Power Quality Disturbances Propagation And Suppression Of Grid-connected Distributed Generation

With the depletion of fossil energy, energy problem is becoming more and more crucial, causing a fast development of dispersed renewable energy source closed to the users and the cogeneration of heat and cool power generations. The installation of distributed generation into the distribution grid can improve the efficiency of energy utilization and increase the energy utilization forms and support high voltage grid. The environmental pollution can also be reduced by the distributed generations.However, due to the natural environmental factors, the output power of distributed generations with renewable energy is random and fluctuant. And the ratio of line reactance to resistance in distribution grid is quite high even though the dispersed sources are quite closed to the load, which will lead to the voltage disturbance caused by the random fluctuation of output power. Thus, for example, the voltage instability problems, which used to occur in the transmission grid, now happen in the distribution grid.The application of distributed generation system makes the renewable energy, such as photovoltaic and wind generator, be a part of the distribution gird. The direct outputs of these sources can be alternating current of which frequency can reach to hundreds and thousands and direct current instead of power frequency-50Hz. Then the power electronic devices are necessary to guarantee the safety of distributed generation system. Thus, the power electronic devices and distributed generators can both emit harmonic currents which will interact with nonlinear load harmonic emissions, which makes the harmonic evaluation in the distribution grid with distributed generation will be more complex.The main research focuses on the propagation characteristic of voltage and harmonic disturbances when the distributed generations are integrated into the distribution grid. And the hybrid energy storage system is used to restrain the power quality disturbances and increase the stability of power supply. The main research achievements are as follows:In order to explore the inherent characteristic of fluctuant output power of distributed generation, Hilbert-Huang theory is applied and the improved empirical mode decomposition method is proposed, which can be also applied to analyze the fluctuation of wind speed and wind power fluctuation. Then the hybrid short-time wind speed forecasting model based on the support vector machine model and improved empirical mode decomposition is proposed. By comparing it with the traditional forecasting method, the effectiveness of the new method is verified.Then the voltage stability index is adopted to evaluate the voltage stability level with the distributed generator integration. The load flow calculating mode of different types of wind turbines is analyzed, and the expanding Newton-Raphson power flow method is proposed to analyze the wind power generation nodes. The comparison of different capacities and installation places, the reasonable installation place is found. The key components of fluctuant output power are also verified by the simulation results.Different to the traditional random harmonic analysis method, the generalized gamma mixture models are proposed to study the random harmonic propagation and interaction by dividing the complicated probability distribution functions of harmonic sources. Then the proposed models are proved to be reliable and accurate by simulating the practical wind farm harmonic emissions. Furthermore, with the combination of the non-uniform distribution of harmonic phase angle, the phasor rotation method is proposed to restrain the summation of harmonic current emissions.By deeply exploring the harmonic emission inherent characteristic of distributed generation, taking double-fed wind generator for example, the characteristic and non-characteristic harmonic propagation of the double-fed wind generator is deduced carefully. Based on the improved generalized gamma mixture models, the harmonic interaction and propagation among small-scale wind farm and nonlinear loads are accurately evaluated and verified in the simulation system.In order to restrain the voltage and harmonic propagation in the distribution grid, the hybrid energy storage management strategy is proposed. Based on the integration standard of distributed generation, the multiple time scales design of low-pass filter is proposed to assign the different capacity for the energy type storage and power type storage system. According to the proposed management strategy, the hybrid energy storage optimization system can both restrain the fluctuant voltage and random harmonic emissions.