Epoxy insulation surface discharge characteristics of metal defect in SF6/N2 gas mixtures under AC/SI superimposed voltages

The combined and composite voltages test is a test method recommended by IEC 60060-1. The combined and composite voltages used in this paper is the alternating current and switching impulse (AC/SI) superimposed voltages. This paper investigates the epoxy insulation surface discharge characteristics of metal defect in SF6/N2 mixture under AC/SI superimposed voltages. The discharge characteristics under SI and AC conditions in AC/SI superimposed voltages are analyzed. The research results are as follows. First, the partial discharge (PD) excitation characteristics under AC/SI superimposed voltages are influenced by the superimposition phase and SI polarity. Under the alternating current and positive switching impulse (AC/PSI) superimposed voltages, the phase range for exciting subsequent AC discharge is 0°∼180°, and under the alternating current and negative switching impulse (AC/NSI) superimposed voltages, th7e phase range for exciting subsequent AC discharge is 180°∼360°. Additionally, the discharge phenomenon under the AC/NSI superimposed voltages is more obvious. Secondly, During the SI wave tail under AC/SI superimposed voltages, there exists a phenomenon of reverse discharge, which is influenced by the superimposition phase. Under AC/PSI superimposed voltages, reverse discharge occurs when the superimposition phase ranges from 90° to 180°, while under AC/NSI superimposed voltages, it occurs when the superimposed phase ranges from 270° to 360°. Finally, A different discharge phenomenon, breakdown discharge, has been observed under both the SI wave front and the AC condition. When the breakdown discharge occurs during the SI wave front, the number of PDs during the subsequent AC voltage increases significantly.