Research And Application Of Three-phase Unbalanced Compensation Device

With the development of China’s low-voltage power supply system,a large number of nonlinear devices and impact power equipment are connected to the system,which makes the problem of three-phase imbalance in the distribution system more and more prominent and seriously affects the power quality of users.In this paper,under the background of specific engineering application,SVG device is used to solve the three-phase unbalanced problem of low-voltage power supply system,and the key technologies are studied,the main work and achievements are as follows:Firstly,aiming at the problem of unbalanced current detection,the zero-sequence current separation method and the time-delay signal separation method are analyzed from the accuracy and dynamic response speed,and an improvement is proposed on this basis,that’s the current detection method based on the dual-synchronous rotating coordinate system can realize the independent detection of the positive sequence,negative sequence and zero-sequence components in the unbalanced current.The simulation model is built in MATLAB/Simulink,and the positive sequence,negative sequence and zero sequence currents are accurately extracted.Secondly,in view of the problem of insufficient phase-locked capability of traditional phase-locked loop when the grid voltage is unbalanced or harmonics,In this paper,the method of decoupling phase-locked loop is designed under the dual-synchronous coordinate system.The phase-locked capability is improved by eliminating the double frequency component of the negative sequence voltage component generated under the positive sequence coordinate,and further ensure the accuracy of the unbalanced current detection.The phase locking effect of DDSRF-PLL and traditional phase locking is analyzed by simulation,and the superiority of DDSRF-PLL phase locking is verified.Then,a method of sequence control is designed in this paper.The positive and negative sequence control is PI decoupling control under the dual-synchronous rotation coordinate,and the zero sequence current is controlled by the quasi-pr controller,and the controller parameters are designed,the compensation instruction current can be tracked without static error.Since four-bridge SVG has many switching states and complex voltage modulation algorithm,so the selection is based on a-b-c coordinates of 3D-SVPWM algorithm,and through the simulation of3D-SVPWM algorithm and the SVG system is verified,the result of simulation shows that the SVG designed in this paper is effective in dynamic control of reactive power and three-phase unbalance caused by unbalanced load.Finally,important parameters of main circuit,hardware circuit and software program of the SVG are designed and applied in the project of impact load of the welding machine.The results show that the SVG device can effectively control the three-phase imbalance and stabilize the power factor of the system above 0.96.