Electric energy demand has been growing constantly as the global population increases.To avoid electric energy shortage, renewable energy sources and energy conservation areemphasized all over the world. The role of power electronics in energy saving anddevelopment of renewable energy systems is significant. Power electronics is applied inwind, solar, fuel cell, and micro turbine energy systems for the energy conversion andcontrol. The use of power electronics introduces an energy saving potential in suchapplications as motors, lighting, home appliances, and consumer electronics.Despite the advantages of power converters, their penetration into the market requiresthat they have a set of characteristics such as high reliability and power density, costeffectiveness, and low weight, which are dictated by the emerging applications. Inassociation with the increasing requirements, the design of the power converter isbecoming more complicated, and thus, a multidisciplinary approach to the modelling ofthe converter is required.In this doctoral dissertation, methods and models are developed for the design of amultilevel power converter and the analysis of the related electromagnetic, thermal, andreliability issues. The focus is on the design of the main circuit. The electromagneticmodel of the laminated busbar system and the IGBT modules is established with the aimof minimizing the stray inductance of the commutation loops that degrade the converterpower capability. The circular busbar system is proposed to achieve equal current sharingamong parallel-connected devices and implemented in the non-destructive test set-up. Inaddition to the electromagnetic model, a thermal model of the laminated busbar systemis developed based on a lumped parameter thermal model. The temperature andtemperature-dependent power losses of the busbars are estimated by the proposedalgorithm. The Joule losses produced by non-sinusoidal currents flowing through thebusbars in the converter are estimated taking into account the skin and proximity effects,which have a strong influence on the AC resistance of the busbars.The lifetime estimation algorithm was implemented to investigate the influence of thecooling solution on the reliability of the IGBT modules. As efficient cooling solutionshave a low thermal inertia, they cause excessive temperature cycling of the IGBTs. Thus,a reliability analysis is required when selecting the cooling solutions for a particularapplication. The control of the cooling solution based on the use of a heat flux sensor isproposed to reduce the amplitude of the temperature cycles. The developed methods and models are verified experimentally by a laboratory prototype.
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