# A Novel Charging Station on Overhead Power Lines for Autonomous Unmanned Drones
## 4 Results
### Energy Harvesting  
Experimental verification was conducted for energy-harvesting technology for aerial drone applications along an overhead power line. The charging station's characteristics were analyzed using the test bench depicted in Figure 10. Testing revealed that the alignment of the core and the gap between the two halves have a significant impact on harvesting capacity. The test bench replicates an ideal scenario with optimal alignment and force to close the core with minimal impact from any resulting gaps. Table 3 defines the energy harvester designed for the charging station and its electronic power specifications. The test bench (Figure 10) is fitted with an autotransformer to control the current induced into the primary coil by the current injector. The charging station harvests energy from the magnetic field, and a rectifier converts induced AC to DC. An isolated DC/DC charger with MPPT control delivers the energy to the battery. A wide range of 50 Hz line current points was generated to assess the amount of energy that can be harvested by the drone charging station, as well as the efficiency of the power electronics (Table 4, Figure 10). \begin{table}
\begin{tabular}{c c c} \hline
**Power Line** & **Value** & **Unit** \\ \hline Current & 0–600 & A \\ Frequency & 50 & Hz \\ \hline Core & Value & Unit \\ \hline Weight & 1089 & g \\ Material & Grain-oriented silicon steel & \\ Width & 40 & mm \\ Height & 45 & mm \\ Length & 60 & mm \\ Thickness & 12 & mm \\ \hline Coil & Value & Unit \\ \hline Turns & 60 & \\ Material & Copper & \\ \hline MPPT Converter & Value & Unit \\ \hline Input voltage & 16–35 & V \\ Output voltage & 20–30 & V \\ Nominal current & 12 & A \\ \hline Battery Pack & Value & Unit \\ \hline Voltage & 24 & V \\ Capacity & 40 & Ah \\ Chemicals & Pb–acid & \\ \hline \end{tabular}
\end{table} 
Table 3: Charging station energy harvester specifications. Figure 10: Laboratory energy-harvesting test bench. The DC/DC converter starts working from 35 A on the power line, providing current to charge the battery. The harvested AC power is rectified, then adjusted to the battery voltage. The MPPT controller adjusts the load impedance to deliver the maximum available power from the energy harvester to the battery under certain conditions. The charging test was conducted during the bulk phase, when the current in the charger is not limited by the battery capacity. Thus, the battery starts charging at 35 A on the AC line. Figure 11 shows a linear dependence between the line current and the energy harvested, which increases linearly from 5 W harvested at 35.6 A to a maximum of 148 W harvested at 606 A on the line. Furthermore, due to non-linearities in the power electronics, the efficiency is lower at lower power levels. However, it can be considered as constant and equal to 80% in the range of 150-600 A AC line current. Efficiency is determined by the rectifier and the DC/DC converter. The passive diode-based rectifier generates a voltage drop, and the DC/DC buck/boost efficiency varies with input and output current and voltage. The overall electronic efficiency is close to 80%. These power losses, of which 42% are attributable to the rectifier stage and almost are attributable 58% to the DC/DC converter, result in efficiencies of 91% and 86%, respectively for these stages. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c}  
Thus, an average 500 Wh battery would be charged from 20% to 80% in just 2 h using the maximum power point of 148 W at 600 A. This charging time may be acceptable for certain maintenance operations or fully autonomous tasks of the UAV. However, there are other tasks that require coordination with operators or other elements for which this charging time may not be feasible. To minimize or even eliminate these charging times, inductive charging could be combined with battery swapping, or a small stationary battery could be incorporated into the charging station.