Non-unit traveling wave protection for flexible HVDC lines based on the ratio of the waveform fitting residual

To solve the problems of sensitivity and rapidity of flexible DC line protection from the perspective of the time domain waveform difference of the fault traveling wave (TW), a mathematical analysis of the waveform char-acteristics of the fault-generated TW of the flexible HVDC line is developed in this paper. Moreover, the dif-ference in the time domain expression between the voltage TW is obtained under internal and external faults. To extract the features of the voltage TW waveform, the Levenberg-Marquardt method is used for data fitting according to two fitting functions corresponding to the internal and external faults. Then, a non-unit travelling wave protection scheme based on the ratio of these two fitting residuals is proposed. Finally, extensive simu-lations based on PSCAD/EMTDC show that the scheme can reliably identify all the faults of the DC line within 1 ms and has a high sensitivity and strong robustness under noise interference and different operation modes.