## 4 USPVPs Control for Applying EL Technique
### Complete System Control Architecture  
In order to perform the EL inspection in USPVPs, a higher automatisation control level should be achieved. On the one hand, it is mandatory to have a system that is capable of taking the images automatically. Nowadays, due to its autonomy and profitability, unmanned aerial vehicles are the most studied inspection system [58; 59], and several studies are focused on the applications of drones to the autonomous monitoring of large-scale PV plants [60]. On the other hand, the direct polarisation  
Figure 3: Main control diagram of a photovoltaic (PV) inverter controller: in blue are marked the high-level control blocks related to the maximum power point tracking (MPPT) and electroluminescence (EL) evaluation working modes.  
of a full USPVP has an energetic consumption so high that it is economically unfeasible. The solution to this problem is to divide the full plant into smaller areas, composed of one or several PV strings whose polarisation power requirements were viable, so that a single area is energised at the same time. To get this, it will be necessary to replace the isolators, located in the combiner boxes, with lower power remote controlled switches.  
The automatisation level discussed above requires a real-time control to be able to complete the inspections as efficiently as possible. Automatic inspections require a control node capable of synchronising the image capture with the sequencing of the strings. The control node needs a communication system to receive images and telemetry from the camera, send the activation and deactivation signals to the breakers and send the current set-point to the inverter, as Figure 4 shows.  
The whole inspection process diagram carried out by the control node to analyse the full plant is shown in Figure 5. First, the inspection order is received, and the control node requests the camera to be in the right position and location. If the camera is near to an un-inspected area, the control node determines the inverter connected to these panels. Second, the selected inverter is stopped, set in EL mode, and all the switches are opened in order to isolate the PV strings from the inverter. In a third place, the camera position is determined accurately and, therefore, the panels that appears in the image. Then, it is possible to close the switch connected to this string and send the current set-point to the inverter. When the camera obtains the images of the entire area connected to the close breaker, the inverter stops the current injection and the switch is opened again, a new string is chosen and the process is repeated. If all the panels connected to this inverter are inspected, every switch is closed and the inverter is restarted in solar inverter mode. When the entire plant is analysed the inspection finishes; otherwise, the camera moves to a new inverter area and the process runs again.  
The hardware plant modifications proposed here, like the AC grid pre-charge system, add on the possibility of new functionalities in the PV plant, for example, reactive power injection at night [61; 62; 63].  
Figure 4: Proposed communication scheme for the implementation of EL maintenance technique in USPVPs.