DISTRIBUTED ELECTRIC PROPULSION [CONTINUED]
• Void Discharge – partial discharges that occur in air voids within an insulation system. The voids can be created either by defect (an air bubble in a molded insulator for example) or by design (clearance zone between contact and insulator cavity). Internal discharges ionize the air, creating byproducts such as ozone, and release UV energy, which causes chemical breakdown of the insulator. • Surface Discharge – partial discharges on the surface of an insulator. Naturally occurring contaminants deposited on the surface of an insulator create a conductive path. Upon conduction, the insulator carbonizes (or “burns”), creating a permanently conductive path. • Corona Discharge – partial discharges which occur in air when the electric field created by a high-voltage conductor is higher than the strength of the air. Similar to void discharges, corona discharge creates byproducts, such as ozone, and in turn releases UV energy, which cause chemical breakdown of nearby insulators. • Treeing – a secondary effect of internal discharges created by void discharge, conductive impurities, or defects, which lead to insulation damage. This is similar to surface discharges, but internal to the insulator. As treeing progresses, a branched network of conductive paths is permanently created within the insulation. Partial discharge testing is an essential performance requirement for airborne applications that, again,
Partial Discharge Partial discharge (PD) in electrical wire interconnect systems refers to a localized breakdown of insulation materials, which does not result in catastrophic failure degrading the insulation between conductors. Simply put, PD is small yet measurable micro-failures that leave the insulation intact, but over time may age and reduce the life of the interconnect system. Partial discharge is a critical issue in advanced eVTOL airborne power applications, given the role of the cabling in distributed electrical propulsion applications, and the higher voltages, current levels, and frequencies typically carried by such transmission lines. Here’s a useful metaphor to illustrate this point, once again using a rope: for a fibrous load-bearing rope, when the load is near breaking strength, individual fibers may fail. Initially, loss of one or two fibers would be negligible, and the rope would continue to support the load. However, over time, as more fibers fail, the accumulative effect would eventually compromise the overall integrity of the rope, and total failure would occur. Similarly, individual partial discharges do not cause immediate failure, but will erode the insulation and eventually lead to failure—a condition that is absolutely intolerable for any airborne system. In terms of FAA and other agency qualification of eVTOL aircraft with distributed electrical power, there are several types of partial discharge to consider:
handle high voltage and/ or high current electrical energy. Frequency Effects Airborne power distribution systems use a wide range of frequencies for various applications. Battery banks provide DC power. In more-electric aircraft (MEA), generators produce power at 400 Hz AC. In all-electric aircraft (AEA), variable- frequency drives (VFD) powering electric motors
QwikConnect • July 2021
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