Research On Coordinated Power And Current Control And Circulating Current Suppression Strategy Of MMC

With the increasing shortage of energy,the new energy has become the research trend.High voltage direct current transmission(HVDC)is indispensable in the development of new energy.Multilevel converter plays a key role in HVDC.Modular multilevel converter(MMC)stands out among many multilevel converters because of its advantages of easy expansion and low switching frequency,which makes it become a hot spot of many scholars.The main research contents are as follows:Firstly,MMC and HVDC flexible transmission technology are introduced,and the research status of multilevel converter is analyzed.The basic topology and operation characteristics of neutral point clamped converter,flyover capacitor converter and cascade bridge converter are introduced in detail.Finally,the corresponding control strategies and methods of MMC such as pre charging,circulating current suppression and modulation mode are summarized.Then,we analyzed the MMC’s topology structure and operating principle.According to the sub module topology,we elaborated the opersting principle of sub module detailly.According to the sub module operation principle,we deduced the MMC’s principle.Then,the mathematical model of MMC is established in different coordinate systems,and two output modulation modes(PWM and NLM)of MMC are proposed,and compared the advantages and disadvantages.Secondly,the coordinated control method of MMC-HVDC power and current in unbalanced power grid is studied.Firstly,the mechanism and components of power fluctuation are deduced in detail under unbalanced power grid.To solve the problem of unbalanced grid voltage,a positive and negative sequence voltage isolation technology based on complex resonant controller(CRC)is gived.This technology uses CRC,no PLL,and can extract positive and negative sequenc voltages.Finally,a general instruction for calculating the positive and negative sequence current is proposed.We can regulate trade-off coefficient in the two-phase static coordinate system,the positive and negative sequence current can be tracked by CRC,so as to achieve the coordinated control of current and power.Furthermore,the MMC circulation suppression strategy is studied.Firstly,the components of circulating current are analyzed in the case of balanced and unbalanced power grid,and the similarities and differences of circulating current components under the two conditions are compared;Secondly,the circulating current suppression strategy based on resonant vector controller(RVC)is proposed and compared with the traditional circulating current suppression strategy based on negative sequence rotating coordinate transformation in the case of power grid balance;Finally,a circulating current suppression strategy based on proportional complex resonant controller(P-CRC)is proposed in the case of unbalanced power grid.The above circulating current suppression strategies can reduce the operation loss of MMC and greatly improve the operation efficiency of MMC.Finally,a simulation platform is built in Matlab / Simulink to verify the above power and current coordinated control and circulating current suppression strategy.The simulation results show the effectiveness and feasibility of the proposed control method.