Research On Simulation Modeling And Control Strategy For Modular Multilevel Converter

As a new type of voltage source converter,MMC is widely applied due to its modular structure,low output waveform,harmonic content,independent control of active and reactive power,and islanding power transmission.At present,the research of MMC is mainly focused on one system modeling,control strategy and so on.There are three main sub modules of MMC:HBSM,CDSM,and FBSM.Aiming at the slow speed of MMC primary device model simulation,a new equivalent method of CDSM and FBSM is proposed based on HBSM variable resistance equivalent method.By analyzing the operation principle of sub modules,CDSM is divided into blocking state and normal running state.FBSM is divided into positive switching state,negative switching state and blocking state.Different operation states are compared with HBSM,and CDSM and FBSM equivalent models under different operation states are obtained.At the same time,in order to optimize the accuracy of the equivalent model,the transition model is added.The device model and the equivalent model are set up in PSCAD/EMTDC,and the accuracy and the improvement of the simulation efficiency of the proposed equivalent model are verified by simulation.At the same time,when realizing FBSM based on RTDS platform,it is found that its blocking state is constantly changing between the big resistance and the charging state,and its accuracy is related to the selection of large resistance value.Aiming at this problem,this paper studies the influence of different series inductance and sub module capacitance value on the steady state accuracy of blocking,and gives the selection method of large resistance under different conditions,which effectively improves the accuracy of equivalent model based on RTDS platform.The effectiveness of the proposed method is verified by establishing the model in RTDS.In this paper,a online redundancy strategy is proposed for the redundancy problem based on the CPS-SPWM.According to certain time interval,the strategy selects sub module of each bridge arm in running state,and inputs the capacitor voltage to the controller,assigns the output signal of the controller to the corresponding sub module.The residual module is in the state of online redundancy,and its control signal is 0.Through this selection,the capacitor voltage of each sub module is at rated value.When the sub module fails,the fault sub module is removed,and the remaining sub modules remain stable.The two ends model built in PSCAD/EMTDC validates the effectiveness of the proposed thermal redundancy strategy.