Whole-line quick-action protection utilising single-end electrical parameters has been studied and verified in alternating current and direct current power systems. This study analyses the origin of fault generated high-frequency harmonics based on the properties of boundary elements and power source during the fault, creating the possibility for the distribution network to achieve single-ended protection. Based on the non-loss transmission line model, the differences of transient harmonic characteristics are explained by the comparison of the stabilisation process of the power system with boundary elements and the one without. It is revealed that the source generated specific high-frequency harmonics and energy storage elements generated high-frequency harmonics contribute to single-ended protection criterion during the fault. As the protection scheme only relying on the source generated specific harmonics is hard to take effect for all segments of the distribution network, the travelling wave generated harmonics ought to be utilised to distinguish between internal and external faults for each segment by adding a proper boundary element in the line.