Static Voltage Stability Analysis And Control For AC/DC System With VSC-HVDC

Power system voltage stability has always been a matter of great concern for power engineers.In recent years,with the increasing of the project number and transmission capacity of VSC-HVDC,the interaction between the AC system and the DC system is increasing.How to establish the static voltage stability analysis method of AC/DC hybrid system and propose a control strategy to improve the voltage stability of hybrid system with VSC-HVDC has become an urgent problem to be solved.Focusing on this issue,the static voltage stability of AC/DC hybrid system with VSC-HVDC is studied in this paper.The basic working principle of VSC-HVDC and the converter station topology are studied in detail,firstly in this paper.On this basis,the mathematical model of converter station and the mathematical model of AC/DC system with VSC-HVDC are established.The power exchange equation between the converter station and the AC system under different control modes is deduced.For the multi-terminal VSC-HVDC system,two kinds of control modes are considered,such as fixed DC voltage control and DC voltage drop control.In this paper,a continuous power flow algorithm for AC/DC systems with VSC-HVDC is developed by combining the continuous power flow algorithm with the alternating solution of AC/DC system.The continuous power flow algorithm effectively takes into account the different VSC-HVDC control methods,adapt to the multi-terminal VSC-HVDC system,and achieve the automatic switching of the control mode when the VSC-HVDC converter station reach the capacity limit.The PV curves of the system is obtained by the continuous flow method of the AC/DC system.And the influence of the VSC-HVDC active power transmission and the reactive power output of the converter station on the static voltage stability of the AC/DC system is analyzed using the load margin index.In order to quantitatively analyze the influence of the change of the control parameters of the converter station on the load margin of the AC/DC system,this paper deduces the sensitivity of the AC/DC system load margin to the control parameters of the converter station using the Jacobian matrix information at the critical point of the system.Based on the sensitivity results,the prediction formula of the load margin is proposed,and a VSC-HVDC control strategy is proposed to improve the static voltage stability of the AC/DC system.Finally,the correctness and validity of the proposed method are verified by analyzing the VSC-HVDC system and the three-terminal VSC-HVDC in the modified 68 nodes and the modified IEEE118 nodes example.