Research On The Control Strategy Of Cascaded Solid State Transformer Under Unbalanced Load Condition

The solid-state transformer(SST),which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology in 2010,has gained increasing attention in the smart grid and energy internet.When the grid voltage or the three-phase load of the solid state transformer is unbalanced,the negative sequence current is generated at the input stage and it will cause pollution to the power grid.In order to improve the ability of SST to cope with unbalanced conditions,an unbalanced compensation control strategy is proposed by this paper based on reduced order generalized integrator(ROGI).The main work is done as follows:Firstly,the research background and significance of the subject are introduced.The development process and research status of solid state transformer technology are reviewed,and the research on the compensation strategy of SST under unbalanced conditions is summarized.On this basis,a typical cascaded SST topology is analyzed,and the input,isolation and output stage are modeled and analyzed.Subsequently,the mathematical model of cascaded SST under three-phase unbalance is established,and an unbalance compensation control strategy based on ROGI is proposed.By embedding a ROGI regulator in the decoupling control of the input stage,the negative-sequence current at grid side is suppressed.Then combined with the transfer function of ROGI regulator,the feasibility of the proposed compensation control strategy is analyzed.Subsequently,according to the control objectives of each stage,the control scheme of cascaded SST under unbalanced conditions is designed.After that,the unbalance compensation range of the control strategy proposed in this paper is calculated based on the limit constraint of the voltage of HVDC capacitor.Finally,the main circuit parameters of the cascaded SST are designed according to the design principle and the function of the parameters.Based on Matlab/Simulink,a three-module cascaded SST simulation model with three-phase star connection is built.The simulation analysis is carried out under three conditions: ideal grid and symmetric load,asymmetry grid side voltage and load,and grid side voltage dip.The validity of the proposed unbalance compensation control strategy is verified by simulation results,and it is proved that the proposed method has better steady-state and dynamic compensation effects than the traditional sequence decomposition and compensation control strategy based on double dq axes.