Power Quality Improving Strategy Of Microgrid Based On Multi-micro-sources Optimization And Control

Along with a continuous adjustment of energy structure in China,the proportion of clean energy generation is increasing year by year.Distributed generation has attracted wide attention because of its flexible power generation and high energy utilization.However,the accommodation problem has been a main contradiction in operation along with the centralized clean energy connected to power grids.A microgrid is a better solution for distributed generation that connects to a distribution grid.Compared with direct grid connection of single or multiple distributed generations,the grid connection in the form of a microgrid overcomes the impact of distributed power supply randomness on the grid and enhances the reliability of power supply.However,the microgrid grid is weak,and the operation is easily affected by micro-sources and loads.Therefore,the power quality problem has attracted considerable attention.The dissertation studies the modeling of multi-source microgrids,the mechanism of power quality,the method of enhancing the active power quality in islanding mode,and the method of power quality improvement combined with active and passive power in grid-connected operation mode,which has important theoretical significance and application value to guiding the operation control of the microgrid,improving power quality,and promoting their development.Through combining practical experience research results in the field of up-to-date clean energy generation and microgrid operation in China,this dissertation firstly summarizes the significance of the development of microgrids and the existing power quality problems.Then it gives the development of microgrid and the current technical difficulties of power quality problems at home and abroad.And thereby,the research work needed to solve the microgrid power quality problem is listed.Multi-source microgrid modeling and power quality characteristic analysis are the basis for the research on improving the power quality of microgrid.Based on a RTLAB hardware-in-the-loop simulation platform,a microgrid model including distributed wind turbines,distributed photovoltaics,supercapacitors,lithium battery energy storage and other micro-sources is established,and the micro-source inverter is modeled in detail.The parameters of the micro-source inverter are continuously optimized through the measured data to improve the accuracy of the model.A small-signal model of a multi-source microgrid is established to provide a basic model for the stability analysis of the follow-up control system.Finally,the mechanism of power quality harmonics,unbalance,voltage fluctuations,frequency stability and circulation in microgrid are analyzed,which provides a basis for the comprehensive improvement of power quality control methods.The control of micro-source inverters is a key technology to improve power quality of a multi-source microgrid.The microgrid power quality problem is greatly affected by its own micro-sources and mixed load with island running.An effective solution to improve the power quality indicators of islanding operations is to improve the micro-source inverter control and strengthen the micro-source active governance capabilities.This dissertation proposes an improved droop control strategy for improving power quality.Firstly,the improved droop control method of adaptive virtual impedance is proposed in view of the circulation problem caused by the unbalanced distribution of reactive power of micro source.Secondly,according to the influence of the unbalanced and nonlinear load on the power quality of the micro source inverter,an improved droop control method for harmonic suppression and negative sequence control is proposed.Finally,a secondary control strategy is proposed,which can stabilize the output voltage and frequency of the micro source inverter.All control methods constitute an overall control strategy.A small signal method is used to analyze the stability of improved droop control strategy with multi-inverters based microgrids.The effectiveness of the control strategy is verified through the experimental method.The results show that the improved droop control strategy has a great performance improvement in stabilizing voltage,stabilizing frequency,power adjustment,and dealing with the influence of nonlinear and unbalanced mixed load.When the main grid is weak or the micro-grid is in islanding modes,it is easy to cause system breakdown due to the fault and power quality problems,since there is no large-scale generator set in the microgrid that suppresses fluctuations and maintains the system.Therefore,it is necessary to introduce "virtual synchronous generator" in the microgrid to simulate the characteristics of synchronous generators and improve the stability of the system.The dissertation presents a virtual synchronous generator control strategy that integrates power quality improvement.Firstly,the energy storage and simulation of the inertial link of the virtual synchronous generator are added to the micro-source inverter.Secondly,the improved droop control strategy of the proposed integrated power quality improvement is optimized,and the improved secondary control and gradient-descent method-based parameter optimization control strategies are added to further improve the performance of the integrated control strategy.Then small signal modeling and stability analysis of the control system.Finally,experimental analysis of the power quality problem in RTLAB shows that the control strategy significantly improves the power quality of the system.In the case of nonlinear,unbalanced loads and sudden changes in the power of the micro-source,the system can effectively reduce the harmonic distortion ratio of the system unbalance,increase voltage and frequency stability.The microgrid is greatly affected by the distribution grid in grid-connected operation mode.The micro-source active control strategy has limited inhibitory effect on the power quality problems caused by the distribution grid.Therefore,this dissertation proposes an integrated active and passive coordinated control strategy for improving power qualities.Not only exerted its own active governance ability,but also reduced the passive management device capacity and reduced costs.First of all,the traditional unified power quality controller is improved,and secondly,the active and passive control strategies are coordinated to ensure the maximum power quality control effect with minimum economic investment,and the last improve the power quality of the microgrid operating in grid-connected mode.