| The large number of power electronic devices connected to the power system will cause the problem of Sub-Synchronous Oscillation(SSO)of thermal power units in the near area.Modular Multilevel Converters-High Voltage Direct Current(MMC-HVDC)as a highly controllable,small output harmonics,active and reactive power electronic devices with flexible control,in the power system large capacity,long-distance transmission advantages are obvious,the actual project more thermal power The situation that the near end of the unit is MMC-HVDC may also lead to the sub-synchronous oscillation of the thermal power unit,which endangers the safe and stable operation of the power grid.The complex torque coefficient method is the simplest and most effective method to analyze the sub-synchronous oscillation of thermal power units,and the risk of sub-synchronous oscillation of thermal power units can be effectively analyzed by the electrical damping magnitude.In this paper,the risk of sub-synchronous oscillation and the corresponding suppression strategy are studied for the system with MMC-HVDC near end of thermal power units based on the complex torque coefficient method.Firstly,the complex torque coefficient and electrical damping curve of the system fed by thermal power unit through MMC-HVDC are derived.Modular Multilevel Converters are used for the MMC-HVDC,and the small-signal model of the dq-axis impedance of the MMC-HVDC is derived by considering the dynamic characteristics of the capacitance voltage of the bridge arm submodules.The six-winding synchronous generator model considering the dq-axis damping winding and excitation winding is adopted for thermal power units,and the generator small-signal model with two pairs of input-output relations of electromagnetic torque and rotor speed,and port voltage and current is established.The canonical tree is selected according to the network structure,and the tree branch capacitance voltage and the connecting branch inductance current are taken as the state variables to establish the network equations.The complex torque coefficients and electrical damping curves of the thermal unit fed through the MMCHVDC system can be obtained by associating the MMC-HVDC,thermal unit and network parts.The correctness of the theoretical derivation is verified in the time-domain simulation.Secondly,it is difficult to understand the mechanism of sub-synchronous oscillation and to formulate suppression measures because the key links and parameters causing sub-synchronous oscillation cannot be determined through the electrical damping curve.To address this problem,the complex torque coefficient transfer function is expanded based on mode decoupling,and the key oscillation modes and fractions causing negative damping are determined by defining the electrical damping extremes.Meanwhile,the electrical damping correlation factor and sensitivity are introduced to determine the key influencing links and parameters of electrical negative damping,and provide guidance for the sub-synchronous oscillation suppression scheme based on the sensitivity analysis results.Finally,based on the key influencing links and parameters of electrical negative damping,the electrical negative damping in the negative damping band is improved by adding sub-synchronous damping control to the MMC converter to suppress the system sub-synchronous oscillation.Then an in-depth analysis of the suppression effect at different additional damping positions is carried out to determine the reasons for the different suppression effects. |
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