# A Novel Charging Station on Overhead Power Lines for Autonomous Unmanned Drones
## 2 Drone Charging Station Design and Development
### Energy Harvester  
The charging station harvests energy from the alternating magnetic field around the overhead power line. This magnetic field is generated by the current flowing through the conductor. The harvester converts the present magnetic energy into electromotive force to power electronic devices or batteries. In order to achieve a reasonable tradeoff between weight and energy harvested, the energy harvesting charging station was designed to deliver up to 150 watts. The harvester (Figure 2) consists of:  
* A ferromagnetic core, which provides a path that concentrates the magnetic flux;
* The primary winding, which is actually the power line, powered by an AC source;
* A coil wound on the core (the secondary winding), which induces the current and receives energy from the primary winding to deliver it to the load. In addition, an auxiliary battery can be connected to decouple the harvested energy from the charging process, which stores energy to ensure a fast charging process. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Aircraft** & **Value** & **Unit** \\ \hline Weight & \(\approx\)15 & kg \\ Payload & \(<\)10 & kg \\ Flight time & 10–50 & min \\ (payload-dependent) & Battery capacity & 250–1000 & Wh \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Professional unmanned aerial vehicle reference specifications. Figure 2: Energy harvester with split core, coil and power line. The materials used in the ferromagnetic core should have high permeability to the 50/60 Hz magnetic field to provide a high level of magnetic induction. They should also have low hysteresis loss to prevent overheating and minimize energy loss. Owing to its high performance, permeability, price and manufacturing flexibility, a grain-oriented silicon steel material was selected for the ferromagnetic core. The core is formed into thin sheets, which are coated to provide high resistance between them to minimize losses due to eddy currents. The core shape was defined to improve magnetic coupling while minimizing overall weight and utilizing the window area for winding placement (patent pending EP 3 605 793 A1). The power cable constitutes the primary winding, which is formed by a single turn, typically using a large-diameter cable. The core is designed with a sufficiently large window area to accommodate a variety of commercial power cables. Conversely, the secondary coil uses an enameled copper wire. The cable diameter was selected to carry the secondary short-circuit current, considering the possibility of its temperature increasing due to the power line overheating. The amount of energy harvested is not affected by the number of turns in the secondary coil; hence, the number of turns was selected to match the induced voltage and the drone's battery voltage over the current range of the power line. A split core was selected to ease the installation of the harvester on the power line. U-shaped cores were determined to optimize inductive coupling while minimizing the window area. A core model derived from [PERSON]'s equations was used for the mathematical representation of the harvester. The primary side current in the overhead power line is denoted as \(I_{p}(t)\) and assuming sinusoidal AC and a secondary side current (\(I_{s}(t)\)) in a coil with \(N_{s}\) turns, the magnetomotive force measured in ampere-turns as seen by the core is expressed as:  
\[AT_{core}(t)=I_{p}\text{sin}(wt)-N_{s}I_{s}(t) \tag{1}\]  
Then the magnetic field strength (\(H(r,t)\)) at a radial distance of r away from the center of the overhead power line is expressed as:  
\[H(r,t)=\frac{AT_{core}(t)}{2\pi\text{r}}=\frac{I_{p}\text{sin}(wt)-N_{s}I_{s}(t )}{2\pi\text{r}} \tag{2}\]  
The magnetic flux density (\(B(r,t)\)) is determined by the \(B\)-\(H\) curve of the silicon steel core. An Arctan function was used to model the \(B\)-\(H\) curve, where \(\text{B}_{sat}\) denotes the saturation threshold of the ferromagnetic material, and \(\alpha\) is a curve-fitting parameter.