Research On Power Flow Algorithm And Control Strategy Of Electromechanical Transient Of VSC-HVDC

Voltage source converter based high voltage direct current(VSC-HVDC)transmission technology has been practically applied in the world.Compared with the traditional direct current power transmission technology,VSC-HVDC has the characteristic of independent control of active and reactive power transmission,the ability of power suppliment to the weak AC power grid and the passive system without the need of communication,and the advantage of DC voltage polarity unchanged when the power flow of the system reverses.In this thesis,the VSC-HVDC power flow control,electromechanical transient simulation and the suppression of inter-area low frequency oscillation of AC system are studied.This thesis firstly analyzes the working principle and the steady-state characteristics of VSC-HVDC,and determines the basic control modes of VSC-HVDC.Based on the nodal current-injection method,a novel alternate solution algorithm is proposed,which is applied to power flow calculation of AC-DC hybrid power system with VSC-HVDC.AC system and VSC-HVDC system are interfaced through the injected currents and realize alternate solution.The bus node type does not need to be changed according to the change of VSC control mode.For constant AC bus voltage control mode,the problem that the reference value of AC bus voltage is not directly given in AC system power flow calculation is solved by using the reactive injected current feedback control method.The simulation results of New England system verify the feasibility and validity of the method proposed in this thesis.And secondly,the electromechanical transient simulation of AC/DC hybrid system containing VSC-HVDC is studied.The outer loop power/voltage controller of VSC-HVDC uses proportional integral(PI)control to produce the inner loop current reference value.Considering AC side mathematical model of VSC-HVDC cannot be accurately decoupled under dq synchronous rotating coordinate,a mathematical model in ?? stationary coordinate is established.The introduction of proportional resonant(PR)control improves inner current controller to realize the zero steady error of tracking the inner current signal.Considering the existence of DC side inductors,and capacitors with fast dynamics,two time-step simulation method is used to simulate the dynamic response of VSC-HVDC accurately.The results of New England system verify the correctness of the VSC-HVDC electromechanical transient model and the validity of two time-step hybrid simulation method proposed in this thesis.In the end,VSC-HVDC is used to suppress the low frequency oscillation of two-area AC network in this thesis.The open-loop transfer function between active power of VSC-HVDC and the corresponding AC transmission line is analyzed through the Prony method.VSC-HVDC additional damping controller is designed by pole assignment method.The simulation results of the 4-machine 11-bus system show that VSC-HVDC with additional damping controller can effectively increase the system damping and suppress low frequency oscillation of tie lines.