SEM and EDX analysis of the needle — Plane and the rod — Plane electrodes for the Partial Discharge Inception Voltage measurement and the arcing test of the mineral oil

The aim of this paper is to describe the Scanning Electron Microscope (SEM) and the Energy Dispersive X-Ray (EDX) test results of the original and tested electrodes used for Partial Discharge Inception Voltage (PDIV) measurement, as well as for the arcing test of mineral oil. The experimental investigations were performed with the mineral oil, Nynas 4000x, with the water content not more than 10 ppm under room temperature. For the PDIV test, the tungsten needle electrodes with the tip radius of 10μm, 20μm, and 40μm respectively were used as the high voltage electrode while the brass plane electrode with 75 mm diameter was used as the grounded electrode with a gap distance of 50 mm. The test experiment was set up according to IEC 60270. The test procedure was performed in accordance with IEC 61294. For the arcing test, the tungsten rod electrodes with the tip diameter of 1 mm, and 2 mm with the curvature of 0.2 mm were used as the high voltage electrode while the brass plane electrode of 75 mm diameter as the grounded electrode with the gap distance of 0.3 mm and 0.8 mm respectively. The test experiment was modified from IEC 60156. The arcing current density was in the range of 5-54 A/cm². The erosion of electrodes used for the oil testing was examined by SEM techniques. SE images, BSE images and EDX spectrum of the original and tested electrodes were produced. The topography, the morphology, and the EDX spectra of the examined electrodes are analyzed. From the test results, there is no evidence to show the erosion of the electrodes after they were used for PDIV and arcing tests. It can also be argued that the investigated tungsten needles, rods and brass plane electrodes can be used for PDIV testing and for the arcing test without the problem of erosion. In addition, carbon is the main contaminant created at the surface of the tested electrodes. The development of carbon was highly possible from the degradation of the mineral oil.