Methodology for Detecting and Interpreting Instantaneous Frequencies in Stand-alone Microgrids - An Application of the Hilbert Huang Transform on Electrical Networks
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Following the tile, this thesis seeks to find a methodology for detecting and interpreting instantaneous frequencies in stand-alone microgrids. This is not a novel exercise for scientists to do but it bears potential to be improved by making use of a new method - the Empirical Mode Decomposition (EMD). The EMD is a non-linear algorithm that separates any oscillating signal into mono-components, leaving space for time-varying amplitude and -frequency of the so called Intrinsic Mode Functions. To start with, a brief introduction to stand-alone microgrids is given. Microgrids are considered to be driven by renewable energy sources such as e.g. solar-panels which make electrical power converter necessary to transform the produced energy from DC to AC. The control of the power converters plays the lead role in shaping voltage and current waveforms of the network while no other traditional type of rotating generators are assumed. The imitation of synchronous machine behaviour, however is a favoured method to control converters because traditional machines have certain inherent advantageous physical properties. The power-balance (load/supply mismatch) and the fundamental frequency of the network (usually 50 Hz or 60 Hz) are set into a backhanded relationship so that changes in the consumed, or generated power leads to a changing fundamental frequency. The impact of fast changing solar power (e.g. reasoned by non-steady solar irradiance - clouds) is obvious and as a follow-up from the converter control it can lead to a supply-fluctuation and violation of the fundamental frequency. Deterioration of the power quality and wrong statements on the actual network status due to improper measurement equipment can follow and are part of the investigation of this thesis. The EMD plays a key-role, as it shows certain advantages against the widely used Fast Fourier Transform in handling non-linear, non-steady signal. The core of the thesis deals with the analysis of voltage and current measurements from stand-alone microgrid, 3-phase systems. Focusing on detecting the true composition of the measured signal, several methods, based on analysis of the raw measurements, combined 3-phase rotating space vectors for voltages and currents and instantaneous power are developed. The developed methods are applied to data, recorded at a marine vessel during sea-voyage. The system on board the ship has certain similarities to previous explained onshore networks, as the balance between generated and consumed power is dynamic. In the ship, the generation is relatively steady but the load (electrical propulsion) varies vastly.