Research On Flexible Interconnection Technology Of Substation Based On Back-to-back MMC

The traditional distribution network is "passive" in terms of power supply mode,operation control,and power supply services.Traditional substations,as an important part of the distribution network to realize the distribution of electric energy,are usually connected by mechanical circuit breakers between the busbars of the main transformer,which have slow response speed and limited number of operations.Moreover,the main transformers usually work in a separate operation mode which can not achieve mutual power support between the main transformers,causing the substation to be passive in terms of structure and operation.It has become a current research hotspot that introducing flexible power electronic switches to replace the interconnection switches between feeders,transforming the traditional distribution network from passive mode to active mode,and forming a new type of flexible interconnection distribution network.In view of this,the article considers the use of a new type of substation with "flexible" interconnection between main transformers to construct the substation as a controllable electrical node of the large power grid in which the section switch between the busbars or the bus tie switch is replaced by a flexible power electronic switch.A small-signal model of flexible interconnected substations is established.Based on this,a load balancing control strategy for substations is proposed.A simulation model for flexible interconnected substations is built to verify the effectiveness and rationality of the proposed control strategy.The specific research content is as follows:Firstly,the basic functions and operations of flexible power electronic switches and the current research status of flexible interconnected distribution network operation control are briefly described;the research status of power electronic distribution network modeling and stability are summarized.Secondly,the concept of Flexible Interconnection Power Station(FIS)based on back-to-back MMC is proposed,and its realization methods and the advantages of flexible and controllable flow in the flexible interconnected distribution network are explained;a controller for MMC,the core component of the FIS system is designed,which concludes an outer loop power controller,a current inner loop controller,and a circulating current suppression controller;an FIS system based on back-to-back MMC on PSCAD/EMTDC software is built,analyzing the power mutual economic characteristics under different control methods.Then,the small signal of the FIS system based on back-to-back MMC was established and realized on MATLAB,which includes the inverter models that consider the internal dynamic behavior of the MMC,the control system model,the AC system and the DC system model.Through comparative analysis with PSCAD transient model,the correctness and effectiveness of the model are verified.Then,the small-signal model of the FIS system of the back-to-back MMC is established,including the inverter models,the control system model,the AC system and the DC system model including the internal dynamic behavior of the MMC,which is implemented on MATLAB.Through comparative analysis with PSCAD transient model,the correctness and effectiveness of the model are verified.Finally,a FIS coordinated control strategy based on main transformer equalization is proposed.Reasonable control parameters are designed by constructing a small signal model of the FIS system with coordinated control strategy,and performing root locus analysis and modal analysis on the system model.The FIS transient simulation platform with back-to-back MMC is built to verify the effectiveness and rationality of the proposed main transformer equilibrium control strategy.