This thesis describes a novel approach that utilizes a special property of the Hanning window to accurately detect the fundamental frequency of the data signal in the presence of harmonic and interharmonic interference. After obtaining the fundamental frequency, the same procedure is applied to all possible harmonics to be filtered for further analysis of the interharmonics. The proposed approach is validated using numerical tests. In the literature, different authors have shown the difficulty of extracting a synchronized sampling frequency from the analyzed signal and discussed the effect of deviation of the fundamental in the presence of harmonics. This thesis suggests a new approach to overcome the difficulties, even if the interharmonics present are in frequency bins near the fundamental. In order to perform the interharmonic analysis, the author followed the IEC standard draft signal processing recommendations, with the exception of using a weighted Hanning window instead of a rectangular window in order to minimize the effect of the spectral leakage, and to minimize the effect of interharmonics on the main harmonics.

Harmonic distortions are inevitably caused by a rectifier and an inverter due to their inherent nonlinearities. An AC-DC-AC converter, configured by the series connection of a rectifier, DC link, and an inverter, induces harmonic distortions at both AC sides and at the DC link. These harmonics can nonlinearly interact or modulate the fundamental frequencies at the AC sides to cause interharmonic distortions. Harmonic and interharmonic distortions can seriously hamper the normal operation of the power system by means of side effects such as excitation of undesirable electrical and/or mechanical resonances, misoperation of control devices, and so forth. This dissertation presents effective methodologies to mitigate harmonic and interharmonic distortions by applying dithered pulse-width modulated (PWM) signals to a voltage-sourced inverter (VSI) type adjustable speed drive (ASD). The proposed methods are also efficient because the dithering applications are performed on control signals without the need for additional devices. By the help of dithering, the rejection bandwidth of a harmonic filter can be relaxed, which enables a lower-order configuration of harmonic filters. First, this dissertation provides a dithering application on gating signals of a sinusoidal PWM (SPWM) inverter in the simulated VSI-ASD model. The dithering is implemented by adding intentional noise into the SPWM process to randomize rising and falling edges of each pulse in a PWM waveform. As a result of the randomized edges, the periodicity of each pulse is varied, which result in mitigated harmonic tones. This mitigation of PWM harmonics also reduces associated interharmonic distortions at the source side of the ASD. The spectral densities at harmonic and interharmonic frequencies are quanti fied by Fourier analysis. It demonstrates approximately up to 10 dB mitigation of harmonic and interharmonic distortions. The nonlinear relationship between the mitigated interharmonics and harmonics is confirmed by cross bicoherence analysis of source- and DC-side current signals. Second, this dissertation proposes a dithered sigma-delta modulation (SDM) technique as an alternative to the PWM method. The dithering method spreads harmonic tones of the SD M bitstream into the noise level. The noise-shaping property of SDM induces lower noise density near the fundamental frequency. The SDM bitstream is then converted into SDM waveform after zero-order interpolation by which the noise-shaping property repeats at every sampling frequency of the bitstream. The advantages of SDM are assessed by comparing harmonic densities and the number of switching events with those of SPWMs. The dithered SD M waveform bounds harmonic and noise densities below approximately -30 dB with respect to the fundamental spectral density without increasing the number of switching events. Third, this dissertation provides additional validity of the proposed method via hardware experiments. For harmonic assessment, a commercial three-phase inverter module is supplied by a DC voltage source. Simulated PWM signals are converted into voltage waveforms to control the inverter. To evaluate interharmonic distortions, the experimental configuration is extended to a VSI-ASD model by connecting a three-phase rectifier to the inverter module via a DC link. The measured voltage and current waveforms are analyzed to demonstrate coincident properties with the simulation results in mitigating harmonics and interharmonics. The experimental results also provide the efficacy of the proposed methods; the dithered SPWM method effectively mitigates the fundamental frequency harmonics and associated interharmonics, and the dithered SDM reduces harmonics with the desired noise-shaping property. / text