Modeling For VSC-HVDC Based On Dynamic Phasor Method And Research Of PLL

VSC-HVDC is a DC transmission system based on voltage source converter, in which the whole switches are full controlled. The new generation of HVDC transmission technology (VSC-HVDC) are based on VSC consisting of full-controlled devices and Pulse Width Modulation (PWM) technology. VSC-HVDC electromechanical transient model can meet the need of the simulation on large power grid, because electromagnetic transient model is not suitable for large power grid. Conventional electromechanical transient model is a phasor model based on "steady state" or "quasi-steady state" assumption. With lots of FACTS devices widely used in power system,"continued rapid dynamic changes "occur in it. In this case, conventional electromechanical transient model is not accurate. Dynamic phasor method is not limited to these assumption, which is more accurate to analyze "continued rapid dynamic changes" in power system.This paper describes the theoretical basis of dynamic phasor analysis method, and analyzes the limitations of traditional phasor to prove the accuracy of the dynamic phasor. This paper introduces the basic institutions and characteristics of VSC-HVDC and build a detailed electromagnetic transient model in time domain using the switch function method.Electromechanical transient model and dynamic phasor model are built,simulation results shows that the accuracy of VSC-HVDC electromechanical transient model and the dynamic phasor model are almost the same but the speed of the dynamic phasor model is much faster.Dynamic phasor model is more accurate than electromechanical transient model in systems containing a large number of power electronic devices considering the dynamic changes of phasor. This paper introduces the VSC-HVDC control system, converter station-level control system is the core of the control system, this paper focuses on the converter station level control system, control methods include indirect current control method and the direct current control method, and then presents a PLL control method based on integration under three-phase unbalanced conditions, which can make VSC-HVDC control system more accurate.