Research On The Design And Control Of Doubly-fed Wind Power Generation System Based On Thyristor Rectifier Singly Connected To DC Grid

As a typical clean energy,offshore wind energy has been widely used with the rapid development of offshore wind power.At the same time,the large-scale trend of wind turbines and the increasing maturity of DC transmission technology have promoted the construction of offshore DC wind farms.Therefore,it is of great significance to study the DC networking mode and control strategy of offshore DC wind farm.In this thesis,a wide voltage-frequency control strategy in the full wind speed range is proposed for the single-machine large-capacity system of doubly-fed wind power generation based on thyristor DC grid-connected.Compared with the traditional doublyfed wind power generation system,the control strategy is under the maximum wind energy tracking control.The rotor-side converter controls the frequency to change proportionally with the wind speed,while keeping the stator flux constant.After the stator side is boosted by the three-winding transformer,the thyristor twelve-pulse is connected for controllable rectification,and the stator voltage can be adjusted by adjusting the trigger angle.Finally,the system wide voltage-frequency coordinated control is realized,and the corresponding simulation model is built in SIMULINK.The proposed strategy can reduce the slip rate and slip power of the doubly-fed generator set,so as to reduce the capacity requirements of the rotor winding and the slip ring brush,and achieve the purpose of reducing the cost of the rotor converter,which is of great significance in solving the bottleneck problem of the large-scale doubly-fed motor.Because the stator voltage and frequency in the voltage-frequency control method change with the change of the speed,the reactive power demand of the AC side of the thyristor rectifier changes under different working conditions.Therefore,this thesis aims at reducing the output of the reactive power of the doubly-fed motor,and studies the reactive power characteristics of the system.Firstly,the reactive power compensation characteristics of fixed capacitors under voltage-frequency control are explored.For the simulation model,the reactive power of the doubly-fed motor is reduced and the operating efficiency of the motor is optimized by selecting the appropriate capacitance value to compensate the reactive power of the system.Then,the reactive power control of the grid side converter(GSC)is added to the system,and the regulation characteristics of the reactive power of the system in the full wind speed range are studied.Then,the voltagefrequency segmentation control strategy and the voltage-frequency segmentation lower limit rotor power control strategy are proposed.The frequency limit and rotor power limit are used to divide the high and low wind speed sections in the full wind speed range and different control methods are adopted in different wind speed sections,which reduces the reactive power output and corresponding loss of the doubly-fed motor,and reduces the cost of system capacitance compensation and the capacity of the rotor converter.The corresponding simulation model is built in SIMULINK to reflect the results of each control strategy.The control strategy of wide voltage frequency operation of doubly fed wind power generation system based on thyristor DC grid connection is built in the laboratory.The experimental platform of 5kW doubly-fed motor is built in the laboratory.In terms of hardware,a whole set of thyristor rectifier device based on STM32 single chip microcomputer is made for DC grid connection.The converter based on DSP control is used to act as the rectifier at the rotor side converter and DC bus.In terms of software,the corresponding program is written and modified to realize the control of thyristor and RSC and the stable output of DC voltage.The experimental results also prove the feasibility of the strategy.