Page 49 - 2021 High-Reliability, Harsh Environment Connectivity eBook
P. 49

Managing high voltage (HV) in the air is more complicated than on the ground. HV can ionize surrounding air, which then becomes conductive to produce a corona discharge. The corona effect is responsible for electrical power losses through voids, cavities, and electrical treeing. A self-sustaining ionizing discharge can initiate electrical arcing and potentially ignite a fire. Selecting materials with dielectric properties well-suited to HV conditions — such as corona-resistant polytetrafluoroethylene (PTFE) — is one way to effectively minimize risks of corona discharge due to insulation breakdown.
2. Critical voltage stresses require techniques that protect against different voltage gradients.
The selection of techniques used to protect conductor parts depends on the characteristics and strength (voltage gradient) of the electrical field around the conductors. Connectors that use conductors with large radii and designs that avoid sharp points and edges help minimize air ionization with relatively low-energy corona effects. To protect against air ionization breaking down dielectric between pins, each pair of conductor pins must maintain an adequate clearance distance as measured through air. These values will change as altitude and temperatures change. Electric discharges can also occur on or close to the insulation surface. A localized, partially conductive path on the insulation surface is called arc tracking. To minimize electric discharges along the insulation’s exterior, each pair of conductive parts — including the binding surface of equipment — must maintain adequate creepage distance as measured on the insulation surface. Creepage distance is typically equal to or larger than clearance distance. Minimum creepage distance can be determined by the insulation’s Comparative Tracking Index (CTI).
» HV connectors protect adjacent pins from arcing by using a male insulation ring that surrounds the female part prior to electrical contact.
3. Avoiding arc tracking damage helps minimize breaches in insulation that can create electrical arcing between wires.
Current on the outer surface of polymeric insulators has the ability to create carbon tracks that damage insulation. Carbon tracks can cause the insulator to lose its dielectric properties and instead become an electric transmitter. Electrical arcing can then occur across the conductive path, resulting in power loss with a high probability of ignition. As previously noted, it is critical to maintain a proper creepage distance and use insulation materials that maintain their dielectric properties in HV conditions.
4. Handling high network operating voltages (>3kV DC) requires the selection of relays and contactors that can withstand extreme electrical stress.
Higher network operating voltages impose significant demands on relays and contactors used for propulsion motor power switching, battery charging management, comfort heating for passengers, and other auxiliary functions. HV relays and contactors are available to meet demanding peak load capacity, operating temperature, coil efficiency, short-circuit protection, breaking capacity, and other critical requirements, such as robust shock and vibration performance.

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