Research On Flexible Dc Transmission And Transformation Technology Of Offshore Wind Power Based On MMC+NLM

Due to the problems of poor economic benefits,difficulty in compensating reactive power on offshore wind power,and the complexity of large-scale system coupling,traditional high-voltage direct current transmission is no longer suitable for the development needs of large-scale and long-distance offshore wind power transmission.As a new type of flexible direct current transmission technology,Modular Multilevel Converter based High Voltage Direct Current(MMC-HVDC)technology has been widely used in the power transmission field due to its advantages such as wide voltage regulation range,high output waveform quality,and stable and reliable operation,against the background of current energy revolution and improvement of power supply.However,as the core device of flexible DC transmission,the stability of capacitor voltage in MMC sub-modules and the suppression of bridge arm circulating current are the key to the stable operation of the entire system.In order to improve the output voltage quality and ensure the stability of the transmission process,this thesis mainly focuses on the research of the modulation method selection of MMC,the equalization control method of bridge arm capacitance,and circulating current control.(1)Two sets of comparison experiments were carried out based on carrier phase shift modulation and nearest level modulation.The number of sub-modules is 6 and 18,respectively,and different modulation methods are used when the number of sub-modules is6,and later analyzed according to the experimental results,also when the number of submodules is 18.The simulation results verify that the nearest level modulation method has the best output when the number of sub-modules is high,and that the carrier phase shift modulation performs better when the number of sub-modules is low.(2)To address the problems of long controller computing time,sequencing responsibility and frequent switching device operation in the traditional voltage equalization control algorithm,an improved control algorithm is proposed to reduce the switching frequency of the device by adding a regulation factor to adjust the sequencing priority.At the same time,the maximum capacitance voltage deviation allowed by the MMC is increased to ensure that its voltage value fluctuates within a certain range.Finally,the proposed method is validated in MATLAB/Simulink and the simulation results show that the proposed method can effectively reduce the number of switching cycles and the switching losses caused by them.(3)During the operation of a modular multilevel converter,the different working states of the sub-modules at different times cause differences in capacitor voltages,resulting in the generation of circulating current and affecting the stable operation of the system.To address the circulating current issue in modular multilevel converters,this thesis proposes the use of a low-pass filter to extract the double-frequency circulating current component,which is then suppressed using a designed PIR.Simulations are carried out in MATLAB/Simulink to investigate the capacitor voltage,bridge arm current and voltage,and active and reactive power.The simulation results show that the proposed controller design can effectively suppress the double-frequency circulating current,while the system exhibits excellent dynamic tracking capability.