107 / 2024-04-12 16:20:37

Influence of AC Conditioning Treatment on Subsequent Vacuum Surface Flashover Process under Impulse Voltage Applications

Surface discharge,flashover,secondary electron emission avalanche,conditioning effect

To effectively suppress surface discharge in vacuum for the development of higher voltage level vacuum circuit breakers, it is crucial to first understand its mechanism. Vacuum surface discharge on an insulator occurs through the process of secondary electron emission avalanche (SEEA). Since SEEA is greatly affected by the surface condition of the insulator, it is necessary to clarify the influence of surface discharge that occurs during the AC conditioning process in the development and manufacturing process of VI. In this study, we have investigated the influence of flashover under AC voltage application on subsequent flashover process and resultant flashover voltage (FOV) under impulse voltage application.

We prepared two cylindrical test specimens designed to mimic actual vacuum interrupter, consisting of electrodes and cylindrical ceramics, One of the two samples was subjected to be pretreated with AC conditioning, following steps: 30 kV, 40 kV, 50 kV and 60 kV for 10 seconds each, and 70 kV for 600 seconds. Inside the sample was retained in a high vacuum less than 10⁻⁴ Pa. For the impulse voltage application, the cylindrical sample was placed inside an acrylic container filled with insulating gas. A negative standard lightning impulse voltage (SLIV, 1.2/50 µs) was applied according to up-and-down method. The applied voltage and current waveforms were measured. The history of FOV during the impulse voltage application process was obtained and compared between the two samples. The current flowing in the process before flashover was measured, which is derived from SEEA and defined as pre-flashover current.

The voltage and current waveforms are shown when applying a negative standard lightning impulse voltage of -155 kV and measuring the pre-flashover current. The pre-flashover current was approximately −50 A for the sample without AC conditioning treatment. On the other hand, in the sample with AC conditioning treatment, the pre-flashover current was −10A at peak. This is because AC conditioning reduced microprotrusions, smoothed the electrode surface, and suppressed the emission of primary electron. Furthermore, the initial flashover voltages were −75 kV and −125 kV, respectively, indicating FOV improvement by AC conditioning. By repeating the flashover by impulse voltage applications, the FOV was improved to –230 kV and –185 kV in both samples, respectively. However, the increasing trend in FOV with AC conditioning treatment was slower upward. This is because the electrode material generated during AC conditioning contaminated the insulator surface, reducing the surface resistivity. This changes the SEEA process along the insulator, resulting in the difference in the pre-flashover current waveform.

In conclusion, the influence of AC conditioning treatment on surface flashover characteristics under subsequent impulse voltage application was investigated. The metal vapor generated by flashover during AC conditioning can contaminate the insulator surface, resulting in the reduction of pre-flashover current waveforms but the decrease in FOV.