多端柔性直流输电系统交流侧故障穿越功率协调控制

在多端柔性直流输电(voltage source converter based multi-terminal DC transmission,VSC-MTDC)系统交流侧发生故障导致电网电压暂降时,为了维持该侧公共连接点(point of common coupling,PCC)电压水平,设计了故障期间各换流器的有功功率和无功功率协调控制策略。在电网故障期间,故障侧换流器由有功电流控制优先切换到无功电流控制优先,根据电网跌落深度发出相应的无功功率,当增发的无功功率未导致交流侧过流时,剩余的电流容量用于维持故障前的有功传输水平。而当增发的无功功率导致交流侧过流时,提出了通过改变交流系统故障对应的换流器的有功功率来避免交流侧过流。同时提出将基于下垂控制的功率控制策略转换为以输出电压为指令的控制策略,当交流系统故障对应的换流器有功功率改变时,有功功率失去平衡,直流电压因电容充电/放电而升高/跌落,此时其余侧换流器在不需要站间通讯的情况下自动随着直流电压的变化而调整自身的有功功率指令,达到自适应调节、自动分配功率的目的,实现自律分散控制。最后在PSCAD/EMTDC上搭建了多端柔性直流输电仿真模型,验证了该文所设计控制策略的有效性。

Power Coordinated Control Strategy for AC-Side Failure Ride-Through of VSC-MTDC System

When AC-side failures of voltage source converter multi-terminal DC (VSC-MTDC) system leads to power grid voltage sag,this paper designs the coordinated control of the active and reactive power of each converter during the fault to maintain the voltage level of related side point of common coupling (PCC).In the duration of the fault,the converter near the fault is switched from preference of active current control to preference of reactive current control and generates reactive power depend on the grid voltage sag.When added reactive power does not cause over-current on the AC side,the extra current capacity is used to maintain the active level before the fault.When added reactive power causes over-current on the AC side,this paper proposes a control strategy to avoid the over-current by changing the active power of the converter related to fault side.Meanwhile,this paper comes up with the control strategy based on the droop control to convert the power into control signals that are instructed by input voltage.With the changing of the active power of the converter near the fault,the active power is out of balance and the DC capacitors rises or falls due to the charge or discharge.At the meantime,other converters adjust their own active power instructions without the communication between stations automatically according to the changing of the DC voltage,which can attain the goal of self-adjustment and power automatic-distribution and realize autonomous decentralized control.In the end,this paper constructs the simulation model of the VSC-MTDC system in the PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.