Bulk power transmission over long distances can be achieved by High-Voltage Direct Current (HVDC) systems due to technical and economic reasons. In order to transmit the electrical energy generated by the offshore wind turbines in the North Sea to the consumption centers located in the south of Germany, HVDC transmission technology will be adopted in the next years. Under DC voltage stress the accumulation of charges arises in the insulation systems in HVDC equipment. Together with critical voltage stresses such as polarity reversal and impulse voltages, this can lead to an excessive electric stress. The aim of this research is the quantification of the relevant phenomena with the consideration of the material properties and their influence on the electrical field distribution in the insulation systems and hence on the insulation strength of HVDC equipment.
Polymeric insulating materials which contain few percent by weight of nanofillers (< 10 wt. %) are called nanocomposites. Such nanocomposites can exhibit improved electrical and dielectric properties compared to unfilled materials and materials with a huge amount (approximately 50 wt. %) of microfillers. This is attributed to a spatially extended interaction zone, the so called interphase, which can be formed between the filler particles and the polymer matrix. Because of the high ratio of surface to volume for nanoparticles the interphase can take a remarkable percentage of the whole composite material in nanocomposites, even for small filling degrees. Therefore its properties can be decisive for the properties of the composite material.
There are several models existing for the description of the interphase. Using electrostatic force microscopy the interphase could be made visible for the first time and its extension could be determined in own research work. In further research projects the interphase shall be characterised electrically and chemically with appropriate methods. With respect to tailor made materials with an extended interphase the factors of influence on the formation of an interphase around nanofillers shall be clarified.
For many decades sulfur-hexafluoride (SF6) has been used as insulating gas with good arc-extinction properties in gas-insulated metal-enclosed switchgear (GIS) and circuit breakers. Presently, the high global warming potential (GWP) of SF6 is motivating a worldwide search for environmentally friendly alternatives. Promising alternative gases with dielectric properties comparable to SF6 have a high dew point, and can be used in GIS as gas mixtures together with carrier gases only. As a precondition for their practical use, characteristic properties of alternative gases and their mixtures have to be examined. The focus of the research project with the new insulating gases is on the insulating capacity of different electrode arrangements with different gas mixtures relevant for high voltage application, the influence of gas liquefaction and the modeling of discharge development.
Partial discharges are the result of overstressing electrical insulating systems. The measurement and the analysis of partial discharges are sensitive diagnosis tools and the base for a condition assessment of high voltage apparatus. They are used in research, during the manufacturing process, after implementation and during the normal operation, too. In the case of alternating voltage, there are hints of the type of fault given by the phasing of the partial discharge versus the testing voltage. In the case of DC, no such a phasing is available. To be able to use the partial discharge diagnosis under DC voltage stress, new methods of partial discharge analysis must be developed.
The aim of this research project is the clarification of the physical mechanism which are active, their modelling and the deductive development of a measurement method including the measurement instrumentation. The research is done for different insulating systems on representative specimen with defined defects.
For further information please contact: Lucas Höfer, M.Sc.