| The rapid development and maturity of medium and high voltage direct current transmission,new energy distributed generation,energy storage equipment and DC load have placed urgent demands on the application of medium and high voltage DC distribution network.As a key link in the application of medium and high voltage DC distribution network,DC Solid State Transformer(DCSST)has received extensive attention from scholars at home and abroad.At present,the topology of DCSST generally uses series and parallel connection of dual active bridge converter(DAB),but the structure can not achieve redundant design and fault isolation,which limits its application range in medium and high voltage DC distribution network.In addition,DAB also has a large power reflow when the high-frequency chain voltage is not matched,and it is difficult to achieve zero voltage switching(ZVS)during light load operation.In order to solve the above problems of DCSST,this paper proposes a cascaded DCSST based on secondary side double winding DAB,which is composed of multiple DC-DC units based on series and parallel connection of half bridge Boost module and secondary side double winding DAB module.In this paper,the main circuit topology of the cascaded DCSST is firstly presented.The working principle and voltage power characteristics of the cascaded DCSST are analyzed based on the half-bridge Boost module and the secondary-side dual-winding DAB module.At the same time,the light-load soft-switching characteristics of the secondary-side dual-winding DAB module are analyzed for the traditional phase shifting and extended phase shifting modulation strategies.Based on these two modulation strategies,the light-load actual soft switching scheme is proposed.Secondly,the voltage and power characteristics of DC-DC unit energy bidirectional transmission are analyzed.According to the power expression,the stability of input voltage equalization and output current sharing control when the DC-DC unit inductance and capacitance parameters are inconsistent is analyzed,and the input voltage sharing and output current sharing control strategy based on the light load actual soft switching scheme is proposed.Furthermore,in order to ensure the reliable operation of the cascaded DCSST under different operating conditions,the fault type and fault characteristics of the cascaded DCSST are analyzed,and the isolation protection schemes under external and internal fault types are given respectively,and the startup process and fault-tolerant scheme of the system are analyzed and designed.Finally,the design process of the magnetic components is given according to the working principle and characteristics of the cascaded DCSST.A 1.2kW experimental prototype was designed to verify the feasibility and superiority of the working principle of the cascaded DCSST,the light-load soft-switching characteristics and the input voltage sharing and output current sharing control strategy. |
