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For more than a century, conventional mineral oil/cellulose insulation is being used as main insulation for power transformers. The life span of these major assets is determined amongst others by the insulation system?s mechanical resistance to withstand short circuit current forces. Since cellulose paper aging is accelerated by temperature, moisture and oxygen, poly-aramid-based synthetic insulation paper, with better thermal stability emerged for application in transformers. However the relatively limited number of studies on hybrid insulation and the cost of aramid paper still limit their use in distribution transformers. This contribution attempts to demonstrate that despite the high cost of aramid paper, hybrid insulation could be used in large power transformers which represent a significant financial investment. To do this, the quality of mineral oil aged with hybrid solid insulation and the thermal aging of the cellulose paper of this insulation are investigated. Comparison was made with traditional mineral oil impregnated cellulose paper to provide a benchmark for comparison. The samples underwent thermal accelerated aging procedure according to ASTM-D1934 standard and, furthermore, were submitted to local overheating. The condition of oil samples collected from each aging vessel was assessed using several diagnostic techniques, namely the relative amount of the Dissolved Decay Products (DDP), Turbidity, and Interfacial Tension (IFT). The Degree of Polymerisation (DPv) and Dissolved gas Analysis (DGA) were used to monitor the thermal aging of the cellulose paper samples. The results indicate that oil aged with hybrid solid insulation yield fewer decay products at some aging stages, especially after the application of a local overheating. The results also indicate that the cellulose paper within solid hybrid insulation is slightly less degraded compared to that of conventional insulation. [ABSTRACT FROM AUTHOR]

This paper presents a study of the impact of two important parameters, moisture and aging of the oil/paper dielectric used as insulation in power transformers.The way in which these two parameters influence different parameters of the Frequency Domain Spectroscopy (FDS) measurements, is emphasized.Different FDS parameters were measured by varying the moisturecontent and the aging degree of the oil impregnated paper.The use of two types of neural networks for analysis of the results was necessary in order to help discriminating the impact of moisture and aging on the FDS measurements and, in some cases, to estimate the aging duration of the paper impregnated with oil. [ABSTRACT FROM PUBLISHER]

Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. Because field measurements, last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care are required to interpret the results when performing tests at relatively low surrounding temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behavior of the insulation system in response to the ambient conditions. In this contribution, the dielectric behavior of a composite oil paper insulation system has been explained from the properties of Debye basic model. A series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. The equivalent circuit parameters of a laboratory made oil paper condenser bushing model were obtained using a non-linear optimization procedure. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. It was shown that the poles can be regarded as parameters able to be used for insulation condition assessment. [ABSTRACT FROM AUTHOR]

This paper presents the analysis of leakage current evolution of an ice-covered station post insulator during a melting period using artificial neural network (ANN) models. The tests, carried out under wet-grown ice regime for different experimental conditions, showed that the permanent establishment of white arcs, identified as "permanent regime" led to flashover in the large majority of the cases,. Based on these observations, the development of a monitoring methodology aimed at forewarning the approach of the leading white arc during melting periods is proposed. The monitoring methodology uses different ANNs in order to predict the appearance of the white arc based on the identification, classification and analysis of the occurrence frequency of electric discharges. The results show that the ANN monitoring model developed is able to predict the onset of permanent regime under various experimental conditions. Hence, it was found that the delay between the permanent regime onset prediction delivered by the ANN model and its realization is 9 minutes on average. These results confirm that the proposed ANN model could be used as part of a monitoring system for post insulators during icing events for protection against potential flashover hazards. [ABSTRACT FROM AUTHOR]

This research is concerned with the analysis of leakage current, LC, of a porcelain standard station insulator during a glaze ice accretion. The results obtained show that the leakage current during ice accumulation goes through two distinct periods characterized by specific LC waveforms and frequency harmonics. Laboratory tests demonstrated that the duration of the first period is directly dependent on the icing rate. Hence, measuring the duration of the first period is equivalent to measuring the icing rate. However, this requires the determination of either the transition between the two periods or the detection of the onset of the ice accumulation. The results show that icing rate measurement can be achieved by using results of leakage current analysis, like the time evolution of the third and fifth harmonic, as well as the phase angle difference between leakage current and applied voltage. This analysis makes it possible to characterize the severity of an ice accumulation on a porcelain station insulator. [ABSTRACT FROM AUTHOR]

In the last two decades, some new methods, based on the characterization of the dielectric response of transformer insulation in both time and frequency domains are being used by power utilities for assessment of the condition of power equipments. From fields and laboratory investigations, these techniques were found to be highly operating conditions dependant. During normal operating conditions, the temperature inside a transformer is much higher than ambient, depending upon the operating condition. Because field measurements last hours after switching off the transformer, the final temperature may be much lower than the initial. Thus at onsite measurements the water migration is commonly running, the transformer is in a non equilibrium state. This transient can lead to mistaken interpretation of insulation condition. In the current research work, a systematic investigation on the influence of thermal transients on the results of polarization and depolarization current measurements is presented. A series of experiments have been performed under controlled temperature conditions on an oil impregnated bushing model. Precautions that can be taken to minimize thermal transient effects are discussed. [ABSTRACT FROM AUTHOR]