Characteristics Of Microbridges Research Articles

Study on micro bridge impurities in oil-paper insulation at DC voltage: their generation, growth and interaction with partial discharge

To investigate the characteristics of micro bridges and their interactions with partial discharge (PD) in oil-paper insulation under DC voltage, an experimental platform with two-copper-electrode models is established. Generation and growth of micro bridge impurities are observed. PDs in experimental process are detected and analyzed. The result shows that the generation and growth of micro bridges have three stages based on their quantity and shape. At first stage PD accelerates the generation of micro bridges. At second stage the grown bridges trigger stronger discharges. At the last stage a so-called "hazardous bridge" that triggers highly intensive PDs emerges, through which breakdown happens. Therefore, bridges and PDs promote each other to cause insulation failure. In addition, impurities at the second stage are observed to transform into pile-like, bridge-like or particle-like shapes. The transformation process is considered to relate with the change of composition force on fiber bundles. Besides, a "cluster discharging" phenomenon is observed at the second stage, which is associated with the generation and disappearance of micro bridge impurities.

Coherence length in deoxygenated (1 0 3)/(0 1 3) oriented YBCO superconductor films

The coherence length ξ c along c-axis direction in (1 0 3)/(0 1 3) oriented YBCO superconductor thin films has been evaluated from the temperature dependence of current–voltage ( I– V) characteristics of micro-bridges in the framework of a model taking in account thermally activated magnetic flux creep in the intrinsic potential of the layered structure of the superconductor. The coherence length gradually increased from the initial value ξ c = 0.14 nm to ξ c = 0.33 nm in the sample subjected to subsequent annealing steps performed in air at increasing temperatures from a range T a = 190–275 °C. Structural modifications of deoxygenated samples were monitored by micro-Raman spectroscopy. The critical temperature T c slightly increases after the initial annealing steps while a significant T c decrease has been observed after T a = 275 °C stage accompanied by appearance of the tetragonal phase in the crystal structure.