Research On The Novel Method Of Fast Protection Of Hybrid Multi-terminal DC Transmission Lines

China’s primary energy centers and secondary load centers do not overlap geographically.Clean energy sources such as wind power,photovoltaics,and hydropower are mostly concentrated in the west and southwest,while load centers are concentrated in the east and southeast.Under the background of the national strategy of "West-East electricity transmission project",large-capacity and long-distance high voltage direct current（HVDC）transmission systems have been widely used.Conventional DC transmission has stability problems such as commutation failure,and flexible DC transmission is less economical.Hybrid multi-terminal direct current（DC）transmission system has become a better solution at this stage,and it has been applied in the "Wudongde" transmission project.Based on the hybrid multi-terminal HVDC transmission line,the following work is done in this paper:1.The principle of ultra-high-speed single-ended travelling wave protection is widely used in the protection of DC transmission lines,but this principle cannot identify the fault direction,and there is a threshold problem,which relies more on the highreliability backup protection principle.Based on this,the work done in this paper for travelling wave protection is as follows: 1)The propagation process of travelling waves in the hybrid multi-terminal network is analyzed,and the reflection and refraction coefficients of travelling waves are calculated;2)Based on the defined fault direction identification coefficient,a fault direction identification criterion is derived,and a complete pilot protection scheme of travelling wave direction is proposed;3)Using PSCAD/EMTDC simulation software to build a hybrid three-terminal DC transmission system to conduct fault simulation research.2.Accurate fault location is an important means to speed up troubleshooting and improve system stability.For the hybrid multi-terminal DC transmission line,how to accurately capture the high-frequency travelling wave signal and propose a travelling wave fault location principle suitable for the hybrid multi-terminal topology needs to be studied urgently.On this basis,the work done in this paper for travelling wave fault location is as follows: 1)This paper studies the transient model of the capacitancedivided electronic voltage transformer and analyzes its differential transmission characteristics of high frequency travelling wave signals;2)Based on the differential voltage travelling wave on the secondary side of the transformer,a novel travelling wave location method suitable for the hybrid multi-terminal topology is proposed;3)The hybrid three-terminal DC transmission system built by PSCAD/EMTDC simulation software is used to conduct fault simulation research.In this paper,detailed simulation verification shows that under the influence of different fault types,different fault distances and different fault resistances,The novel travelling wave protection method proposed in this paper has high reliability,and the proposed novel travelling wave positioning method has high positioning accuracy.