A fault location method for extra-high voltage mixed line based on variation of sequence voltage

For extra-high voltage mixed line, the parameters of cable segment and overhead line segment are different, so the fault location method of uniform line cannot be used. Based on the characteristics of mixed line parameters, a fault location method for extra-high voltage mixed lines based on variation of sequence voltage is proposed. A precision, fast calculation method of voltage along the line is researched by using linear equivalent. Through simulation tests the precision of method is high. In terms of frequency domain transfer equations, sequence of the amplitude attenuation and phase delay along fault lines are calculated. Extra-high mixed line fault types can be divided into two categories: the asymmetric fault and the symmetric fault. For the asymmetric fault, before fault happened, the negative sequence voltage along mixed lines is 0. So the fault negative sequence voltage variation is the current time negative sequence voltage. Negative sequence voltage amplitudes along mixed line are calculated by using two-terminal electric parameters of line and compared for asymmetric fault. The fault location is the point with equal negative sequence voltage amplitudes. For the symmetric fault, the line will not appear negative sequence voltage. There is positive sequence voltage only. But, for three phase fault with transition resistance, the fault point voltage is not the lowest. The calculated voltage amplitudes along the line have two intersections: fault point and false point. The intersections have almost equal amplitude, so using numerical magnitude of point amplitude to false identification is hard. To solve the problem what has mentioned above, this paper uses the positive sequence voltage variation for fault location. Before fault happened, the positive sequence voltage along the line is not 0, so the positive sequence voltage variation is the difference of current and previous positive sequence voltage amplitude. Then, for three-phase symmetry fault, the fault location is the point with equal voltage amplitudes variation of positive sequence. Finally, the simulation is carried out to verify the algorithm. The proposed algorithm is computationally efficient with high precision. It is not affected by transition resistance, and no false identification problem.