| Modular Multilevel Converter(MMC),as a new generation of voltage source converter topology,has the advantages of low switching loss,large capacity,low harmonic content and modular design,and is widely used in flexible HVDC transmission system.MMC-based multi-terminal direct current(MMC-MTDC)system not only has the technical advantages of MMC,but also has the characteristics of flexible operation of MTDC system,multi-power supply and multi-point power supply.It is widely used in islands power supply,new energy distributed generation system and other fields.This thesis mainly studies the control strategy and stability analysis of MMC-MTDC system.Considering the strong coupling,non-linearity and uncertainty of flexible HVDC transmission system and the poor stability of MMC converter station-level control with traditional PI double closed-loop control,a sliding mode control strategy based on a new variable exponential reaching law method is proposed.In this strategy,the DQ axis current error is taken as the sliding surface,and a sliding mode controller based on the new exponential reaching law is designed to replace the inner loop current controller in the traditional PI double closed-loop system,which improves the convergence speed of the traditional exponential reaching law and weakens the chattering phenomenon of the system.Then the Lyapunov second method is used to analyze the stability of the system.Finally,a two-terminal MMC-HVDC is established on the PSCAD/EMTDC platform.In the simulation model,single-phase grounding fault,three-phase short-circuit fault and positive-pole grounding fault on AC side are simulated respectively.The simulation results of traditional PI control and sliding mode control strategy based on new exponential reaching law show that the proposed algorithm not only has good convergence effect,but also improves the dynamic response speed of the system.The master-slave control strategy,voltage droop control strategy and voltage margin control strategy for multi-terminal MMC-MTDC system are studied.By comparing the advantages and disadvantages of the three control strategies,aiming at the voltage stability of MMC-MTDC system and the power balance between converter stations,an improved droop control strategy is proposed in this thesis.The droop coefficient is designed by comparing the average value of multi-terminal DC voltage with the reference value of voltage.It can realize that the system has a shorter time to reach stability,a smaller voltage fluctuation and a louder noise when the steady-state power flow flips.It should be faster;when the main station fails to exit the system,the active power can be reasonably allocated to ensure the normal operation of the system.Small signal model is an important method for system stability analysis.Considering the dynamic characteristics of internal harmonics in MMC sub-module,this thesis establishes small signal models of MMC converter station,control system and AC/DC network,and compares them with electromagnetic transient simulation model to verify the accuracy of the small signal model. |
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