Due to the overload limitation of the converters, the main protection for DC lines of High Voltage Direct Current based on Modular Multilevel Converter (MMC-HVDC) requires ultra high speed for fault detection. The protection principles of single-terminal traveling-wave can meet this requirement. Such protection principles which are usually constructed by transmission theory of fault traveling-wave have a clear physical concept and do not depend on boundary components. Almost all of these principles are dependent on accurately detecting the arrival time of the traveling-waves. Thus, it is urgent to research a new time-frequency analysis method with a high time resolution to identify the arrival time of traveling-wave. This paper introduces Transient-Extracting Transform (TET) to detect traveling-wave and improves it fully considering the fault traveling-wave characteristics of DC lines. The time resolution of improved TET is greatly increased

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