# What is the best way to transmit 1 kW (power supply) to a tethered drone so that the cable thickness and weight is minimized?

I am delivering 1 kW power to a tethered drone. To minimize the wire weight and maximize maneuverability, I would like to keep the wire as thin and lightweight as possible.

The motors on my drone have a peak current of 70 A. However, a wire capable of supporting 70 A will be pretty thick.

The way around this is to step up the voltage on the ground to say 10 kV, thereby limiting the current to 0.1 A (to deliver 1 kW power). The drone will then have a transformer to step down the voltage along with an AC-DC converter.

From what I understand, the issue with high voltage is the possibility of a dielectric breakdown and arcing. Since I will be running two wires (live/ground) to the drone in a single cable, I will need to insulate the wires sufficiently to prevent breakdown.

How much insulation (say PVC) will I need to handle 10 kV?

Will it make a difference if I transmit DC instead of AC? Which one would be better?

Original question: Calculate the amount of insulation thickness needed for handling V volts across 2 copper wires.

• I would suggest 300-400 V DC, then you can use standard twin mains-rated 3 A flexible cable, and a more or less standard SMPS down to your required 14 V, which will be the lightest way to transform 1 kW. You can't lighten the cable down to 100 mA, it would not be robust enough to survive handling, and the 10 kV would be hard work to transform. Oct 9, 2022 at 15:39
• Even ignoring the wires, a power supply capable of stepping down 10 kV to something reasonable for your application will be far too bulky and heavy for a drone. Not to mention that things rated for handling 10 kV are expensive. Oct 9, 2022 at 15:43
• The reason for the high cost is in large part due to insulation and creepage requirements, but also due to the fact that very few people need it, because the average engineer would avoid working with voltages that high whenever possible. And with good reason; not only is it expensive and difficult, it's dangerous. Oct 9, 2022 at 15:52
• @PragyAgarwal A bachelor's program in electrical engineering? Oct 9, 2022 at 15:56
• Switching from 2 to 3 wires to use a 3 phase supply doubles the amount of power you can transmit over the same weight of metal. In a scenario like this where cable weight is critical, you should seriously consider this option. And 240v 3 phase AC is a standard off the shelf option. Oct 9, 2022 at 16:28

The idea is not to touch high voltage with semiconductors.

Instead, generate three phases of low-voltage high current 20kHz square wave, using H bridges. Use at least 120VDC as the input supply voltage. That feeds three step-up high frequency transformers loaded for resonance tuned at 20kHz. You want to suppress the harmonics since they'll suffer heavy losses in the long wires.

Connect the secondary neutrals together, and feed the three phases only (no neutral) up the tether in a delta configuration. Those can be three separate hook-up wires twisted together - or even hanging free, for better air cooling. Target about 1A current per phase maximum, AWG 20 wire, 1kV or less AC voltage (that's 2.8kVp-p and it's what the insulation in a cable would have to withstand).

For AWG 20 or 0.5mm2 wire size, 20kHz is about the maximum to keep the conduction across the entire cross section of the wire and not suffer from skin effect losses.

On the top, use three single-phase transformers in a delta-star arrangement to step down the voltage, and three half H-bridge synchronous rectifiers to feed the DC link capacitor. The DC link is the DC source for the BLDC inverters etc.

You'll be wasting some energy in the wire, but that can be acceptable since the wire is hanging in free air. To keep the wire cool, the spool on the ground has to be a very long drum with forced air cooling through the inside of the drum, and only a single wire layer. You'll most likely need three drums in parallel, one for each phase.

AWG 20 hook up wire is less than 1kg per 100m, so for three phases you'll need say 2kg drone load capacity for each 100m of tether length.

I'll try to provide an answer, although it consists of questions and suggestions. The title of this question should probably be "What is the most efficient and practical means for delivering 1 kW power to a tethered drone". There should be a "sweet spot" combination of wire gauge, insulation, and voltage. Perhaps look at test leads typically rated 5 kV, and rather flexible. They often use silicone insulation, which is very flexible.

I agree with comments suggesting 3 phase power at high frequency, and it should be possible to use commonly available ferrite cores and bobbins to make a small three phase transformer. It might be easier to use two separate transformers.

If efficiency is not a major issue, it will probably be best to match the impedance of the cable to the load, so that a 2 kW ground station could deliver 1 kW to the drone, "wasting" 1 kW in the tether. In free air, with the heat distributed along a considerable length, heating should not cause a problem. I would suggest using 1 kV three phase which would result in 500 V phase to phase at the drone, with a current of about 1.2 amps per phase. Use the length of the tether to determine the wire size that will drop 500 V over its length. It might even be possible to use aluminum conductors, although perhaps with some steel strands (or maybe Kevlar) for strength.

You can probably find off-the-shelf 480 VAC (720 VDC) switching supplies or DC-DC converters rated 500 watts, and bypass the input rectifiers and H-bridge, using one phase of the three phase supply.

A quick search for "flexible thin high voltage silicone wire" came up with this supplier, and the smallest available is #30 AWG rated 3 kV, and 1 mm diameter. It has about 0.1 ohm/foot resistance, so for 1000 ft, it's 100 ohms, which is a drop of about 120 volts at 1.2 amps. That's just about 10% for a 1200 volt supply. Weight of the copper is about 0.0003 lb/ft, so for three wires a 1000 ft tether would weigh about 1 pound. You can figure the weight of the silicone insulation knowing the total volume and density of silicone.

Silicone has a density of about 100 lb/ft^3, or 2 Mg/m^3 and a 1 mm diameter wire 1000 m long (for 3 strands) is 78 cm^3 or 0.00275 cubic feet. So weight would be about 0.275 lb or 124 grams.

• Why would you want to waste a kW in the transmission? You can probably do a lot better than that without losing too much power. Oct 9, 2022 at 22:13
• The primary advantage to inefficiency in transfer is allow you to use smaller conductors, with a higher voltage drop/unit length. Exactly where that sweet spot will lie will depend on the length of the tether, temperature rating of the insulation, and current required, vs voltage used. It may potentially make sense to operate the tether at up to 70C at peak load. Oct 9, 2022 at 22:26