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Study on AC withstand characteristic of air gaps in switchgear at low air pressure condition

The bare conductor in switchgear constitutes complicated air gaps, and their AC withstand characteristic will obviously affect switchgear insulation performance. In this paper, AC withstand tests of various types of air gap were carried out. The air gap samples were rod-plane air gap and 4 typical types of air gap from the bus chamber of 40.5 kV metal-enclosed switchgear. The samples were tested in climate chamber where high altitude environment was simulated. And correction of withstand voltage for switchgear air gaps at low air pressure were conducted. Test results indicate that, the withstand characteristic of 4 typical types of air gap in switchgear are different from that of rod-plane air gap. The relationship between withstand voltage Uw and air gap distance d is nonlinear. The influencing characteristic exponent n of air pressure on Uw is related with air gap type. Linearity value m and air pressure index n are independent to some degree. An equation was proposed to express the relationship of Uw with air gap distance d and altitude H, and this equation can be used to calculate the safety distance of air gaps in switchgear under low air pressure situations. The results are significant for the design and selection of switch devices.