## 1 Introduction
### Substation Maintenance  
The assets of the facilities are subject to different types of maintenance. Corrective maintenance is the one used exclusively in low voltage, consisting of repairing or replacing the damaged element, depending on its cost. Preventive maintenance is the one that is carried out every certain interval of time: a series of established activities are carried out on the element in question. In this way, possible defects that are in the early stages can be identified and repaired before they get worse. On the contrary, it has the drawback that, if the equipment is intervened unnecessarily, a defect may be introduced during handling. Condition-based maintenance analyzes the main parameters of the element and acts on it only if it is considered necessary. In this case, many parameters must be monitored, which can be expensive. Reliability-centered maintenance, in addition to considering the condition of the element, takes into account its importance and the economic consequences that the failure may cause.  
Substations, as one of the most critical facilities of electric companies, must be maintained in accordance with the required quality criteria. Within them, batteries, circuit breakers, and power transformers are the most important elements. The former, because they are the ones that feed the protection and telecommunications circuits; the second, because they open and close the energized circuits, with the consequent arc created in these maneuvers; and the latter, because they are the essential components of the substations, since they are responsible for the voltage changes in the circuits, and this is the goal of most substations.  
Battery maintenance is carried out, considering it as a single element due to its small size [7] and analyzing its internal functioning by thermography [8]. Unlike the battery, the circuit breaker is a more complex piece of equipment, so its components are examined in its maintenance: SF\({}_{6}\), which is used as a dielectric to extinguish the arc [9], although other types of nonpolluting compounds are also being studied [10]; the state of its mechanical parts, through speed and pole displacement [11] and coil consumption [12]; or detecting the presence of hot spots [13]. The power transformer is the most complex element of the substation and, in its maintenance, its components are also examined: the state of the oil, both in its main tank [14], or as a consequence of having been contaminated from the on-load tap-changer (OLTC) [15]; its mechanical behavior, both of the main coils [16] and of the OLTC contacts [17]; the existence of partial discharges [18]; detecting turn-to-turn faults [19] and using acoustic methods [20]; looking for the presence of the possible hot spots [21]; or assessing the technical condition of the transformer [22]. A state of the art of maintenance in power transformers is shown in [23].  
The maintenance techniques applied to each piece of equipment are usually specific to each one. However, thermography is common to all. Moreover, it is also used for the maintenance of facilities [24], usually carried out with human resources, although it can be also carried out with fixed cameras [25] or autonomous systems [26]. In addition, when reviewing the maintenance of electrical installations, thermography is one of the most widely used techniques for all equipment and facilities [27].