Presently, maintenance management of assets in infrastructure utilities such as electricity, gas and water are widely undergoing changes towards new working environments. These are mainly driven against the background of stringent regulatory regimes, an ageing asset base, increased customer demands and constrained financial resources. Therefore, it becomes imperative for infrastructure utilities to strive towards more effective and efficient operation and maintenance approaches. In this light, new asset management approaches such as risk management are gaining more interest worldwide. In this research the focus has been on the framework for introducing modern maintenance management in electricity network organisations. It covers the development and practical application of enabling factors for maintenance management such as organisation capabilities, maturity models, structured and comprehensive frameworks for assessing risk based maintenance policies and decision-making tools and technologies. Thus, an integral, multi-disciplinary, approach towards maintenance management needs to be established, which is a challenge that has been taken up in this thesis for electricity network organisations.@en

Worldwide there is an increasing need for a more sustainable form of electrical power delivery with a growing share of renewable energy generation. In the distribution and transmission network, large-scale and small-scale wind and solar power plants will be introduced, in proportion to the annual economic growth. The transmission and distribution network will be expanded, focusing on the electricity transport, however, there will also be a need for exchanges with neighboring countries. Alternative solutions are needed in order to support the changes of the future grid. High temperature superconductors are an alternative to conventional conductors, due to their high current density and very low AC loss, and therefore deserve more attention. The purpose of this study is to explore ways to integrate high-temperature superconducting cables in a future network and to compare their favorable technical properties with, e.g., the conventional XLPE cable. The development of 2nd generation high temperature superconducting tapes results in a high tape quality, making it very attractive for use in superconducting power transmission cables. At the same time, the network requirements placed on the grid, based on society needs are changing, such as low magnetic field emissions, reducing space requirements, lower losses, minimizing visual intrusion, etc. Our study shows that superconducting cables compared to conventional cables score better on these societal requirements. From our comparison of three practical low and high temperature superconductors we can conclude that Yttrium Barium Copper Oxide is the most suitable superconductor for use in transmission cables. Our techno-economic analysis shows that superconducting cables become already competitive with conventional cable in the AC transmission, such as XLPE cables. Possible future problems concerning the transport capacity in the power grid where high temperature superconducting cables can offer a solution have been identified. For one promising location, we have formulated the requirements for the design of a high-temperature superconducting cable. Next, we propose two types of cable systems (with cold and warm dielectric). For both types we describe the core, the electrical insulation, the screen, the cryostat, the cooling system, etc. Also for the distribution grid a techno-economic investigation is conducted. To assess the feasibility of the application in distribution grids, we have experimentally demonstrated a reduction of AC conductor losses from 1 W/m to 0.1 W/m. We also carried out an experimental investigation to improve the developed cryostat design for a 6 km long cable connection. Despite these substantial technical improvements our economic study showed that the high-temperature superconducting distribution cable is not yet competitive with the present conventional distribution cable systems except for niche locations where additional advantages e.g. magnetic emission, reduced space usage, power density weighs more heavily. Besides the HVAC grid the Netherlands has HVDC interconnections with neighboring countries and there are initiatives for the use of DC high voltage connections to wind farms further out to sea. In our study, we make reference to a suitable location, where the above mentioned attractive features of the superconducting cable are applicable as well. We advised a modified design of a HTS HVDC cable which enables a possible upgrade of the transmission capacity of the HVDC link at such location. The main results from the investigation are that: • Based on our techno-economic analysis HTS cables offer the most competitive solution in transmission grids. Introduction of such cables will reduce HTS tape price, which in turn will stimulate further applications. • Conceptual designs of competitive HTS AC and DC transmission cables are formulated along with that for HTS AC distribution cable. Novel designs allow for much longer length between cooling stations. • Our experimental research has shown that HTS cable core losses may be reduced by a factor 10 (down to 0.11 W/m/phase at 3 kArms, 77 K, 50 Hz). • Dedicated low friction cable cryostat was developed and successfully tested for 47 meters length. Patented multi-layer thermal insulation improves the cable cryostat heat leak from 1 W/m to 0.1 W/m.@en

The addition of microsized fillers to polymers, in order to tailor their properties, has been extensively used in many industrial applications since the 1960s. The same approach applies to the field of electrical insulation. Epoxy resin is a widely used polymer in the electrical power sector, but it is usually loaded with microsized fillers, such as aluminum oxide and silicon dioxide, mainly to increase its thermal conductivity, improve its mechanical properties, and to decrease cost. Polymers with microsized fillers are called microcomposites. In the mid-1990s, a new type of polymeric composites for high voltage applications, the so-called nanocomposites, emerged. The main characteristic of these composites is the small filler size, which is smaller than 100 nm at least in one dimension. Since then, there has been a growing interest in the performance of polymeric nanocomposites for high voltage applications, including epoxy nanocomposites. The performance of nanocomposites is mainly related to the tremendous effective internal surface area of these materials because of the high surface-tovolume ratio of nanofillers. After 20 years of research, a significant amount of data has been generated which reflects the potential of nanodielectrics. It has been shown that nanofillers are capable of contributing to the improvement of both the thermal and electrical properties of polymers. However, the laboratory performance of nanocomposites is inconsistent and unpredictable. These are the main factors which inhibit the applicability of nanodielectrics. Important challenges in the field of epoxy nanocomposites should be overcome before nanodielectrics can be produced on an industrial level. The most important challenge is related to the dubious reproducibility of the nanocomposite performance which is closely related to sample homogeneity. Thus, the effectiveness of separating the nanoparticles from each other and the homogeneous incorporation of them into the polymer matrix are expected to affect the performance of nanocomposites. However, the extent to which the behavior of nanocomposites is influenced by sample homogeneity is not well defined. In this thesis, a number of epoxy nanocomposites and mesocomposites were synthesized aiming at the analysis of the parameters which influence their thermal and electrical properties. The analysis includes the thermal and electrical conductivity, dielectric response, and breakdown strength under both AC and DC electric fields. The experimental results demonstrate the important role of interfaces in the behavior of epoxy nanocomposites. Based on the experimental results, important parameters for determining the performance of nanocomposites are suggested to be the polymer re-organization and water uptake. The former is related to the influence of nanofillers on the polymer structure, i.e., the areas in the vicinity of nanofillers are assumed to exhibita different behavior from the rest of the polymer matrix. The uptake of water is related mainly to the hydrophilic nature of nanofillers and plays a significant role in the electrical performance of nanocomposites. Apart from the aforementioned mechanisms, the presence of structural imperfections should not be neglected as they affect both the thermal and electrical properties of epoxy nanocomposites. Additionally to the experimental part, models were developed for both the relative permittivity and thermal conductivity of nanocomposites. The models are based on the two aforementioned parameters; polymer re-organization in the vicinity of nanofillers and water uptake due to the hydrophilicity of nanofillers. The main characteristic of both models is the use of the same structure which strengthens the validity of the assumptions. The experimental results are in good agreement with the model results. Also, a large part of the thesis is devoted to the evaluation of the influence of sample homogeneity on the performance of nanocomposites. For this purpose, nanocomposites with different synthesis techniques were fabricated. The results suggest that the thermal conductivity, dielectric response, and breakdown strength (AC and DC) of epoxy nanocomposites are not significantly influenced by the nanoparticle distribution. This observation suggests that high levels of reproducibility can be achieved when the particles are similarly dispersed and differently distributed. Finally, hybrid composites which combine both microsized and nanosized fillers were fabricated, tested, and analyzed. This type of composites is more likely to be employed in industry as epoxy resin in its pure form is rarely used for high voltage applications. It is usually reinforced with high loadings of microparticles. Microcomposites reinforced only with a small amount of nanofillers, i.e., less than 1 % by volume, show a significant thermal and electrical improvement.@en