Research On VSC-HVDC Control Strategy Under Voltage Asymmetry Of Power Grid

With the development of power electronic devices such as high power full-controlled switching devices(IGBT),voltage source converter based high voltage direct current(VSC-HVDC)has been widely recognized in the society.Compared with line-commutated current source converter based high voltage direct current(LCC-HVDC)transmission,VSC-HVDC has significant advantages and it has a wide range of application prospects in distributed energy grid-connected,urban grid DC power supply,island system power supply,passive system power supply and other fields.In actual project operation,due to the asymmetry of grid voltage caused by three-phase components,line parameters,load asymmetry,voltage dips etc.It is very important to study the dynamic characteristics of VSC-HVDC power transmission system under the condition of voltage asymmetry.This paper mainly focuses on some problems under asymmetric working conditions,and conducts some research on the control issues under symmetrical working conditions.The main research contents are as follows:First of all,According to general topology structure of two level of three-phase voltage source inverter is established in different coordinates system,the transient mathematical model is analyzed in the symmetry of the VSC-HVDC transmission system under the condition of classic current control strategy;This paper presents an improved fuzzy adaptive PI controller for the characteristics of poor dynamic performance and weak anti-disturbance performance of the classical double closed-loop PI control strategy under symmetric conditions.Aimed at the problem of initial parameters PI controller is not easy to select,this article selects the particle swarm optimization(PSO)algorithm,the PSO optimization algorithm in combination with fuzzy adaptive PI controller is obtained a kind of VSC-HVDC compound double loop controller.Secondly,the mathematic model and controller strategy of VSC-HVDC under the unsymmetrical condition of power grid are analyzed in detail.Compared to the traditional double sequence dq vector current control strategy,improved in ?? coordinates vector current control strategy,effectively reduce the number of the controller and the complexity of the design;In order to improve the stability of VSC-HVDC power transmission system under asymmetric operation,model predictive control method is applied to improve a new VSC-HVDC network voltage unbalance composite controller,which is a proportional integral reduction order quasi-resonant(PI-ROQR)current controller.Finally,under the MATLAB/Simulink environment,the control strategy of the proposed voltage asymmetrical condition is simulated and verified.Combined with the laboratory three-phase two level voltage source converter through the back of a chair(BTB-VSC-HVDC)system platform for this design of the power grid voltage control strategy under symmetric and asymmetric state experiment validation.