Fault Analysis And Protection For Local Coupling Doublecircuit Transmission Lines With Closed Loop Network

Because of power geographical conditions and shortage of transmission corridor and effect of some other reasons, engineering appears a large number local coupling circuit transmission lines. Partial same-tower transmission line exists a complex inter-line mutual inductance between the lines on the same tower. Unsame-tower part transmission line doesn’t exists mutual inductance between the lines and at least one side of the lines don’t have public bus. Therefore, when fault happens, the structure of zero-sequence network and zero-sequence current distribution are more different with completely same-tower transmission line, so existing fault calculation method cannot meet the requirements of partial same-tower transmission line. When partial same-tower with closed loop network operate, ground fault causes zero-sequence circuit conduction, mutual and through ring line zero-sequence current in superposition cause zerosequence magnitude and direction are both changed and may cause zero-sequence current protection tripping or malfunction. Therefore, this dissertation researches on fault analysis method of partial same-tower line with consideration of closed loop network, zero-sequence current protection and ground distance protection in-depth study, the main contents are as follows:1. According to the structure of local coupling circuit transmission lines. Use the phase component method establish each part line voltage and current interface equation, based on port network theory each part line equation will be established contact, combined with fault boundary condition equations to build fault calculation model, in order to realization a fault method of local coupling circuit transmission. Based on the ATP/EMTP build a simulation model to verify, the results present proposed method can quickly and accurately calculate. In addition, calculation program based on proposed method provide an accurate and efficient tool to research operating characteristics of protection.2. Select typical local coupling circuit transmission lines, based on proposed fault analysis method to research the effect of single-line fault and inter-line fault, respectively, on characteristic of zero-sequence current protection. And for the malfunction of protection by single-line fault and the rejecting act of protection by inter-line fault, this dissertation proposed a new protection setting strategies, thus ensuring reliability and sensitivity of zero-sequence current protection, for cross-country fault, traditional zero-sequence directional elements are present tripping possible problems on same-tower side. This paper proposes an improved criteria: Firstly, based on two loop voltage and current distinguish system side from line fault, according to the three-sequence phase angle difference of co-vectors and reverse vector to discriminate single line fault from cross-country fault. If single line fault happens, and then use traditional zero-sequence directional element to achieve fault line selection. This dissertation verifies proposed criterion at different fault conditions, the results show that regardless of the type of fault and fault location proposed criterion all can accurately determine system side fault from line fault, and can select fault line when single line fault happens.3. Due to the structure of partial same-tower, completely ignore or consider mutual coupling can neither accurately reflect the grounding impedance. In this paper, based on the voltage and current equation when fault located behind cutoff point get a calculation formula of fault distance. Compare this calculation result with the length of front of cutoff point to correct calculation process, in order to realize a correct calculation method. And using ATP/EMTP software to build a comprehensive simulation model to verify, validation results show that the proposed method can accurately calculate the fault distance at different fault conditions.