The Multi-level Simulation Modeling And Control Research Of Modular Multilevel Converter

The status of flexible HVDC in the field of HVDC transmission technology now is the traditional technology can not be replaced.But if applicate flexible HVDC system based on two or three level in the actual HVDC system,there are many defects,especially in the problem of voltage levels,loss and covering area.After carefully research,the modular multilevel converter--MMC based on DC transmission system.It has many advantages such as high output level and flexible level regulation.With the modular design,it can reduce switching frequency effectively.It can bring huge economic benefits when used in HVDC transmission system especially in the large power condition.The topology and mathematical model of MMC are studied.Main works are as follows:1.The block,half block and de-block states are studied based on the topology of sub-module and sub-modules group.The equivalent circuit and mathematical model of the sub-module and sub-module group are put forward.On this basis,the block,half block and de-block states can be simulated using the equivalent circuit and mathematical model of the sub-module and sub-module group.Furthermore,four types of simulation models with different simplification degrees are set up.In the study of the MMC,the simulation model with different simplification degrees can be chose according to the different simulation purpose.2.Based on the simulation model with different simplification degree,Carriage Phase Shift PWM(CPS-PWM),Nearest Level Modulation(NLM)and the modulation of sub-models grouping control are studied.One this basis,the floor modify number shift algorithm are put forward.It can realize the grouping step control.With using different modify numbers,the similar effect of the Carriage Phase Shift PWM can be achieved.Every sub-module group can be regarded as an independent controlled voltage source by this method.Using the DC component added by the floor modify number shift algorithm,the voltage balance control between the sub-module groups can be achieved.3.The cause of circulation current is studied particularly.The circulating current suppression controller is designed based on the equivalent circuit of this double frequency component.The simulation proves that the designed controller can restrain the circulating current effectively.