A significant practical benefit of inverter-based modulation is the ability to take DPL images of all modules that are connected to the same inverter, either sequentially and with high spatial resolution (close-up imaging), as shown in Fig.2, or simultaneously in a single overview image, with reduced spatial resolution. The larger field of view required for an overview image can be achieved either by moving the camera further away from the target or by using a camera objective lens with a shorter focal length. Fig.4 shows a DPL image of the rooftop system that was acquired with a 12.5mm focal length lens. The acquisition of the full roof in a single DPL image could not be achieved in this study, simply due to limitations with respect to the positioning of the camera using our improvised camera mount (i.e. the camera could not be moved further away). The image contains a few artefacts, including the effect of optical reflections of surrounding trees that were moving in the wind (green arrow in Fig.4). Note that reflections of stationary objects do not cause such artefacts, since they cancel out in the image difference calculation. We also observe an artefact associated with the shadow cast by the (shaking) mounting pole and the camera (blue arrow). The latter effect causes the top third of one module to appear darker. The shading effects by the camera can be avoided by more careful placement of the camera, in particular, when images are acquired using an RPA (as demonstrated in the following section). Artefacts associated with objects such as plants, buildings or other architectural features are generally not a concern for large utility-scale systems and may be avoidable in applications on residential PV systems with careful planning and timing of experiments.

Utility-scale System

Utility-scale inverter-based DPL imaging was demonstrated at a 149 MWDC solar farm in Australia in October 2023. The farm became fully operational in March 2021, utilising 400 W monofacial half-cell modules on single-axis trackers. The modules feature PERC solar cells with a nominal open-circuit voltage of 690mV per cell according to the module specifications. The farm uses 2.75 MW SMA central inverters (Sunny Central 2750-EV), with approximately 9,000 modules connected to each inverter. For the proof of concept of large-scale DPL image acquisition using inverter-based modulation reported here, only one 2.75 MW inverter was switched. As a precaution, only <60% of the inverter’s rated DC power was switched at any time (e.g. between 90% and 30% of maximum power) to avoid potential risks of thermal cycling on the power electronics. DPL image acquisition was achieved using the same InGaAs camera (1280 x 1024 pixel resolution) that was used for the data acquired on the rooftop section discussed above. The camera was mounted on a customised DJI M600 hexacopter RPA, the latter running custom image acquisition software on an on-board computer. A Gremsy gimbal was used to stabilise the camera, enabling image acquisition with up to 50 ms exposure time for the individual image without significant loss of image sharpness due to camera shake. Images were acquired using a 50 mm focal length objective lens equipped with the same bandpass filter described above.