Research On Low-Frequency Oscillation Suppression Of Vehicle-Grid System Based On Static Synchronous Compensator

In recent years,low-frequency oscillation(LFO)events sometimes happened in the vehicle-grid system of electrified railways,which could result in traction blockades and affect the normal operation of railway systems.To solve this problem,this paper proposes a suppression scheme of LFO considering a static synchronous compensator(STATCOM)that is applied to the traction network.The active disturbance rejection control(ADRC)and modular multilevel converter(MMC)are introduced to optimize the STATCOM,which effectively suppress the LFO of the vehicle-grid system.Firstly,the mathematical modeling of single-phase STATCOM is built in the dq-frame.Considering that STATCOM is a coupled two-input and two-output nonlinear system,the feedforward decoupling control based on current indirect control is adopted to realize the separate control of active power and reactive power.Simulation experiments are carried out by connecting the RL load and the RC load,which shows that the STATCOM has a fast response speed and good compensation performance.In addition,the dynamic performance of the whole system when the STATCOM is connected to the vehicle-grid system is also investigated.Simulation results show that the proposed scheme can suppress the LFO.Secondly,considering the sensitivity of traditional PI control methods to system disturbances,the ADRC technology is applied to enhance the robustness and anti-interference ability of STATCOM.Based on the ADRC theory,a first-order nonlinear ADRC controller for STATCOM is designed.Compared with the PI controller,it is verified that the ADRC controller has the better control performances and better suppression for the LFO.Thirdly,in view of the defects of the traditional two-level STATCOM in topology,the MMC technology is introduced.The simplified single-phase MMC equivalent circuit is obtained by establishing the MMC single-bridge arm mathematical model.The LC filter loop is designed to suppress the circulating current by analyzing the internal circulation components of the MMC.By designing the grid-connected control and DC-side voltage control,the MMC-STATCOM can compensate the reactive power for the vehicle-grid system.The simulation model is built to analyze the electrical performance of the whole system when the MMC-STATCOM is connected to the vehicle-grid system.The results show that the MMC-STATCOM has excellent characteristics in suppressing the grid voltage fluctuation and reducing harmonic contents of the vehicle-grid system.Finally,a semi-physical experimental platform based on dSPACE real-time simulator is built.Based on this platform,the LFO reproduction experiment and the LFO suppression experiment based on MMC-STATCOM are completed,which verifies the effectiveness of the proposed suppression method.