Phase-selection Study For Double-circuit Lines

To save resources of power transmission corridor, reduce the power construction cost, improve transmission capacity and land utilization, the double-circuit lines on the same tower has become the primary choice on 220 kV and above voltage grade in China. The double-circuit lines as the trunk line usually undertake super power transmission and regional power grid connection. Therefore, the safe and reliable operation matters a lot to the power grid.Because the span between the parallel lines is small, it is more likely to lead to the crossing-line faults. What’s worse, the mutual inductance coupling between lines makes the defaults more complicated. The phase selector of the traditional single-circuit line operates incorrectly when it is applied in double-circuit lines, especially in crossing-line faults. Therefore, this paper analyzes the fault feature, extracts the characteristic signal of the fault current, verifies the analysis results with PSCAD/EMTDC, and discusses the effect of the fault type, location and transition resistance on the characteristic signal of the fault current. Based on the above analysis, a new fault phase selection scheme has been proposed. This scheme is implemented by two steps. Firstly, take double-circuit liens faults as the single-circuit system faults based on the currents of the same-name phases. Then, select the fault phase according to the electrical quantities of the equivalent single-circuit system. Secondly, determine the specific circuit of the fault phase according to the ratio regularity of the same-name phase current. The simulation results show that the two-step fault phase selection is not affected by fault type, transition resistance and so on. Double-circuit lines with narrow span can ensure reliable phase selection on 80% of the lines, but the phase selection may select wrongly in lines near-termimal. With the aid of other criteria, it can be further improved.Because the present researches are mostly based on the power frequency and there are few researches about transient quantity, this paper studies further on the transient phase selector, in order to adapt to meet the requirements of new transient protection. The transient phase selector which is on the basis of the two-step fault phase selection utilizes the wavelet transform tool to extract the same-name-phase currents, takes the transient energy as phase selection criteria to build a new principle of transient fault phase selection. The simulation results show that the fault phase selection based on transient quantity not only makes up the phase selection drawback, but also obviously improves the phase selection performance. In addition, the paper takes comparision the research on the power frequency and transient phase selection, and analyses their respective advantages and disadvantages. It is of great importance for the comprehensive and systematical research on the fault phase selection of double-circuit lines.