On Harmonic Emission, Propagation and Aggregation in Wind Power Plants

Abstract: The increasing use of wind energy is a global trend as part of the overall transition to a more sustainable energy system. By using modern technologies, the wind energy is converted into electric power which is transported to the consumers by means of the electric power system. The use of these technologies, in the meantime, plays a significant role in maintaining power quality in the electric power system; including positive as well as negative impacts. This thesis emphasises on harmonic distortion within a wind power plant (WPP), for a wind turbine and for the plant level.The harmonic study presented in this thesis has been based on field measurements at a few different individual wind turbines and at a second location in one WPP. In general, the levels of harmonic distortion as percentage of the turbine and WPP ratings are low. Among the frequency components, even harmonics and especially interharmonics are present at levels comparable with the levels of characteristic harmonics. The measurements show that both harmonics and interharmonics vary strongly with time. Interharmonics further show a strong dependence on the active power production of the turbine, while characteristic harmonics are independent on the power production. The even harmonics and interharmonics may excite any resonance in the collection grid or in the external grid.The origin of interharmonic emission due to power converters has been verified through a series of measurements over a two-week period. The interharmonic emission originates from the difference between the generator-side frequency and the power system frequency. A series of interharmonic frequencies are produced and they vary in accordance with the generator-side frequency. Both these interharmonic frequencies and the magnitudes are related to each other, and the theoretical relations have been confirmed through the measurements.The harmonic propagation in a collection grid has been studied by using transfer functions. Without the need to know the harmonic sources, the characteristics of harmonic propagations are quantified through transfer functions. The method has been used to estimate the total harmonic level in a WPP, by combining knowledge of the transfer function with information from harmonic emission of the individual wind turbines. The harmonic aggregation of the emission from the individual turbines towards the point of connection (PoC) has been studied as well. From the studies it was found that interharmonics show a stronger cancellation compared to harmonics, especially compared to lower-order harmonics.According to the object of interest and the harmonic propagation, a distinction has been made between primary and secondary emission. A more detailed classification of the different propagations within a WPP has been proposed. A systematic approach for harmonic studies in association with WPPs has resulted from this. The harmonic voltages and currents at any location are obtained as the superposition of the contribution from different emission sources to this specific location. This location can be either within the WPP or in the external grid. The studies presented conclude that all the contributions should be included to get a reasonable overview of the harmonic distortion in the WPP.

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