Research On Power Quality Analysis And Governance Of Grid-connected Microgrids Connected To Impulsive Loads

In recent years,due to the problems of energy shortages,environmental pollution caused by the rapid growth of the global population and the rapid development of the economy and poor flexibility and reliability of traditional power grid,the microgrid constructed from renewable energy sources has attracted much attention and has broad development prospects owning to its cleanliness,environmental protection and sustainability.With the rapid development of science and technology,and the continuous advancement of industrial processes,the number of non-linear and impact loads in the microgrid has increased dramatically,which not only seriously damages the power quality of the microgrid,but also reduces the life of electrical equipment in the system and malfunctions.Therefore,it is very innovative and challenging to analyze the power quality of the microgrid connected with impact loads.This thesis takes the microgrid power quality requirements as an opportunity to analyze in detail the power quality problems caused by impact loads connected to wind-solar storage microgrid grid-connected systems,and adopts static synchronous compensator(STATCOM)to govern effectively the power quality problems caused by impact loads.The main contents of the thesis are as follows:1.A grid-connected system model of wind-solar storage microgrid with the impact load is established in accordance with actual working situation.Photovoltaics adopts PQ control strategy for grid connection.Doubly-fed fans use"back-to-back"converters for grid connection.The energy storage system adopts bidirectional DC-DC control with charge and discharge function to smooth the power fluctuations of photovoltaics and wind turbines.Arc furnace model as a typical impact load was established,two parameters of arc radius and arc voltage were introduced to improve the time-domain differential model of the arc,and the accuracy of the improved arc model was verified by simulation.2.The problems of three-phase imbalance,harmonics,reactive shock,and voltage fluctuations caused by the impact load(arc furnace)connecting to the grid-connected microgrid are analyzed.When one phase arc length changes,the corresponding current and voltage changes are the most obvious.Due to the existence of the three-phase arc coupling relationship,the current of the remaining two phases also changes,resulting in the occurrence of three-phase unbalance problems.During the melting period,the arc cannot be stably burned due to the lower furnace temperature,the harmonic currents of all phases of the grid-connected bus of the microgrid seriously exceed the standard.When the arc furnace works in the three-phase ground short circuit,it causes serious reactive impact problems,and voltage fluctuation far beyond national standard.3.The STATCOM with improved sliding mode control strategy is adopted to govern the power quality of the microgrid.STATCOM’s control is a double closed-loop control structure of the voltage outer loop and the current inner loop.The voltage outer loop adopts classic PI control.The current inner loop adopts a new type _di-_qi current detection method,and an improved sliding mode control strategy based on the double power approach rate is proposed,which has high current detection accuracy,response speed and anti-interference ability.The simulation results show that when the STATCOM is put into operation,the bus current harmonics at the grid connection point of the microgrid caused by the impact loads are effectively suppressed,current harmonics of all phases are controlled within the national standard limits.At the same time,the problems of three-phase imbalance,reactive impact and voltage fluctuation are also significantly improved.