技术分享｜柔性直流输电网线路保护配置方案

在传统的交流系统中,线路保护对保障系统的安全稳定运行具有重要作用。但在现有的直流输电工程中,直流线路的保护的重要性有所下降。现有的直流输电工程多为双端直流输电工程,对于基于晶闸管的常规直流输电系统,当直流线路发生故障时,由于其定电流控制的存在,可以通过增大晶闸管的触发角来降低线路电压从而将故障电流降低到较低水平,系统中的设备不会遭受稳态大电流的冲击[29-30];对于双端柔性直流输电系统,由于直流断路器不成熟[31-33],因此直流线路发生故障时往往只能通过开断交流侧的交流断路器来切断故障电流[21, 34]。但是对于柔性直流电网来说,由于其自身的故障特点以及直流断路器难以做到大容量等原因,线路保护的重要性被提升到前所未有的高度。可以说,柔性直流电网的线路保护技术是保证柔性直流电网能够得以真正实现的关键技术之一[35-36]。因此,柔性直流电网线路的主保护在满足对动作速度要求的同时,应当具有较高的可靠性、足够的灵敏性和绝对的选择性,不能再简单地直接借鉴常规直流输电系统的线路保护。

3.3 后备保护的比较

柔性直流电网线路的后备保护应当采用纵联保护技术。纵联保护相对于单端量保护往往具有更好的选择性和灵敏性,当单端量主保护受其灵敏度限制不能识别经大过渡电阻接地故障时,纵联后备保护可以良好地作为单端量主保护的补充和配合。现有的常规直流输电系统的线路后备保护往往采用纵联电流差动保护,但受故障后暂态过程的影响,电流差动保护往往引入较大的延时,动作速度较慢。对于柔性直流电网来说,其直流线路的后备保护动作时间应少于20~30 ms,以保证在直流侧发生故障时直流线路后备保护能够先于交流侧保护动作,进而在时序上更好地配合超高速的直流线路主保护和交流侧的保护。

4 结论

柔性直流电网的故障处理方式应当与传统的交流电网类似,即通过保护和直流断路器实现对故障元件的快速隔离或切除,以充分发挥柔性直流电网的输电灵活性和可靠性优势。

柔性直流电网线路主保护应当采用单端量保护算法以保证保护具有极快的动作速度,并需要采取适当的解耦措施和故障选极方法,以及噪声、雷击等干扰的识别方法,以保证保护的可靠性;后备保护则应当采用纵联保护原理以作为主保护的补充,保证保护具有足够高的灵敏度;同时后备保护也应当具有较快的动作速度以在时序上配合快速主保护和交流侧保护,尤其是在直流侧发生故障时,后备保护要能够先于交流侧保护动作。

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