Research On Application Of SVG In Three-Phase Unbalance Regulation Of Low-Voltage Distribution Network

The three-phase four-wire power supply system is widely used in the low voltage distribution network.Due to the spatiotemporal non-uniformity of the load,the three-phase unbalance in the distribution area has become increasingly serious in recent years.The three-phase unbalance not only affects the quality of power supply,but also brings hidden dangers to the safe and stable operation of the system.Given the dynamic response requirements of the system,conventional methods such as the capacitor switching and commutation no longer apply.This thesis manages the three-phase unbalance of low voltage distribution network by means of three-phase four-leg static var generator.An appropriate control strategy is designed for the SVG system to realize reactive power compensation and three-phase unbalance regulation.The system designed in this thesis can improve the power quality and save investment and rebuild cost which is of great theoretical significance and practice value.This thesis first describes the causes,hazards and regulation methods of low-voltage distribution network three-phase unbalance and analyzes that three-phase four-wire SVG system is suitable for solving three-phase unbalance problem.Then two common topologies of the three-phase four-wire SVG system are compared and the three-phase four-leg topological structure is chosen in advantage of more simple and practical.The sequential equivalent model of the three-phase four-leg SVG system under unbalanced loads is obtained by mathematical modeling in different coordinate systems.The three-dimensional space vector modulation algorithm based on the abc coordinates is introduced since the conventional modulation methods are no longer applicable for the three-phase four-leg SVG system with one more degree of freedom compared to three-phase three-leg SVG system.To realize the function of SVG,the primary task is to obtain the sequence components of the load current.A T/4 time delay algorithm is proposed in the load side current separation and a single phase locked loop based on second-order generalized integrator is used to insure phase information accurate.In order to improve the tracking effect of instruction current,an improved control strategy of the three-phase four-leg SVG system is designed,in which the positive and negative sequence components are decoupled in the corresponding dq rotating coordinate system and an improved PR controller is used in the zero sequence component regulation.The main circuit model of SVG is established and simulation tests are done with the help of MATLAB/Simulink simulation software.The simulation results show that the three-phase four-leg SVG system designed in this thesis has both three-phase unbalanceregulation and reactive power compensation functions with good dynamic performance.Finally the hardware circuit and the software program of the control system are designed based on the TMS320F28335 digital signal processor,and the system compensation performance is verified by building the experimental platform and testing.