Energy-based control design for a solid state transformer

This paper addresses the control design for a three-stage solid stage transformer (SST). When two or more independently controlled converter stages are cascaded, which is the case for the three-stage SST, the interaction among the stages can lead to instability. Normally, the input and output impedances of the cascaded stages need to be examined according to the impedance criterion to ensure stability. In this paper, a novel energy-based control design method is proposed for the three-stage SST that enables decoupled control design for the rectifier and DC-DC stages, which avoids the need for the impedance checking and hence significantly simplifies the control design. The energy-based concept also works well with the power control methods commonly used such as those based on the DQ frame or the instantaneous reactive power (IRP) p-q theory, which allows for simple reactive power or power factor control. The design of the various control loops of the SST is described in detail in this paper and validated through simulations.