Studies On High-frequency Oscillation And Transient Stability Of Wind Power Integrated MMC-HVDC System

With the continuous increase in wind power penetration,modular multilevel converters(MMC)based high voltage direct current(HVDC)technology has become an effective solution for largescale wind power transmission over long distances due to its unique advantages in new energy integration.However,due to large number of power electronic devices and complex control loops in wind farms and MMC stations,wind power integrated MMC-HVDC system presents complex characteristics such as multi-time scale,nonlinear,coupled,harmonic interaction and transient switching.In this case,it is prone to instability following disturbances,which threatens the safe and stable operation of power system.On the one hand,wideband oscillations from subsynchronous frequency range to mid and high frequency ranges have occurred in several wind power integrated MMC-HVDC projects at home and abroad,causing wind turbine(WT)shutdown,converter blocking,equipment damage and power transmission interruption,severely restricting the efficient and reliable consumption of wind power.On the other hand,unlike the inherent power synchronization characteristics of synchronous generators,the synchronization mechanism between WTs and wind farm-side MMC(WFMMC)is determined by control loops.After an AC short-circuit fault occurs,the synchronization between WTs and WFMMC may be lost,which poses a risk of large-scale WT trip-off.Therefore,the wideband oscillation and transient synchronization stability problems of wind power integrated MMC-HVDC systems require further in-depth research.In addition,the development of MMC-HVDC for wind power delivery is moving towards MMC based multi-terminal DC(MMC-MTDC)grid with more flexibility and reliability.However,the existing simulation models of MMC-MTDC grid is still inadequate,which severely restricts the transient stability simulation analysis of wind power integrated MMC-MTDC system.Therefore,this dissertation focuses on the high-frequency oscillation and transient stability issues of wind power integrated MMC-HVDC systems,and conducts research from the aspects of dynamic modeling,stability mechanism analysis and stability enhancement.The main achievements are as follows:(1)High-frequency oscillation mechanism and influencing factors analyses of WFMMC under no-load charging condition.First,the harmonic state space(HSS)model of a WFMMC station considering converter transformer stray capacitance is established.By analyzing the root locus of the system with increasing converter transformer stray capacitance,the necessity of considering converter transformer stray capacitance in the high-frequency oscillation analysis is demonstrated.Then,the participating factor analysis of the high-frequency oscillation modes of WFMMC station under no-load charging condition is carried out.The results show that,unlike the power controlled or DC voltage controlled MMC whose outer loop dynamics have negligible influence on the high-frequency oscillation,the AC voltage control outer loop of V/f controlled WFMMC has a significant influence on the high-frequency oscillation.By ignoring the dynamics of the elements with low participation in the high-frequency oscillation modes,a reduced-order model suitable for high-frequency oscillation analysis is proposed.It accurately retains the high-frequency characteristics of the system.Using the reduced-order model,the high-frequency oscillation mechanism of the system is revealed by impedance characteristic analysis.The results show that the impedance of converter transformer is capacitive in the highfrequency range due to the stray capacitance while large time-delay makes the impedance of WFMMC present inductive negative resistance in the high-frequency range.Therefore,there is a risk of high-frequency oscillation when the MMC charges with converter transformer under no-load condition.Finally,the effects of system parameters on the high-frequency oscillation are analyzed by calculating the root locus of the system,providing a theoretical basis for WFMMC parameter design and high-frequency oscillation suppression strategies.The correctness of the analysis results is verified by electromagnetic transient simulations.(2)Transient synchronization stability analysis of wind power integrated MMC-HVDC systems considering the dynamic interactions between WTs and WFMMC.On the one hand,a fourth-order synchronization stability model of wind power integrated MMC-HVDC system is established,taking into account the PLL dynamics of WT grid-side converter and the AC voltage control(AVC)dynamics of WFMMC.By using the fourth-order model and numerical integration method,the interactions between the PLL of WT and the AVC of WFMMC is revealed.Besides,the effects of system parameters on the transient synchronization stability are analyzed.The results show that the PLL of WT and the AVC of WFMMC are coupled through the voltage drops on grid impedance and fault resistance.Appropriately increasing the PI parameters of AVC and the current limit of WFMMC can effectively improve the transient synchronization stability of the system.Meanwhile,when the fault location is closer to the wind farm or the fault resistance is smaller,there is a higher risk of transient synchronization instability of the system.On the other hand,the transient synchronization stability mechanism of wind power integrated MMC-HVDC system is revealed by using the equivalent swing equation and equal area criterion(EAC).Different methods to improve the transient synchronization stability are summarized.Based on this,the potentials of WT and WFMMC to improve the transient synchronization stability of the system are deeply explored.The optimal currents of the WT and WFMMC to improve the transient synchronization stability of the system following faults are analyzed,providing a theoretical basis for the design of transient synchronization stability enhancement strategies.The simulation results show that when the output currents of WT and WFMMC meet the optimal currents after faults,the WT and the WFMMC can still keep synchronized even under severe faults.(3)Electromechanical-electromagnetic hybrid simulation method of bipolar MMC-MTDC grid applicable to the transient stability analysis of wind power integrated MMC-MTDC system.First,the dynamic models of AC/DC equivalent circuits and control loops of an MMC are established.By discretizing the components of bipolar DC grid and further expressing them in Norton equivalent circuits,the companion circuit model of bipolar DC grid is proposed,which can accurately reflect the unbalanced operation of a bipolar MMC-MTDC grid.The dynamics of both inductance and capacitance are considered in the DC line model,improving the simulation accuracy of DC faults in MMC-MTDC grid.Then,the implementation method of the electromechanical-electromagnetic hybrid simulation of MMC-MTDC grid in existing electromechanical simulation program is proposed.The multi-rate method is used to ensure simulation efficiency.The proposed electromechanical-electromagnetic hybrid simulation method of MMC-MTDC grid provides an effective tool for the transient stability simulation analysis of power system with wind power integration via MMC-MTDC grid.Finally,the proposed method is used to realize the electromechanical-electromagnetic hybrid simulation of the North China Power Grid with new energy integrated Zhangbei MMC-MTDC grid in the power system analysis software package(PSASP).The transient interaction characteristics of AC/DC grid under different faults and operation modes are revealed.The simulation results verify the correctness and effectiveness of the proposed method.