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   Lightning Strikes on Flygen AWECS 

Lightning strikes are a common well understood threat to radio towers & power lines. Billions of dollars in damage result yearly. The engineering issue applies particularly to high-altitude conductors proposed in flygen AWECS.

The basic tools of mitigation are old- Franklin bypass conductors (lightning rods) & Faraday cages. Additionally, surge arrestors like Metal Oxide Varistors can be placed at key electrical components. Airplanes have special features such as "static wicks" to bleed charge. All means involve trade-offs in weight, cost, & complexity.

In a flygen AWECS the conductive tether is already a lightning rod. Aerostat tethers use a conductive jacket over the tensile, power, & data lines with claimed success, but a flygen is a more marginal opportunity to specially a dedicated shield due to higher airspeed drag & weight criticality. Flygen tether designers using power coax are obliged to consider the outer conductor as the primary lightning current conduit. There will be strong inductive coupling with the center conductor when a strike breeches the thin metallic outer conductor & surface charge then runs inside the outer tube. This will cause new problems. The polymer tensile/insulation component cannot tolerate any real heating while tens of thousands of degrees (C or F) are typical lightning effects.

All critical airborne electrical components require shielding. The multi-flygen AWEC power wiring harness, if uncaged, can act as an antenna network developing high internal voltages & currents, even if not directly struck. MOSFETs as used for motor/gen control are very susceptible to high voltage transients. Delicate avionics & associated sensor nets may need to be completely opto-isolated.

An acute challenge to a flygen AWEC is a strike with coiled turns still on the winch drum. The impedance of the coil backs up discharge & the conductor likely vaporizes, perhaps resulting in a runway kiteplane without motor power supplied from the surface.

When a lightning strike is brought down to an AWEC ground-station, the charge must be passed to ground harmlessly. All conductive structure must be bonded to eliminate killer-ground potentials. The slip-ring conductor may arc-weld itself or maybe fail open, if not designed for the event.

Forced landing during lightning risk eats into capacity factor. Avoiding regions where lightning is common limits market. Certifying lightning fail-safe airworthiness will prove slow & costly. Even successful lightning-strike survival can require flygen AWECSs to be landed & carefully inspected for damage, often as subtle as bit-flip in microelectronics.

Groundgen AWECs face far less lightning hazard as the tethers are nonconducting. Salt residue can build up in a marine application & should be rinsed out. A bypass conductor clamped a short height up a tether will provide good protection to ground equipment & personnel. In the event of a lightning-cased crash, consequences are less severe.


~~ Dave Santos              

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