The influence of the test methods on the Partial Discharge Inception Voltage value of the mineral oil using the needle — Plane electrode configuration

The aim of this paper is to describe the study of the test procedureś effect on Partial Discharge Inception Voltage (PDIV) of mineral oil. This paper represents the experimental investigation of the PDIV of the mineral oil, Nynas 4000x, with the water content less than 10 ppm. The PDIV test was experimented by using tungsten needle electrodes with the tip radius of 10 μm, 20 μm, and 40 μm respectively as the high voltage electrode while the brass plane electrode with 75 mm diameter was used as the grounded electrode. The gap distance of the electrode system was fixed at 50 mm. The test experiment was set up according to IEC 60270. Five types of the PDIV test procedures were studied as the following: The first test procedure was performed in accordance with IEC 61294. Each needle was tested ten times in succession. Three needles of each needle tip radius were tested. In the second test procedure, the test voltage was applied to the electrode system with a rate of 1 kV/s from 0 to 65% of the PDIV value obtained from the first method. Then, the test voltage was increased in steps with 1 kV per step with a step duration of 1 minute until PDIV, the first voltage that PD ≥100 pC, was detected. Each needle was tested five times in succession. Three needles of each needle tip radius were tested. In the third test procedure, the test procedure was relatively similar to the second procedure except the step duration was adjusted to 5 minutes. In the fourth procedure, the test procedure was relatively similar to the second procedure except the test voltage was increased in steps with 1 kV per step with a step duration of 1 minute until the first PD pulse was detected; at this voltage level, the test voltage was recorded as the PDIV. In the fifth procedure, the test procedure was relatively similar to the fourth procedure except the step duration was adjusted to 5 minutes. The PDIV mean value of each electrode system obtained from each test procedure was calculated. It was found that the PDIV value depended strongly on the test procedure and the needle tip radius. The PDIV obtained from the first test procedure gave the highest PDIV which was higher than the PDIV value from the second, the third, the fourth and the fifth PDIV test method about 8%-18%, 12%-24%, 20%-28%, 36%-41%, respectively. The absolute peak value of PDIV pulse current was about 0.4-5 mA. PDIV pulse current amplitudes depended on the needle tip radius as well. The test results elucidate that the procedure for applying the test voltage and the stressed voltage - time interval firmly affect the PDIV of the insulation system.