Study Of Control Strategy And Grid Connection Characteristics Of Voltage Control Inverter

At present,most grid-connected inverter devices are current-controlled devices.When they are connected to the grid,they perform as controlled current sources and hardly provide beneficial support to the power grid.As their penetration rate in the power system is increased,the difficulty of voltage and frenquency regulation increases.The grid-connected inverter based on droop control uses voltage-type control algorithm to simulate the sagging characteristics of the synchronous generator,and has the function of frequency regulation and voltage regulation when the inverter is connected to the grid.Therefore,it can provide beneficial support for the grid.However,due to the scattered distribution of distributed generation,it needs to be connected to the power grid via long-distance power transmission lines.As more and more distributed power generation system connected to the power grid,the impedance of the grid can not be neglected.In this case,the interconnected system formed by the power electronic device and the power grid may easily cause harmonic oscillation problems and affect system stability.The impedance analysis method is a relatively simple method of grid connection stability study.Based on the impedance analysis method,this paper studies the control strategy and grid connection characteristics of voltage-controlled inverter(VCI).In this paper,the output impedance model of a single-phase VCI is established and compared with the output impedance model of a conventional single-phase current-controlled inverter(CCI).It is pointed out that the characteristics of low-frequency band in the single-phase VCI output impedance are opposite to CCI.Then,by using the method of harmonic linearization,the positive and negative sequence impedance models of three-phase VCI are established.The differences between the output impedances of single-phase VCI and three-phase VCI are analyzed and the causes of the differences are explained.Considering that the power loop of the three-phase converter will cause mirror frequency coupling effect,the paper explains the mechanism of mirror frequency coupling effect,derives a more complete converter impedance model with power loop and analyze the influence of parameters on the model,Based on this model,the output impedance between VSG and droop control is simply compared,which indicates that the two are equivalent under certain conditions.Finally,the stability analysis of interconnected system composed of VCI and power grid is analyzed by impedance analysis.The virtual impedance method is proposed to improve the phase margin of interconnected system and increase the system robustness.The paper verifies the correctness of the impedance model by simulation and experiment.The stability of the inverter connected to the grid is verified by RT-LAB hardware-in-loop experimental platform.It is proved that the introduction of virtual impedance helps to improve the system robustness.