Particles in transformer oil are harmful to the operation of transformers, which can lead to the occurrence of partial discharge and even breakdown. More and more researchers are becoming interested in investigating the effects of particles on the performance of insulation oil. In this paper, a simulation method is provided to explore the motion mechanism and accumulation characteristics of different particles. This is utilized to explain the effects of particle properties on the breakdown strength of mineral oil. Experiments on particle accumulation under DC voltage as well as DC breakdown were carried out. The simulation results are in agreement with the experimental results. Having a DC electrical field with a sufficient accumulation time and initial concentration are advantageous for particle accumulation. Properties of impurities determine the bridge shape, conductivity characteristics, and variation law of DC breakdown voltages. Metal particles and mixed particles play more significant roles in the increase of current and electrical field distortion. It is noteworthy that cellulose particles along with metal particles cannot have superposition influences on changing conductivity characteristics and the electrical field distortion of mineral oil. The range of electrical field distortion is enlarged as the particle concentration increases. Changes in the electrical field distribution and an increase in conductivity collectively affect the DC breakdown strength of mineral oil.
Natural ester is a good substitute for mineral oil. It has been successfully applied in power transformers, and its application scale will continue to increase. With the aging of oil-paper insulation in transformers, moisture and impurity particles will inevitably be generated. In this paper, dry and wet mineral oil and natural ester samples, as well as dry and wet cellulose particles, were prepared. The accumulation behavior of the cellulose particles and the conductivity current in mineral oil and natural ester under a non-uniform dc electrical field were compared by using eight combinations of dry or wet cellulose particles and dry or wet insulation oils. The results show that the speed of the cellulose particles and the growth of impurity bridges are faster in wet mineral oil and natural ester, especially when the cellulose particles are also wet. The increase of electrical field strength accelerates the appearance of this phenomenon. The cellulose particle bridging phenomenon in the mineral oil is more significant than that in the natural ester. Under the same dc voltage, the saturated conductivity current in every mineral oil sample decreases as the electrode distance increases; however, the opposite phenomenon is observed in the natural ester samples. Moreover, the oil moisture content increase has a more significant influence on the saturated current and the dc breakdown voltage than the particles' moisture content increase. For the same particle concentration level, the natural ester has a lower conductivity current and a larger dc breakdown voltage than the mineral oil. The natural ester presents better resistance to particle pollution and better insulation properties than the mineral oil.INDEX TERMS Natural ester, mineral oil, moisture, cellulose particles, conductivity current, breakdown.
Although highly porous carbon electrode materials from biomass wastes for high-performance electric double-layer capacitors (EDLCs) have attracted great attention recently, the fast charge−discharge performance under ultrahigh current density (100 A g −1 ) still remains a challenge. Herein, we develop a promising route to massively prepare honeycomb-like activated carbon (AC) with hierarchical porous features from spent lotus stems (SLSs) exhibiting an ultrahigh specific surface area of 4190 m 2 g −1 . The preparation process of the SLSAC samples includes carbonization at 400 °C in argon (denoted as c-SLS) and followed a KOH chemical activation using various KOH/c-SLS mass ratios at 800 °C for 1 h. The SLSAC-5-based electrode (KOH/c-SLS = 5/1) displays a good gravimetric capacitance of 330 F g −1 at 0.5 A g −1 with a superior high-rate capacitance of 243 F g −1 at 100 A g −1 and presents remarkable cycling stability with a capacitance retention near 100% over 10,000 cycles at 2 A g −1 using 6 M KOH aqueous electrolyte. Additionally, the SLSAC-5-based sample delivers an energy density of 18.6 W h kg −1 at 199.2 W kg −1 with 1 M Na 2 SO 4 electrolyte. Herein, we reveal a simple, promising route to massively generate SLSAC-based samples and investigate prominent electrochemical properties in ultra-fast charge−discharge EDLCs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.