Research On STATCOM Based On MMC In The Medium Voltage

With the rapid development of the power system, the influence of the reactivepower in the medium voltage power system is getting attention by people. At ratedvoltage, the stability of reactive power in power system is the key indicator to measurethe quality of power grid. When reactive power is inadequate, it is very importmant forreactive power compensation in time. This paper compares the advantages anddisadvantage of existing reactive power compensation devices and describes theadvantages of static synchronous compensator reactive (STATCOM). STATCOM is akind of new type dynamic reactive power compensation device. It has the advantagesof rapid response, good filtering, high control accuracy, suppress voltage fluctuationsand flicker and so on, which makes it become a hot topic of modern reactivecompensation technology.Firstly, the paper analyzes the research situation of STATCOM in electrical powersystem. And it elaborates the basic principle and mathematical model of MMC. Weintroduce three different modulation methods such as Nearest Level Modulation(NLM), Decimal Pulse Width Modulation (DPWM) and Carrier Phase-Shifting PulseWidth Modulation (CPS-SPWM) aimed at the complex topology of MMC. The paperdesigned a21-levels single phase inverter based on MMC using the NLM methodunder the MATLAB. And the simulation results verified the correctness of modulationmethod.Secondly, the paper introduces the basic principle of STATCOM. It also analyzesthe topology of STATCOM including delta connection and star connection andestablishs the equivalent mathematical model. Then the design formulas of inductor,DC capacitor and IGBT parameters in STATCOM main circuit are deduced. Then wehave a research on the control strategy of STATCOM. The paper using the double loopcontroller, the outer loop controller is DC voltage controller and reactive powercontroller and the inner loop controller is current controller based on the currentdecoupling, so it implantation of active power and reactive power independentcontrol.Then adopting the voltage balancing control based on the sort method to control the arm capacitor voltage.Lastly, the paper compared the delta connection of STATCOM with startconnection of STATCOM in technical and economic performance. Then simulation onthis two connection ways at different situations including normal situation, the step ofreactive power, the different intial value of three-phase capacitor voltage and threephase current unbalanced fault. Compared with the simulation results, we concludedthat the voltage of sub-module in delta connection is1.73times than the voltage ofsub-module in star connection when the number of sub-module in one arm and theoutput ac voltage and current are the same. The star connection STATCOM is moresuitbable for the mid-voltage distribution network according to the cost of a signaldevice. And the control effect of delat connection is better when it occurred a step ofreactive power. And it also not increased the cost. And the compensation effect of deltaconnection is better than the star connection when the three phase current unbalancedfault occurred in AC grid.