Research On Current Differential Protection For EHV/UHV Transmission Line

As the development of the power system, the EHV and UHV transmission line are appeared and increased rapidly, which elevates the performance requirements of the protective relay equipments. Currently the micro-computer based protection devices are widely used in EHV/UHV lines, the effect are good as whole, but some limitations are also emerged through long-term practical operations. In order to solve these problems, comprehensive and systematic researches are discussed in this thesis.Based on the capacitance current steady-state compensation and time domain transient compensation, the compensation scheme using single-terminal voltage is described in this thesis in view of TV wire-break or capacity limitation of communication channel. Since teed lines are increased, the compensation method suitable for teed lines is also presented. Large numbers of simulation results for two topology lines show that the steady-state capacitance current could be calculated well in two kinds of compensation method, however, the transient capacitance current is difficult to compensate fully. Therefore, the traveling-wave differential relay is proposed for its immune to capacitance current.4 kinds of differential current expressions are presented in this thesis based on the basic characteristics of traveling-wave and principle of traveling-wave differential protection. Further more, in order to seek the optimal differential current expression in different conditions, the sensitivity and operation speed for in-zone fault are compared and analyzed respectively. With respect to actual three-phase line, the fault phase selected trip could be realized by split-phase criterion from decoupling transformation.Traveling-wave differential protection possesses a special advantage in EHV/UHV transmission line protection. However, unbalanced differential current is brought under normal conditions due to the interpolated truncation error, the data synchronization error, the line resistance model error, the unequal traveling speed between earth sequence and spatial sequence, the uncertainty of line parameters and the transient noise of capacitor voltage transformer(CVT). These causations of unbalanced differential current afore-cited are analyzed theoretically and detailedly, and then draw some important conclusions, which provide a good foundation for the application of traveling-wave differential relay. Referring to the conventional current differential protection, an applied scheme and its specific implementation technique for traveling-wave differential protection are presented in this thesis. Theoretical research and simulation study show that the relay scheme operates sensitively, rapidly and selectively, and possesses a good capacity for application.Several specific transmission lines such as lines with shunt reactors, teed lines, lines with series compensated capacitor and double-circuit parallel line on the same pole are widely used in the power system. When applying traveling-wave differential protection to these particular transmission lines, it will confront with some new problems. Based on ilka equivalent model, alternative forms of the differential current expressions suitable for each specific transmission line are described in this thesis which makes it possible to apply such schemes to the protection of such lines.The rapidity of conventional switching into fault protection is decelerated heavily because of the serious transient of unloaded switching in EHV/UHV transmission lines. Based on analyzing the difference of the measurement impedance angle between switching into normal and fault state, comparing the level of harmonic noise under the two conditions, the thesis presents a novel scheme of unloaded switching into fault protection which is assisted by speedy distance relay with diviation characteristic. Theoretical analysis and simulation shows that the scheme can distinguish fault state from normal line quickly and accurately. Accordingly it possesses a good engineering application prospect for its sensitivity and rapidity.