Steady current differential protection is the currently most commonly used practice for series compensated transmission lines. The current differential protection criteria for transmission lines at home and abroad are summarized

and their sensitivity in series compensated transmission lines

as well as the influence of system impedance and the degree of series compensation on protection sensitivity are analyzed. Analysis shows that the steady state of current differential protection may refuse to move for lack of sensitivity. The improvement algorithm of current differential protection using steady current can effectively solve this problem. The simulation of 500 kV series compensated ultra-high voltage transmission lines in Sichuan grid with PSCAD/EMTDC software has verified this conclusion.