Research On Control Strategy Of Static Synchronous Compensator Based On MMC

One of the most important problems in modern electric power industry is the stability of power system.If the power system wants to operate reliably and stably,it needs to ensure that the system voltage runs within the required range.Because of the characteristics of transmission,the voltage can only be controlled effectively through devices that are spread across the entire system.Static Synchronous Compensator is an important part of flexible DC transmission technology and has good dynamic compensation performance.Modularized multilevel converter is the latest achievement of multilevel technology.Its highly modularized structure has the advantages of reducing switching frequency and improving output performance.Therefore,MMC structure as the main topology of STATCOM,as a new type of dynamic reactive power compensation device(MMC-STATCOM),has a good development prospects.This paper focuses on the research of modularized multilevel STATCOM control strategy.The main contents of the research are as follows:1)The reactive power compensation principle of STATCOM is analyzed,and the mathematical model of three-phase stationary coordinate system is established.The direct control and indirect control are introduced by mathematical model,and the current detection technology is analyzed.2)As the main topology of MMC-STATCOM,MMC control strategy is very important.In this paper,the modulation strategy of MMC,the capacitive voltage sharing strategy of submodule and the loop suppression strategy are discussed.In the aspect of modulation strategy,the carry phase shift sinusoidal pulse width modulation(Carrier Phase Shifting SPWM,CPS-SPWM)is selected by comparing and analyzing several common modulation methods.In the part of capacitive voltage control,the stepwise voltage balancing method is used to realize the capacitor voltage balance of the submodule.In the aspect of circulating current suppression,a circulating current suppressor without phase decoupling is designed on the basis of analyzing the shortcomings of the traditional PI-based circulating current controller.Finally,the simulation is carried out on the matlab/simulink platform and the comparison is made.3)Combined with the former two parts,the mathematical model of MMC-STATCOM is established,and the whole control structure of the system is built.In the whole control strategy,the conventional double closed loop control method is used,the DC side voltage of the outer ring is stabilized,and the current control of the AC side of the inner ring is used.However,the traditional inner loop controller has a complex structure and a large amount of calculation.In order to simplify the structure of the controller,an inner loop controller with PR control is designed,which can directly track the AC current without dq transformation and decoupling control.Finally,the simulation model is built on the matlab/simulink platform.In the constant reactive power control mode,the simulation verifies that the system can effectively absorb(emit)reactive power.In the load reactive power control mode,simulation verifies that mmc-statcom can effectively compensate for reactive power.