Empirisk bestämning av nollföljdsimpedans hos transformatorer med utdragen deltalindning

Abstract: The main goal of the thesis is through available data, empirically determine the ratio betweenthe zero and positive-sequence impedance of different transformers with accessible-winding, i.e. when the -winding is loaded. Reasons for using a -winding couldfor example be to generate a specific phase shift in the voltage between the primary andsecondary side, or because it is economical at high current and low voltage. The thesisalso include an analysis on dependencies to tap-position, transformer construction andconnection type (auto connected or fully wound), as well as if variations can be related tomanufacture, type denotation, production year, rated power and rated voltage.Information of the ratio between the zero- and positive-sequence impedance are used tocalculate, for example, fault currents and are important when setting the protection system.A transformer’s sequence impedance is usually determined at the end of productionthrough a number of tests. However, there are several transformers in the power systemwithout information of the zero-sequence impedance and thus, this thesis has been performedsince necessary tests cannot be performed when the transformer is in operation.The data has been divided into six groups: shunt reactors, two-winding transformers witha -winding, three-winding transformers with one -winding, three-winding transformerswith two -windings, two-winding transformers with shorted Y-winding and withouta -winding and three-winding transformers with shorted Y-winding and without a -winding.To analyse the available data, an algorithm has been created in MATLAB to find possiblecorrelations. Relations has been linear modelled and are presented visually and numericallywith prediction intervals to be used in future network calculations. The mean andthe confidence interval of the ratio between the zero- and positive-sequence impedance,as well as the ratio between the zero- and positive-sequence resistance has been calculatedfor each group, depending on the number of core legs of the transformer.The most important results include the mean (and the confidence interval) for the ratiobetween the zero- and positive-sequence impedance, as well as the ratio between the zeroandpositive-sequence resistance for transformers with three and five legs. A correlationwas found for the ratio between the zero- and positive-sequence impedance and the tapposition.Furthermore, it was found that the ratio between the zero- and positive-sequenceimpedance has had an upgoing trend over the years, while the ratio between the zero- andpositive-sequence resistance has had a downwards trend. This is especially true for twowindingtransformers with an -winding. Following, a correlation was found betweenthe zero-sequence resistance and the positive-sequence impedance, where the data pointswere divided into two groups, where one of them was gathered around transformers withfive-leg core. A possible explanation could be that these transformers are equipped withmagnetic shunts. However, this was not possible to investigate in this study.