| With the development of China's power grid and the extensive construction of smart grids,flexible DC transmission has gradually gained more and more attention,and its role is also growing.The emergence of modular multi-level converters(MMCs)in current flexible HVDC systems improves output voltage waveform quality,reduces operating losses,and costs.Modular multilevel converters have low rate of change and low output harmonics compared to other types of converters in terms of voltage and current.The modular design facilitates the expansion of the converter and the capacity of the flexible transmission system.development of.Therefore,the multi-level converter has been widely recognized and has been applied in several fields,which has a good application prospect.This paper addresses the problems in the MMC control system,such as the design of the control system,the slow response of the system,the presence of harmonics between the bridge arms,etc.,which may threaten the stability of the system and the safety of the equipment.Starting from the principle of the converter,through the analysis of the structure and operation principle of the multi-level converter,the related research on the control system of the MMC-HVDC system is carried out.The specific contents are as follows:(1)A set of MMC-HVDC inverter control system was designed.Based on the knowledge of the multi-end MMC-HVDC system,this paper establishes the corresponding system model based on the basic structure and operation principle of the MMC-HVDC system,and designs a double closed-loop control system based on state feedback for MMC-HVDC..In the control system,a control method based on state feedback and phase-locked loop is proposed for the cross-coupling problem of active power and reactive power in the coordinate system.The coupling problem between active power and reactive power is effectively solved,thereby achieving powerless control of power.(2)A simplified small-signal model of MMC-HVDC is established.Based on the design of the state feedback control system,this paper uses the modular modeling idea to model the small signal model of the MMC-HVDC system.Firstly,the MMCHVDC system is divided into several small modules according to functions,and respectively modeled.The complexity of modeling the internal modules of the converter is simplified.Since the DC side voltage of the converter changes little during normal operation,it is approximated to the rated value,and the internal modeling of the converter is simplified by a simplified idea that the power standard value and the current value are approximately equal.Finally,the models established by each small module are interfaced to complete the modeling of the entire MMCHVDC system.The simplified and small-signal model of MMC-HVDC is simulated and compared by simulation software,and the accuracy and effectiveness of the simplified model are verified.The invention solves the problems of complex modeling of MMC-HVDC converter and large workload of modeling.(3)The optimization strategy of MMC-HVDC control parameters is proposed.The small signal model and the firefly optimization algorithm are used to optimize the control parameters in a large number of PI controllers in the state feedback double closed loop control system to improve the system performance and improve the system response speed.Firstly,the evaluation criteria corresponding to the objective function and the objective function for simplifying the small-signal model optimization are designed.Secondly,the firefly algorithm is used to optimize the objective function,and the optimization result is filtered by the simulation method,so as to obtain the optimal result that satisfies the condition.Finally,the simulation results show that the feasibility of the optimization scheme and the effectiveness of the optimization results are verified by electromagnetic simulation. |
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