PD-source localization in the insulating system of large power transformers

PD-source localization in a complicated multi-material insulating system like that of a power transformer is still a challenge for all testing engineers. Additionally to the measurement of apparent charge with a conventional PD-system (according to the Standards), detection of acoustical PD-signals can be used to localize the PD-source. In many cases however, especially for PD-sources hidden in the solid insulation or in the main insulation of the transformer, there are no acoustical PD-signals detectable. In such a situation the consequent application of an advanced PD-system performing both statistical analysis of the PD-signals (PD-pattern) and analysis of PD-current impulses in time- and frequency-domain is the next step in the localization procedure. Here localization is based on the comparison of the results delivered by the real PD-source with characteristic responses of the RLCM-network of the specific transformer to calibrating impulses applied at the respective bushing. Results of these measurements and a detailed analysis of the transformer design reveal the areas of high electrical field in the insulating system where the PD-source is located. In this contribution both the localization procedure and the application to the real case of 340 MVA, 236/21 kV three phase transformer are discussed