Suppression Strategy On Bridge Arm Circulation Of MMC-STATCOM In Three-Phase Four Wire System

Electric energy is an indispensable energy for people's production and life.With the development of power system technology,the problem of power quality is becoming more and more prominent.Modular Multilevel Converter(MMC)is a new type of topology applied to the Static Synchronous Compensator(STATCOM),which can improve the stability of STATCOM switching and the small compensation capacity.However,there is a problem of circulation inside MMC.In this paper,MMC-STATCOM is introduced in detail and the suppression strategy of circulation is put forward.In this paper,firstly,the topology of three-phase four leg MMC,the output characteristics of the sub module,and the relationship between the voltage and current are introduced in detail,and the equivalent voltage equation and topology of MMC are obtained.In view of the MMC common three kind of modulation strategy and two kinds of charging ways are introduced and the simulation analysis,choosing the Carrier Phase shift modulation(CPS-SPWM)as MMC modulation strategy and self-excited charging as charging way of MMC.Secondly,the MMC topology is applied to the STATCOM for the purpose of compensating the harmonics,reactive current and asymmetrical current.In combination with the CPS-SPWM modulation method of MMC,the reference compensation current is calculated by using the instantaneous reactive power,and deduce the MMC-STATCOM upper and lower leg reference voltage calculation formula,and then track the compensation current reference value.Then,the structure and mathematical model of MMC-STATCOM internal circulation are analyzed,and two circulation suppression strategies based on small signal model and sliding mode control are proposed to suppress the circulation.In order to reduce the sub-module voltage fluctuation,a stable capacitor voltage strategy based on voltage equalization control and voltage equalization control is proposed.Considering the fluctuation of circulation and sub-module,a comprehensive compensation strategy of MMC-STATCOM is proposed.Finally,in order to verify the correctness of the proposed control strategy,a MMC-STATCOM simulation model was built in the MATLAB/SIMULINK software.Through the simulation analysis,to achieve the harmonic and reactive current integrated control and control of the circulation.