| There are two kinds of control strategies for parallel converters HVDC transmission system: parallel converters control and parallel MTDC system control. In this paper, the application of parallel MTDC system control strategy in the parallel converters HVDC system is theorectically analyzed and studied through simulation research.In chapter1, the configuration of parallel converters system is introduced first and the research and application of the system at home and abroad are analyzed. Then the differences between parallel converters system and parallel MTDC system in system configuration, operation modes, control modes and application aspects are summarized. In chapter2, the main parameter mathematical model of parallel converters system is presented. The basic operation modes and control modes are proposed and the parallel-converter power control mode used in parallel converters system is designed. In chapter3, some common faults, e.g. losing one firing pulse of an inverter, DC line faults and emergency stop of converters, are analyzed and corresponding control methods are proposed. In chapter4, the connection configuration and control strategy of Xiangjiaba-Shanghai project are briefly introduced. Some technical problems occurred during the commissioning test are analyzed and solution suggestions are provided. In chapter5the main circuit model and control block models based on the Qinghai-Tibet Project II are built in PSCAD. Simulation studies of system start/stop, steady state control, transient control and DC protection are proceeded by this model. The simulation results show the validity of the control strategies of steady state and faults. Chapter6is the conclusion and prospect. The Qinghai-Tibet HVDC transmission project I, whose aim is to solve the problems between power supply and demand, has already been put into operation by the end of2011. Project I is a normal2-terminal HVDC system with one12-pulse converter per pole in each station. According to the plan of State Grid Corporation of China(SGCC), in Qinghai-Tibet HVDC transmission project II, another12-pulse converter will be paralleled to the original converter of project I per pole in each station. Moreover, the Xiangjiaba-Shanghai UHVDC project has already been in normal operation. In the hybrid-deicing mode operation of this project, the parallel converter configuration is adopted, where pole1and pole2high-voltage converters are parallel. Besides, in the12th5years plan, plenty of UHVDC transmission projects are programmed, all of which can operate in hybrid-deicing mode. Therefore, it is meaningful to study the operation modes and control and protection techniques of the parallel converters HVDC transmission system. |
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