A travelling wave differential protection scheme for half-wavelength transmission line

Half-wavelength transmission technology is getting more and more attention due to its excellent characteristics such as large transmission capacity and long transmission distance. Traditional differential protection is not applicable to half-wavelength transmission line because of the impact caused by large distributed capacitance current. Travelling wave (TW) differential protection is not affected by distributed capacitance current, so it can be applied to long EHV/UHV transmission line. However, when directly applied to half-wavelength transmission line, large unbalanced differential current will appear during external faults, since the resistance of the line and the frequency-dependent feature of the line parameters which are neglected by TW differential protection have such great impact that they cannot be neglected anymore. In order to improve the performance of TW differential protection, this paper proposes a scheme which has taken the resistance into account and uses power-frequency component of the differential current to identify fault for the purpose of avoiding the influence of the frequency-dependent feature of the line parameters. Simulation results verify that the proposed scheme is effective in reducing the unbalanced differential current, and as a result, ensures the selectivity of the protection. The scheme also has high sensitivity during internal faults with large fault resistance.