Research On Optimal Control Strategy Of MMC Without Arm Current Sensors

Recently,with the increasing proportion of new energy generation in power system,HVDC Flexible suitable for large-scale long-distance transmission and distribution has been widely studied.Voltage-source converter(VSC)is the core component of HVDC Flexible,which is responsible for energy conversion and control.Among many kinds of converter topologies,modular multilevel converter(MMC)becomes the preferred scheme of HVDC Flexible because it is easy to expansion and has the ability of unbalanced operation.In this paper,MMC is taken as the research object.Aiming at the problem of cumbersome control process and large number of sensors needed for converter,an optimized control structure is proposed,which omits arm current sensors,phase-locked loop and coordinate transformation.The research content includes the points:First,this paper researchs the operation mechanism of the MMC,establishs the model of MMC switch function,deduces equivalent circuit of AC side and DC side of MMC.These lay a theoretical foundation for the research of optimizing control structure.Then,the modulation strategies of multilevel converters are classified,including carrier phase-shifted(CPS),carrier level-shifted(CLS),nearest level modulation(NLM).Based on NLM modulation,an NLM method is proposed without using arm current sensors.This method can judge the input direction of submodules without using arm current sensors,which simplifies the system structure.During the operation of MMC,the arm current signal is mainly used to judge the charge and discharge state of the capacitor voltage of the submodule and realize circulation suppression.Therefore,omitting the arm current sensors in the modulation will increase the difficulty of circulating current suppression.To solve this problem,this paper studies the mechanism of circulating current in converter according to MMC’s switching function model,deduces the calculation expression of circulating current,and analyzes that the unbalanced voltage dominated by double frequency is the main cause of circulating current.In order to achieve circulation suppression while omitting the arm current sensors,the existing circulation suppression methods are studied in this paper.On this basis,a circulating current suppression strategy based on unbalanced voltage is proposed,which suppresses the circulating current indirectly by suppressing the unbalanced voltage.The results show that the control effect of this method is good and the circulation is obviously restrained,which is basically consistent with the traditional sensors based method.Finally,embarks from the control structure optimization,this paper studies the classic inverter control strategy,an unit vector method is proposed based on abc coordinate system,this strategy can realize the inverter control without phase-locked loop and coordinate transformation.The unit vector method combined with no arm current method,the optimization of MMC without arm current control structure is realized.