## 1 Introduction  
Energy use worldwide is changing quickly due to the need to mitigate climate change. Renewable energies (RE) have a crucial role in helping the world to meet its energy needs, with the possibility to supply four-fifths of the world's electricity by 2050, massively cutting down carbon emissions. Nowadays, new legislation is promoting the installation of RE power plants to increase their penetration level in the power generation system [1,2]. However, photovoltaic (PV) and wind power have to exploit their full potential, decreasing the installation and maintenance costs of power plants. Currently, the proportion of electricity generation of the European Union (EU) from renewable energy sources has increased from only 12% in 2000 to more than 30% in 2020 [3]. This represents an average growth of 1.7% per year. Moreover, RE power installation worldwide has changed from a 20% of the shared electricity net in 2010, to a value over 30% in 2020, and the forecast is to reach the 50% by year 2050 [4]. In particular, wind and solar plants dominate the growth in renewable energy generation. Nowadays, the reduction of needed investment cost for new utility-scale photovoltaic plants (USPVPs), up to 6 times in respect to one decade ago [5], has placed the PV technology in a competitive scenario regarding other generation sources, both conventional and renewable. This situation is accelerating the rate of PV power penetration into the international power system, turning solar power generation into an important actor in the transformation of the future electric market [6,7]. In addition, in order to substitute conventional power plants, the size and rated power of USPVPs have increased exponentially in the last decade, recently reaching rated powers up to 2.2 GW [8]. The area needed to place USPVPs grows linearly related to the rated power capacity [9], and therefore it is crucial to develop efficient and optimum maintenance techniques for USPVPs, which can cover huge extensions of terrain in a short time at a competitive cost. One of the most promising maintenance techniques in the current state-of-the-art is the evaluation method of PV panels damage through the study of images. This method covers up two different technologies: the study of infrared thermography (IRT) images and the study of electroluminescence (EL) images. On the one hand, the study of IRT images analyses hot and cold spots on the PV cells while they are generating power during the daylight [10]. On the other hand, the study of EL images analyses the PV panels while they are in the direct polarisation working zone, detecting the damaged modules from the emitted spectrum in the non-visible zone [11]. The paper focus on the latter category, which will be explained in detail in the following section. The main objective of this paper is to present a new method to reduce the maintenance costs of EL testing, using the hardware provided by the PV solar inverters and a sequencing of the PV strings to polarise them accordingly. The paper analyses hardware and software used in current and future USPVPs, and points out the required changes in order to apply the developed technique conveniently. Finally, the paper provides a discussion about the expected outcomes and costs of the new method. This paper is organised as follows. First, Section 2 describes the principles of EL technology applied to PV panels and the current maintenance procedure, as well as a comparison with the most commonly used image processing analysis: the IRT technology. After that, Section 3 describes power inverters used currently in USPVPs and how to adapt them to run EL tests. Section 4 introduces the control technique added to the power inverter to induce a direct polarisation of the PV panels, as well as how to synchronise the whole plant in order to perform the EL maintenance action in the proper way. Following this, Sections 5 and 6 show the built prototype, the demo-plant and the results obtained from the experiments.