| Modular multilevel converter(MMC)has been widely applied in the Voltage Source Converter based HVDC area all over the world for its unique technical advantages.Based on which,the MMC projects have witnessed an increasement in voltage,capacity and terminals,showing great promising prospect.By now,researching on MMC is a definite hot topic,which is mainly focus on mathematical modeling,modulation mothods and control strategies.While,platform of real time simulation or dynamic simulation of lower voltage prototype is much closer to the practical projects since it can implement experiments on physical control systems before the projects estabilished,which has significant value to practical projects.In addition,the research about precise simulating and modeling of muti-terminals MMC systems or DC grids based on MMC was barely mentioned.As to the problems above,this paper proposed an efficient real time simulation model of MMC in RT-LAB and the model is appliable for super high-level MMC systems.Comprared with the MMC model developed by Opal_RT Corporation,the proposed model took less resource in real time simulation.Besides,this paper proposed an approach to obtain the optimal MMC level by both considering the relative errors and computation efficiency.Finally,a three terminal MMC system was built in PSCAD/EMTDC to verify the adaptability and effectiveness of the approach.Firstly,the development of MMC-HVDC and recent research on MMC modeling were introduced.This paper also analysed the basic topology,opearation principle,mathmatical model,modulation strategy and steady control method of MMC.Secondly,the traditional Thevien equivalent MMC model was proprerly analysed.Based on which,this paper developed an efficient real time simulation MMC intergal model of improved Thevien equivalent.The proposed model optimized the converter both from circuit simplifying and sorting algorithm,which made the computation complexity asend linearly with the MMC levels increasing.In addition,the optimal ranking method was improved and expanded to traditional MMC model for capacitor voltage balancing which led to great reduction in computation complexity of sorting algorithm.The State Space Nodal method was applied to devide the converter into multiple sub-networks for solution,which brought the real time simulation of two-terminal MMC system over 601 level into reality.Finally,based on the traditional Thevien equivalent MMC model,an approach to precisly simulate the muti-terminal MMC system was proposed by considering both the accuracy and computation complexity.A novel calculation method for solution step to simulate various level MMC models precisely was obtained.In addition,applying the Improved Analytic Hierarchy Process(IAHP)to analyse the relative errors of MMC model while the simulation time of various levels MMC are measured.The optimal MMC level obtained after the data processed by normalization and curve fitting.What's more,considering the different weight between accurancy and effiency,the comprehensive optimal MMC level could varies according to the different creteria of accurancy's corresponding weight.The simulation results verified that the optimal MMC level was reasonable and effective. |
PDF Full Text Request