Research On Analytical Calculation Of Traveling Wave And Novel Traveling Wave Protection Principles For Multi-terminal Hybrid DC Transmission Lines

Multi-terminal hybrid DC(MTHDC)technology combines the advantages of LCC-HVDC and MMC-HVDC,which can not only achieve larger capacity and more economical multiterminal power transmission,but also avoid the risk of commutation failure.It becomes a better replacement of traditional two-terminal LCC-HVDC.However,MTHDC consists of two different types of converters and two segments of overhead lines with different parameters.Thus,MTHDC is significant different from traditional two-terminal LCC-HVDC in traveling wave characteristics of transmission line fault.Therefore,the existing travelling wave analytical calculation methods cannot be applied in MTHDC.In respect of this,research on the fault characteristic analysis,travelling wave analytical calculation method and new travelling wave protection scheme are significant for ensuring reliable operation of MTHDC.This paper focuses on the research of MTDC travelling wave protection and the research work can be listed as follow:(1)Based on the response characteristics of two different converters at the traveling wave stage,the travelling wave equivalent model of LCC and MMC are derived;The complexdomain expression of characteristic impedance and propagation function are derived by loworder approximation of the frequency-domain parameter of transmission line;The travelling wave equivalent model of MTHDC is established by combining the equivalent model of converters and transmission line.Finally,the travelling wave response curve of equivalent model is verified using time-domain simulation of PSCAD.(2)Based on the equivalent model,the expressions of the initial travelling waves,the attenuation and distortion coefficient of transmission line and the refraction coefficients of the T-connect DC bus and the converter stations can be derived,so as to obtain the travelling wave analytical calculation model of forward travelling wave at the converter stations.(3)Based on the proposed analytical calculation method,the setting calculation and sensitivity analysis method of travelling wave protection are presented for an example system.The proposed method and the analytical results are verified by PSCAD simulation.And the results show that the proposed method is correct and superior in computation efficiency.(4)The phase characteristic of initial travelling wave caused by inner-zone and outer-zone fault,the phase shifting characteristic of T-connect DC bus and overhead line are analyzed based on the proposed analytical calculation method.The analysis results supply theoretical fundament of new travelling wave protection principle of MTHDC.(5)Based on the phase characteristic of travelling wave caused by inner-zone and outerzone fault of MTHDC system,a new travelling wave protection principle is proposed using Morlet wavelet transformation.The proposed protection principle includes start-up unit,direction unit,faulting pole selection unit and fault detection unit.The proposed protection principle is verified using a great number of simulation cases concerning different faulting locations and faulting resistances.The result shows that the proposed protection principle can correctly trip,and has superior performance under high-impedance transmission line fault.