Showing papers on "Pressboard published in 1991"
TL;DR: In this paper, a Couette charger was built to simulate flow electrification processes in transformers where transformer oil filled the annulus between coaxial cylindrical electrodes, and a charge injection model was refined to examine the migration of double layer and injected charge in the imposed sinusoidally timevarying electric field.
Abstract: A Couette charger (CC) facility has been built to simulate flow electrification processes in transformers where transformer oil fills the annulus between coaxial cylindrical electrodes. Transient measurements with a step change in temperature have shown the charge density to change from an initial value to a new steady-state value, including cases of polarity reversal, both values dependent on the equilibrium moisture levels in oil and pressboard. Through a model that accounts for diffusion of charge from the paper-oil interface into the bulk of the oil, these measurements are used to deduce the equilibrium wall charge density. To help understand the effects of energization and to scale laboratory measurements at low voltages and low frequencies to operating transformers, a charge injection model was refined to examine the migration of double layer and injected charge in the imposed sinusoidally time-varying electric field. Good fits between this model and measurements are achieved. >
72 citations
TL;DR: In this article, a low-permittivity pressboard obtained by blending polymethylpentene fiber with cellulose fiber is described, which has a permittivity of 3.5.
Abstract: The development of a low-permittivity pressboard obtained by blending polymethylpentene fiber with cellulose fiber is described. The permittivity is 3.5, which is lower than that of conventional pressboard. With this pressboard, insulating characteristics of intercoil models were investigated under lightning impulse and AC voltage conditions. It is demonstrated that a spacer fabricated from this pressboard material increases the partial discharge inception and breakdown voltages up to 30% compared with conventional pressboard spacers. >
24 citations
08 Jul 1991
TL;DR: In this article, a Couette facility was developed, where transformer oil fills the annulus between coaxial cylindrical electrodes which can be bare metal or covered with transformer pressboard.
Abstract: To simulate the flow electrification processes in transformers, a 'Couette facility' has been developed, where transformer oil fills the annulus between coaxial cylindrical electrodes which can be bare metal or covered with transformer pressboard. Rotation of the inner cylinder provides controlled turbulent flow. This turbulent mixing transports electric charge from the electrical double layer at the liquid/solid interfaces to the oil volume. Measurements of the volume charge density are performed using an absolute charge sensor, which brings a sample of fluid from the turbulent core into a Faraday cage. In this facility, the temperature can be controlled from 15 degrees C to 140 degrees C and vacuum drying is used to reduce the moisture level below 1% in the pressboard. The oil conductivity and moisture content can be monitored continuously. Transient measurements with a step change in temperature have shown that the volume charge density follows the temperature. >
20 citations
08 Jul 1991
TL;DR: In this paper, a three-wavelength interdigitated flexible sensor has been developed for measuring the heterogeneous properties of transformer pressboard, where the imposed spatial wavelengths are 1 mm, 2.5 mm and 5 mm, and the temporal frequency ranges from 0.005 Hz to 10000 Hz.
Abstract: A three-wavelength interdigitated flexible sensor has been developed for measuring the heterogeneous properties of transformer pressboard. The imposed spatial wavelengths are 1 mm, 2.5 mm, and 5 mm, and the temporal frequency ranges from 0.005 Hz to 10000 Hz. Because the penetration depth of the electric field is approximately 1/5 of the wavelength, the sensor can be used to observe spatial as well as temporal changes in the dielectric loss of transformer pressboard. Experimental results indicate that the measured response is sensitive to changes in the moisture content of the pressboard. In addition, the sensor can be used to monitor the absorption of moisture into pressboard as a function of time. This measurement provides an estimate of the diffusion coefficient of moisture in oil-free transformer pressboard at room temperature of 3.5*10/sup -11/ m/sup 2//s. >
14 citations
08 Jul 1991
TL;DR: In this paper, three typical electrode systems immersed in transformer oil, i.e., needle-plate, cylinder-plate and sphere-plate electrode systems, have been used to study the characteristics of impregnated pressboard under AC voltages.
Abstract: Three typical electrode systems immersed in transformer oil, i.e., needle-plate, cylinder-plate, and sphere-plate electrode systems, have been used to study the characteristics of impregnated pressboard under AC voltages. According to careful observations, the six stages of the discharges which occur when the applied voltages are increased can be distinguished. The inception electric field strengths of the surface discharge in the three electrode systems were computed. The results of experiments and calculations may be useful for analyzing the surface discharge failures of pressboard barriers in power transformers which have occurred in recent years in China. >
11 citations
08 Jul 1991
TL;DR: In this article, angularly resolved measurements of the electric field in transformer oil around two overlapping pressboard barriers, a geometry relevant to HVDC transformer design, were performed at room temperature and at 80 degrees C and the data are compared with the results of electrostatic calculations.
Abstract: The authors report angularly resolved measurements of the electric field in transformer oil around two overlapping pressboard barriers, a geometry relevant to HVDC transformer design. Measurements have been performed at room temperature and at 80 degrees C and the data are compared with the results of electrostatic calculations. It is concluded that the Kerr electro-optic field mapping technique has been developed into a versatile engineering tool for investigations concerning HVDC converter transformer insulation. The present study of oil/pressboard insulation shows that traditional electrostatic field mapping gives a reasonable picture of stress levels, provided the resistivity ratio between oil and pressboard is not chosen too high. >
4 citations
TL;DR: In this article, the surface strength of processed transformer pressboard was determined in a practical configuration under additive polarity composite DC plus switching impulse and DC plus lightning impulse voltages, with specimens impregnated with field-aged oil.
Abstract: The surface strength of processed transformer pressboard is determined in a practical configuration under additive polarity composite DC plus switching impulse and DC plus lightning impulse voltages. Results obtained with specimens impregnated with field-aged oil are presented. With both types of composite voltage, most breakdowns occurred in the oil away from the interface. There is no significant influence of gap length on the location of the breakdown. With increasing DC bias fewer breakdowns occurred on the surface. >
2 citations
Patent•
06 Nov 1991
TL;DR: In this paper, the hot pressboard of the utility model was changed to steam heating; a steam inlet and a steam exhaust port were arranged; and the steam passage was arranged in a utility model to communicate the steam in-let and the exhaust port together.
Abstract: The utility model relates to a leather ironing machine. The ironing machine used presently is heated electrically; the production process is complicated, and a great amount of electrical energy is consumed; simultaneously the steam of present tanneries can not be utilized sufficiently. The hot pressboard of the utility model is changed to the steam heating; a steam inlet and a steam exhaust port are arranged; a steam passage is arranged in the utility model to communicate the steam inlet and the steam exhaust port together; the production process of steam heating hot pressboard adopted is simple; a great amount of electrical energy is saved for nation; simultaneously the steam of present tanneries is fully utilized, and the production cost is reduced.
1 citations