Study On Relay Protection For UHV Transmission Lines

In order to meet the continued load growth in the future and improve the efficiency of energy utilization in our country, a development goal to accelerate the construction of a UHV backbone network, comprised by1000kV UHVAC transmission systems and±800kV UHVDC projects was proposed by the Sate Grid Corporation of China (SGCC). In early2009, the first1000kV UHVAC Jindongnan-nanyang-jingmen pilot demonstration project in China was put into operation, which indicated that China has achieved a breakthrough in the UHV core technologies and equipment localization, and had an important significance to ensure national energy safety and reliable power supply.The UHVAC long distance transmission lines have characteristics of huge transmission capacity, small characteristic impedance and large distributed capacitance. The large capacitive current would cause malfunction, sensitivity decrease and other issues of relay protection devices in some cases. Two measures can be applied to solve these problems. One is a compensation of the capacitive current, and the other one is a new protection principle not affected by the capacitive current. A depth study on the solution of influences on the protection device caused by the capacitive current is carried on. On the basis of simulation and analysis on the transient characteristics for the UHVAC pilot demonstration project, a novel principle of current differential protection based on Marti model applying for the UHVAC transmission lines is presented. The proposed principle does hardly affected by the capacitive current. Moreover it ensures both sensitivity for internal faults and reliability for external faults of the protection devices. A method to calculate reactor current in the time domain is put forward when circuits are installed shunt rectors. The Implicit Trapezoidal Integration Method and the Backward Euler Method are integrated to obtain the stable and accurate numerical solution. Then the novel principle of current differential protection based on Marti model is applied in the double circuit UHVAC lines, and a capacitive current compensation algorithm for double circuit UHVAC transmission lines is also presented. The detailed research work is shown as follows:(1) The line parameters of UHVAC pilot demonstration project in China are calculated according to the original conditions. Varieties of transient components emerging during faults and breakers operation are simulated and analyzed, which lay a firm foundation for the later research.(2) The current differential protection principle based on Marti line model applied in the UHVAC transmission lines is proposed, which does not affected by the capacitive current. More accurate results will be obtained by way of the principle taking account of frequency-dependent characteristics of line parameters. A simple and practical criterion is also given. The expressions of operating current at the conditions of various short-circuit faults are deduced. The high sensitivity and allowable transition resistance ability of the proposed protection principle are verified by simulations.(3) A method to use current differential protection principle based on Marti model into the UHV transmission lines equipped with the shunt reactors is put forward. The reactor current is calculated in the time domain by way of Implicit Trapezoidal Integration Method on the normal time, and the Backward Euler Method on the network jumping moment. This disposal can not only avoid the oscillation of numerical calculation, but also improve the simulating accuracy as much as possible. At last the modified reactor current will be counted into the protective criterion.(4) The simulation model of the Huainan-Wannan UHVAC double circuit lines in China are constructed after the calculations of line parameters. Simulation on the conditions of various short-circuit faults, especially faults between differential lines are carried out. At the same time the impact on the relaying protection is also analyzed.(5) In order to use the aforementioned current differential protection in the UHVAC double circuit lines, the phase-mode transformation matrix is extended firstly. In the operation, the double-circuit six lines are regarded as a unified protected object. The working steps are also listed in the dissertation. The simulation results verify the correctness of the protection principle applied to the double circuit lines.(6) When an internal fault occurs on the double circuit lines, the transient current of non-fault-phase distorts greatly. At this time the traditional half-wave Fourier Transform Algorithm does not function properly. For these reasons, an improved half-wave Fourier Transform Algorithm is applied to filter. Simulations show the algorithm has good precision and speediness.(7) A capacitive current compensation algorithm for double circuit UHVAC lines is presented. On the basis of the transmission line model with distributed parameters, the current sample values are compensated after calculating the capacitive currents. The algorithm does not require excessive high sample frequency and has a wide practical value.The above researches and results show that the proposed current differential protection principle based on Marti model is suitable for the UHVAC single circuit or double circuit transmission lines. This principle can solve the difficulties in the traditional relay protection, and related study has more significance in theory and practice.