Study On Fault Characteristics Of Multi-send HVDC System And Its Impact On Protection

HVDC/UHVDC transmission system has been widely used to transmit the power from western energy area to eastern load center in China, for its many advantages in the long distance and high capacity power transmission. With the rapid development of power resources, more HVDC projects will be built in some area to transmit the power to remote load center in the near future, which forms multi-send HVDC system. When fault occurs in the sending-end AC system of multi-send HVDC system, strong interaction not only exists between AC and DC systems, but also exists between various HVDC systems, so the fault characteristics of the multi-send HVDC system are more complex than the AC-DC interconnected system. The fault characteristics of HVDC system will influence the operating characteristics of line protections in the sending-end AC system. Therefore, the dissertation studies the fault characteristics of the AC-DC interconnected system and the multi-send HVDC system, and analyzes its impact on the line protections of the sending-end AC system. This study can provide important theoretical and practical value for the safe and stable operation of the multi-send HVDC system.This paper studies the rectifier side of the AC-DC interconnected system. Based on the different response characteristics of HVDC control system, a steady-state analysis model of the AC-DC interconnected system is built, the steady-state characteristics under different fault conditions are analyzed. Then a multi-send HVDC system based on CIGRE HVDC benchmark model is built. And the fault transient process is simulated. For the main HV/UHV line protections, containing distance protection based on variation of power-frequency components, current differential protection and distance protection, the impact of HVDC equivalent characteristics on the operating characteristics of line protections is theoretically derived. A simulation model containing line protection device based on PSCAD/EMTDC is built and the operating characteristics of AC line protection are simulated. The results show that the sending HVDC affects the operating characteristics of distance protection based on variation of power-frequency components obviously, makes the range of protection reduces greatly or loses protection range under certain conditions, but its impact on the current differential protection and distance protection is not significant. At last, The AC-DC interconnected system and the multi-send HVDC system model based on RTDS are built, and the operating characteristics of AC line protection based on real protection device are simulated. The simulation results testify the correctness of the theoretical analysis in this paper.