Research On Control Strategy Of Flexible Hvdc Transmission System Based On MMC

Compared with AC transmission,flexible DC transmission has the advantages of high power and low loss,and it has advantages in long-distance transmission,island power supply,urban distribution network upgrade and new energy grid connection.The flexible DC transmission technology based on Modular Multilevel Converter(MMC)adopts modular design,which is highly scalable,and has the advantages of high waveform quality and low switching frequency.It is more suitable for High Voltage Direct Current transmission.In this paper,the converter voltage equalization control strategy and the internal loop suppression control strategy that directly determine whether MMC can operate normally were studied.The power control strategy of the converter station of MMC-HVDC system was also studied.(1)The topology and working principle of MMC and its sub-modules were introduced.Then,the selection of system parameters was analyzed and the modulation strategy suitable for MMC was expounded.(2)Due to the voltage imbalance problem in the modular multilevel converter,the interphase circulation increases.the traditional capacitor voltage sequencing equalization method is inefficient.This paper proposed a split bubbling voltage equalization control strategy based on bubbling principle.In order to reduce the switching frequency of the sub-module,a capacitor voltage equalization method with a retention factor was adopted.The simulation results showed that the proposes method can effectively reduce the switching frequency while improving the capacitance equalization efficiency.(3)Aiming at the problem of circulation in the operation of MMC,the mechanism of circulation generation was analyzed firstly,and the main component of the circulation is the double frequency component.Secondly,the deficiency of the traditional double frequency circulation suppression strategy was analyzed.Then this paper proposed a loop suppression control strategy combining a notch filter and a quasi-proportional resonance controller.This method did not require current coupling and accelerates the response speed of the system.The simulation results showed that the method effectively suppressed the circulating current and effectively reduced the distortion of the bridge arm current.(4)The working principle of MMC-HVDC system and its control strategy were introduced and the mathematical model of the system's three-phase stationary coordinate system,dq coordinate system and ?? coordinate system was established.The outer loop power controller,the inner loop current controller in the dq coordinate system,and the inner loop current controller in the ?? coordinate system were designed.The simulation results were compared between the two inner loop controllers.The results showed that when the controlled amount of the rectifier side or the inverter side changes,the system can quickly track the reference value and reach a steady state.The simulation verified the effectiveness of the control strategy...