| Modular multilevel converter with its highly modular design,power / voltage level easy to expand,high output waveform quality,low harmonic content,strong fault handling ability,low system loss and other technical advantages,has aroused the attention of specialists from domestic and abroad,and has played a huge role in the field of high-voltage and high-power flexible DC transmission.However,in the medium and low voltage DC power grid,due to the significant reduction of the number of MMC sub modules,there will be problems such as high voltage harmonic content and serious current distortion,which will lead to the deterioration of power quality and make it difficult to meet the requirements of the grid.Thus,the modulation and circulation suppression strategies for MMC in the field of middle and low voltage are studied and analyzed,and this paper proposes a hybrid modulation strategy and circulation suppression method based on quasi proportional resonance controller.the main research of this paper is as following:1.The topology and working principle of MMC are introduced and analyzed.Then,the mathematical models of MMC in abc and dq coordinate systems are derived and analyzed in detail.Firstly,the internal and external dynamic characteristics of MMC are analyzed in abc coordinate system.It is proved theoretically that the inner loop current control and outer loop power control of two-level VSC can be applied to MMC topology.Then the mathematical model in abc coordinate system is transformed to dq coordinate system by using Park transform,which lays the foundation for the design of MMC internal and external loop controller.2.The MMC modulation strategy suitable for low and medium voltage applications is studied.This paper points out the problems existing in the application of NLM and CPS-PWM in the field of medium and low voltage,and then proposes a hybrid modulation strategy which combines the advantages of NLM and CPS-PWM.Finally,the above modulation strategies are compared and analyzed in terms of output voltage and current waveform quality,total harmonic distortion rate of voltage and current,circulating current waveform and switching frequency.The simulation results show that when the number of MMC sub modules is small,the hybrid modulation strategy can not only improve the output waveform quality of MMC,but also reduce the requirements of system control accuracy,and then reduce the loss of converter to ensure the stable operation of MMC.3.This paper studies and analyzes two kinds of MMC sub module capacitor voltage equalization control strategies,which are the traditional sub module capacitor voltage sorting algorithm and the sub module modulation wave longitudinal fine-tuning voltage equalization method.The results show that the hybrid modulation strategy can limit the capacitance voltage fluctuation of the sub module to a lower range than the NLM.In order to solve the problem that the capacitance voltage of the sub module fluctuates greatly in the CPS-PWM strategy,the method of adjusting the longitudinal voltage of the sub module modulation wave is studied.The capacitance voltage equalization control algorithm can significantly reduce the capacitance voltage fluctuation of the sub module.4.The vector control strategy and circulation control strategy of MMC are studied.The double closed-loop decoupling controller based on dq coordinate system is designed.Then,through the mathematical model analysis of the causes of the internal circulation,it is concluded that the circulation is mainly composed of the negative sequence component of the second harmonic;then,for the second harmonic component of the circulation,the adaptive notch filter is used to extract the second harmonic circulation,and the quasi proportional resonance controller is used to suppress it.The loop suppression method is simple in principle and easy to implement.It can basically eliminate the second harmonic loop of MMC without additional cost,and at the same time reduce the capacitance voltage ripple of sub module. |
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