Research On Fault Steady-state Analysis For Common-tower Double-circuit Lines And Its Fault Phase Selection Using One-terminal Electric Quantities

Common-tower double-circuit line has been widely used in power system, due to the advantages of low occupancy of land, short constructing period, high transmission capacity and saving money. But the span between the parallel lines is small, which may more likely to lead to the interline fault. The mutual inductance coupling between parallel lines is obvious, and the fault characteristics are more complicated under various faults. Therefore, when the phase selector for traditional distance protection which only uses single-circuit line’s one-terminal electrical quantities is applied to parallel transmission lines, it may lead to mal-operation. The dissertation begins with fault steady-state analysis and focuses on phase selection research using parallel transmission lines’one-terminal electrical quantities. They are mainly including:Aiming at the parallel lines which have narrow line-to-line distance have more risks on interline fault, a qualitative analysis approach for fault on parallel lines using average mutual inductance has been proposed based on the tower configurations of these double-transmission lines. Through this approach, all the simple faults and interline faults can be anglicized and calculated. Using the average mutual inductance substitutes for mutual inductance between lines and mutual inductance between phases which are little different, the parallel lines’model can be simplified using the average mutual inductance. Via equivalent simplification, the interline fault can be simplified to a virtual single-circuit-line fault form for calculation, and the achieved electric quantities can be used in the original network for calculating those of the parallel lines. It solves the problem that the qualitative analysis for fault on these parallel lines is difficult.Aiming at in the open literature the fault analysis for fault on parallel lines is not complete and lack of deducing electrical quantity expression, based on the method of six sequence fault component, all the20kind of fault on normal parallel lines has been analyzed. The analysis process considered the transition resistance, and deducing integrated electrical quantity expressions which are related to protection. Via analyzing the achieved electrical quantity expressions, the fault characteristics of six sequence currents at fault point and the measuring point have been researched, so have those of same-direction currents (T-currents). opposite-direction currents (F-currents) and phase-currents’phase components at the measuring point. Besides, the sequence voltages and phase voltages at the measuring point have been studied also. All the theoretical analysis and research provided theoretical foundation for proposing new phase selection scheme for double-circuit line in later study.A new fault phase selection scheme using parallel lines’one-terminal fault components of phase currents has been proposed. Firstly, select the fault phase using the sum-currents of parallel lines’same-name phases. Then, judge the fault is at the parallel lines near-terminal or not. If it is true, select the fault phase lines via comparing the fault component values and phases of same-name-phase currents; if it is false, select the fault phase lines via comparing the closeness of fault phases’ same-name-phase currents, or in other words identify the fault phase lines using the phase component value of F current/T current. The scheme is simple and easy to carry out with no need for long data channels. It can serve for the relay protection’s first section. Moreover, it can deal with all kind of short faults on same-tower double circuit lines, especially has better performance for interline fault. It is better than other phase selection methods applied to double-circuit line, which can’t deal with complex interline fault even lead to select wrong phases.A novel comprehensive phase selector which is suitable for applying in on-the-spot application has been presented. Firstly, phase selection with assistant voltages has been brought in, namely after fault happenning phase selector selects the fault phases using voltages and currents separately and gets a result by cooperation, then using currents to judge the specific fault phase lines. In this way, it solves the problem that phase selector which only uses currents misses fault phases because of CT saturation. Secondly, an adaptive mechanism of using the ratio of F current and T current has been proposed and brought in for selecting fault phase lines using currents when the fault is not at the near-terminal. It improves the adaptability and reliability of the phase selection scheme. Thirdly, the rule for judging near-terminal fault has been improved, avoiding unwanted phases selected when the fault point is at the end position of the near-terminal. In general, the presented comprehensive phase selector can fit the complicate working environment well and has remarkable ability for phase selection, without influence of mutual inductance between lines.