ELECTRICAL METHODS FOR VERIFYING INTERNAL-PRESSURE OF VACUUM INTERRUPTERS AFTER LONG-TIME SERVICE

The application of vacuum circuit breakers in medium voltage switchgea r is steadily increasing. It is common practice to perform pressure me asurements in the factory on new vacuum chamber with sophisticated equ ipment based on the magnetron principle. However, for re-examining the internal pressure after several years of service, other and more simp le procedures suitable for application at the site are needed to check whether a given pressure threshold value still exists. In this paper, results of recent investigations will be presented, which deal with t he fundamentals of two new methods to check the vacuum inside the cham ber, using only electrical parameters. With increasing pressure, the p rebreakdown current changes in such a way that not only the power freq uency emission current patterns change but also high frequency current pulses, with a frequency range of approximately 30 to 300 kHz, appear superimposed on the emission current. This paper reports under which conditions these pulse currents appear, how they can be detected, and how they correlate with the pressure. Furthermore, in case the current is interrupted at a frequency range of approximately 10 to 90 kHz, th e current switching capability and the reignition voltages change sign ificantly with increasing pressure inside the vacuum chamber. It was f ound that the vacuum chamber loses its high frequency current switchin g capability when the internal pressure is higher than approximately 1 0(-2) to 10(-1) Pa. At the same time, the breakdown voltage and reigni tion voltages are not yet appreciably decreased even though the pressu re rises to approximately 1 Pa.