Research On Stability Analysis And Control Method Of VSC-HVDC System Based On Im-pedance Method

With the development of Voltage Source Converter based High Voltage Direct Current transmission(VSC-HVDC)technology,the voltage level and system capacity of VSC-HVDC projects are also increasing,which has been widely used in many scenarios,such as AC grid interconnection,island power supply and new energy access.At present,the AC/DC system oscillation often occurs in the operation of VSC-HVDC project,which threatens the safe and stable operation of VSC-HVDC system.Based on the impedance modeling method,this paper studies the oscillation phenomenon in the operation of VSC-HVDC project,aiming at explaining the mechanism of oscillation,proposing the analysis method of oscillation characteristics with engineering applicability,and designing the damping controller to suppress the oscillation,which has important theoretical and practical value for improving the safety and stability of VSC-HVDC system.The main work of this paper is as follows:(1)This paper studies a modeling method which is suitable for different controller structures and can unify the modeling of VSC AC/DC side impedance.The AC/DC impedance models of two-level VSC and modular multilevel converter(MMC)are derived.The accuracy of the model is verified by comparing with the sweep frequency model of electromagnetic transient simulation.In this paper,the dynamic equations of the converter is divided into two parts: electrical part and controller part.By defining different input and output variables,the AC/DC impedance model of the converter are obtained by matrix operation.The internal dynamic characteristics and multi harmonic coupling characteristics of MMC are considered in the electrical part;the closed-loop dynamic characteristics of time delay,phase locked loop,main controller and circulating current controller of MMC are considered in the controller part.This modeling method takes into account the accuracy and flexibility,and can quickly and efficiently establish the AC / DC side impedance model of different types of converters under different control modes,which provides a theoretical basis for the impedance stability analysis of VSC-HVDC System.(2)The impedance model of VSC-HVDC system with different topologies is established,and the application method of impedance analysis method in VSC-HVDC system is expounded.The equivalent source-load subsystem modeling and analysis method of VSC-MTDC system is proposed: the DC voltage controlled converter is equivalent to the source subsystem,and other active power controlled converters and DC lines combined are equivalent to the load subsystem.The stability of the system is judged by Nyquist criterion.The method is applied to the stability analysis of multi terminal VSC-HVDC(VSC-MTDC)system.The influence of DC controller,power flow and DC lines on the stability of VSC-MTDC system are explained by impedance theory.Aiming at the phenomenon of DC oscillation caused by the negative damping characteristic of DC voltage controlled converter’s DC side impedance when outputing active power,an improved virtual impedance control strategy is proposed.The proposed strategy can eliminate steady-state error and improve system stability and the influence mechanism of the proposed strategy on system stability is explained by impedance theory.The accuracy of the analysis method and the effectiveness of the proposed damping strategy are verified by simulation.(3)Combined with practical engineering,the stability of AC/DC side of back-to-back VSCHVDC system are studied by impedance stability criterion.Aiming at the phenomenon of high frequency oscillation occurred in Chongqing Hubei back-to-back VSC-HVDC project,the impedance model of MMC connected to AC system is established.The influence of controller parameters on the impedance characteristics of MMC and the influence of grid impedance on system stability are studied.It is pointed out that delay is the main cause of high frequency oscillation between MMC and external circuit,and the high frequency oscillation waveform is reproduced by simulation.The DC side impedance model with two terminal back-to-back MMC-HVDC is established,and the stability of the system is analyzed by the impedance stability criterion.The influence of electrical parameters and controller parameters on the DC side impedance of MMC is studied,and the DC oscillation problem that may exist when the parameters of converter station and controller are unreasonable are analyzed.The accuracy of the analysis of AC/DC side oscillation impedance is verified by electromagnetic transient simulation.(4)The DC side impedance model of MMC considering the dynamics of power grid and wind farm converter is derived.The stability of MMC-MTDC system with wind farm connected is studied by using the equivalent source-load subsystem modeling and analysis method proposed in this paper.The impedance of equivalent load subsystem will show capacitance effect in a certain frequency band due to the capacitance effect of DC lines,which is related to DC line and DC reactance parameters.When the capacitance effect region coincides with the inductive negative damping region of the source subsystem,the phase margin at the impedance intersection point of the interactive system may be insufficient which may destabilize the system.Aiming at the phenomenon of DC oscillation caused by the interaction between DC reactance,DC lines impedance and converters impedance,damping control strategies based on DC current feedback are designed in DC voltage controlled converter and active power category control converters respectively.The proposed controller strategies can damp DC oscillation and the mechanism can be explained by impedance theory.The effectiveness and dynamic performance of damping control of single or multiple converters are verified and reflected by electromagnetic transient simulation.