Investigation of distortions in microgrids
MetadataVis full innførsel
This thesis presents a validation study based on data analysis aimed at identifying the cause of atypical behavior observed in a microgrid's data. The data of a stand-alone PV microgrid in Bhutan is analyzed. The analysis of the voltage waveform distortions found in the data is utilizing the Hilbert-Huang Transform (HHT) and periodograms. The Hilbert-Huang Transform shows an oscillating frequency with fluctuations of 10 ms, while the fundamental frequency component's period is 20 ms. As we couldn't attribute this distortion to a nonlinear load, we first argue that the most probable cause of the distortion is a 100 Hz oscillation commonly known to exist on the dc bus. The Periodogram shows a dominance of odd harmonics, with particular emphasis on 50 and 150 Hz. The analysis reveals that the dominance of the third harmonic can be explained by multiplication in the microgrid's controller. There are two interpretations for the other odd harmonics. The pulse-width modulation and unideal low pass filtering cause odd harmonics on the ac side of the inverter. The same does a harmonic feedback from the ac bus to the dc via the inverter. The harmonic feedback creates a positive feedback loop through the controller resulting in odd harmonics on the ac side and even harmonics on the dc side. The thesis will also compare the voltage data measured on a physical grid with a mathematical model of the system and a simulation to test the hypothesis of the distortion's origin. This test goes far in confirming the hypothesis of the source of the distortions.