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

This paper discusses the recently developed analysis method for service aged power transformers equipped with an OLTC: dynamic resistance measurements (DRM). Because the on-load tap changer contributes to many power transformer failures the emphasis is on OLTC diagnosis. Off-line diagnosis of on-load tap changers during regular maintenance can detect maintenance errors and assess the condition of OLTC parts not accessible for inspection. DRM can be valuable to on-load tap changer (OLTC) condition assessment because of the large variety of defects and OLTC degradation mechanisms that can be found. This paper deals with the results that can be measured with DRM. Besides undisturbed DRM-data, most common deviations originating from degraded and defect tap changers will be discussed. It is explained how these different types of defects appear in the DRM results. The location and shape of the deviation can be used to find the cause and importance of the defect. A subdivision is made between OLTC's in good condition, OLTC's with increased contact resistance, OLTC's with open contacts and OLTC's with deviating switch times. By doing so, an overview of possible DRM results is gained. The seriousness of the defects is also discussed in this paper. Differences between the DRM results of different types of OLTC's will be discussed by showing results of DRM tests. Most graphs presented in this paper are recorded on service-aged OLTC's in the Dutch utility grid. Some measurement results are performed during experiments in a high voltage laboratory.@en

Diagnostics of HV power cables are playing an important role in the meaning of two aspects: 1) on-site diagnostic of new installation, where a commissioning test has to prove the absence of any defects in the cable insulation and accessories, after transportation and installation. 2) On-site diagnostic of a serviced aged power cable where the condition after a period in service or information about workmanship of repairs has to be known. As it is known some installation defects can result in breakdown even after a successful after-laying test. Obtaining more diagnostic parameters like; Withstand Voltage, Partial Discharges (PD) and Dielectric Loss (tan ¿), providing a indication of insulation condition and accessory performance. In this paper several examples of on-site tests on new and serviced aged XLPE HV power cables will be discussed. Examples of: after-repair, after-laying, and condition assessment tests will be presented as performed with Damped AC diagnostics.@en