Pressboard

About: Pressboard is a research topic. Over the lifetime, 1123 publications have been published within this topic receiving 9089 citations.

Weibull Statistics in Dielectric Strength of Oil-impregnated Pressboard under Ramped AC and DC Voltages

Abstract: AC and DC breakdown tests were performed on large populations of oil-impregnated pressboard samples. The effect of voltage ramp rate on dielectric strength has been investigated. A statistical analysis of breakdown data was carried out using the Weibull distribution. The 90% confidence intervals of Weibull graphs were calculated. The study shows that dielectric strength and shape parameter change versus ramp rate. The variations are attributed to the size and number of defects. Discharges occur from the oil to the oil-pressboard interface and lead to breakdown. DC dielectric strength is larger than that corresponding to AC voltage. This is ascribed to the dissipated energy difference under the two types of field and the fatigue produced by the alternating voltage. This phenomenon is related to space charge. Under DC stress, dielectric strength is higher under negative polarity. It is assigned to the different quantities of space charge accumulated under the two polarities.

Space charge development measurements inside oil impregnated pressboard

Abstract: Electrification due to oil flow inside transformers induces charge accumulation at the pressboard interface. A part of this charge migrates inside the impregnated pressboard, which leads to a space charge development inside the pressboard. Then this accumulation may be responsible of electrical failure. In order to predict the phenomenon it is important to know the charge evolution and its variation with the temperature gradient and concentration of additives, considered to be used to prevent such failures. An experimental setup has been perfected in which an impregnated pressboard stack is submitted to a potential difference and a temperature gradient. From measurements of potential on special electrodes inserted in the pressboard stack it is possible to obtain the space charge density development. We have made experiments for several potential difference and temperature gradient. The evolution of the space charge density development is observed in terms of time. Then the effect of the Benzotriazol (BTA) concentration has been tested.

Easy-to-operate bag sealer

Abstract: The item is an easy-to-operate bag sealer that can inflate as well as vacuum, for normal household use, to keep foodstuff fresh. In the front portion of the surface of the body is the bottom frictional pressboard bar that has a frictional surface on one side. Under the cover, facing the frictional surface of the bottom frictional bar is fitted with the upper frictional pressboard bar for vacuuming or inflating. The diagonal frictional force produced by the frictional surface of the bottom frictional pressboard bar and that of the upper frictional pressboard bar is enough to enable the cover to stay closed during the process of realizing the vacuuming and inflating functions. A heat wire is located in parallel to and in the front of the bottom frictional pressboard bar and under the cover is fitted with another coordinating pressboard bar for bag sealing. Also included is an indirect switch to be pressed down to activate the vacuuming and inflating functions control, placed underneath the body, through a hole opened on the body at a corner position formed by the bottom frictional pressboard bar and the surface of the body. Under the cover, at the exact position facing the hole on the body through which the indirect switch is protruding, a cave-like opening is made on the upper frictional pressboard bar or wherever deemed appropriate. The coordinating factor of this cave-like opening and the indirect switch has a procedural function to control the activation of vacuuming and inflating.

Increase of Static Electrification in Aged Transformers

Abstract: Increase in static electrification and discharge generation was experienced during the service operation of aged power transformers of both shell and core-form. However, the electrical characteristics of the oil did not demonstrate increase in static electrification. The investigation on the aged oil by measurement of electrostatic charging tendency suggested the possibility that some deteriorated compound in oil was adsorbed to the pressboard and increased the static electrification. However, the measurement of electrostatic charging tendency with the aged pressboard did not demonstrate enough increase, whereas accumulated charge measurement of the aged pressboard confirmed strong increase in static electrification. On the other hand, the measurement of accumulated charge by potential measurement confirmed strong increase of charge accumulation by the pressboard aging. It was suggested that measurement of accumulated charge was suitable for the assessment of static electrification in aged transformers.

Transformer insulation structure for dielectric liquids with higher permittivity

Abstract: Insulation structures based on cellulosic materials and mineral oil have been the standard solution for transformers for the last hundred years, both in distribution and power transformers. These structures typically consist of a solid-liquid arrangement, aimed to divide larger voltage drops in smaller steps, creating small volumes of liquid with high dielectric insulating performance and reaching the most compact design. The total potential difference is borne by sequential voltage drops defined according to the geometry and the permittivity of the materials. Standard arrangements and configurations have been developed by transformer manufacturers according to the voltage levels, as the analytical solution of the distribution of the electrical field may be extremely complex. Even nowadays, where numerical calculation models, mostly based on the Finite Elements Method, are available, the use of standardized solutions is not uncommon, as the detailed calculation may be highly time consuming. When the dielectric liquid is changed, replacing the traditional mineral oil by natural ester liquid, the distribution of stress in the insulation system is no longer the same. The permittivity of natural ester liquid is around 40% higher than that of mineral oil, while the permittivity of pressboard and paper made from kraft pulp, impregnated with natural ester liquids is just slightly higher. The result is a different distribution of voltage drops in the insulation system, which must be taken into consideration when designing a natural ester filled transformer. At first glance, the effect is positive, since the critical parameter is typically the stress, the field strength, in the liquid insulation and the higher permittivity shifts the stress from the liquid into the solid insulation. This modification of electrical field distribution may allow, in some cases, increasing the average field strength and thus a more compact design, as there is a better balance between the stress in the solid and liquid portions. However, the modification of the electrical field distribution may lead also to new critical regions. Corners and labyrinth constructions may have higher superficial dielectric field gradients. Additional edge protection may be required in regions where they were not required with mineral oil, as the stress in the paper will be increased. Less specialized engineers tend to take the breakdown voltage as the only relevant parameter for the insulation design. A large quantity of test data has indicated that the breakdown voltage of natural ester liquid, excluding the undesirable highly divergent field (needle to sphere), is similar to mineral oil. But this does not mean that adjustments in the insulation system design would not be required. This paper will present some common configurations, potential optimizations and areas requiring attention for designing insulation systems using cellulose and natural ester liquids.