Research On Topology Optimization And Control Of Modular Multilevel Solid State Transformer

Power transformer is the key equipment in power system.The conventional line frequency power transformer can only realize electrical isolation and voltage matching,which can not meet the future needs of power system in some applications.Solid state transformer(SST)has more advanced functionalities with the help of power electronics technology.It has the ability of port voltage and current regulation,fault ride through and isolation,reactive power and harmonic compensation,and also has a rich variety of AC and DC ports.Therefore,SST is widely used in distributed renewable energy generation system,traffic electrification,large scale power supply and so on.SST is usually constructed with the idea of modularization.On the one hand,SST needs to be connected to the medium voltage power grid.The modularization idea divides the voltage and current stress by connecting submodules in series and parallel,so as to increase the voltage and current capacity of SST.On the other hand,the modularization idea can realize the standardized production of SST and reduce the manufacturing,maintenance and redundant configuration costs of submodules.Among the modular SSTs,the SST based on modular multilevel converter(MMC)has many unique advantages.Its characteristic of centralized energy processing simplifies the transformer design proccess and reduces the transformers number.Its independent submodule structure facilitates the access of distributed renewable energy and energy storage system.Its virtual DC bus provides accessable port for the high-voltage DC grid,and the arm inductors can limit the fault current and strive time for fault protection during DC short-circuit.However,the conventional MMC-SST realizes power electronic conversion by connecting multiple conversion stages in cascade,which requires more active and reactive devices and results in large volume and weight and low power density.Although increasing the operation frequency can reduce the volume of magnetic components such as inductance and isolation transformer,there is a bottle neck when the operation frequency is increased to thousands of Hertz.In order to further reduce the volume of MMC-SST,it is necessary to excavate more useful characteristics of MMC in topological level.Therefore,this thesis systematically proposes a set of topology optimization methods of MMC-SST in the view of power loop analysis,and optimizes SSTs in medium voltage DC and medium voltage AC applications,respectively.The research contents are as follows:(1)A power loop analysis and optimization method of MMC converter is proposedThis thesis reviews the operation principle of the conventional MMC converter,and puts forward a power loop analysis method of MMC converters with differential and coommon common mode concepts.Firstly,various specific components in MMC are abstracted into "source" and "impedance",and the expression of MMC power conversion stage is simplified in the form of abstract circuit;Secondly,by regarding the differential and common mode circuits as power loops with different frequencies(DC,line frequency or medium frequency),the abstract circuit of the MMC power conversion stage is further extended to many different versions;After analyzing various power loop allocation methods,a series of abstract circuit of MMC power conversion stage are finally obtained.A series of MMC power conversion stage can be obtained by restoring the components in the abstract circuit to specific components according to certain rules.The proposed power loop analysis method is helpful for researchers to evaluate the feasibility and practicability of various MMC power conversion stage;Based on the analysis results,this thesis selects one of the "DC-MFAC" conversion stages and one of the "LFAC-MFAC" conversion stages for further optimization.(2)A series of optimization schemes for SSTs in medium voltage DC applications is proposed according to the power loop analysis results of "DC-MFAC" conversion stage.According to the power loop analysis results of "DC-MFAC" conversion stage,a topology with integration potential is selected,a class of highly integrated medium voltage DC SST is constructed,and the optimal design scheme of this kind of SST is proposed.A modulation method based on “square-voltage-trapezoidal-current” is proposed,which realizes zero voltage switching for all switching devices in submodules,effectively reduces the switching loss,increases the operation frequency of the SST and reduces the volume of passive components.The auxiliary voltage-balancing circuit between submodules is introduced to realize self balancing of the capacitor without voltage sensors.The voltage sampling circuit and communication circuit in submodules are omitted,and the volume of submodules is thus reduced.By reasonably configuring the auxiliary voltage-balancing circuit parameters,the zero current switching of the diodes in the circuit is realized,which makes the circuits qualified for the high operation frequency.Simulation and experimental results verify the effectiveness of the proposed schemes.(3)A class of highly integrated SSTs in medium voltage AC applications is proposedaccording to the power loop analysis results of "LFAC-MFAC" conversion stage.A topology with integration potential is selected from the power loop analysis results of "LFAC-MFAC" conversion stage,and a class of medium voltage AC SST is proposed.Compared with the traditional MMC type SST,the proposed SST omits the intermediate DC link,and can realize direct conversion from LFAC to MFAC,which can reduce the number of conversion stages.This thesis introduces the operation principle and parameter design method of the proposed SST in detail.The comparison results demonstrate that the proposed SST can effectively decrease the number of sub modules,reduce the volume of magnetic components and improve the topology integration.Simulation and experimental results verify the rationality and feasibility of the proposed SST.(4)A control system design method for the highly integrated MMC-SST is proposed.According to the characteristics that MMC-SST needs to realize multiple control objectives at the same time,this thesis proposes a control system architecture with "port current control" as the inner loop and "port voltage/power" and "internal energy balancing" as the outer loop to realize multi-objective cooperative control.According to the coupling characteristics of different frequency components in MMC-SST under high integration,a controller design method based on multi-coordinate-transformation is proposed in this thesis.When designing the port current controller at a certain frequency,each state variable is transformed into a rotating reference frame synchronized with the port frequency.In this way,the dominant component of the port current is transformed into DC terms,and no steady-state error control can be realized by using PI controller.The interference terms are transformed into high-frequency terms,whose impact can be avoided by reasonably designing the controller bandwidth.On this basis,this thesis takes the proposed medium voltage AC SST as an example to show the design process of the control system.The simulation and experimental results show that the proposed control method can effectively realize decoupling control of port current at different frequencies,and can effectively realize the port power control and internal energy balance.