| As the capacity of offshore wind farms continues to increase,higher requirements are put forward for the power collection system and transmission voltage level of large offshore wind farms.Compared with the traditional AC power collection system based on voltage source converter and high-voltage DC transmission technology,the all-DC wind farm has the advantage of eliminating large-scale AC Transformers,improved efficiency and power density,reduced cable size and weight,and other advantages,wind farms with DC current collection and transmission systems have become the focus of recent research.Compared with traditional single-winding permanent magnet synchronous generators(PMSG),the modular multi-winding PMSG-based wind farm connected in series and parallel DC can provide a higher voltage to meet the requirements of high-voltage power transmission.The parallel connection of clusters of wind turbines can increase the capacity of wind farms.From the perspectives of power loss,component cost,power margin and reliability,the series-parallel topology of wind farms can reduce the grid loss of the collection system and eliminate offshore converters,thus having the advantage of reducing construction and maintenance costs.It has good application prospects in large-scale wind farms.Based on this background,this article takes the series-parallel DC offshore wind farm based on multiwinding permanent magnet synchronous generators as the research object,and its mathematical model was established and coordinated voltage control strategy was studied.This article is aimed at the modular multi-winding generator seriesparallel DC grid-connected offshore wind farm system,combines the research on modular multi-winding generator and series-parallel DC grid-connected offshore wind farm,focuses on the mathematical model and topology of multiwinding permanent magnet synchronous generator,efficient control strategy of multi-winding permanent magnet synchronous generator,coordinated voltage control strategy of series-parallel DC grid-connected offshore wind farm,the optimal operation control strategy of series-parallel offshore wind farms,etc.The main research contents are as follows:(1)The aerodynamic model and mechanical mathematical model of wind turbine are constructed,and the power output characteristics of wind turbine are analyzed.Then the speed control strategy of PMSG based on the optimal tip speed ratio is proposed.And through coordinate transformation,the motor equation was transformed from the static three-phase stator coordinate system to the two-phase synchronous rotating coordinate system,and the PMSG vector control strategy was raised.The mathematical models of generator side converter and grid side converter of permanent magnet direct drive wind turbine are established respectively.The decoupling control of active power and reactive power is realized by controlling the grid side current.Finally,the model is built by Matlab/Simulink to verify the effectiveness and correctness of the fan power output characteristics and the control strategy at the generator network side.(2)To solve the problem of limited capacity and poor transient performance of permanent-magnet direct drive wind turbine,the topology of modular permanent magnet direct drive generator is constructed.According to the speed requirements of the optimal tip speed ratio,the speed and power predictive control strategy is proposed by designing a torque observer.Compared with the traditional proportional-integral control,the proposed speed model predictive control method improves the steady-state and transient performance of the system.Aiming at the shortcoming of model predictive control(MPC)with large harmonics,a new modulation method considering the current saturation effect is proposed.By adding a current limiting link,the harm caused by over-current can be avoided.Aiming at the shortcoming that predictive control method depends on parameter accuracy,a robust predictive control method based on parameter identification is proposed.The proposed parameter identification method combines the structural characteristics of modular motor,improves the robustness of the control system without increasing torque ripple,and the identified parameters can also monitor the working state of wind turbine in harsh environment.(3)This paper proposed a new series-parallel DC connection topology for offshore wind farms based on multi-winding PMSG.The characteristics of series-parallel DC wind farms based on multi-winding PMSG were analyzed,and the mathematical model of multi-winding PMSG wind power generation system was formed.In addition,for series-parallel DC wind farms based on multi-winding PMSG,a coordinated control strategy considering the power flow of DC wind farms was proposed to coordinate the active power output of the wind turbines in the wind farm,suppress the overvoltage of the wind turbines,and ensure the safe operation of the wind farm.Finally,a WF model with three clusters in parallel and each cluster consisting of 6×5MW modular multi-winding PMSG in series was built to verify the effectiveness of the proposed power and voltage coordinated control method in suppressing the overvoltage of the wind turbine.(4)This paper proposed a coordinated voltage control strategy for radial DC offshore wind farms based on Model Predictive Control(MPC).The proposed scheme designed two control methods.In the economic operation mode,the wind farm controller generated the optimal active power reference value and medium voltage collector bus voltage reference value.The control objective was to minimize the power loss in the wind farm and to ensure that all busbar voltages in the wind farm were within a feasible range while tracking the power reference value issued by the dispatching department.In the voltage control mode,the main control objective of the wind farm controller was to minimize the deviation between the wind turbine terminal voltage and the rated voltage.Taking into account the dynamic response of the DC wind farm system,the control objectives and operating constraints were clearly expressed in the optimization problem of MPC control.In addition,the calculation of the sensitivity coefficient of the DC radiation network has been developed to improve the calculation efficiency.In the Matlab/Simulink simulation environment,a DC wind farm containing 20 multi-winding PMSG based wind turbines was built to prove the effectiveness of the proposed method. |
