Effect of Temperature on Main Partial Discharges Phenomena Under DC Voltage Stress

This work is aimed at characterizing the main partial discharges phenomena under HVDC stress and with the presence of thermal gradient. Specifically, the increase in discharge activity without changes in applied voltage is investigated. Even the supply and disconnection phases are excluded from the analyzed acquisition range. In this way, any variation in discharge activity can be attributed only to thermal effects. A setup consisting of a pair of specimens connected in series and supplied with a voltage of 20 kV DC has been used for the tests. One specimen is defect-free and immersed in a tank containing silicone oil. The other is designed to give rise to certain discharge phenomenon. Three tests were carried out using specimens for internal, surface, and corona discharges. For each test, two acquisitions, lasting 30 minutes, have been made. One with both specimens at room temperature and another with the healthy specimen heated through a resistive element in order to obtain a discrete thermal gradient. Thus, a total of 6 acquisitions have been made and analyzed. The results show that for all three specimens the discharge activity undergoes a significant increase in the transition from the roomtemperature condition to the thermal gradient condition. The increase in the amount of detected discharge is greater in tests with corona and surface specimens. In general, an increase in average discharge amplitude is also observed. The observed changes can be attributed to a different distribution of the applied voltage between the two specimens due to changes in the conductivity of the defect-free specimen caused by heating. This behavior is also found in variations in the electric field profile in HVDC applications due to the presence of heat sources that generate thermal gradients. For example, load currents for cables.