UHVDC分层接入系统的线路后备保护优化

Transmission Line Backup Protection Optimization in UHVDC Hierarchical Access System

UHVDC分层接入交流电网的形式在我国已经屡见不鲜。由于分层以及交直流之间的耦合关系,混联系统的故障暂态过程与纯交流系统有所不同。其中,交流故障引发直流换相失败会给交流系统继电保护带来一定影响。关注距离保护在经过渡电阻接地故障时的动作性能,基于电压相量平面,分析了距离保护测量阻抗不准确的原因和相关因素。然后,结合换流变压器绕组的接线形式,利用对称分量法的序网图进行分析,给出了线路两端零序电流相位的解析表达式;提出了基于零序电流相位的故障判据,并分析了该判据与过渡电阻、负荷电流以及分布电容的关系。最后,仿真验证了所提判据的正确性和有效性。

UHVDC hierarchical access into AC power system has become common in China. Because of the coupling relationships between different voltage AC systems and between AC and DC systems, transient process of hybrid power system differs from AC system. Commutation failure of DC system caused by AC faults may do harm to AC relay protection. Distance protection behavior during single-phase ground fault via transient resistance is studied in this paper. Based on voltage vector diagram, the reasons of incorrect actions of distance protection and its influencing factors are analyzed. Then, characters of different winding connections of convertor transformer are summarized. Using the sequence network of symmetrical component method, zero sequence current analytic expressions of both ends of the transmission line are derived. A backup protection criterion is proposed based on zero sequence current angles. Its relationship with transient resistance, load flow and distributed capacitance is studied. Finally, simulations using PSCAD software verify correctness and effectiveness of the proposed criterion.