Voltage Stability Analysis Of VSC-HVDC Systems

Voltage stability is a slow development branch in the field of power system stability.The study of voltage stability has become a very important subject in the field of power system research since the occurrence of many voltage collapse accidents at home and abroad in twentieth Century.With the development of grid connected large-scale distributed energy and large capacity long-distance transmission,VSC-HVDC(voltage source converter based multi-terminal direct current)system is research hotspot in recent years..It is imperative to study the voltage stability of the VSC-HVDC system.PV curve is the important tool for power system static voltage stability analysis.In the pure AC system,the continuous flow method or its improved method are often used in the PV curve.But for the VSC-HVDC system,the method of multiple power flow calculation for solving the PV curve of vhvdc transmission system is undoubtedly a very large computation project because of its complex structure and more parameters.In this method,the independent variable of the PV cure are changed into voltage modulus,and the power parameters are introduced as dependent variable.Then the method expanded the load power as Taylor series of voltage modulus in the ground state power flow by applying the Taylor series.This method can get the results of voltage stability critical point quickly and avoid repeatedly power flow calculation.Simulation result proves that the proposed method has higher calculation efficiency and accuracy.The analysis of small disturbance stability is an important method for dynamic stability analysis of power system.For the analysis of small interference voltage stability,the key is to establish the linearized differential algebraic equation of dynamic components and to obtain the state matrix of the whole system.In the flexible DC transmission system,the parameters of the system and the network structure are changed because of the addition of the flexible elements.In power system small signal stability method,the node voltage equation is the general form used to describe the interface equation of power system and dynamic components.Considering the large correction calculated amount of the state matrix when dealing with the system whose load adopted the voltage static model and the operation mode is constantly changing,a new method that the interface equation use power flow equation to carry out small signal stability analysi s is proposed in this paper.Based on the linear differential equation of the original dynamic components and injection power used as the substitution variable,the power linear algebra equation of synchronous generator is deduced.And thus the new whole s ystem state matrix and the correction of the voltage static load characteristic model to state matrix are obtained.Analysis shows that the state matrix has the characteristics of contract network and its modification formula has the advantages of simplifi ed network computing.The numerical simulation results verified the correctness of this method.