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This article discusses an advanced method for estimating dc conductivity of oil using parameters evaluated under switching impulse voltage application. Firstly, the effectiveness of switching impulse application for assessing the condition of oil–paper insulation is investigated. Thereafter, the evaluated parameters under switching impulse voltage are fitted by Havriliak and Negami (HN) model for estimating the oil conductivity. The method is applied on oil-impregnated pressboards containing different moistures and a distribution transformer for experimental investigation. Moreover, the limitations of the proposed method are also discussed in this article. [ABSTRACT FROM AUTHOR]

This paper presents an advanced technique for calculating activation energy of oil-impregnated pressboard through dielectric response current measurement under switching impulse voltage at different temperatures. The proposed technique analyzes the measured impulse voltage and current signals to formulate its equivalent circuit at different temperatures. The equivalent circuit parameters are utilized to determine the shifting of ac conductivity curve along frequency axis with variations in temperatures. Based on the shifted ac conductivity curves at different temperatures, the corresponding activation energy of the insulation is estimated. The effectiveness of the proposed technique is investigated through a few laboratory prepared test samples. [ABSTRACT FROM AUTHOR]

This study experimentally investigated and theoretically analysed the evolution of gas bubbles in a wet oil-paper insulation system. Water vapour bubbles significantly decrease the dielectric withstand strength of insulating oil. The influence of moisture content on paper, quality of oil, quality of paper, ageing state and temperature increase on this 'Bubble Effect' was systematically investigated. New thermally upgraded paper combined with new insulation oil shows the best resistibility against bubbling because of its microstructure. Paper ageing increases the bubble inception temperature, whereas oil ageing decreases this parameter. Bubbles appear only for a steep temperature rise of more than 3 K/min; for lower values the moisture diffuses into the oil only. An empirical equation that calculates the inception temperature with respect to paper and oil quality and ageing is derived. The evolution of gaseous bubbles constitutes a risk for transformer operation if a steep temperature gradient occurs, if the moisture in paper is above 2% and if bubbles move into an area with high dielectric stress. [ABSTRACT FROM AUTHOR]

The ability of a detailed RLC model of a winding to calculate the transfer function (TF) and to demonstrate the effects of various winding damages on TF has been proven on small laboratory windings by previous studies. However, this ability is not adequately known for real high-voltage windings. Hence, a 400 kV disc winding consisting of 86 discs is used in this work to study, whether the detailed model is able to represent the behaviour of large-power windings in a TF form. In addition, disc space variation (DSV), known as a frequently occurring mechanical damage in transformers, is analysed experimentally and mathematically. The accuracy of the model to identify the DSV is verified with the help of the measured TFs. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the influence of surface charges on the discharge development across particle contaminated spacer surfaces for lightning impulse voltages. Results; Discussion; Conclusions.

This contribution describes an evaluation method for phase resolved patterns of partial discharges over time. Therefore, their specific shape characterizes patterns in question. The method searches for these characteristics within PRPD patterns by comparing characterization and PRPD using 2-dimensional cross-correlation. Crosscorrelation represents a measure for the similarity between different PRPD patterns. A use case determines the usability of the method in practice. The result is the time dependent occurrence of specific patterns, respectively specific PD sources. Long-term monitoring data of an aged generator step-up transformer with several PD sources of varying activity is used as PD dataset. PDs are recorded by the use of the electro magnetic PD measurement method, also known as the UHF-method. [ABSTRACT FROM AUTHOR]

In the context of thermal performance and thermal design, it is of significance to predict the magnitude and the location of the 'hot-spot' temperature inside a power transformer. In an attempt to accurately predict this hot-spot in an oil-immersed transformer, various numerical modelling approaches have been developed for calculating the cooling oil flow distribution, which are generally categorised as either 'network models' or the methods that incorporate forms of computational fluid dynamics (CFD). In network modelling, the complex pattern of oil ducts and passes in a winding is approximated with a matrix of simple hydraulic channels, where analytical expressions are then applied to describe oil flow and heat transfer phenomena. On the other hand, CFD models often adopt discretisations of much higher fineness, which can be expected to offer a higher order of accuracy but also comes with a large increase in the required computational resources. In order to compare both modelling approaches, the network model implementation TEFLOW and a commercial CFD package, ANSYS-CFX, were applied on a typical 'zigzag' oil channel arrangement of a disc-type winding to predict oil flow distribution and disc temperatures; experiments on hydraulic models have also been performed to validate the models. The principle work of this study is then comparing the results and concluding recommendations to industrial practices. [ABSTRACT FROM AUTHOR]

The paper, prepared by CIGRE WG D1.25, presents the experience in practical application of sensitivity verification of the UHF partial discharge detection system for GIS after its introduction by ELECTRA Report in 1999. A detailed description of the two-step procedure, Step 1 in laboratory and Step 2 on-site, is given and supported by examples. Guidelines will help manufacturers and users in the effective application of the UHF method for PD detection on GIS and will permit to avoid technical misinterpretations. The procedure is described in details in CIGRE Technical Brochure. [ABSTRACT FROM PUBLISHER]

The aim of this paper is to present the first full-scale experimental validation of the very fast transient (VFT) wave damping capability of high-frequency resonators in gas-insulated switchgear (GIS). In this regard, the numerical eigenvalue analysis method implemented in COMSOL is used to design and tune the HF resonators. The accuracy of the numerically computed resonance frequencies using this method is validated against the alternative finite-difference time domain method implemented in CST Microwave Studio and against the experimental data from low-voltage measurements. The designed resonator was then implemented in the test setup of a realistic 550-kV GIS where it was subject to VFT waves resembling the disconnector switching operations. The damping mechanism is achieved by using a lossy cylinder in the resonator gap. It is shown that the presence of the lossy resonator results in damping of the VFT wave total energy by 27% even though its first peak is slightly enhanced. The damping efficiency in terms of the voltage amplitude at the resonance frequency is about 60%. [ABSTRACT FROM AUTHOR]

The paper, prepared by CIGRE WG D1.03 (TF 09), presents the guidelines for risk assessment procedure on defects in GIS based on PD diagnostics. The procedure, described in detail in CIGRE Technical Brochure 525, starts with sensitive PD measurement to detect the critical defects and follows with identification of the type of the defect and its location inside the GIS. This information taken together with other essential data from laboratory measurements, manufacturer's experience, design aspects and trend analysis of the PD activity, are the base for the estimation of the criticality of the defects. Finally, the risk assessment is performed based on the estimated dielectric failure probability and failure consequences that can be different in case of on-site testing or in service activity. [ABSTRACT FROM PUBLISHER]