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The vacuum degree of Vacuum Interrupter (<italic>VI</italic>) will deteriorate because of gases emitted by arc heat, and leakage through the joints. It is important to select an appropriate maintenance criteria for a <italic>VI</italic> by monitoring the vacuum degree. However, monitoring of a <italic>VI</italic> installed inside a solid insulation apparatus in a transmission system is impossible. Only studies on the insulation deterioration because of the decrease of the vacuum degree have been conducted, and important factors such as the maintenance criteria have not been investigated. Moreover, most studies have only been performed using AC voltage, not DC voltage. Therefore, in this paper, AC and DC partial discharge were compared by monitoring of the vacuum degree in a distribution class <italic>VI</italic> to propose maintenance criteria according to voltage type. Through this, for efficient maintenance, maintenance criteria of <italic>VI</italic> for each voltage type are presented. The vacuum degree at which the dielectric strength rapidly decreases was confirmed according to voltage type. The coupling capacitor was installed directly on the floating shield of <italic>VI</italic>. Moreover, to measure the pattern and apparent charge of partial discharge according to the vacuum degree, phase resolved partial discharge (PRPD) for AC partial discharge and pulse sequence analysis (PSA) for DC partial discharge were adopted. It is found that maintenance should be performed when the vacuum degree is in the range of <inline-formula> <tex-math notation="LaTeX">$10^{-3} \boldsymbol {\sim } 10^{-2}$ </tex-math></inline-formula> torr regardless of the voltage type. Moreover, the usefulness of the technology is higher at AC voltage than at DC voltage.

Comparison of AC and DC Partial Discharge by Monitoring of Vacuum Degree in a Distribution Class Vacuum Interrupter