Research On Model And Modeling Method Of Flexible Dc Transmission System Under Asymmetric Operation Of Ac System

Multi-Terminal Flexible DC Transmission System Based on Modular Multilevel Converter(MMC-MTDC)is a new high-voltage long-distances DC transmission technology,which has the advantages of high flexibility,low electromagnetic interference,high modularity,low harmonic content,high waveform quality and high reliability.Therefore,it is widely used in power transmission scenarios such as large(asynchronous)power grid interconnection,urban power supply,large-scale new energy grid-connected,and power transmission of island.So it has gradually become the main development trend of HVDC transmission engineering technology.Although the MMC-MTDC system has many advantages,its dynamic performance will also be influenced by the variation of the operating state of the AC system connected to it.The unbalanced AC voltage,sudden-connection of renewable energy,and unbalanced load all will cause asymmetry in the AC system,and the dynamic characteristics of the overall power grid will be more complicated.In the case of asymmetric AC system,the AC current on the MMC valve side will have a negative fundamental component,which may cause double-frequency fluctuations of the instantaneous AC active and reactive power.Meanwhile,the internal arm current of the MMC converter will appear the zero sequence double-frequency component,resulting in double frequency fluctuation of the DC system voltage and current.Therefore,the research about the control optimization and small-signal modeling of MMC-MTDC system under the condition of asymmetric AC system has important value on theoretical analysis and engineering application.Firstly,the operation mechanism of the MMC-MTDC system under AC asymmetry conditions is studied in detail,and the main focus is on the unbalanced charging/discharging mechanism of sub-module capacitors and traditional control strategies of converter stations.In order to improve the control ability of the MMC system under AC asymmetry conditions,the MMC control optimization method is proposed,which can achieve multi-objective control of suppressing imbalance AC current,instantaneous power fluctuation,and stabilizing the DC voltage and current;meanwhile,an adaptive phase-locked loop(PLL)based on multiple complex coefficient filters(MCCF)is constructed,which can realize fast tracking of the grid phase under asymmetric AC voltage conditions,thereby improving the accuracy of system control.Then,based on the MMC bridge arm circulation suppression strategy,the intersite coordination method and the inherent electrical characteristics of the system,combined with the characteristics of the optimized control system and the capacitor unbalance charge/discharge mechanism of the MMC-MTDC system under the condition of AC system asymmetry,the state-space matrices of the AC system,DC network,MMC electrical and control system are derived,thereby a detailed small-signal model covering MMC converters,AC systems,and control systems under asymmetric AC systems has been established,which is applicable to MTDC transmission systems with multiple renewable energy application scenarios and DC network topologies.Meanwhile,considering different controller parameters and droop control strategies,the impact on the small-signal stability of the MMC-MTDC system is discussed by eigenvalue analysis,and the overall dynamic performance of MMC-MTDC systems under the condition of transmission power fluctuation is analyzed,which can provide design basis for the characteristics research,parameter tuning and controller optimization under AC asymmetric conditions.Finally,based on the OPT-5600 real-time simulation platform,a 5-terminall MMC-MTDC transmission system model is constructed,the accuracy of the proposed model is verified against the electro-magnetic transient model and traditional small-signal model.