| MMC has the advantages of low operation loss,flexible expansibility and high reliability,and it is more suitable for high voltage and high power occasions,so it is extensively applicated in HVDC transmission field.Not only its study has achieved a lot,but also developed rapidly in engineering application.However,there are still many aspects to be studied,such as the optimization of control strategy,the improvement of voltage balance strategy and the method of circulating current suppression.This thesis takes MMC-HVDC transmission system as the control object,and the above problems are studied as follows:Firstly,the basic circuit topology of MMC is introduced,and the working principle of its sub-model is discussed under different working occasions.Then this thesis describes the work flow of two common modulation technologies.Because of the influence of the structure of MMC,the capacitor voltage of sub-model is unbalanced.The modulation strategy needs to cooperate with the appropriate capacitor voltage equalization strategy.However,the traditional voltage balancing strategies have the defects such as complicated calculation,high switching frequency,so the sorting algorithm is optimized to reduce the number of its switching,and through simulation to verify the improvement effect.Secondly,its mathematical model is established.This thesis summarizes its three different levels of control strategies,as system level control,converter station level control and valve level control.Then designs the overall control strategy of the system under the active network.To solve the circulating current problems in operation,the circulating current suppressor is designed.The dual-terminal 21-level active power MMC-HVDC simulation model is built,and the simulation results show that it has a good control effect.Finally,traditional PI control has some defects,such as slow dynamic response speed,easy overshoot and difficult parameter setting.Combined with grey predictive control,fuzzy control and PI control strategies,this thesis designs a dual-terminal active MMC-HVDC transmission system based on Grey Fuzzy PI control strategy.To compare the control effects of the MMC-HVDC system based on Grey Fuzzy PI control strategy and the MMC-HVDC system based on PI control strategy,the simulation is implemented under three working conditions of active power step,reactive power step and active power reversal,and its results show that the former not only can track the change of reference value faster than the latter,but also can reduce overshoot more effectively,and it has higher control accuracy. |
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