The measurement of partial discharges (PD) is a non-destructive and sensitive diagnostic tool for the condition assessment of insulating systems. Two major tasks of PD measurements may be distinguished, (i) PD detection, hence providing evidence and the type of the PD and (ii) the location of the PD. The question "Where is the PD source?" is amongst others tremendously important for scheduling and starting maintenance/repair actions cost and time efficiently or to perform a risk analysis. Here the possibility to geometrically localize the flaw, by means of arrival times of acoustic PD signals, gets an extremely interesting option. Precise acoustic arrival times are consequently essential to accurately locate PD in a power transformer. The averaging of acoustic PD signals helps to enhance the acoustic sensitivity. The acoustic detection limit is lowered significantly and the determination of the arrival times is made possible for weaker PD. Supplementary steps, like automatic objective arrival time determination or additional wavelet-based de-noising further improve the overall location accuracy. A new location approach works with pseudo-times and allows for the use of robust direct solvers instead of the previously used iterative algorithms.

This paper discusses the suitability of vegetable oil as an insulating medium in power transformers. A high flash point and very good environmental compatibility compared to conventional mineral oil are good reasons to use vegetable oil in power transformers. In this paper, the physical and electrical performance and the aging behaviour of three natural vegetable oils are compared to one synthetic and one traditional mineral oil. The breakdown voltage of new, dry, natural vegetable oil is more than 80 kV, which is above the breakdown voltage of mineral oil. Regarding the aging of paper, natural esters are more friendly toward paper than mineral oil. The water solubility clearly exceeds that of mineral oil but the same dependence of dielectric parameters on relative moisture saturation was observed for all liquids. Aging of vegetable oils under the presence of air leads to oxidation and, thus, to a strong increase of viscosity. Therefore, transformers filled with these liquids need to exclude oxygen from air. The conducted experiments show vegetable oil to be applicable for use in power transformers.

The paper provides essentially a summary of PD measurements applying the UHF range in order to increase the detection threshold, to improve the localization accuracy and to perform on-line measurements of Partial Discharge (PD) in noisy environments. The electromagnetic UHF technique offers good signal to noise ratios, because external PD signals and disturbances can be shielded effectively. A new developed method allows the localization of PD in gas-insulated substations (GIS) by frequency domain measurements. The basic idea is the displacement law of Fourier transformation. The interference phenomena of superposed signals from two sensors give information about the time delay of the sensor signals. On-site PD measurements are made at cable connectors by means of monopole antennas housed in a barrel sleave, while the cable is in service. Thus a sensitive PD measurement even in noisy environment is possible. PD measurements on several 72 kV cable connectors were performed in an unshielded laboratory. On-site measurements during operation showed the great potential for condition assessment. For decoupling sensitive UHF PD signals from the inner of a power transformer tank UHF sensors applied through drain/oil valves are used. Experimental studies indicate that all relevant types of PD possibly occurring within a transformer emit high frequency spectra to be detected with UHF sensors. Furthermore in laboratory experiments and on-site measurements very moderate UHF signal attenuations have been observed.

With the increasing age of the primary equipment of the electrical grids there exists also an increasing need to know its internal condition. For this purpose, off- and online diagnostic methods and systems for power transformers have been developed in recent years. Online monitoring is used continuously during operation and offers possibilities to record the relevant stresses which can affect the lifetime. The evaluation of these data offers the possibility of detecting oncoming faults early. In comparison to this, offline methods require disconnecting the transformer from the electrical grid and are used during planned inspections or when the transformer is already failure suspicious. This contribution presents the status and current trends of different diagnostic techniques of power transformers. It provides significant tutorial elements, backed up by case studies, results and some analysis. The broadness and improvements of the presented diagnostic techniques show that the power transformer is not anymore a black box that does not allow a view into its internal condition. Reliable and accurate condition assessment is possible leading to more efficient maintenance strategies.

The transfer function (TF) these days is a well-known method to detect different types of mechanical damage in power transformers. The most important mechanical faults mentioned by the authors and researchers, which are most likely to be detected using the TF and occur frequently in transformers, are disc-space variation, radial deformation, and axial displacement. These faults are investigated in this paper using three different similar-size test objects. Since the TF method is a comparative method and the measured results should be compared with the reference results, some mathematical methods are studied to compare different TFs. A complete fault detection, which means determining the type, location, and level of the faults by using TF analyses is the main aim of this paper.