A rotating cylindrical electrode apparatus, which provided cylindrical Couette flow, was used to simulate flow electrification in an electric power transformer. The apparatus had Shell Diala A transformer oil filling the annulus between coaxial cylindrical stainless steel electrodes that were either bare metal, or covered by a thin copper sheet and/or EHV-Weidmann HiVal pressboard insulation. Extensive experiments characterized the time transient and steady state behavior of the electrification through measurements of the volume charge density, the terminal voltage, and the terminal current as the system was driven out of equilibrium by changes in the flow rate (inner cylinder rotation rates of 100 to 1400 rpm, Reynolds numbers of 5 x lo3 to 5 x lo5), temperature (15 to 70'0, insulation moisture content (0.5 to 20 ppm in the oil), applied voltage (0 to 2 kV dc), and concentration of the non-ionizable antistatic additive 1,2,3 benzotriazole (BTA, 0 to 60 ppm). Generally, the electrification increased with flow rate and temperature but the BTA appeared to cause competing effects: it decreased the volume charge density on the liquid side of the interface by a factor of 4, which reduces the electrification, but also decreased the oil conductivity by a factor of 10, which enhances the electrification. A critical oil BTA concentration of 5 to 8 ppm minimized the electrification. The volume charge density was sensitive to the current flowing through the interface and surface charge accumulation. With pressboard covering the cylinders, the electrification effects of the temperature and moisture were decoupled during the transient following step reductions in the temperature. The oil moisture content did not affect the oil conductivity significantly, but the volume charge density varied inversely with the moisture content during an initial equilibration period just after the oil had been added to the system. The highest electrification was observed during this equilibration period.

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Sinusoidal steady state and transient decay measurement techniques were used to monitor the dielectric properties of electrical insulation, Impedance measurements of an air-gap capacitor, immersed in Shell Diala A transformer oil and driven with a 1 V peak sinusoidal signal (5 mHz-10 kHe), showed that the oil conductivity increased with temperature ( 15-70°C) and aging in air but was independent of the oil moisture content (2-30 ppm). Interfacial double layer effects were also observed.Surface charge effects were studied in a cylindrical electrode apparatus with EHV-Weidmann Hi-Val pressboard on the inner cylinder and transformer oil filling the gap. The pressboard dielectric properties were obtained from time transient decay measurements of the open-circuit voltage after disconnecting a DC voltage source from across the cylinders. These measurements indicate that the conduction mechanism through the pressboard follows a drift dominated unipolar conduction law rather than ohmic conduction.Further frequency sweep dielectrometry measurements using a three-wavelength interdigital electrode sensor confirm the dispersive properties of oilimpregnated pressboard. This sensor was also used to measure the time and space distributions of moisture diffusion into oil-impregnated pressboard.

Many aircraft components originally designed to last the design life of the aircraft without experiencing cracking (i.e., safe-life components) are now failing in service due to multiple site damage in locations not considered critical in original fatigue evaluations. For safelife components that must now he managed by damage tolerance methods, periodic inspections are generally far more costly than for components originally designed with planned inspection% Often. the high cost of these inspections is associated with disassembly and surface preparation. This paper describes (1) surface mountable eddy-current sensors for periodic or continuow monitoring of difficult-to-access locations (e.g.. in fuel tanks): (2) high resolution scanning eddy-current sensor arrays for imaging of subsurface cracks and corrosion using inductive sensing elements; and (3) a new giant magnetoresistive sensor with a shaped field dri\je winding for sensitivity to deeply buried flaws in aircraft structures.