Constantly growing amount of renewables and storage installed in the power system results in an increased interest in the power transfer under Direct Current (DC), especially in the low and medium voltage (LV and MV) networks. This is valid for both already existing as well as for newly installed cable systems. Although there is virtually no experience with MV DC networks and accessories, it is widely known that the electric stress distribution within insulation is different for AC and DC voltage. Liquid filled joints utilize an insulating liquid to fill the inner volume of the joint. A moisture sensitive, silicone based liquid can be taken as one of the examples. Beside all dielectric and thermal properties, such liquid has a property of hardening when getting in contact with moisture. By measurements of such material, it has been confirmed that the dielectric permittivity for solid and liquid state is of the same value. Thus the hardening process does not have influence on the field distribution under AC stress. However, the resistivity of the material changes when the hardening starts. This in turn, has an influence on the field distribution under DC. In order to investigate the criticality of liquid-solid interfaces, the DC breakdown testing was performed. More specifically, the testing focused on the interface being normal and tangential with respect the electric field. The literature states that the interface of two different insulating materials is an electrically weak spot. In our experiments, the contrary has been observed. The interface between liquid-solid silicone materials is at least as strong as the liquid form of the dielectric. In the current contribution, we will also discuss the implication of the mentioned findings on the feasibility of utilizing a silicone liquid filled AC MV joint under DC stress

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In recent years, the architecture of the power system has been the subject of a major change. The commonly known structure with centralized generation and one directional power flow, gives gradually the place to the new concept of the power network where both the energy generators and consumers are distributed. Particularly, the continuously increasing environmental concerns as well as the legal regulations result in a widespread installation of renewables. In addition, storage devices are incorporated in the network in order to cope with sudden increase of the generated energy. To facilitate the energy flow from the renewables towards the power network, the proper voltage level is maintained by means of transmission controllers. Such controllers consist of high frequency switching elements and the operation of such devices introduces high frequency harmonics that are propagated in the vicinity of the installed controller. On the one hand, a high content of harmonics is an emerging issue for the power network. On the other hand, some researches show the deleterious effect of high frequency signals on the reliability of electrical insulation. Finally, for many years, the harmonic content was not considered neither during the design nor the component’s testing phase. In our paper, we will firstly review the published researches regarding the influence of harmonics on the reliability of cables, terminations and transformers. Further on, an approach to model the influence of high frequency on the electric field and temperature distribution in two types of medium voltage cable termination, will be made. 1 INTRODUCTION @en

Bentheimer sandstone is a quartz-rich permeable hard sedimentary rock used for core flooding experiments. When fired to stabilize clays (subjected to high temperatures), pyrometamorphical phase changes induce texture and pore framework alteration. As a consequence the new dielectric response may influence wettability. The literature regarding pyrometamorphical behavior during and after thermal treatment is ambiguous, so we evaluate desirable effects (fixation of clay minerals) and undesirable effects (dielectric surface changes) in the matrix. Porosity, permeability, surface charge, specific surface area and dielectric respond were measured before and after firing of samples up to View the MathML source∼1000°C under oxidizing and non-oxidizing conditions. The matrix properties were determined using X-ray diffraction and X-ray fluorescence, scanning electron microscope imaging, and thermomechanical-, and thermogravimetric analysis with differential scanning calorimetry.

Firing causes dehydration, dehydroxylation and irreversible transformation of original clays, organic matter, and carbonates to glass, oxides and feldspars. During heating quartz transfers from α- to β -quartz and back during cooling. This changes the grain volumes and consequently reduces the matrix integrity. The sandstone has a slight porosity and permeability increase (∼5%∼5%). Further, a shift in the point of zero charge toward a higher pH may result in wettability alteration from strongly water-wet to oil-wet. Additionally, a decrease in the permittivity value and marginal dispersion of the dielectric constant (∼5%∼5%) between the high and the low frequencies was observed. Due to firing and related dispersion of the iron oxides within the matrix framework, Bentheimer sandstone becomes a weaker insulator.
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An on-load tap changer (OLTC) is prone to many different defects and degradation mechanisms due to its continuous operation under load condition. A high load current and high operation temperature accelerate the process of contact wear. Contacts erode due to the fact that under certain conditions the contacts develop a resistive layer due to hot insulation oil. As a major cause of OLTC failure, these defects might take place after some years of operation and their frequency increases with the operation time. Regular maintenance is applied due to degradation of the insulation oil. Dynamic resistance measurement (DRM) can be conducted during this maintenance to measure the resistance of the OLTC when it operates. The measurement results provide important information for assessing the actual condition of the OLTC. This paper aims to analyze the effect of test parameters on DRM results from service-aged OLTCs and describes the implementation of DRM in OLTC condition assessment. Two test parameters were selected for observation: the test current amplitude and the circuit resistance. Their amplitude was varied during laboratory experiments as well as on-site after regular maintenance. To quantify the various measurement results, an analytical tool using 4 performance parameters has been developed. Finally for interpretation, with the help of a three dimensional classification system which is presented in this paper, the actual condition of OLTCs can be identified during maintenance activities.@en

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