| With the rapid development of the national economy, great progresses have been made in the electric power industry. However, the geographical distribution of load and energy in China shows an extreme inconsistency, which brings about ever higher demands for the transmission capacity of power grid. Hence, construction of nationwide energy transmission channels becomes an urgent need in order to achieve long-distance large-capacity power transmission. As to Ultra High Voltage Direct Current(UHVDC) transmission system, it has huge transmission capacity and great transmission distance, and is therefore quite suitable for long-distance and high-power transmission project. In China, a couple of UHVDC transmission projects have already been completed, and Yun-Guang ?800kV UHVDC transmission project is one of them. Yet for UHVDC transmission project, the voltage withstand ratio of its devices are significantly lower than that of HVDC transmission project. Once there is a protection malfunction and so forth, there could be severe overvoltage, even causing an abnormal outage of the transmission system. Therefore, with respect to the Yun-Guang UHVDC transmission project, analyzing and optimizing the last circuit breaker protection is of great importance for the operation safety and stability of the power grid.Firstly, characteristics of Yun-Guang UHVDC transmission project are studied and an appropriate simulation model of the transmission system is established based on PSCAD in this paper. Also, the principles of both conventional and arrester based last circuit breaker protection are further analyzed, and the simulation model of arrester based last circuit breaker protection is established.Secondly, in light of the problem that the conventional last circuit breaker protection would reject when the long transmission line is under no-load condition and would malfunction when the AC line is under light load condition, an improved criterion on determining the line isolation condition is proposed by combining the AC line switch status, the head end power injection and the head end voltage. Moreover, in light of the problem that the arrester based last circuit breaker protection would malfunction when the AC line is under light load condition, countermeasures are proposed. That is, based on the improved line isolation condition determining criterion, combining with the line status to decide whether to reduce power emergently. In this way, protection malfunction under light load condition can be avoided and the system can be ensured to operate stably and continuously. The validity of the improved methods above is verified via simulation verification.Furthermore, based on the negative-sequence theory, the recognition mechanism of short circuit capacity and topological structure of AC system are proposed. On the basis of this mechanism, prognosis of AC system bus overvoltage is achieved and improved strategy of arrester based last circuit breaker protection is proposed. On the other hand, focusing on the issue that the prognosis of overvoltage may fail when AC system is operating under certain special conditions, the arrester based last circuit breaker protection is further improved and simulation is carried out for verification.Finally, with regard to the issue that the arrester based last circuit breaker protection may malfunction due to overvoltage when heavy load is shed in the AC system, a corresponding protection coordination mechanism is proposed by combining the previous improved line isolation condition determining criterion and AC bus overvoltage prognosis method. Besides, focusing on the issue that overvoltage may occur on the AC bus when the AC system has only one line left and suffers from a high-impedance-grounding fault, a series of measures, e.g. blocking line protection timely and so on, are designed to reduce the system overvoltage. Simulation is carried out to verify the validity of the coordination mechanism above. |
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