Study On The Distance Protection For Parallel Transmission Lines On The Same Tower

Parallel transmission lines on the same tower has been widely used in practical, due to conservating the land, reducing the investment, and improving the transmission capacity of the unit corridor. The span between the parallel lines is small, which may more likely to lead to the cross-country fault. And the fault charateristics is complicated by the mutual coupling of parallel lines. Therefore, when the traditional distance protection is applied to parallel transmission lines, it may maloperation. Because of the large transmission capacity of parallel lines, the maloperation of the protection have a great impact on the power system stability. The dissertation focuses on the distance protection of parallel transmission lines. They are mainly including:Aiming at the normal operation of parallel lines, the phase selection method based on one-end data from parallel-lines and based on one-end data from single-line are proposed respectively. The phase selector based on one-end data from parallel-lines is proposed according to the correlation analysis of the same phase, in combination with the amplitude result of the fault components of unilateral phase currents. When one-end data from parallel-lines can not be obtained, the phase selection is proposed based on the sequence voltage to determine the fault phase, then the relationship between the superimposed positive-and negative-sequence impedances to distinguish the fault line, to determine whether the fault occurs on line â…  or line â…¡. It solves the problem that the traditional phase selector cannot correctly judge the fault phase of the adjacent lines.For parallel transmission lines under the condition of the single line in operation, the novel phase selectors are proposed, when the applied traditional phase selector based on the super-imposed components or the sequence components in combination with the impedance may be maloperation in complex condition. Firstly, a phase selector is proposed based on multi-parameter comparison to overcome the single-phase high resistance grounding fault. Secondly, an additional phase selector is proposed according to the corresponding relationship between the zero, negative sequence current components and the asymmetric fault types, to overcome the effect of the system oscillations. Thirdly, an additional phase selector is proposed based on phase superimposed component and the phase selector based on sequence component as supplementary method, to overcome the phase-phase grounded fault via asymmetric high-resistance.A new algorithm of first-zone distance relaying is proposed for single-phase grounding fault of parallel transmission lines. Based on the case of single-phase grounding fault of parallel lines, the relation between the zero-sequence currents of parallel lines is derived. Then, the zero-sequence current of the adjacent parallel line is expressed by the zero-sequence currents of the concern parallel line when a fault location is obtained from the traditional distance relaying. Finally, an accurate fault impedance can be calculated by one-end sampling data from single line with the consideration of the impact of mutual coupling effect. The algorithm is insensitive to the accuracy of the estimated fault location and have a good adaptation capability to the variation of the operating system, have a good performance for the accuracy of the calculated impedance.The novel distance protection algorithms for first-zone distance relay of parallel transmission lines are proposed for cross-country grounded faults and cross-country ungrounded faults, respectively. After analysing the cross-country ungrounded fault, the relation between six phase currents and six sequence currents of parallel lines is derived. Moreover, the sequence currents of the adjacent parallel line at relay location are expressed only by the sequence currents of the concerned line. Then, a novel first-zone distance relaying algorithm of parallel lines using only single-line sampling data is proposed for cross-country ungrounded faults. When a cross-country grounded fault occurs, a ground point appears. The zero-sequence currents of parallel are not equal. It greatly increases the difficulty of the realization of the distance protection. An attempt is made to express the zero-sequence current of the adjacent parallel line at relay location by the zero-sequence current of the concerned line in combination with the fault characteristics of the system model, by which the impact of the mutual coupling between the lines can be eliminated. And thus the distance algorithm for cross-country grounded faults is presented. The above methods solve the problem that the traditional distance protection based on the single line electrical quantities applied to parallel lines may malfunction, because of the measured impedance affected by the mutual inductance between parallel lines.