Study On Protection And Control Strategy Including RTDS Simulation Model In MMC-HVDC

As the growing demand of electricity, the pressure of power supply in the power grid increases day by day. This requires us to develop scale of the power grid towards larger capacity and closer connection, and to apply High Voltage Direct Current (HVDC) transmission system more and more. Because of its advantages such as flexible controlling and strong adaptability, flexible High Voltage Direct Current (HVDC-flexible) transmission technology has wider prospect than traditional HVDC transmission system. However, the research about HVDC-flexible in China is still at the stage of beginning, and the time of HVDC-flexible projects appliances is shorter, thus we lack the corresponding operating experience. How to guarantee the safe and stable operating of HVDC-flexible which exploits the advantages of convenience and flexiblity in operating? It deserves our discussion.Firstly, this paper analyzes the correspoonding current research situation about the operating, protection and control of HVDC-flexible both domestic and overseas. It concludes the advantages of HVDC-flexible compared with traditional of direct current transmission. It particularly explores the protection and control of this system from the research method and research thought.Secondly, this paper introduces several kinds of HVDC-flexible topology and its converter structures, and it particularly talks about the modular multilevel converter(MMC). This paper anzlyzes the operating principles of this system, and on this account, the paper compares the advantages with shortages of this structure in flexible direct current transmission. On the platform of RTDS, we built two-ends MMC-HVDC system. Drawing lesson from the realities of HVDC-flexible demonstration project in Nan’ao, we set the system parameter of the simulation module. The module shows its state correctness in each process such as starting and running after testing, which lays foundation for the following work.Thirdly, by analyzing the fault characteristics of different faults happen under the HVDC-flexible, this paper studies the strategies on protection configurations of it. These strategies include:dividing the protected areas in converter stations; putting important protective elements and allocating protective ways for the above elements; designing complete protection configuration scheme establishing simulation model of HVDC-flexible including protection strategies and verifying the protection configuration scheme by simulation.At last, by imitating the situation which AC-fault happens outside the HVDC-flexible, this paper analyzes the influence on HVDC-flexible caused by unsymmetrical fault. Applying the instantaneous symmetrical components method, this paper puts forward a kind of negative sequence current suppression strategy, thus the safe and stable operating of the system is guaranteed. This instantaneous symmetrical components method increases the calculation in dual-loop control system, instead of adding PI control unit. This kind of negative sequence current suppression strategy can restrain the converters’ negative-sequence current under the condition that unsymmetrical faults happen in the alternating current system. In this way, we keep the threshold current symmetrical, and keep the rejection away converters and guarantee the sustaining operating of the system.