There is an increasing demand for compact onsite dielectric test systems of medium and high voltage equipment. Though there are commercially available mobile high voltage (HV) test sources, they have some important drawbacks such as limitation in voltage capability, complexity in building up the test setup and lack of versatility to test different equipment. Alternatively, the present challenges in these conventional test sources could be resolved by constructing a power electronics based test source. Amongst various converter topologies, Modular Multilevel Converter (MMC) is considered as a suitable topology for this application because of its high efficiency, modular structure, and reduced filter requirement. In order to achieve a compact solution, an oil immersed design of MMC, instead of the typical air insulated converter design is proposed in this master thesis. Oil can act both as a coolant and an effective insulating medium, thereby reducing the dimensions of the test system.
This master thesis demonstrates the feasibility of oil immersed MMC based HV test source, covering both system level and component level aspects. The extent of compactness of the test source in oil was estimated at a system level. It was found that for the given trailer dimensions, the voltage capability of test source is around 4 times higher in oil when compared to air, This shows the effectiveness of oil as an insulating medium. The feasibility of oil immersed converter design depends on the degree of compatibility of its components under oil. Hence, experimental investigation was conducted on Insulated Gate Bipolar Transistor modules (IGBT) under oil as they are considered to be a pivotal component in MMC. Preliminary results show the penetration of oil into the IGBT chip surface. Despite oil migration, the operation of IGBT was not hindered during the test. Based on the results obtained from experimental work, a road map of future tests is suggested that need to be performed to realize an oil immersed HV mobile test source.