# Assessment of Partial Discharges in the Air by Application of Corona Camera
## 2 Measurement System, Characteristics of UV Cameras, Measurement Range  
Measurements were conducted in the Laboratory of High Voltages using a standard high voltage cell. The experimental system comprised a control panel and a single-phase test transformer with an upper voltage of 110 kV. The test transformer also included a protective resistor. PDs were generated on prepared spark gaps, which consisted of a system of two electrodes. Spark gap was supplied with alternating voltage (AC). The voltage supplying the spark gap was measured on the high voltage side using an electrostatic voltmeter. All measurements were taken and recorded in daylight, without obscuring the area of the laboratory. The UV DayCor Superb UV camera was applied for the measurement and registration of ultraviolet radiation originating from the formed PDs (Figure 1). The camera was installed on a tripod. Its distance from the source was controlled in accordance with the adopted course of the measurement schedule. This camera can be employed for monitoring of PDs found on insulators, in intermediate, high and ultra-high voltage power lines. The measurement principle is based on the use of the count of total of impulses generated by light detectors. The camera has built-in detectors that register photons within two radiation ranges: UV (250-280) nm and in the visible light range (380-780) nm. The camera counts the number of photons emitted by PDs and as a result displays the average value of the number of counts over a period of about 5 s. The gain parameter can be adjusted from 0 to 250 with gradation in steps of 10. The camera software carries out an overlap of the count with the number of photons in the UV radiation range on the photograph with the image of the source of PDs. The photo is taken using a standard CCD matrix that is sensitive to visible light. After merging both images, a precise image with the intensity of PDs is created that is visible on the examined sources of PDs. The second part of the image takes the form of a photograph with the examined object in the daylight. The camera lens and optical filter used in the camera are optimized to visualize the effect of PDs, along with the surrounding background in the visible light. The images of phenomena occurring during measurements are displayed online on a color LCD screen, following by recording them and a subsequent analysis carried out in order to interpret the phenomena. Figure 2 shows a photo of the camera screen with the recorded image of signals generated on the basis of the emission spectra of PDs. The signals originate from the PDs generated in the needle--needle spark gap. Table 1 contains a summary of the principal technical parameters of the DayCor Superb camera that were applied throughout the measurements. The measurements involved emission spectra of PDs generated in two types of spark gaps: needle-needle and needle-plate. Three types of thin electrodes with the diameters of 0.23 mm, 0.40 mm and 0.50 mm and a spark gap with conical electrodes with a needle angle of \(\alpha\) = 20\({}^{\circ}\) were applied in the needle spark gaps. The electrode systems adopted for the study generate non-uniform distribution of the electric field. Needle-needle electrode systems can be adopted as a point fault model and a needle-plate system can model a point fault with a grounded surface (e.g., in the case of a high voltage device). Figure 1: Diagram of the measuring system. Figure 2: Image from camera screen with emission spectra signals originating from PDs: space of PD’s formation (**1**); high voltage electrode (**2**); gain parameter (**3**); grounded electrode (**4**); number of counts (**5**). The measurements involved three types of tests. The first was concerned with the measurement of the number of UV impulses emitted as PDs, and generated in the needle -needle system, as a function of PD generation voltage. For these purposes, thin needle spark gaps with three diameters were applied (Figure 3a) in the measurements: 0.23 mm, 0.40 mm and 0.50 mm.