Pressboard

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

Pressboard and process for its preparation

Abstract: High temperature resistant pressboard having a desirable combination of compression set values and oil absorption is prepared by a process whereby a low density pressboard is first prepared by forming a wet lap of multiple layers of a waterleaf containing 50-95% by weight water and pressing the wet lap at 100-200° C. under a pressure of 10-60 kg/cm 2 , drying, ultimately at 270°-320° C. until substantially no further moisture is evolved and finally pressing at 270°-320° C. under a pressure of 8-350 kg/cm 2 and optionally cooling under restraint.

Investigation of the impregnation of cellulosic insulations by ester fluids

Abstract: This paper confronts the practical problems associated with ester fluids, due to their higher viscosities, for the impregnation of cellulosic insulation systems used in power transformers. The impregnation mechanism by an insulating fluid was studied using an analytical model and then laboratory tests were carried out to investigate the impregnation process of cellulosic materials in the forms of paper, pressboard and laminated pressboard blocks by various insulating fluids. The impregnation of both papers and pressboard barriers brings no practical problems due to their thinness; however the impregnation procedure for thick laminated pressboard blocks by esters needs to be established. It was shown that by increasing the impregnation temperature to 60degC, the viscosities and the capillary action of esters can achieve a similar impregnation to that of mineral oil at room temperature and consequently a full impregnation can be achieved for the ester filled power transformers in the same length of time.

A mathematical model to measure instantaneous moisture content in transformer insulation cellulose

Abstract: Solid transformer insulation is mainly composed of cellulosic materials, in the form of pressboard, Kraft paper and wood. Oil is used to impregnate voids in the cellulose. The water content of cellulose usually increases over the life of the transformer, caused by both moisture ingress and the gradual degradation of cellulose, which produces moisture as an aging by-product. Equilibrium curves are used to estimate the moisture content of cellulosic insulation using the vapor pressure of water dissolved in the oil. However, a very long duration is required for water to attain this equilibrium partition, much longer than the usual daily cyclic temperature profile of a transformer, and thus it is very unlikely for the insulation to reach this state. In this paper a novel method to estimate the moisture in cellulose during non-equilibrium conditions is presented. The water adsorbed by the cellulose migrates in and out the oil, and the diffusivity depends on the temperature and the concentration of moisture in the cellulosic insulation. This diffusion is mainly governed by Fick's law. For a transformer, the change in moisture content of oil in a small duration is mainly due to the moisture content in the cellulosic insulation. This paper presents a method to estimate the moisture content of cellulosic insulation of the transformer at that instant from the change in the moisture content of the oil.

Effect of moisture migration on surface discharge on oil-pressboard of power transformers under cooling

Abstract: In this paper, the PD properties of an ester oil/PB and a mineral oil/PB system undergoing a moisture transient are investigated using a needle-plane electrode under natural cooling. The temperature, moisture concentration and water activity of the oil are continuously monitored. Moreover, an improved moisture diffusion model is developed by using COMSOL Multiphysics to verify the experimental results. The measurement results show that there is a sharp reduction of partial discharge inception voltage (PDIV) or a sudden increase of PD intensity in the process of natural cooling for both ester oil and mineral oil system. However, the PDs extinguish gradually after the occurrence of the minimum of PDIV. For the cooling mode selected in this study, the minimum of PDIV for the ester oil system appears in the third hour after cooling begins, while it is about in the second hour for the mineral oil system. In addition, the simulation results are consistent with the experimental data.

Charge behavior in Palm Fatty Acid Ester Oil (PFAE) / pressboard composite insulation system under voltage application

Abstract: Palm Fatty Acid Ester Oil (PFAE) is one of the most promising environment-friendly biodegradable insulating oils for transformers. PFAE has significant advantages compared to other vegetable origin insulating oils and mineral oil, e.g. not only good insulation performance, high permittivity and high moisture tolerance, but also excellent cooling ability due to low kinematic viscosity, high thermal and chemical stability and high productivity. However, since the molecular content of PFAE is quite different from mineral oil, the fundamental charge behavior in PFAE has not been clarified yet. To clarify the temporal and spatial charge behavior in PFAE, we directly measured the electric field in oil duct model with PFAE / pressboard (PB) composite insulation system under dc voltage application and charged flow condition by using Kerr electrooptic field measurement technique. By the comparison of electric field and charge behavior characteristics with fresh and degraded mineral oil, we revealed that the essential mechanism of charge behavior is similar, but the difference in temporal and spatial characteristics is attributed to the higher intrinsic ion density and higher permittivity of PFAE than that of mineral oil.