Constant extinction angle control for the inverter will cause a rather strong coupling effect between rectifier and inverter but a few actions of the tap changer on both sides. On the contrary

when constant voltage control is adopted a weak coupling effect but more times of the tap changer action. Based on an analysis of the advantages and disadvantages of constant extinction angle control and constant voltage control

an improved control strategy is proposed. With the improved strategy

the DC voltage will be controlled by the extinction angle of inverter while the DC voltage order should be calculated based on power order. The tap changer of the inverter converter transformer will be controlled so that the extinction angle stays between 17. 5° and 21. 5°. The improved control strategy retains the advantage of constant extinction angle control and constant voltage control. A control strategy is also provided to coordinate the tap changer control and reactive control. With the control strategy

the repeated action of tap changer will be prevented. An example based on a plan project shows that the improved control strategy can reduce the action times of reactive power sub-banks and tap changer while providing a reference for long distance HVDC projects.