Influence of Parallel Double-Circuit Transmission Lines on Inverse-Time Zero-Sequence Current Protection and Its Improved Algorithm

The selectivity of inverse-time zero-sequence current protection during earth fault occurred within parallel double-circuit transmission lines is analyzed.Research result shows that as for other transmission lines adjacent to both sides of the parallel double-circuit transmission lines the zero-sequence current in the formers is not always less than that in the parallel double-circuit transmission lines,so that the non-selective action of inverse-time zero-sequence current protections for the adjacent transmission lines may occur.For the faulty line itself the zero-sequence current at the end that is more distant to the fault point end may be larger than that at the opposite end nearer to the fault point,so the protection equipped at the more distant end may act earlier than the protection equipped at the nearer end;the protection equipped for the non-faulty line of the double-circuit lines will not act earlier that that equipped for the faulty line,so its selectivity can be ensured.Therefore,the possibility of misoperation of the protection is dependent upon zero-sequence parameters of transmission lines adjacent to both sides of the double-circuit transmission lines and the position of fault point,and is not dependent upon the zero-sequence parameters and zero-sequence mutual inductance of double-circuit transmission line themselves.Meanwhile,a wide area information-based inverse-time zero-sequence current protection algorithm is put forward,which can improve the speed of the protection while the selectivity of the protection is enhanced.The correctness of above-mentioned analysis is verified by simulation results.