Study On Protection Principle For Half Wavelength AC Transmission Line Based On Distributed Parameters

For the reverse distribution of the energy bases and the power load centers in China, energy transmission becomes a key problem which is related to the national economy and the people’s livelihood. Ultra-high-voltage (UHV) transmission is an effective solution to the problem of China’s energy delivery, and the research of Ultra-high-voltage half-wave-length alternating current transmission (HWACT) is of great significance to our energy problems. The paper mainly studies relay protections of HWACT, and the research includes fault characteristic analysis, current differential protection and impedance pilot protection of HWACT.Firstly, the distributed parameters model of single-phase HWACT was established according to the transmission line theory. Secondly, fault calculations of transmission line were done based on symmetrical component method. The mathematical expressions of bus voltages and bus currents were built for different type of faults, which was the theoretical basis of fault characteristic analysis. Thirdly, output curves of electrical quantities were drawn for all sorts of faults on the basis of above conclusions of theoretical analysis, including curves of phase voltage, phase current, zero sequence voltage, zero sequence current and measured impedance. The fault law was extracted which is important to the fault analysis of HWACT. Finally, current differential protection and impedance pilot protection of HWACT were put forward in the paper.The distributed capacitive current along HWACT has a significant effect on the relay performance and should be taken into account in the protection principle. The study results in the two new protection principles based on the steady state transmission line equations, in which the distributed capacitive current is inherently represented. For the current differential protection, the fault phase selector should be used to realize segregated phase current differential protection, and the compensation current should be added to each phase difference current. For the impedance pilot protection, the fault location should be measured by the measured phase and distance. The experimental verifications show that the new principle can truly reflect the fault condition.