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For the majority of proposed wave- and tidal current-driven power generation applications, the electrical generators are submerged in sea water, frequently at many metres of depth. The environment places significant stress on the rotating or translating seals between the driven shaft and the electrical generator leading to reduced reliability and lifetime. A potential solution is to eliminate the seal, thereby flooding the generator and allowing sea water to circulate around the shaft, windings and rotor of the machine. The impact of immersing the windings of the machine in sea water is assessed here. Specifically, the impact that the insulation has on the leakage capacitance as well as the consequent impact the leakage capacitance has on current and voltage oscillations in the switching converter used to excite the winding is assessed. Thermal tests are conducted to assess the impact of the insulation on the thermal conductance of the coil-insulation system. Experimental evidence is provided

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Despite recent advancements in material technology, oil-cellulose insulation systems in form of so called Transformerboard combined with paper and mineral oil are still one of the backbones of high voltage technology. This insulation system is widely used in the high voltage industry, especially in high voltage transformers for AC and HVDC applications, e.g. converter transformers. The space charge behaviour of Transformerboard has rarely been investigated, causing the presumption oil-cellulose insulation systems would not collect significant amounts of space charges. This exploratory work investigated oilcellulose insulation in new condition and at different aging stages.Main objective of this series of measurements was to investigate the influence of isothermal aging on the charge behaviour of Transformerboard. To achieve this, a number of oil-impregnated samples were measured with the pulsed electroacoustic (PEA) method. The non-destructive PEA method has the advantage of enabling multiple measurements on the same sample. The surface of the Transformerboard samples has been grinded to eliminate possible surface effects here. These samples had a thickness of 1 mm and they have been impregnated with mineral insulating oil. Space charge profiles were recorded during polarisation and depolarisation at 20°C and 60°C. New samples were recorded first and compared to samples, which were aged for 11 weeks at 135°C. A preliminary explanation for the charge mechanisms involved is given in the analysis.