How to design drone power tether system? (high power with ultra light cable)
Let's check out their website:

They seem to use several solutions:


High voltage DC with motors in series (that's kinda crappy)
High voltage DC with voltage "regulators"
High voltage high-frequency AC


This last one is interesting. They use 7 kHz AC high voltage on the cable, and a tiny transformer (due to the high frequency) in the drone.

The choice between DC and AC would mostly depend on which results in the minimum weight converter in the drone to get the desired power.

So, AC on the cable allows to use just a transformer and rectifiers in the drone, but you can get skin effect problems on the cable. I guess that's why they chose 7 kHz and not higher. It would result in a rather small transformer...

DC would require a converter in the drone. The lightest would probably be a resonant one. Some weight can be saved by downsizing the input caps.

I wonder why they didn't just use a balloon though.