Research On Stability Of AC/DC System Containing LCC-VSC Hybrid Hvdc Transmission

The hybrid High Voltage Direct Current(HVDC),which employs line commutated converter(LCC)at the rectifier station and voltage source converter(VSC)at the inverter station,combines the advantages of LCC and VSC to transmit high power of long-distance with no risk of communication failure,thus having wide application prospects.Therefore,both the research on the stability of the LCC-VSC hybrid HVDC system itself or the analysis of its influence on the stability of the overall AC/DC hybrid system after being connected to the power system are of great significance.First of all,in order to provide a good theoretical and model basis for the subsequent stability analysis,this paper derives the mathematical model of the LCCVSC hybrid HVDC transmission system and direct-drive wind power in detail,and establishes its small signal model by means of modular modeling.It includes LCC converter and its control system,VSC converter and its control system,phase-locked loop,DC network and AC system,as well as permanent magnet synchronous generator model,shaft model,machine side and grid-side converter model,phase-locked loop,DC network and the direct-drive wind power part of the wind power grid connection link.The accuracy of the small signal model is verified by comparing with the time domain model built in the Matlab/Simulink simulation platform.Then,aiming at the problem of improving the stability of the hybrid HVDC system,this article gives a general parameter coupling analysis procedure.First,adopt the modal analysis principle of eigenvalue calculation and participation factor analysis to identify the main circuit parameters that affect the dominant mode,and then perform root locus analysis on the coupling relationship between the main circuit parameters and the most relevant controller parameters.The two optimization parameter fitting method based on Lagrange fitting method is proposed,so as to obtain the optimal value of the controller parameters under different circuit parameters,and verified the effectiveness of the fitting curve by time domain simulation.Finally,the interaction of the direct-drive wind power transmission scenario through the LCC-VSC hybrid HVDC transmission is studied.In terms of research methods,in order to avoid the risk of dimensionality hazard in small signal analysis due to the increase of the system scale,this paper adopts a new type of stability analysis method,namely the open-loop resonance method,and proposes the stable judgment method suitable for dual-input dual-output open-loop interconnection systems which can effectively predict the position of the closed-loop characteristic root based on the open-loop system.Based on this,a universal stability analysis procedure that can analyze the interconnection characteristics of any two subsystems is summarized,and the concept of resonance domain is proposed.Through the study of the stability of the LCC-VSC hybrid HVDC system and direct drive wind power under the open-loop resonance condition,the dynamic interaction between the hybrid HVDC transmission and the original system after being connected to the power system is explained in principle,and the effects that cause the system to oscillate are explained in principle.The parameter conditions are summarized.Finally,time-domain simulation verification proves the correctness of the above-mentioned research ideas.