| High voltage direct current(HVDC)convergence and transmission system is one of the most effective solution to solve the problem in long-distance large-scale offshore wind farms.Direct drive permanent magnet synchronous generator(PMSG)and three-level converter have the advantages of simple structure,high efficiency,low failure rate and high withstand voltage.Furthermore,it is an effective equipment to connect the large offshore wind turbine and the HVDC collection grid,and it is an advanced technical for the wind power generation system.This paper studies the key technologies of 5MW PMSG,three-level AC/DC converter and modular three-level DC/DC converter.The main research contents and innovation results of this thesis are listed as follows:1)The conventional two-level converters cannot meet the requirements of the withstand voltage and capacity of the high-voltage high-power offshore wind generator.In order to solve this problem,the topology and control system for 5MW PMSG is proposed in this paper.The input and output voltage levels and the number of submodules of the modular DC/DC converter are determined.Furthermore,control objectives of the designed system are proposed.The proposed method is simple and effective,and it can be applied to large offshore wind turbines with rated voltage of 3k V and rated capacity of 5MW or more.2)The clamped capacitor voltages of the neutral point clamped three-level(3L-NPC)converter cannot be controlled very well when the space vector pulse width modulation(SVPWM)strategy is adopted.Virtual space vector pulse width modulation(VSVPWM)strategy is the most effective way to solve this problem.However,the amplitude of virtual vector in the conventional VSVPWM is not flexible,which limits its ability to control the clamped capacitor voltages.In order to solve this problem,an improved VSVPWM with variable amplitude of the virtual vectors is proposed.Based on the proposed VSVPWM strategy,the relationships between the modulation coefficient,amplitude of the virtual vectors,line voltage harmonics and common mode voltage are studied.The conventional VSVPWM strategy is a special state of the improved VSVPWM strategy proposed in this paper.When the proposed VSVPWM strategy is implemented,the voltage difference of the divided capacitors is smaller than that in the conventional VSVPWM.Furthermore,when the modulation coefficient is equal to the ratio of the virtual medium vector amplitude to medium vector amplitude,the minimum harmonic of the line voltage and minimum common mode voltage are achieved.The proposed VSVPWM strategy and the conclusion are suitable for rectifier and inverter.In addition,it is suitable for unit power factor operation and non-unit power factor operation.The proposed VSVPWM is finally verified in MATLAB/SIMULINK.3)The accuracy of the conventional MPPT method based on hill climbing is low.In order to solve this problem,based on estimated wind speed,an improved optimal gradient variable step hill climbing is proposed in this paper.Firstly,the high gain observer is used to obtain the observed mechanical torque of the wind turbine.Second,the effective wind speed is estimated based on the observed mechanical torque and Newton-Raphson method.Finally,the given speed of the generator is obtained according to the estimated wind speed and the proposed improved optimal gradient variable step hill climbing method.The proposed method does not require wind speed sensor.Furthermore,more wind energy could be captured and the stability of the MPPT control system is higher.In addition,the speed of the PMSG can be controlled to optimize the harmonic of the AC line voltage.The proposed MPPT control strategy is finally verified in MATLAB/SIMULINK,and the results show that the proposed method can achieve good control performance.4)The conventional control strategy for the clamped capacitor voltages of the FBTL DC/DC converter cannot meet the requirements of different working conditions.Furthermore,the clamped capacitor voltages cannot be self-balanced in the conventional FBTL DC/DC converter.In order to solve this problem,an intermediate level voltage modulation(IMLVM)strategy is proposed.The self-balance of the clamped capacitor voltages can be achieved.In addition,compared with the conventional disturbance rejection strategy of the clamped capacitor voltages,the proposed scheme has smaller static error and stronger control ability.In order to improve the efficiency of the FBTL DC/DC converter,the optimal control of the two duty cycles of the FBTL DC/DC converter must be achieved.This paper studies the relationships between the two duty cycles and the harmonic loss of the output voltage of the FBTL inverter,the transmission power of the converter.Then an optimal control strategy is proposed for the two duty cycles.The proposed strategy can minimize the output voltage harmonic of the FBTL inverter.A FBTL DC/DC converter based on MATLAB /SIMULINK and a FBTL DC/DC converter experimental prototype are built and tested,and the results verify the theoretical analysis.5)The conventional control strategy for the input and output voltages of the modular DC/DC converter has low reliability,and the quantitative relationship between the two voltage loops is not clear.In order to solve this problem,a series connection double voltage closed-loop control strategy for the input and output voltages is proposed in this paper.Small signal model is the important part to analyze the control system of the DC/DC converters.However,the conventional small signal models of the DC/DC converters are complex and difficult to understand.In order to solve this problem,a small signal model for the FBTL DC/DC converter is established,and the correlation transfer functions are derived.Based on the established model,a series double voltage closed-loop control system is designed by means of root locus and bode diagram.Compared with the conventional FBTL DC/DC converter model,the established model in this paper is simple and effective.Compared with the conventional parallel double voltage closed-loop control strategy,the proposed control strategy can quantitatively analyze the input and output voltage loops.A FBTL DC/DC converter based on MATLAB/SIMULINK is built and tested,and the results verify the theoretical analysis.6)A 5MW PMSG based on modular DC/DC converter proposed in this paper is built on MATLAB/SIMULINK.When the wind speed changes according to different rules,the waveforms of the key variables of the generation system are recorded and analyzed in detail.The simulation results verify the correctness and effectiveness of the proposed topology and its control strategy. |
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