Research On Control Strategy For Compound Fault Of DC Transmission System Based On MMC

As the raise of Global Energy Interconnection, as well as the need to build a Strong Smart Grid, It will promote the position of HVDC to unprecedented heights. The reason of selecting ultra-high voltage AC/DC hybrid power grid as the strong grid is the HVDC has the following characteristics. First of all, it can improve the transmission capacity of the AC power line under the status of synchronous operation and improve stability level of power system as well. Then, It can connect two major regional power system under asynchronous operation, while the short-circuit capacity isn’t increased. Next, its transmission power is great and transmission direction can adjust quickly, for above reason, it can meet the needs of Power Transmission from West to East. Finally, compared with the AC transmission line of same voltage level, the width of DC transmission line corridor is approximately half, this can ease the tension of land resources which brought by the development of power system. From the above characteristics of MMC-HVDC, it becomes a new hot topic.Currently, the study on MMC-HVDC focuses on its modeling and simulation, control and protection strategies. Single-line to ground fault of DC side and sub-module fault are typical faults of MMC-HVDC, when each of the above faults occurs, common practice is tripping circuit breaker, interrupting transportation of power, and the stability of the system is greatly reduced. When the DC side using large resistors to clamp voltage level instead of capacitors, if single-line to ground fault of DC side happens, only the potential reference point of DC side is changed, system’s power transport normally; When sub-module goes wrong, redundancy fault-tolerated control strategy can s uppress the fluctuation of direct current, which replace the failed sub-modules with equal number of redundant sub-modules. When these two faults occur simultaneously, we can realize the control of compound fault based on above two control strategies, this will help to improve the reliability of the system.A two ends and 21 voltage-level MMC-HVDC system simulation model is set up in PSCAD/EMTDC, simulation results verify the correctness of control strategy for above compound fault, the results also show that the strategy can make the system transport power stably under the circumstances of failure, and can achieve rapid recovery after troubleshooting, effectively improve fault tolerance of the system.