Aiming at the problems of ambiguous fault characteristics

weak anti-interference ability and complex threshold setting of protection criteria for high-resistance grounding faults in flexible DC power grids

a protection scheme for four-terminal flexible DC power grids based on the impedance polarity difference of current-limiting reactors at both ends of the line was proposed. Firstly

by taking advantage of the significant fluctuation characteristic of the impedance of current-limiting reactor after fault occurrence

the impedance change rates of positive and negative sequence current-limiting reactors were adopted to construct the protection starting criterion. Secondly

the cumulative sum algorithm was used to extract the initial fault wave signal

and the dynamic time warping algorithm is combined to realize dynamic signal alignment. Based on the impedance polarity difference of current-limiting reactors at both ends of the fault line under different fault types

the polarity of the cosine value of the impedance angle of current-limiting reactors at both ends of the DC line was utilized to establish the internal and external fault identification criterion. Finally

the ratio of the impedance integral values of positive and negative sequence current-limiting reactors was employed to design the fault pole selection criterion. Simulation results show that the proposed protection scheme can accurately identify fault types

avoid complex threshold setting

and no misjudgment occurs under 1 500 Ω high-resistance grounding

10 dB noise interference and various complex operating conditions

which meets the requirements of flexible DC grid for protection selectivity.