| With the depletion of fossil energy and environmental pollution,clean energy sources,such as photovoltaic power generation and wind power generation,are attracting more and more attention and achieved certain development.But its development is still limited by the fact that China's clean energy is mainly distributed in the west regions and the load is mainly distributed in the central and eastern regions.The situation makes the demand for high voltage direct current transmission increasingly urgent.Traditional two-level?three-level converters based DC transmission system are difficult to meet the needs of long-distance?large-capacity and ultra-high-voltage DC transmission.Modular multilevel converter(MMC)has broad development prospects due to its characteristics of high conversion efficiency?low total harmonic distortion and low system loss.However,the structure and working principle of MMC is more complicated than traditional converters,and the control methods of traditional converters are no longer suitable for MMC.Therefore,further research on the operation characteristics and optimal control strategy of flexible DC transmission system based on modular multilevel converter(MMC-HVDC)is of great significance for the clean energy consumption and Energy Internet construction in China.The main research contents of this paper are as follows:1.The structure and working principle of the MMC-HVDC system and its sub-modules are analyzed in detail.The mathematical model of the system is derived and the composition of circulating current is analyzed.2.The time complexity of the sub-module capacitor voltage equalization control strategy based on sorting is high,and the capacitor voltage equalization control do not need to determine the order of the voltage of all sub-modules.So a sub-module capacitor voltage equalization control method based on improved bucket sorting is proposed to reduce the time complexity by reducing unnecessary sorting and achieve a good voltage equalization control effect.3.When the model predictive control is applied to the MMC-HVDC system,there are a great number of control options,the control system has a large amount of calculation,it is almost impossible to realize real-time control.The cost functions' weight selection of multi-controlled variables lacks the uniform standard and fixed weight does not have good applicability.So a hierarchical model predictive control method based on sub-module capacitor voltage bucket sorting is proposed.The hierarchical model predictive control and bucket sorting algorithm are used to reduce the computing tasks.By separately controlling the controlled variables,it is no longer necessary to select the weight of the cost function,so that the control system has better adaptability.4.The outer loop PI controller is also needed in the improved model predictive control,which limits rapid response characteristics of model predictive control,and outer loop PI controller parameter setting requirements are high.A model predictive control strategy based on outer loop fuzzy PI controller is proposed to solve the above problems.According to the characteristics of PI parameters,the outer loop fuzzy control system is designed to realize the real-time tuning of controller parameters,which can improve the transient performance of the system and reduce the accuracy requirements for the initial parameter tuning of the system.5.A back-to-back MMC-HVDC simulation system with 200 sub-modules in every arm is set up in PSCDA/EMTDC,and the effectiveness and superiority of the proposed control methods are confirmed in the simulation system. |
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