On high-frequency distortion in low-voltage power systems
Abstract: Power quality is a subject that has received a lot of attention during the last 10 to 20 years, both in industry and in academia. Power quality concerns interaction between the power grid and its customers and between the power grid and equipment connected to it, reflected in voltages and currents. Research and other developments in this area have to a great extent concentrated on relatively slow and low-frequency phenomena, with the main emphasis being on voltage dips (reductions in voltage magnitude with duration between about 50 ms and several seconds) and low-frequency harmonics (waveform distortion by frequency components up to about 2 kHz). These phenomena are reasonably well understood and several standards cover the area. For higher-frequency phenomena, above 2 kHz, there is no such general understanding, nor is there anything close to a complete set of standards covering this area. Modern energy efficient equipment connected to the grid, like fluorescent lamps but also solar panels, often uses switching technology, with switching frequencies that can range from a couple of kHz up to several hundreds of kHz. The grid is also used for communication of e.g. meter readings, system controls etc. This so-called power-line communication is using the same frequency range. The main frequency range of interest for this thesis has been the range from 2 to 150 kHz. There are two completely different measurement methods covering this frequency range: time-domain based and frequency-domain based. Time domain based measurements are used throughout the thesis. This gives an opportunity to choose between different analysing tools where among others the joint time-frequency domain has shown to be a useful tool for describing waveform distortion in our frequency range of interest. The majority of the measurements presented in this thesis have been directed towards fluorescent light powered by high frequency ballasts. This type of load has been, due to stringent harmonic limits, one of the first to use a more advanced switching technology called active power factor correction. This technique is also getting more frequently used in other small-power equipment, like computers. Installations of lights in stores etc. normally contain a large number of ballast connected together and the interaction is of importance, for example for setting emission and immunity standards. The measurements on ballasts presented in this work have shown that distortion in the frequency rage 2-150 kHz comes in three types: narrowband distortion; wideband distortion; and recurrent oscillations. The recurrent oscillations are a new type of powerquality disturbance that had not been recognized as such before. The measurements further have shown that the three types of distortion spread in a completely different way from the individual devices to the grid. This knowledge is essential for the setting of emission requirements on energy-efficient equipment.
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