Research On Hierarchical Control Strategy Of An Improved MMC-STATCOM Based On CPS-PWM

Converters suitable for high voltage and high power become the focus of attentionsof scholars, because of the industrial development and users of electricity required toimprove the power quality. As a kind of cascaded multilevel converter, the modularmultilevel converter（MMC）have the common advantages of multilevel converter. At thesame time, itsapplication in flexible HVDC field is widespread concern because of its special commonDC energy storage device. However, its application in high voltage and high powerSTATCOM is also have considerable potential for its modular design, multi-level output,etc. In this paper, study of MMC-STATCOM is expanded from mathematical modeling,the carrier phase modulation and hierarchical control strategy.Firstly, the common DC energy storage device of traditional MMC converter isanalyzed and an improved MMC converter without common DC energy storage device isproposed. The comparative analysis of traditional MMC converter and improved MMCconverter is expanded from the sub-module switching state, sub-modules modeling, legmodeling, modeling of the AC side and common DC link energy device modeling. It canbe fot from the comparative analysis that the time domain mathematical relationshipbetween the relevant physical quantities. And the advantages of improved MMC convertercompared to traditional MMC converter are found which includes less mathematicalconstraints and more sub-modules switching state combinations.Secondly, the carrier phase shift pulse width modulation (CPS-PWM) methods forthe specific application of the two topologies are analyzed. It is found that the maximumnumber of output voltage level is N+1when CPS-PWM is used in traditional MMCconverter. And when CPS-PWM is used in improved MMC converter, the maximumnumber of output voltage level is2N+1when N is an even number and it is N+1when N isan odd number. Then, the harmonic characteristics of output voltage when usingCPS-PWM and the effect of modulation ratio on the output level are analyzed and thecommon law is summarized by the simulation results.Finally, a kind of hierarchical control architecture for improved MMC-STATCOM isproposed, which contents pre-charge control of sub module, current control and DCvoltage equalizing control of sub module. The pre-charge control of sub module isincluding two steps. The first step is charging by the non-controlled rectifier bridge andthe second step is charging by controlled rectifier, which transformed the DC voltage control into active current control of AC-side. Four compensation conditions ofMMC-STATCOM can be got from the vector analysis for voltage and current of AC-side.The current loop PI controller is designed under dq coordinate system. The DC voltagecontroller and current loop PI controller form the upper controller, which is designedbased on the ideal condition that all DC voltages of sub module are equal. However, inpractice devices, the sub modules cannot be exactly the same, so equalizing control forsub module is necessary. The sub module DC voltage controller, which is the lowercontroller, is designed based on power balance of each sub module. The final modulationsignal is added by the modulation signal of upper controller and the modulation signal oflower controller. All above analyses and control architecture are verified by simulation andexperiment results.