| In order to respond to China's "West to East power transmission" strategy and solve environmental pollution problems,it is imperative to construct the UHVDC(Ultra High Voltage Direct Current)transmission project for large-scale,long-distance power transmission.It is expected that within 5 years,China's DC transmission project will reach around 50,among which UHVDC transmission project accounts for 60 percent.However the voltage level of the UHVDC transmission system is high and the transmission distance is long which may cover some provinces.Besides the transmission corridors are always built in remote and rugged areas.All of these conditions may cause high probability of failure,which will seriously affect the safety and stability of the DC transmission system.Therefore,the circuit protection is increasingly important for long-distance UHVDC transmission line in the project.At present,the main protection of the transmission ling of the UHVDC transmission system is traveling wave protection which has good selectivity and quick motion and is not easily affected by the system fluctuation during the transmitting process.However,in actual project,many kinds of factors will cause malfunction of the traveling wave protection device.In theoretical way,the current traveling wave protection emphasizes new algorithms and improves its reliability by mathematical means.In practical application,these algorithms have some limitations and they have high requirements for the system operation,which impede the promotion of protection criteria in certain degree.In this paper,the current status and the development prospect of UHVDC transmission system are analyzed.Based on Yun-Guangdong UHVDC transmission system,the PSCAD/EMTDC simulation tool is used for model construction.And then this paper introduces the traveling wave theory,researches its transmitting characteristics and analyzes the existing wave protection criteria,thus the advantages and disadvantages of the traveling wave theory is summarized.On this basis,the fault voltage is decomposed into two parts:line mode components and ground components.Then compare the two kinds of parts and find the differences between on-line fault and out-of-line fault as well as the differences between negative line and positive line.And then according to the differences,simplify the original criteria and set the action equation and finally analysis the impact of transition resistance.The simulation shows that this method simplifies the data processing in traditional traveling wave protection and that it is free form the influence of transition resistance.The ways of the power line protection should not be less than two,because when the failure occurs,other uncertain factors may cause main protection—traveling wave protection refusing to move.Therefore this paper proposed a backup protection program—differential protection considering the line distribution parameters.This protection scheme optimizes the current compensation for the transmission line distributed capacitance and eliminates the influence of distributed capacitance discharge and thus avoids the protection malfunction of out-of-line fault. |
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