多端柔性直流电网线路故障暂态等值电路及暂态特征分析

Study on the Transient Equivalent Circuits and the Transient Characteristics ofMulti-terminal Flexible DC Grids After Line Faults

线路保护问题是柔性直流电网进一步发展的瓶颈,故障后的等值电路及暂态特征作为线路保护的理论基础亟待系统性分析。该文利用换流器与输电线路的暂态模型和故障点的边界条件,建立线模–零模解耦后的暂态等值电路,得到故障行波传递到故障线路端口、直流母线和非故障线路端口的传递函数,分析初始故障行波的暂态变化特征。此外,通过分析故障行波在感性线路端口和故障点间的多次折射和反射,得到故障行波在线路端口反射和故障点折射、反射后波形的方向和幅值变化规律,并分析在故障点线模和零模之间的混叠现象。最后,在PSCAD/EMTDC搭建四端仿真模型,通过理论计算值和仿真模拟值的对比分析验证正确性,并分析换流器、限流电抗器等器件参数变化时波形的变化特点。

The problem of line protection is the bottleneck of the development of a flexible DC grid.As the theoretical basis of the line protection,the equivalent circuit and transient characteristics after line fault need systematic analysis.This research will provide the cornerstone for the design of new line protection.Using the transient model of the converter,model of transmission line,and the boundary conditions of the fault point,the transient equivalent circuit after decoupling into line-zero mode is established.The transfer functions of the fault traveling wave at the faulty line terminal,the DC bus,and the healthy line terminal are obtained,and the transient characteristics of the initial fault traveling wave are analyzed.Besides,the multiple refractions and reflections of fault traveling waves between inductive line terminal and fault point are analyzed,the change law of the waveform after refraction and reflection is obtained,and the aliasing phenomenon between line-mode and zero-mode at fault point is analyzed.Finally,A four-terminal simulation model built in PSCAD/EMTDC demonstrates the correctness through the comparative analysis of the calculation and the simulation value.The characteristics of the waveform are analyzed when the parameters of the converter and current limiting reactor are changed.