Control Strategy Design and Parameter Selection for Suppressing Circulating Current Among SSTs in Parallel

The circulating current among the paralleled solid state transformers (SSTs) originated by the line parameter and mismatched output impedance comes into appearance in the islanded operation of the novel microgrid, the future renewable electric energy delivery and management system. In this regard, the aggregated control strategy of both virtual inductance and external inductance is proposed in this paper to suppress circulating current. The "matching concept" regarding the parameter selection of virtual and external inductance is proposed to ensure the reliable operation of the system through constructing the complete small-signal model of SST. On the basis of aggregated control strategy and small-signal model, the adaptive virtual impedance control strategy is finally proposed by introducing the feedback of circulating current into the conventional virtual impedance control. The virtual impedance is adjusted adaptively according to the judgment algorithm to regulate the equivalent output impedance of SST. With this modification, the adjustment ability of virtual impedance control against sudden disturbance is improved and the circulating current is further reduced. The simulation results with nonlinear loading are presented to prove that the proposed control strategy, and the parameters determined by the matching concept substantially reduce the circulating current among SSTs and improve the accuracy of load sharing among SSTs.