The Research On Control Strategies Of Modular Wind Power Grid-connection Converter

With the increasing scale of offshore wind farms,DC wind farms become the development trend of offshore wind power.MMC has the advantages of high modularization,easy expansion of voltage level and flexible con trol.Modular wind power grid-connection converter based on MMC is able to connect the wind turbine to medium or high voltage DC power grid directly,which is suitable for offshore DC wind farms.In this paper,several key problems in the application of mo dular wind power grid-connection converter are studied.Firstly,based on the operation characteristics of wind turbine,the influencing factors of wind energy utilization coefficient are analyzed,and the mathematical model of wind turbine power characteristics is established.According to the operation characteristics of PMSG(permanent magnet synchronous generator),the mathematical models of PMSG in different coordinate systems are established.After introducing the topology of MMC and the characteristics of SM(sub module)in different operation modes,the mathematical model of MMC is established.Secondly,starting from the energy fluctuation of bridge arm,the voltage fluctuation expression of SM is derived.Aiming at the problem that SM voltage fluctuates greatly at low frequenc y,the principle of high-frequency injection method is analyzed in detail.Meanwhile,the expressions of common mode voltage and circulating current under two different injection methods are derived respectively.On this basis,the control strategy of converter is designed in detail,and the control effects of different injection methods are compared through simulation.Thirdly,the principle and implementation of the hybrid modulation method is introduced in detail.Meanwhile,the method of accumulating SM voltage to approach the modulation wave is proposed,in order to eliminate the modulation deviation caused by the fluctuation of SM voltage.After analyzing the reason of high switching frequency of power devices with hybrid modulation method,an improved method is proposed to reduce the switching frequency of power devices.The method use s right triangle carrier in PWM part and maximum voltage deviation sequence of SM in NLM part.The modulation effects of different modulatio n methods are compared through simulation,which verifies the validity of the proposed modulation method.Finally,a topology for series-connected single-phase MMC for wind power systems is proposed.In order to inject high frequency common mode voltage,a new single-phase MMC is designed,and the principle of high-frequency injection method to suppress the fluctuation of SM voltage is introduced in detail.The control method of the proposed topology is designed in detail and the validity of the proposed topology is verified through simulation.