接入弱交流系统的背靠背直流输电系统控制策略改进

背靠背直流工程逆变侧采用定熄弧角控制时,整流侧和逆变侧耦合关系较强,但换流变压器分接开关动作次数少;而采用定直流电压控制时,耦合关系较弱但分接开关动作次数多。在分析相关控制策略优缺点的基础上,提出了改进控制策略,逆变侧通过熄弧角控制直流电压为指令值,直流电压指令值随直流功率变化,分接开关控制熄弧角在正常范围内。该控制策略保留了定熄弧角控制和定直流电压控制的优点。提出整流侧和逆变侧在分接开关动作时预估无功小组是否会因分接开关动作而投切,进而确定分接开关和无功小组动作的先后关系,防止了分接开关的反复动作。算例表明该控制策略可以减少背靠背工程无功小组投切和分接开关动作次数,减小逆变侧无功小组投切电压波动,适用于接入弱交流系统的背靠背工程,长距离直流输电工程也可以借鉴文中控制策略。

Improvement of Back-to-back DC Transmission System Control Strategy for Weak AC System

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.