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Stator coil end of high-voltage rotating machines is equipped with stress grading system (SGS), which consists of two dielectric materials (conductive slot coating and stress grading tape/paint (SGT/P)). SGS grades surface potential curve on coil end and prevents surface partial discharge. Improving SGS and raising voltage limit of rotating machines are key to produce more efficient and durable products...
In order to clarify breakdown mechanisms in insulation oil / pressboard composite dielectric system, it is effective to measure the electric field distributions. We have investigated the electric field distribution in transformer oil using Kerr electrooptic effect. At first, Kerr constant of the transformer oil was measured using the optical system. The constant was determined to be 2.0×10−15[m/V2...
In order to improve the insulation performance of oil-immersed power transformers, the characteristics of the creeping discharge under impulse voltage have been investigated. In this work, by using streak camera, investigations of the influence of the white discharge trace on the creeping discharge characteristics were implemented. The results showed that the velocity of the discharge developing along...
To understand the properties of composite insulating materials, dc breakdown testing and the thermal conductivity measurement of polymethylmethacrylate (PMMA)/aluminum oxide (alumina) composite material created using the electrostatic adsorption method were performed. In the PMMA/alumina sample, which has an alumina as a main particle and a PMMA as an adsorption particle, the dc breakdown strength...
Carbon-based semi-conductive materials (corona armor tape or CAT), and SiC-based nonlinear grading materials have been used widely to reduce the electric field in the region where the stator bar exits from the slot in form-wound rotating machines. Recent studies show that electric field and local heating in the end-turn stress grading system become severe under PWM waveforms and high frequency repetitive...
SiC-based nonlinear resistive grading materials have been used for end turn stress grading of form-wound rotating electrical machines, such as high voltage motors and turbine generators, over a long period. Although these materials work well under power frequency, the electric field and local heating become problematic under PWM repetitive pulses for the stress grading systems of converter-fed rotating...
This paper deals with the discharge characteristics in the oil/Pressboard composite insulation system under the application of a standard lightning impulse voltage. Discharge images were observed using a digital camera. We investigated the barrier effect on partial discharge inception voltage (PDIV) and breakdown voltage (BDV). Furthermore, we observed the discharge generation across the insulation...
To understand the electrical treeing properties at the interface, the treeing breakdown test was performed. The effect of the filler added into one side of the insulation layers on the treeing breakdown was investigated. Time to breakdown of the epoxy/epoxy sample including the silica fillers at upper epoxy resin layer became shorter than that not-including the silica filler as the filler content...
This paper deals with the impulse PD (partial discharge) characteristics in oil/pressboard composite insulation systems. We examined the PD starting point in test samples with the oil gap length varied from 1.6 mm to 9.6 mm, which simulated the oil duct in an oil-immersed transformer. Consequently, we found that when the oil gap length was below 4.8 mm, the PD starting point shifted from the oil gap...
SF6 gas has been widely used in high voltage power equipment (HVPE) because of its high insulating and arc-quenching performance. However, the reduction of use of SF6 is highly required, because SF6 is a potent greenhouse gas. Thus, the improvement of SF6 gas insulation system is indispensable in order to further downsize HVPEs with reduced SF6 usage. In this work, we applied a dielectric coating...
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