03_kao_guarded_needle - F E A T U R E A R T I C L E High...

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High Field Measurements with a Guarded Needle Key Words: Space charge, protrusions, guarded needle manufacture, threshold field, degradation, conduction models T he electrical degradation of solid dielectrics often originates in the vicinity of defects (impurities, protrusions, or inclusions). The degradation in- volves a high field region of localized high carrier mobil- ity that leads to the formation of a space charge limited field region in which charge cycles during application of anacfield[1], [2]. Thiscycliccarriermotion resultsinhot electrons and ultra-violet (UV) photons produced by car- rier recombination,both of which can degrade the dielec- tric. Carrier mobility-related prebreakdown phenomena can only be studied within microscopic dimensions, as the power dissipation would cause thermal runaway for a macroscopic geometry. The guarded needle apparatus is designed for such measurement, using a sharp metal nee- dle to create local high electrical field, which induces car- rier mobility. The guarded needle measurement technique was first developed at the Corporate Research Laboratories of ABB [3], [4]. The implementation described here was de- veloped with the help of ABB, which provided detailed schematicsfor themeasurementapparatus.ABBalsopro- vided a general description of the needle construction; however, the authors requireda good 12 months of effort before “good” needles could be made routinely. Given the difficulty in fabricating such guarded needles, the au- thors have published detailed fabrication details [5]. The “resistive” current is measured by guarding the needle to within 30 µ m of its tip and compensating elec- tronically for the residual displacement current [3], [4]. However, due to the difficult nature of such a measure- ment, only limited results have been reported [4], [6], [7]. The fabrication of the guarded needle electrode has been the main obstacle to this experiment. “Charge Injection” Measurements What is Measured? In the guarded needle measurement, the needle is in- serted into an insulatingsample, the other side of which is supported by a plane electrode. A time-varying voltage is applied between the needle (at ground potential) and the plane electrode. The variation in the charge induced on the very small unguarded tip of the needle, less the charge caused by displacementcurrent, is measured as a function of time. The residual capacitive (displacement) current betweenthe needletip and the planeelectrodeis canceled May/June 2003 — Vol. 19, No. 3 0883-7554/03/$17.00©2003IEEE 19 FEATURE ARTICLE Yang Cao and Grace Ge Jiang Electrical Insulation Research Center University of Connecticut Steven Boggs Electrical Insulation Research Center Electrical Engineering and Physics Depts. University of Connecticut Carrier mobility-related prebreakdown phenomena can only be studied within microscopic dimensions.
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03_kao_guarded_needle - F E A T U R E A R T I C L E High...

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