The Static Voltage Stability Analysis Of AC/Hybrid MTDC System

With the increasing of HVDC projects, HVDC has played a pivotal role in state grid of china. The CSC-HVDC and VSC-HVDC can be combined by different topologies to form a hybrid HVDC system which has complementary advantages and can improve security and stability of grid. Most of the static voltage stability analysis is about a single CSC-HVDC or VSC-HVDC system. Due to the diversity of operation mode and complexity of control mode, static voltage stability analysis of AC/Hybrid MTDC system is more complex. Little study has made in this field.This article is to study static voltage stability of AC/Hybrid MTDC system.Firstly, as to AC/Hybrid MTDC system, the paper analyzes the relationship between the AC and DC subsystem through establish the mathematical model of VSC, CSC and DC network equation. A power flow algorithm which takes the coupling term into account is formed. After improved on continuation power flow,the paper proposes a static voltage stability analysis method of AC/Hybrid MTDC system in fixed load increase way. The method takes the DC control mode conversion and the transformation of the converter turnsratio into consideration.Secondly, according to the load margin differ greatly in different load growth direction, in order to grasp the emergency situation, a algorithm to seek the most dangerous load growth direction and the closest voltage instability point of AC/Hybrid MTDC system according to the mathematical principle of the shortest path is proposed. Meanwhile the paper raises measures to accelerate the convergency. This algorithm is applied to analyze the wind farm, and the paper analyzed the effect of wind power fluctuation.The power flow algorithm and static voltage stability of AC/Hybrid MTDC system which uses voltage droop control strategy is studied. A comparative analysis of the static voltage stability between voltage droop control strategy and master slave control strategy is done. The paper studies the influence of DC voltage response coefficient to static voltage stability.Finally, based on the static voltage stability of AC/Hybrid MTDC system, the paper studies the optimal control vector. And the paper proposes AC, DC measures to increase the static voltage stability, such as increase the reactive power compensation capacity of converter station, change control mode and given value of converter.