## 2 Performance of a Correct Thermography
### Factors Influencing Thermographic Inspection  
To carry out a correct thermography inspection, a few factors that influence the accuracy of the results obtained must be considered. These factors can be procedural, technical, and environmental [28]. The procedural factors are those that are a consequenceof the thermography inspection. It must be performed by a qualified operator with sufficient experience [29]. The most important technical factors are the emissivity of the inspected elements, the current that circulates through the circuit, the distance to the target element, and the specifications of the camera used. Regarding environmental factors, the most influential are the ambient temperature, rain, the wind, and the solar radiation [30]. With respect to technical factors, a sufficiently precise camera must be used to perform the thermography at a certain distance. It must be borne in mind that the operator cannot approach the elements to which thermography must be performed, as they are energized equipment located at a certain height. For this reason, a thermographic camera with a lens that allows sufficient resolution and image increase should be used. The resolution will allow sufficient image quality and precision to detect the hot spots at the safe distances to which the thermography must be made. In addition, thermal imaging cameras compensate for these factors to provide the correct values of the measurements. To do this, it is necessary to introduce the required parameters with values that are as close as possible to reality, since otherwise the results obtained may differ greatly from the actual values measured. It must be considered that an exact emissivity value for the equipment inspected in a substation cannot be obtained. This is because of the presence of very diverse elements whose emissivity will change over time as it ages, until it stabilizes. In addition, they are equipment subjected to high voltage, and, for that reason, they are not normally accessible. Therefore, it is not possible to use any of the methodologies that allow for obtaining an exact value. Furthermore, it would be necessary to obtain this value for each element, and for each time the thermography inspection was to be carried out (which can happen several times a year). For this reason, when thermography inspections of all substations of an electrical utility are to be carried out, it is impossible to carry out all the preparations prior to the inspection. In fact, thermography cameras have an emissivity table for the operator to include the exact value of the element, if known, or assign the value of the group to which it can be associated, such as matte, semi-matte, semi-matte glossy or glossy. Wind is one of the environmental factors that influence the measurement during thermography inspections. To correct its influence, methods that manage to obtain the exact value have been designed. However, some of them are not applicable in substations. Other weather conditions, such as rain, hail, fog, or storm, are not considered in this work. The reason is that substations are facilities with energized equipment in a delimited area and subject to safety distances. For this reason, and due to safety reasons, when those atmospheric conditions exist, maintenance work cannot be carried out. Therefore, thermographic inspections cannot be carried out under such conditions. International standards [31; 32] include parameters with conservative values that are recommended for use. Subsequently, these values are included in the procedures of each of the electrical utilities. Although these standards are not specifically for substations, their values apply to them because they are defined for electrical equipment subjected to similar conditions. Regarding the value of the emissivity, although 0.5 values have been used in North America, these values have subsequently changed to values between 0.7 and 0.9, finally setting at the same value recommended in [32]: 0.8.