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Nowadays, reliability of power system is of great importance. Therefore, the condition of transformers as the expensive and critical components of a power system should be monitored and assessed. The dominant factor that causes limitation in transformers lifetime is the paper insulation. This paper aims to investigate the conspicuous influence of moisture content, especially in low oxygen and temperature on lifetime estimation under constant conditions, based on the results of previous experiments. Degree of polymerization (DP) is employed to quantify the effects of detrimental parameters with respect to their time-varying nature. This paper proposes a novel lifetime estimation-approach considering a yearly moisture growth for transformers. The results indicate different trend of DP profiles, which lead to an accurate lifetime estimation in comparison with DP obtained through ideal conditions. Furthermore, oscillations in the hot-spot temperature of transformers are demonstrated to have a negative effect on the lifetime. [ABSTRACT FROM AUTHOR]

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures. [ABSTRACT FROM PUBLISHER]

In this paper, the feasibility of inkjet printing of circuit and microwave structures on paper-based substrates is investigated for the first time in the implementation of a complete low-cost wireless platform for sensors. First, the system-level design of the module including the amplifier characterization were carried out to ensure optimum performance of the sensor modules in the UHF bands used in RF identification communication. These results were then used to design two different antenna structures, which are printed on paper along with their respective circuit layouts using inket-printing technology. Different techniques were investigated for the assembly of circuit components on the silver printed layouts. Finally, wireless link measurements on the assembled prototypes verified the good performance on the wireless and sensing sides. [ABSTRACT FROM AUTHOR]

The lifetime of a power transformer is typically defined by the lifetime of the cellulose insulation in the transformer. As the paper ages, there is a reduction in its mechanical strength properties. Dakin [1] was the first to show that the temperaturerelated chemical changes to the paper followed an Arrhenius relationship. The Arrhenius relationship shows, for example, life can be decreased to half for every 6 to 7°C rise in temperature in the hot spot temperature range of 80 to 100°C; the doubling rate can range from 5.9 to 9.7°C in the 80 to 180°C range [2]. It has also been long understood that the aging of cellulose is influenced by the amount of oxygen or water in the paper and the surrounding insulating medium [3], [4]. Paper and pressboard consist mainly of cellulose fibers composed of a polymeric chain of cellulose, which is formed from D-glucose monomers. The reduction in mechanical strength described above is due to splitting of the cellulose polymer by the action of oxygen, water, temperature, and likely acids. The number of D-glucose monomers in the cellulose fiber is termed the degree of polymerization (DP). New oil-impregnated cellulose typically has 1,000 to 1,100 individual monomers in the cellulose polymer. The DP of cellulose at its traditionally accepted end of life is 150 to 200. This article will first provide a review of data from aging studies performed on cellulose (both Kraft paper and pressboard) impregnated separately in mineral oil and BIOTEMP (ABB, USA), a high oleic sunflower based fluid. Next, possible mechanisms that explain the differences in aging behavior will be discussed. The article concludes with recommendations for new temperature limits for BIOTEMP-impregnated transformers in comparison with mineral-oil-impregnated transformers. [ABSTRACT FROM PUBLISHER]