The Improvement Of VDCOL Control Function In High Voltage DC System

With the more and more prominent status of HVDC transmission in the power system,the research on the control and improvement of HVDC system becomes the hotspot in the power engineering and academia.The voltage dependent current order limiter(VDCOL)in the HVDC control system can restrict the DC currents at low voltages and reduce the reactive power demand during failure and recovery,which contributes to restoring the system stability.This paper summarizes the VDCOL function,analyzes the factors that should be considered in the HVDC transmission project aiming at the VDCOL setting method,by analyzing the constraint relation between the dynamic reactive power and the VDCOL function,the improvement of the VDCOL function is carried out,and here are the following researches :First,this paper studies the setting method of VDCOL parameter from the viewpoint of reactive power characteristic of HVDC transmission system.Based on the VDCOL voltage and current characteristics,the operation characteristics of HVDC system are analyzed in detail;Then,with VDCOL parameters and AC voltage as independent variables,reactive power exchange as dependent variable,the range of VDCOL parameters is parsed;it is advantageous to maintain the voltage stability and the fast recovery of the voltage after fault by adjusting the VDCOL parameters in the specified area,.The simulation verification in the CIGRE DC transmission standard test system proves the correctness and validity of the above-mentioned analytic method.Then,the method of fitting formula and quantitative calculation is proposed in this paper from the point of VDCOL function,despite the limitations of VDCOL parameters,based on the AC-VDCOL control link,by improving the VDCOL function through the reactive power control.Both methods use the constraint relationship between AC bus voltage,DC current and reactive power to improve the VDCOL function.Corresponding to the degree of voltage drop of AC bus,and set the DC converter station reactive power exchange,the corresponding DC current is obtained,which improves the system recovery performance after the failure.This method can not only realize the quantitative control of reactive power,but also avoid the problem that the converter reactive power control function directly controls the rectifier side trigger angle or the inverter side turn-off angle to cause the unstable operation of the DC system.At the same time,the improved methods are verified by CIGRE DC transmission standard test system and a ± 500 kV actual DC engineering PSCAD model.The results show that the two methods can both realize the quantitative control of the reactive power.When the converter station is used as the dynamic reactive power source to inject the reactive power into the AC system,it is beneficial to the rapid recovery of the AC bus voltage after the fault and improve the system failure restore performance.The research results have significant reference for improving the steady state and dynamic performance of HVDC transmission system.Finally,the above researches are summarized and forecasted,and further research works are put forward.