# Assessment of Partial Discharges in the Air by Application of Corona Camera
## 1 Introduction  
The detection, measurement and location of partial discharges (PD) without interfering in the normal operation of high voltage power equipment provides considerable diagnostic opportunities. This applies particularly to the assessment of their insulation systems' performance. Early detection and analysis of PDs offers an early identification of faults that have developed in insulation systems [1, 2, 3].  
The use of specific diagnostic methods for PD detection depends on whether there are discharges occurring inside the insulation or discharges occurring in the air or on the surface of the insulation systems [4, 5, 6, 7, 8]. For detection and measurement purposes involving internal PDs, electrical methods are appropriate, consisting mainly of measuring their apparent charge Q\({}_{\text{p}}\)[9, 10]. The acoustic method is also used, based on the measurement of the acoustic signal generated by the PDs [11, 12, 13, 14] and the chromatographic method (DGA) that is applied to identify by-products of insulation failure resulting from the effects of PDs [15, 16, 17, 18]. These methods are mainly used for diagnostics of faults found in the isolation of large power transformers and high voltage cables.  
The second group of PDs involves examples that occur in the air or on the surfaces of solid dielectrics in the gas environment. Examples of installations that may have this type of PD include power line insulators and numerous spark gaps installed in power stations. In the insulation systems of these devices, there is always a non-uniform distribution of the electric field, which in turn is related to the occurrence of PDs [19]. This issue also applies in power circuits such as power modules [20].  
Research is also being conducted into a method of estimating the level of PDs on the basis of radiometric measurements and linking the emitted power to the intensity of registered PDs [21]. Appropriate algorithms for locating PD sources based on analysis of the received signal strength are also being developed [22].  
The classic parameters characterizing PDs include their characteristic voltages: corona voltage V\({}_{\rm o}\), sparking voltage V\({}_{\rm s}\). and breakdown voltage V\({}_{\rm b}\). However, the visual and acoustic methods of determining these voltages are inaccurate, as they depend on a variety of external factors; therefore, they can be applied only for approximate technical evaluation of diagnosed insulation systems [23].  
Use of a UV camera is increasingly important in monitoring various types of environmental parameters, e.g., in atmospheric science, volcanology and surface smoothness measurements [24; 25; 26]. Dual-band imaging in visible light and UV range can also detect and locate corona discharge sites [27]. For these reasons, UV cameras are increasingly being employed in the diagnostics of air insulation systems for the precise and objective determination of these voltages. In the literature, they are also referred to as corona cameras [28; 29; 30]. They can be employed for the detection of PDs occurring in high-voltage elements of power equipment throughout their normal operation. UV cameras also allow the assessment of the levels of PDs expressed in terms of the registered count of UV impulses.  
Additionally, mobile solutions with a UV camera for the inspection of power lines, which are based on unmanned aerial vehicles (UAVs), are already being developed and implemented [31; 32].  
Unfortunately, the number of UV impulses registered in this way, which characterize the PDs, depends not only on the intensity of discharges but also on external conditions. The list of them includes:  
* adopted level of signal gain set on a camera,
* distance between the camera and source of partial discharge.  
The identification of the impact of these factors on the recorded values of UV emissions forms is the subject area discussed in this paper.