Research On UPFC Function Based On Back-to-back VSC-HVDC Composite System

The interconnection system of power grid requires higher security,stability and controllability of power grid operation.As the most potential power electronic device in the power system,unified Power Flow Controller(UPFC)based on modular multi-level(MMC)structure is mostly suitable for high voltage and high power occasions and its powerful functions are of great significance to improve the transmission capacity and transient stability of power grid,enhance the controllabili'ty of the system and optimize the operation of power grid.In this paper,based on the back-to-back VSC-HVDC grid-connected device,combined with the structure characteristics and working principle of MMC,a method of converting the MMC grid paralleling device into MMC-UPFC is proposed.After the grid parallel device completes the synchronous parallel operation,the state of the circuit breaker is used as the characteristic signal to block the converter of parallel side and series side,and the conversion of UPFC device is realized through the operation of related electrical equipment,which makes the same device realize different functions under different control modes and improves the utilization ratio of the device.The feasibility and validity of this method are verified in PSCAD/EMTDC simulation software.Based on the analysis of the mathematical model of MMC-UPFC,the coupling term of the current inner loop which is used as the internal disturbance and external disturbances are observed and compensated with linear active disturbance rejection technology.A second-orderlinear active disturbance rejection current inner loop controller is designed,and a control strategy of MMC-UPFC based on linear active disturbance rejection is proposed.A 13-level MMC-UPFC model is built in PSCAD/EMTDC simulation software,and the system with MMC-UPFC is simulated,which the simulation results show that MMC-UPFC can effectively adjust the line current.According to the anaylsis of waveform of dynamic response of inner loop current in the process of power flow regulation,the LADRC MMC-UPFC has the advantages of fast response and no overshoot.By analyzing the voltage and current waveforms of single-phase grounding and three-phase short-circuit faults,the LADRC MMC-UPFC has the function of suppressing voltage sags,and enables the system to quickly return to stable operation after fault removal.