Small-Signal Modeling of Three-Level Boost Rectifier and System Design for Medium-Voltage Solid-State Transformer

This paper presents small-signal modeling of three-level boost (TLB) converter for active front-end power-factor-correction in medium-voltage (MV) solid state transformer (SST). The analysis has been conducted for continuous conduction mode (CCM) in two different voltage modes of TLB to derive system dynamics and to confirm that transfer functions in the voltage modes are identical to one another. Based on the derived models, this paper introduces MV-SST system design guidelines including controller and input filter, considering limited selection width of off-the-shelf high-voltage-rating capacitors and nontrivial source impedances. Digital feedback controller and input filters are designed based on the derivations and implemented to a 4-module-stacked SST system prototype of which TLB plays a role of active front-end PFC. In order to verify the effectiveness of small-signal analysis and system design, experimental verifications of the prototype have been conducted for single phase 3.8-kV rms input voltage to 400-V de bus voltage conversion and 15-kW full load condition in the existence of 65-mH source impedance.