Research On Relay Protection Of Four Parallel Transmission Lines Different Voltage Grades

Four parallel transmission lines are used in power transmission network currently due to its characteristics of saving transmission corridors, improving power transmission capacity and decreasing construction costs. Because all the single lines of four parallel transmission lines are set on the same tower in parallel way, mutual electromagnetic effect exists not only between different phase of the same line but also between different lines of different grade, which makes the relation of voltage and current more complicated. Generally, four parallel line of different voltage grade consists of two parallel lines of different voltage grades, which cannot be decoupled with twelve-sequence component method. Additionally, there are lot of fault types, besides the single circuit fault, over-line fault occurs occasionally. There are many problems when traditional relay protection method is applied on four parallel transmission lines, such as, transmission lines cannot be decoupled, zero sequence mutual inductance influences the measure of inductance between protection location and fault location.Based on detailed analysis of four parallel transmission lines of different grades and sequence component method, a new method to calculate fault current with the boundary condition and relationship between sequence networks is proposed in this paper. The traditional single-phased current differential protection is seriously affected by distributed capacitive current of transmission line. In order to eliminate the influence of distributed capacitive current, a novel principle of current differential relay protection of transmission line based on inverted sequence component which only occurs in the inner of four lines in this paper. At the same time, a zero-sequence current compensation coefficient of ground-fault distance protection for four-parallel transmission line is discussed especially and algorithm of relay settings is presented for correct operation of ground distance protection. Measures of increasing the ability of anti-transition resistance were proposed in this paper.A great many of simulation results indicate that these methods proposed in the paper are correct. Considering coupled inductance when calculating fault current is necessary. The short-circuit current calculation method, the current differential protection scheme and grounding distance protection scheme are very effective for the further research on four parallel transmission line of different grades.