The transformers are very complex devices, consisting of different subcomponents e.g. winding, tap changer, bushing etc. During operation, all of the subcomponents are stressed and subjected to aging. When the aging reaches certain level, the subcomponent may fail. By application of statistical tools it is possible to investigate the failure occurrence in transformers. That means, that by fitting the mathematical model to the data, the parameters of the transformers' population can be estimated. By doing so, it is possible to see what is the stage of life of transformers, and what can be expected in terms of coming failures. However for a proper analysis, it is important to know how to deal with failures that do not seem to follow the statistical distribution chosen. Such outliers may affect the analysis considerably. In the paper, the influence of the outliers on the results of expected failures is presented. The latter is very important for the utility, as such information can be used as an additional information when deciding about replacement of transformers. Also, when deciding about spare transformers, knowledge about failure expectation is very valuable.@en

Static resistance measurements on power transformers give an indication about increased contact resistances of the OLTC. Increased contact resistances can also be measured by means of a dynamic resistance measurement (DRM). This paper elaborates on condition diagnosis of electrical contacts of in-tank high-speed resistor tap changers using DRM. DRM uses a low test current through the on-load tap changer to detect increased contact resistances and current interruptions. The difference with static resistance measurements is that the tap changer is switched through all its tap positions during the test. The test current gets no change to fully stabilize due to the inductance of the transformer windings. DRM is therefore less accurate then static resistance measurements but contains more information about the type and location of tap changer defect. This contribution will show that more defects on OLTC's can be found with DRM compared to static resistance tests: the additional value of DRM compared to static winding resistance measurements is discussed. Secondly, important topics that should be considered when performing dynamic resistance measurements are discussed, like the effect of the test current amplitude, the secondary short-circuit and circuit resistance. Simulation results and experimental data are presented to make the reader familiar with the pros and cons of dynamic resistance measurements.@en