| The voltage source converter（VSC）,which is developed based on fully controlled power electronic devices,is the main component of the high-voltage direct current（HVDC）transmission system.VSC mainly has three topologies.i.e.,two-level,three-level and modular multilevel converter（MMC）structures.The MMC topology has become the mainstream topology of flexible HVDC system due to its advantages of low harmonics,modular design,low-switching frequencies and low power losses.There have been extensive studies on the control strategy,circulation suppression strategy and submodule capacitor voltage equalization strategy of the MMC.However,research on the key technical issues such as the modeling of two-terminal MMC-based DC（MMC-HVDC）system and the stability study under small disturbances is still insufficient and deserves further investigation.In this paper,research is conducted from three aspects: MMC operating principle and control strategy analysis,mathematical modeling of the MMC with detailed internal dynamic,small signal modeling and stability analysis of a two-terminal MMC-HVDC system.MMC working principle and control strategy analysis.Firstly,the MMC topology,submodule working principle and multi-level generation mechanism are analyzed,the MMCHVDC system control strategy is more complex than the two-level VSC,including dq decoupling control strategy,submodule capacitive equal pressure strategy,inter-phase circulation suppression control strategy and modulation strategy.The one-terminal MMCHVDC model is established on the PSCAD/EMTDC simulation platform,and the system model of P/Q control strategy is verified,and the simulation results show that the system model and control strategy have good dynamic performance.Detailed dynamic modeling inside MMC.Small signal model is of great significance to predict the stability of the system under small interference,and for the small signal modeling problem of MMC,most of the literatures reduce the complexity and modeling difficulty of small signal model by simplifying the internal dynamic phase volume,but the simplified model ignores many MMC internal characteristics,which will inevitably cause some errors to accurately describe the whole system dynamics when carrying out stability analysis.In view of this problem,the small signal model considering the detailed dynamic characteristics of MMC is established,the small signal model considering the internal characteristics of MMC is more complex,and the MMC-HVDC system is divided into three modules by modular method,which deduces the characteristic matrix of each module in detail,and then unites the three modules into a single overall feature matrix.By comparing the dynamic calculation results of the small signal model with the simulation results of electromagnetic transient PSCAD/EMTDC,the accuracy of the model is verified.Further,the single-terminal MMC-HVDC small signal model under different control strategies(P/Q and Vdc/Q)is established,and the simulation results of the electromagnetic transient model are compared,the correctness of the model is verified,and the foundation is laid for the establishment of the small signal model of the multi-terminals MMC-HVDC system.Small signal modeling and stability analysis of a Two-terminal MMC-HVDC system.With the increase in DC grid ports,the modeling requirements of multi-terminals DC networks are increasing,the modeling difficulty is becoming more and more complex.in view of this problem,first,the dc port interconnection line model of the two-terminal system is established,combined with the system model under P/Q and Vdc/Q control strategies,the overall state space model of the two-terminal MMC-HVDC system is formed.The system state space model is then linearized at steady-state operating points,the small signal model and state matrix of the two-terminals system are deduced in detail,and the general solution of the multi-terminals MMC-HVDC system state matrix is further deduced on the basis of the two-terminals system’s overall state space model.The two-terminals system is characterized by root analysis and modal identification,and the weak damping modality with great influence on system stability is analyzed to find out the main participating variables and pay attention to them.In view of the effect of system parameters on stability,the effects of the two-terminals MMC-HVDC system AC system short-circuit ratio（short-circuit ratio,SCR）,main controller parameters,circulation suppressor parameters,locking phase ring parameters and DC line distance on the system damping characteristics are studied,and the corresponding root trajectory is made,and the external ring controller parameters and circulation suppression controller parameters are optimized according to the root trajectory analysis results.The proposed method has good theoretical significance and engineering application value. |
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