Research On Nonlinear Robust Control Method For Voltage Source Converter Multi-terminal HVDC System

At present,with the development and utilization of large-scale wind energy,solar energy and other new energy sources,voltage source converter based multi-terminal HVDC transmission system(VSC-MTDC)has become an effective way to establish wind farm and AC grid platform to realize power transmission.Because the VSC-MTDC system has a flexible form of power transmission and it’s active and reactive power can be controlled independently.It also can supply power for passive network with compact structure.Because the wind farm has the random fluctuation characteristics of power and frequency,the common control methods of converter station are difficult to achieve the stable control of power and voltage.In order to realize the efficient and reliable access of large wind farms to power grid,it is necessary to further study the control methods and control strategies of converter stations in VSC-MTDC system.In view of the difficulty in reflecting the internal characteristics of the VSC-MTDC system with wind farms in traditional modeling methods,an integrated transient simulation model based on the dynamic model of VSC-MTDC system with wind farm is established,which based on the dynamic characteristics of DC network and connected AC system and the power characteristics of wind farm.The establishment of the model provides theoretical basis and basis for accurate selection of electrical parameters of the system,steady-state analysis of system operation under small disturbance and stable and effective control of converter station.Aiming at the dynamic disturbance factors in the transient process of converter station operation of VSC-MTDC system and the influence of the change of electrical quantity after disturbance on the operation of the system,a new time-varying parameters extended state observer based on hyperbolic sine function and hyperbolic tangent function is proposed,which can be used to accurately observe the state variables and the state variables of the system.It can be used to accurately observe the state variables and non-linear disturbances of the system,so as to restrain the influence of system disturbances on the system control and operation.The observer can effectively suppress the differential peak phenomena when there is a large error between the initial value of the system and the initial value of the observed system state,and accurately observe the uncertain disturbances and measurement errors existing in the wind farm and the VSC-MTDC system,so as to improve the control effect of the converter station controller.Aiming at the influence of uncertain disturbance of wind farm on the VSC-MTDC system,from the point of view of the shortcomings of the existing controller design and the improvement of the robustness of the system,the terminal sliding mode control reaching rate under different sliding mode reaching laws is studied.And a double-power sliding mode reaching law with variable parameters is proposed.The reaching law can quickly converge to the neighborhood of equilibrium zeros under uncertain disturbances.The sliding mode control system based on variable parameter double-power sliding mode reaching law has high tracking accuracy for given signal and given differential signal,and effectively reduces the high frequency chattering of control input signal.Aiming at the problem of slow dynamic response of converter output voltage and high waveform distortion rate,a terminal sliding mode control method based on extended state observer and variable parameter double power reaching law is proposed for converter station of the VSC-MTDC system.The extended state observer(ESO)can accurately estimate the system uncertainty and external disturbance,and compensate dynamically in the controller.It can effectively reduce the switching gain in sliding mode control and improve the robustness and stability of the system.The control method can stabilize DC voltage in the presence of uncertain disturbances,so that the system can restore stability faster and the static stability is better.In the process of power transmission of converter station,the output deviation of DC voltage is too large due to power fluctuation,and the capacity of converter station is over-limited and over-current.Based on the analysis of the constraints of the traditional droop control method,a DC voltage coupling control strategy using power coupling factor is proposed,which realizes the adaptive dynamic adjustment of the droop coefficient.The control strategy can maintain the stability of DC voltage and improve the voltage quality,while ensuring the effective follow-up of power transmission.When the DC voltage is high,the system can achieve power redistribution and effectively avoid the overcapacity and overcurrent of the converter station.When the system is disturbed by large power,the system can track the given system effectively and quickly,so that the system has good stability and robustness.