DISTRIBUTED Electric PROPULSION
FREQUENTLY-ASKED QUESTIONS
What is the difference between “breakdown voltage,” “DWV,” and “voltage rating?” Breakdown voltage is the voltage required to cause failure and permanent damage to electrical equipment. Dielectric withstanding voltage, or DWV, is a proof- test voltage performed to ensure the electrical equipment is free from defects. Voltage rating, AKA working voltage, is the operating voltage of the equipment. To provide safety margin, there is always a difference between operating voltage and DWV test voltages. In practice, Glenair provides DWV test data for our interconnect technologies to ensure defect- free material makeup, but it is understood to be the customer’s ultimate responsibility to derate DWV according to their exact requirements. For situations in which an actual qualified voltage rating is essential—again as opposed to a DWV test voltage—additional testing and evaluation may be completed by Glenair. What does “current rating” mean? Current rating is the maximum allowed current before failure, often defined as 42.8°C temperature rise for a single conductor at sea-level. It is acceptable to use a conductor for currents greater
than the current rating if the system can tolerate the corresponding temperature rise. However, increased altitude and wire bundle size will reduce performance, and should be derated accordingly. Any application requiring currents in excess of the standard current rating should be carefully evaluated on an application-by-application basis. Glenair engineering may assist evaluation in accordance with SAE AS50881. What is partial discharge, and how does it affect performance? Partial discharge is localized breakdown within the insulation which does not fully bridge between conductors. Small defects within the insulation, such as air voids or contaminants, are the primary cause. While partial discharge does not result in immediate failure, it will accelerate insulation aging and shorten the interconnect life expectancy. Partial discharge takes many forms, including void discharge, surface discharge, corona discharge, and treeing.
How does altitude affect voltage performance?
In general, clearances between components in connector insulation are filled with air. The air pockets are electrically insulating, acting as virtual “components” within the insulation system. Increased altitude reduces air pressure, which reduces the electrical (dielectric) strength. As the air’s dielectric strength is reduced, the total combined strength of the insulation system is reduced as well. How does source frequency affect voltage performance? Increasing the frequency of the applied power source increases stress on the insulation, reducing the breakdown voltage (and therefore DWV and voltage rating) of the interconnect. Direct current (DC) sources are the least stressful on insulation, allowing much higher voltages than alternating current (AC). The interconnect testing standard is power-frequency AC (50/60 Hz). Increasing frequency will reduce strength and the interconnect should be derated accordingly. Any application requiring high frequencies (>800 Hz) or optimized DC performance when only AC DWV is known should be carefully analyzed for safe performance.
Corona Discharge on a 500 kV power line Photograph by Nitromethane via Wikipedia
QwikConnect • July 2021
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