Multidimensional Analysis of a Real Transformer Fleet Based on the Evaluation of Oil Properties
1HTWK Leipzig, Leipzig University of Applied Science, Faculty of Electrical Engineering and Information Technology, Department of electric power systems; 2FTZ Leipzig e.V., Research and transfer centre at the HTWK Leipzig, Germany; 3ABB AG, Halle/Saale, Germany; 4Leibniz University Hannover, Institute of Electric Power Systems, Division of High Voltage Engineering and Asset Management, Schering-Institute, Germany
Transformers belong to the most important and most elementary elements in the power system. Because an unplanned outage of a transformer can lead to interrupted power supply or the stop of production, the related costs can be much higher than the costs of the transformer itself. Therefore it is of elementary interest for operators of transformers to know the condition of the transformers, identify risky units and to avoid unplanned outages. Therefore different methods of condition assessment were developed. The most important groups of those methods are electric and dielectric measurements of the transformer and the different analysing techniques on the insulation liquid (oil tests). The different oil tests can be separated into two main groups of parameters: Firstly the parameters of the standard oil test (SOT), which show how far the oil is still able to withstand the thermal and the dielectric stresses occurring during the operation. Secondly, failures at the active part can be indicated with the help of the results of the dissolved gas analysis (DGA). This method is based on two steps: In the first step it must be checked if the results show typical or untypical gas concentrations. If untypical or increased gas concentrations are found, the type of the failure can be identified by the gas ratios in the second step. In this contribution both groups of parameters - the results of the standard oil test as well as the results of a dissolved gas analysis - were analyzed in a multidimensional investigation to observe if dependencies between both groups of parameters can be found. Therefore it is investigated if typical increased gas contents can be found depending on oil properties or if both groups of parameters can influence each other. These investigations were based on a huge number of oil test results of real transformers of different voltage levels, applications, age, and manufacturers.
Estimation of the Ageing Condition of Oil-filled Transformers based on the Oil Parameters Using a Novel Fuzzy Logic Algorithm
1Leibniz Universität Hannover, Germany; 2School of Electrical Engineering and Informatics, Institute of Technology Bandung, Indonesia; 3GRIDINSPECT GmbH, Feldatal, Germany
The service availability of transformers is vital for the continuity of electricity supply. The condition of the transformer insulation determines the remaining life of the transformer. Over the transformer service life, both liquid and solid insulation undergoes a continuous aging process under electrical, chemical mechanical and thermal stresses. The insulating liquid of a transformer can be reconditioned or replaced; however, this is not the case for cellulose insulation. Therefore, the condition of paper insulation is the major factor determining the aging status of a transformer. To assess the paper condition, the common method is to measure the degree of polymerization (DP) of the paper insulation as a significant parameter. This method is destructive as it requires a sample of paper from the inside of the transformer. Therefore, it could not be applied for operating transformers. There are several approaches to calculate DP value without direct measurement of paper samples. Our previous studies revealed the correlation between DP of paper insulation and dissolved gases, electrical and chemical parameters.
This research presents an improved method based on a fuzzy logic system for estimation of DP of paper insulation based on dissolved gases and physico-chemical parameters of oil insulation. A special algorithm is developed to create the rules for the use of fuzzy sets based on the information gain extracted by the entropy of the laboratory measurement data. This algorithm employs entropy to examine the sample homogeneity. Entropy is a measure of information theory that can determine the dataset characteristics concerning impurity and homogeneity. The novel algorithm uses fuzzy sets of oil parameters like Acidity, Interfacial Tension (IFT), Carbon Dioxide (CO2) and Carbon Monoxide (CO), and the breakdown voltage (BDV) for determination of the DP value.
Correlation of Dielectric Parameters and Insulation Properties of Power Transformers
Oil-paper insulated power transformers exist since more than 125 years and since nearly a century, electrical measurements are in use for the assessment of their insulation condition. For a long time, high voltage DC resistance measurements and high voltage dielectric loss measurements at power frequency have been the commonly used methods. Frequency variable high voltage dielectric measurements, Recovery voltage measurements (RVM) and polarization-depolarization-current measurements (PDC) were developed towards the end of the last century in order to gain more information about the insulation condition. Modern technology enables more advanced analysis and with the use of broadband dielectric measurements, also known as dielectric frequency response (DFR), it is possible to determine the absolute moisture content in the cellulose insulation as well as the oil conductivity. Nonetheless, the old measurement methods are still widely used for condition assessment of power transformers but often without a profound knowledge of the exact meaning of the measured values. This paper uses DFR measurements on more than 100 power transformers and tries to correlate the different measurement values and the condition of the insulation, represented by the moisture content in the cellulose and the oil conductivity. It shows that most measurement values obtained by classical measurements are dominated by the influence of the oil conductivity and that the moisture content only has minor influence.
Analysis of the Calculation of Translating the Dielectric Spectroscopy of Major Insulation of Transformer to Oil-impregnated Paper Using XY Model
State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China
The FDS (Frequency Dielectric Spectroscopy) Analysis has good application prospects in transformer insulation detection due to its non-destructive measurement and robustness against noise. Two steps are needed when analysing transformer insulation using FDS method. First translate the dielectric spectroscopy of major insulation to oil-impregnated paper. Then analyse aging and moisture degree of oil-impregnated paper based on the dielectric spectroscopy. Using a commonly used model named XY model, the complex permittivity of paper can be calculated by solving a quadratic equation, which has two solutions. Define these two solutions respectively as solution1 and solution2. In this paper, the characteristic and distribution of these two solutions are studied. Firstly, introduce the calculation process of translating dielectric spectroscopy of major insulation to oil-impregnated paper. Then do the calculation for the cases of measured complex permittivity of oil-impregnated paper with different moisture content. The results show that the correct solution of most conditions is solution1. With the moisture content of paper decline, the range of frequency where solution2 is correct reduces. Furthermore, do the calculation for universal cases and analyse the influences of the relevant five variables. The results show that for all the situation, either the real or imaginary part of one of the solutions is negative, which is unreasonable and easy to be excluded, indicating the impossibility that both solutions are reasonable. Besides, the distribution of the two solutions is studied. Only when w<0.01 rad/s, part of the correct solution becomes solution2 and enlarges with the decrease of w. And the increase of X and Y shrink the outlines of area which solution2 is correct and move in positive direction of X-axis.
Moisture Saturation as a Universal Evaluation Criterion for Water Contamination in Insulation Systems
1Magdeburg University of Applied Sciences, Germany; 2University of Stuttgart, Germany
This paper discusses the theoretical foundation and proposes the practical application of moisture saturation in oil and also in paper as a parameter, which correlates directly with the detrimental effects of moisture in oil-paper-insulations: decrease of breakdown voltage, bubble evolution and accelerated aging. Moisture migration and equilibrium are determined by differences of water potential. Considerations about thermodynamic equilibrium provide the theoretical background for moisture measurements through equilibrium. The relationship between moisture saturation and detrimental effects of moisture in oil-paper-insulations are discussed in detail. The practical implementation of measurement and evaluation of water saturation concludes the paper.