Pressboard

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

Insulation Condition Evaluation of 750 kV Power Transformers Based on Frequency Domain Spectroscopy

Abstract: The reliability of oil-immersed power transformers depends on the insulation system to a large extent,and the lifetime decrease with the deterioration of the insulation systemTherefore,insulation diagnosis and lifetime evaluation for oil-immersed transformers are necessaryIn this paper,diagnostics of transformer insulation are analyzed,and a new diagnostic technique based on frequency domain spectroscopy(FDS)is presentedThe 750 kV transformer operated in Lanzhou East Substation is measured on-site with the FDS methodThe experimental results show that the moisture content in transformer pressboard is eligible,and its insulation condition is wellAnd the experimental results are consistent with those of the conventional preventive testThe validity of FDS method is proved,and it is promising as a novel preventive test method

Study of the Dielectric Response of Ester Impregnated Cellulose for Moisture Content Evaluation

Abstract: Knowledge about moisture content in impregnated cellulose insulation is essential for power transformers when estimating remaining lifetime, planning operating conditions and drying procedure. Frequency dielectric spectroscopy (FDS) is non-destructive, convenient method to asses moisture content of insulation. It is important to study dielectric response of ester-cellulose insulation due to growing number of transformers using synthetic ester MIDEL 7131 as insulation fluid [1, 2]. The paper presents study of dielectric response of synthetic ester-pressboard samples with various moisture content (from dry to 5%) measured in wide temperature range (from 5 ˚C to 50 ˚C). Measurement results for pressboard impregnated with synthetic ester are compared to data of pressboard impregnated with mineral oil. The aim of presented work is to provide patterns of dielectric response of synthetic esterpressboard for accurate moisture content evaluation.

Improving the modelling of the water content of power transformer insulation in the frequency domain

Abstract: One technique to measure the water content of the insulation of an operating transformer is to use online water activity sensors with cellulose adsorption isotherms. In a cellulosic pressboard/paper insulation system, the water migrates between the cellulose and oil until a known equilibrium partition is reached, when the cellulose adsorption isotherms can be used to estimate the water content of pressboard from the concentration of water dissolved in the oil. However, a significant drawback is that the time constants involved in the diffusion of water through pressboard are very high, and it is very unlikely that the transformer will be at one temperature long enough for the water partition to reach equilibrium. In the past this has been resolved by using long term averages. However, an obvious question is for how long a long term average should be applied for before it filters out actual features of the measurement data, and how short it may be to suppress the short term fluctuations. We set out resolve this question by analysing the frequency spectrum of the water content of pressboard calculations for an operating power transformers. This showed a difference between the very low frequencies attributable to the migration of water, and the higher frequencies caused by the daily temperature cycling of the transformer. A low pass filter was then applied to the measurement data. This technique was found to be more effective than using a long term average, although further research is required to optimise such a filter.

Experimental study on moisture dynamics in transformers insulated with natural-esters

Abstract: In the last years great attention has been paid to ester fluids as an alternative to mineral oil. At the present the use of these liquids is becoming an habitual practice in distribution transformers and some experiences are also starting to be reported about their application to power transformers. One of the main differences between ester fluids and mineral oils is that those are able to absorb much greater amounts of water when they are in contact with cellulosic insulation. Moisture is one of the key variables that must be kept under control for a safe transformer operation. As is well known the distribution of moisture between transformer solid and liquid insulation depends on transformer temperature, and thus a continuous migration of moisture between both materials takes place during transformer normal operation. In the case of transformers isolated with ester fluids, the equilibrium points, and probably the time constants of the migration processes, would be affected by the different properties of these materials with regard to moisture. In this work the moisture dynamics in ester fluids is compared with that in mineral oils. Drying processes of pressboard has been experimentally simulated using mineral oil and a natural ester as insulating fluid. The results of these experiments will be used in the future to develop dynamic models that will allow to simulate the moisture migration processes in transformer insulation during normal operation as well as transformer drying processes.