Study Of Direct Current Transmission System Based On Modular Multilevel Converter

Flexible HVDC technology is better than the traditional dc transmission technology.It can solve the problem of the interconnection of sync,and remote isolated load power supply,etc.Application of two level,or the level of flexible HVDC system has a lot of shortage,such as the volume of the filter is too big,the voltage level is very low,low le vel topology harmonic content's loss will be very big,so these shortcomings make the topology of the low level can't further application in dc transmission sys tem.The application of the new modular multilevel inverter dc transmission system has the inverter modules cascade.Because the number of inverter level is high,so the flexible control on-off of each module,output the required level,to adapt the different power levels and voltage grade requirements.Compared with the two or three level inverter,modular inverter not only reduce the switching frequency of the sub module,but also need increase the high power ac filter equipment,so it is suitable for application in high power dc power transmission.This paper firstly introduces the modular multilevel converter modules cascade topology structure and working principle of inverter,on this basis,this paper expounds the modular multilevel converter modulation method.In view of the capacitor voltage balance traditional sorting method,this paper proposes a new sort of introduction of maintain factors,this method can effectively reduce the module of the switching frequency.After exchange device inside the flow circulation mechanism is analyzed,and designed the negative sequence twice frequency circulation inhibition of controller.In this paper,the whole control system,has based on modular multilevel inverter dc transmission system set up the simulation c ircuit.For the proposed modulation method,sub modules of capacitance voltage control strategy and restrain circulation module are simulated respectively,comparing to verify the effectiveness of various control methods.And ultimately from the output three-phase voltage and current waveform,the electric current on the dc bus,etc.,to verify the stability of the dc transmission system operation,to verify the effectiveness of the whole control system.