Control of Utility Interfacing Modular High Frequency AC Link Converter Based on Fundamental Current Estimation

This work proposes a cell-to-cell voltage balance and flexible power sharing control strategy for 1-phase utility interfacing 1-phase modular high frequency AC link (HFAC-L) converter. Input series-output parallel (ISOP) configuration having cascaded multilevel front end converter (CMFEC) in the first stage and dual active bridge (DAB) converter in the second stage is the topology of focus in this work. This topology is extensively used in the solid state transformer (SST). Flexible power sharing is accomplished by estimating the high frequency link (HF-link) fundamental current component of each cell. No high bandwidth current sensors are used. Moreover, this scheme also incorporates parametric estimation of each HF-link inductor. This feature makes it robust to gradual ageing related parametric variation. Irrespective of the number of cells, this strategy requires only two low bandwidth current sensors, for grid current and load current measurements. Compared to erstwhile approaches, this flexible power sharing strategy allows for controlled zero power (phase-shedding/plug-out) operation of a cell while still regulating its DC link voltage. The proposed strategy also makes the subsequent phase-addition/plug-in process seamless without the need for pre-charge circuits. This scheme can also be reconfigured to operate in output current regulation mode. These features are validated through experiments performed on a 1.6 kW laboratory prototype.