Modeling And Control Of DC Grid Integrated With Large-scale Wind Power

By implementing DC grid,there are many inherent advantages in large-scale wind power integration and transmission such as no power angle stability problem,flexible power flow control,but the fluctuation and intermittence problems caused by high permeability wind power installation still bring severe challenge to the security and efficient operation.Therefore,adapting key device of the DC grid,making voltage stable and power flow distribute rational are core issues of DC grid control.To overcome limitation of power device,modular multilevel converter(MMC)is widely used in multi-terminal direct current(MTDC)system.As a junction between wind farm and DC grid,the converter plays a role as a valve,and thereby is the most direct means to regulate voltage fluctuation and power flow distribution.According to the three-level control structure of power flow divided by mainstream researches,the control strategies from primary level and secondary level are studied in this dissertation.The specific contents are as following:1,In order to reveal the mechanism of voltage fluctuation introduced by wind power integration,and provide model foundation for simulation and controller design from different levels,modeling of wind power generation,MMC and DC grid are studied.Aerodynamic model,rotor dynamics model and drive-train model are derived,and the active output calculation and correction methods considering vibration and yaw error are proposed.Mathematical model of MMC sub module is calculated,and simplified model are put forward by neglecting switching device and neglecting sub module capacitor.Grid connected devices are divided into three types as V-type,P-type and Z-type according to the effect towards DC grid,and simplified models of each type used for system level simulation are built.Utilizing bond graph method,the equivalent model of six terminal tree topology DC grid is built and state space equation is derived,and output equation as well as constraint condition are set up according to the general requirement of grid operation.The accuracy of the models and simplified models are verified via simulation studies.2,Control strategies are studied from primary level.According to the instantaneous energy balance on both sides,the transfer function under double frequency negative sequence rotational frame is derived,thus internal model control(IMC)based circulating current suppressing strategy is presented.Considering line impedance,the P-Vcharacteristic curve under multi point voltage controls are analyzed,hence selection criteria of voltage margin and droop coefficient are pointed out.Subsequently,an improved variable droop coefficient method is put forward,taking advantages of both master-slave and droop control,meanwhile a virtual inertia additional control method is also applied to wind turbine generator(WTG).Simulations are carried out and the effectiveness of proposed control strategies are verified.3,Control strategies are studied from secondary level.The controllability and observability of state space equation of the six terminal DC grid are analyzed,distribution of closed-loop poles varing with line parameters are studied,so that simplified model suitable for secondary controller is presented.Then,adaptive PI controller is designed for the variation parameter DC system,and robust PI controller is designed based on LMI as well.In order to solve steady-state deviation issue,aiming at minimize the grid loss and DC voltage deviation rate,an optimal power flow(OPF)controller based on PSO is proposed,furthermore,a fuzzy control is introduced to improve convergence speed.Eventually,simulation studies are carried out,the results show that adaptive PI controller has ability to suppress voltage fluctuation effectively,and,the OPF controller is able to take account of both efficiency and safety.4,Six terminal DC grid physical simulation platform is designed and built.It includes hardware parts,specifically are reconfigurable grid structure,line equivalent parameter,solid-state circuit breaker,DC connection of WTG.And software parts,include three level control structure,startup and shutdown logic.Based on this platform,functional verification experiments are carried out,inspect the platform has ability in topology,fault protection and operation researches.Besides,performance verification experiments are carried out,the results proved that the proposed PSO method is able to achieve OPF distribution.