Effect of Foreign Matters Attached to The Switching Core of Vacuum on Load Tap Changer on Oil Gap Breakdown Voltage

Carbon powder and iron powder are common in the inspection of tap changer hanging core, selected the common powder to study the breakdown characteristics of vacuum on load tap changer oil gap under the environment of 30.5mm ball electrode. The results shows that: 1. Under the static test, the foreign matter will move close to the electrode under the action of electric field; the breakdown voltage drops approximately linearly before the critical point due to the accumulated mass of foreign matter, the breakdown voltage drops exponentially after the accumulated mass of foreign matter breaks through the critical point; if the foreign matters that can move directly by electric field force, the particle size is positively correlated with the breakdown voltage drop rate. 2. Under the dynamic test, with the increase of standing time, the oil sample is gradually clear and the recovery voltage is gradually restored; compared with iron and carbon powder, carbon powder can affect the recovery ability of oil gap breakdown voltage for a relatively long time; within 5 minutes of standing, both foreign matters can lead to oil gap breakdown voltage lower than 4KV, which is similar to that tap changer accidents often occur after shifting. The relevant conclusions of this paper can provide reference for tap changer operation and maintenance.