Mätning av krafttransformatorers nollföljdsimpedans

In the E. ON sponsored research project, Distibution LABoratory, at the Department of Industrial Electrical Engineering and Automation at LTH the impact of the increased move towards use of cables in the Swedish distribution grid is examined. The project also built a model of a small distribution grid. The model was used to simulate a Petersen coil earthed grid system. Petersen coil earthing lowers the fault current by compensating the capacitive currents. Since the share of cables in the medium-voltage networks have increased so have the capacitive currents. In order to minimize the fault current the inductive compensation currents need to be increased. When calculating the compensation current an uncertainty is found as an unknown zero sequence impedance in the Yyn-connected transformers. The purpose of this master thesis is to measure the zero sequence impedance of the power transformers, investigate the transformer tank's impact on it, and to develop a method that can be used for measurement on the full-scale transformers. The work shows that the zero sequence impedance in Yyn-connected transformers is not infinite as many textbooks show. For phase-to-ground faults the zero sequence magnetic flux is forced to flow into the air and the transformer tank which greatly affects the size of the zero sequence impedance. As a consequence, the zero sequence impedance should be measured on site and after installation to obtain an more accurate result. The measurements of the zero sequence impedance in transformers gives some uncertainty that makes it difficult to draw definite conclusions. Measurements made on full-scale transformers where raw data is saved would increase the possibility to process the information and to exclude the influence of the Petersen coil earthing used in the distribution model.