Research On The Key Technologies Of Direct Current Transformer Based On Multiple Submodules

With the rapid development of dc transmission and distribution system,in order to realize interconnection of different voltage level dc power systems,and access to distributed new energy power supplies,energy storage systems and dc loads,dc voltages matching and energy conversion will be inevitable.In this thesis,some key technologies of dc transformer based on multiple submodules are studied in order to promote the development and application of dc conversion technology.In view of votages mismatch,the high frequency link circular current of dc transformer based on multiple dual active bridges(abbreviated as DAB)increases rapidly,which leads to the increase of power loss,and even to harm the safe and stable operation of the dc transformer seriously.Two schemes based on switched capacitor access and improved switched capacitor acess are put forward respectively in this thesis.By adding switched capacitor access interface(abbreviated as SCAI)or auxiliary switches in the dc capacitor branch,and by voltage matching control or short circuit control,high frequency link voltages matching can be achieved,and circular current can be obviously reduced.In addition,the faulted submodule can be bypassed or be cut off from dc power system by turning off the SCAI switches or auxiliary switches,when some submodules fault or external short circuit fault occurs,and the reliability of the dc transformer is improved obviously.In order to solve the problem that the series number and parallel number must be equal,the design flexibility is insufficient,and the rated voltage and current of switch is difficult to be fully utilized,a dc transformer scheme based on multiple high frequency links is proposed and its characteristics are studyed in this thesis,and the results reveal that the square wave modulation is the b est and the triangular modulation is the worst modulation strategies in the dc voltage utilization,active power transmission capability,circular current characteristics and soft switching characteristics,and this contribute to the theoretical research a nd pratical application of high frequency link operation of the converter.In the practical application,the submodule control system is supplied by submodule capacitor of the dc transformer based on multiple DABs.The unbalance principle in the pulse locked state is analyzed and a voltage balance control scheme based on minimum voltage tracking balance control and auxiliary voltage control loop is proposed in this thesis,and it can deal with the voltage unbalance of starting process,standy pulse locked state and operation pulse unlocked state.The prototypes of dc transformers based on switched capacitor access and improved switched capacitor access,dc transformer based on mult ple high frequency links,and dc transformer based on multiple DABs are built respectively,and the experiments validate the effectiveness of the proposed schemes and the correctness of the theoretical analysis.