Model verification and common mode current analysis of grid connected three phase two level converters

Abstract: In this Master’s thesis an in-depth model of a shunt active filter for high frequency analysis is constructed. The purpose is to enable analysis of higher frequency behavior reaching 1MHz. Using the high frequency model, common mode currents can be simulated. The model is built using high frequency impedance measurements of the internal components in one of Comsys ABs active filters. The measurement is done up to 10MHz and equivalent circuits are constructed to mimic the parasitic components. The impedance is measured over the components and from the component terminal to ground. The model containing the equivalent circuits is made and calibrated to match the device under test. Using the measured component values and the measured parasitic elements, a simulation model could be designed. Equivalent circuits were used to model the non-idealities in the components and thus their high frequency behavior. An initial model was run and the simulated signals were compared to the lab setup measurements. The component values were tuned to better match the measured frequencies and damping resulting in a final simulation model. The outcome of the thesis is a model that simulates the overall frequency behavior up to 2 MHz. The major oscillations caused by switching IGBTs and how they are extended through the machine can be observed in the simulation. Fast Fourier transform analysis supports the similarities between measurements and simulations where peaks at relevant frequencies are observed. Common mode currents flowing through parasitic couplings can be approximated using the model. Discussions concerning the frequency and damping adjustments are made where areas such as eddy currents and measuring uncertainties are touched upon. A comparison between reality and simulation is done where differences are analysed and discussed. The thesis provides a model that can be used up to 2MHz where common mode currents can be approximately simulated, therefore the purpose was achieved. It lays a foundation for continued work concerning EMC and high frequency behavior in active filters. Further work to improve upon the model could be simulating non-ideal IGBTs and locating the large damping resistance present in the lab setup. To use the model in deeper analysis the model should be run in different operation states to ensure the performance of the model is satisfactory.