

Comparison of power frequency and impulse based Partial Discharge measurements on a variety of aerospace components at 1000 and 116 mbar

Operation of aerial inspections vehicles in HVDC environments Part A: Evaluation and mitigation of high electrostatic field impact

Impact of thermal cycling on high voltage coils used in marine generators using FEA methods