Active Filter Capability of a Voltage Source Inverter in Marine Applications
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- Institutt for elkraftteknikk 
This thesis is centered around the possibilities of utilizing the active filter capability of a Voltage Source Inverter (VSI) to mitigate harmonic distortion in a marine power system. Hotel loads in marine vessels constitute an increasing part of the installed power onboard, as ships are increasingly electrified. Hotel loads represent the loads that do not contribute to the propulsion of the vessel, e.g. lighting or cranes. The system studied in this thesis consists of a power supply supplying hotel loads in the MW range. The power supply, which is connected to the internal DC-grids of the ship, consists of a VSI and a LCL-filter. The problem is related to the highly distorting hotel loads propagating harmonic distortion into the system. The harmonic distortion is mainly produced from the diode rectifier stages in Variable Frequency Drives (VFDs). The VFDs are used to control AC-motor torque and speed by varying frequency and voltage at the motor terminals. The main objective of this thesis is to ensure a voltage waveform at the terminals of the power supply within the requirements on Total Harmonic Distortion (THD) set by Det Norske Veritas (DNV). The worst-case THD obtained in the system exceeds the requirements. Measures must be taken in order to lower the harmonic content at point B and fulfill the requirements. An active harmonic filter integrated to the control of the VSI was developed in order to mitigate the harmonic distortion. Three cases have been simulated, with different degrees of harmonic filtering. The applied active filter features great performance and obtains a low THD well within the demands. However, it comes with a price in terms of increased power losses and the current rating of the components. Efforts were therefore made to decreasing the power losses and the system currents. Case 2 was devoted to finding the minimum power losses when still operating within the THD requirements. In the last case, a proposed degree of harmonic mitigation was simulated. This case yielded a worst-case THD well within the requirements, while still keeping both power losses and system currents within acceptable limits. It was concluded that the active filter capability of the Voltage Source Inverter is sufficient to mitigate the harmonic distortion in the system. However, the results are strongly dependent on the parameter data; further research should therefore put more efforts into gaining more precise estimations of these data.