| With the advancement of science and technology and the requirements of sustainable development policies,a large number of distributed generation is connected to the power grid in the form of micro-grids.This type of distributed power supplies based on power electronic interfaces does not have damping and inertia.The high proportion of these will exacerbate the adverse effects on the operation of the micro-grid.Recently,the Virtual Synchronous Generator(VSG),which is widely mentioned,is a type of technology that enables the output of the inverter on the grid side to have similar characteristics of synchronous motors,which is used in inverter control can improve the operation of the micro-grid.Besides,the frequency and voltage of the micro-grid will fluctuate in the island operation mode.The modulation effect of the distributed generation inverter and the existence of non-linear loads will increase the voltage harmonic pollution of the micro-grid and affect the user’s power quality.Therefore,designing effective control strategies to improve the frequency and harmonics in the operation of the micro-grid is of great significance to the stable operation of it.In summary,this paper studies the key technical issues such as micro-grid control operation stability,VSG control strategy stability,frequency-no error adjustment control,and harmonic detection,and completes the micro-grid operation optimization control strategy research with virtual synchronization technology.The main research content and results of this thesis are as follows:(1)Expound the development background and research progress of the micro-grid.On this basis,a three-phase power electronic inverter model and a micro-grid distributed generation model are established.Design the grid structure of the AC micro-grid.Then build a simulation model verified the voltage,frequency changes,and voltage harmonics when switching its operating mode.(2)Aiming at the problems of micro-grid operation,such as the lack of inertia of the output of the power electronic interface inverter on the side of the distributed generation,the micro-grid cannot achieve frequency no error adjustment.On the one hand,it is proposed to use the VSG control strategy to increase the virtual impedance to make the active-frequency and reactive-voltage in the microgrid satisfy the droop relationship.Further simulating the second-order equation of the synchronous motor to make its output damping and inertial,and analyze the impact of the damping coefficient and rotational inertia on the output frequency.On the other hand,the principle of primary and secondary frequency modulation of synchronous motors is analyzed.By improving the active-frequency control loop in the VSG control strategy,proposed the frequency improved VSG control strategy to achieve t no error adjustment of the frequency of the micro-grid.(3)Aiming at the problem of non-linear load and harmonic pollution caused by power electronic equipment during microgrid operation,an improved second-order generalized integral harmonic frequency division detection method is proposed to achieve the frequency extraction effect.This method can effectively filter the high-order harmonics amplified during the operation of the micro-grid island status,and at the same time,it can overcome the interference of low single-line voltage and load disturbance on harmonic detection.And on this basis,frequency division suppression is used to effectively suppress the 5th and 7th voltage harmonics in the island micro-grid. |
